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Sangean ATS-909 external antenna impedance??
Lostgallifreyan wrote in
: He wrote that. I didn't. Sorry amdx, potential for confusion there... I mean the guy who wrote what you linked to.. |
Sangean ATS-909 external antenna impedance??
On Sat, 02 Jan 2010 17:20:24 -0600, Lostgallifreyan
wrote: I haven't a clue about intermod, yet. One thing at a time. The term Intermod is probably mis-direction if you research it. Basically, if an nearby AM/FM/TV transmitter (and nearby can be on the scale of several miles) happens to excite your antenna; then its developed voltage will overload the frontend (Intermod follows, but the products are not what I am emphasizing here). This overload can be many, many kHz, or MHz from the intended and tuned signal; and yet this frequency remote signal will develop an AGC that drives down gain on your intended signal. This characteristic is VERY common for untuned frontends in modern receivers. It is not often noted for poor antennas (those whips, when they are used for SW), but when a real antenna is attached *BINGO* sensitivity goes down the toilet. By providing a tuned input, the side-signal that would otherwise silently drive AGC is attenuated, and AGC is developed only by the in-band signals. Right now I see at least three contradictions (re ground rods, transformers, and feedlines) with advice from several people, one of which (the guy who wrote the description of the antenna and balanced line I mentioned) is part of a group of hams who is turned to for advice by the others. No guarantee of correctness, perhaps, but if I keep on being told I'm wrong when my stuff is coming as directly as I can get it from others with experience, then as far as I'm concerned I'll do what I think best and get out of the crossfire. A reasonable posture. Specifically, many times I've seen advice that service grounds are not adequate because of common mode noise and local currents, hence the ground rod you vehemently negate. I don't negate its use, I say that it is NOT RF ground. If you tie this ground rod to the service ground, then that wire will probably act more in your behalf than either "ground." There is a world of difference between safety grounds (what those rod-thingies are) and RF grounds (which often don't go into ground at all). Ground is a long and rich story that has been celebrated in this group for years. It deserves respect and attention well beyond these few words. I can ground to service ground at near end but if the receiver is on batteries, not connected to anything except a transformer coupling RF from the antenna, then the ground only needs to be at the antenna end, according to advice I've seen in several places. To your specific arrangement - quite true. However, many who have claimed to have made every precaution then connect their receiver to an amplifier, computer, what-you-might-call-it and a new path to ground winds its way through interesting environments that are RF rich. Even if I do ground to a water pipe or other local ground, all advice I see until now insists on having a ground rod as close to the antenna as possible, no matter what else I do, yet now you urge against this. I urge against mixing grounds. Such things arrive by the most benign and seemingly inconsequential actions. I will stop asking for advice if all I see is vigorous contradiction between people who claim knowledge I do not have. Diverting that disagreement to one with me doesn't alter this, I did not originate the info behind the choices I am considering. Even if all the various contributors come here and duke it out between them it appears I'll be none the wiser. Attention to one detail at a time helps, but a lot of this arrived through responding to the query for antenna port Z. Those adjuncts that massage input/output Z also fold in the discussion of ground. Convention has it that you start a new thread for each side-topic that drives you into conniptions. Asking about the facts and foibles of ground would be a good start on a new thread - especially when Art's wet-dreams descend into discussion of particle duality self annihilation driving all participation away from antenna design. For instance "Why are ground rods considered insufficient for RF application?" I am content to respond to either discussion. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
On Sat, 02 Jan 2010 17:33:03 -0600, Lostgallifreyan
wrote: To save time: "http://www.kongsfjord.no/dl/Antennas/The%20Best%20Small%20Antennas%20For%20M W,%20LW,%20And%20SW%20rev%202.pdf An example of invention driving the discussion rather than the need being satisfied. Simply put, there is absolutely no reason to use a "balanced" line. It is window dressing for the circuit which IS balanced (and balanced for no apparent reason for this unbalanced source). Metaphorically, it is like adding a clutch to an automatic shift. Yes, you can do it. It might appear to be elegant. It will certainly work. But why? Try asking why the trappings of this novel design don't bring some solution in a new thread. You might stumble at offering the problem it pretends to solve. (I will anticipate it has something to do with noise, THIS will certainly raise a lot of catcalls.) 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
"Lostgallifreyan" wrote in message . .. Lostgallifreyan wrote in : He wrote that. I didn't. Sorry amdx, potential for confusion there... I mean the guy who wrote what you linked to... The Sangean ATS-909 along with similar radios are designed to resolve signals from the whip antenna or in built ferrite antenna. Attaching 8 to 10 feet of wire to the whip will bring in more stations but depending on location may well pick up so much extra signal as to cause intermodulation and AGC limiting preventing reception of the weak signals you want to receive. As stated earlier, the front end of these receivers is wide open and the front end is exposed to the complete spectrum of transmissions received by the antenna. There is nothing inherently wrong with the receiving system you have decided upon but it will undoubtably overload your receiver with signals and you will be puzzled as to why the reception seems poorer with more noise pickup rather than less. As Richard has stated you need some form of preselection to filter out the unwanted signals before they get into your radio. Basically this is a tuneable filter which only allows through a single band of frequencies at a time. The following site explains the essentials. http://www.dxing.com/tnotes/tnote07.pdf You can buy commercial preselectors but they will probably cost as much as your radio. As they are generally passive devices built from a set of switched coils and a variable capacitor they last forever and old ones do come up from time to time at junk sales and the like. It is possible to make a simple filter to cover just one or two bands that interest you. By all means, try the external antenna system but be prepared to buy a 'better' receiver with front end band pass filters or a preselector. You can have too much of a good thing when it comes to receiving antennas. A bigger receiving antenna won't bring in signals from further away. If they are there, the receiver is probably sufficiently sensitive to pick them up already. What the bigger antenna will do is raise the level of all the signals it is picking up and feeding into the receiver and that includes noise, and other unwanted stations. That is why you need additional filtering to cut down the unwanted signals and allow your receiver a fair chance of demodulating what you actually want to hear. Regards Mike G0ULI |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Sat, 02 Jan 2010 17:20:24 -0600, Lostgallifreyan wrote: I haven't a clue about intermod, yet. One thing at a time. The term Intermod is probably mis-direction if you research it. Basically, if an nearby AM/FM/TV transmitter (and nearby can be on the scale of several miles) happens to excite your antenna; then its developed voltage will overload the frontend (Intermod follows, but the products are not what I am emphasizing here). This overload can be many, many kHz, or MHz from the intended and tuned signal; and yet this frequency remote signal will develop an AGC that drives down gain on your intended signal. This characteristic is VERY common for untuned frontends in modern receivers. It is not often noted for poor antennas (those whips, when they are used for SW), but when a real antenna is attached *BINGO* sensitivity goes down the toilet. By providing a tuned input, the side-signal that would otherwise silently drive AGC is attenuated, and AGC is developed only by the in-band signals. Ok, this is cool, I understand that, and I also see that it doesn't really concern intermodulation products as the initial problem is a bigger one if it occurs. Can't help wondering why a receiver doesn't do some tuning before the AGC for exactly this reason, but never mind... Right now I see at least three contradictions (re ground rods, transformers, and feedlines) with advice from several people, one of which (the guy who wrote the description of the antenna and balanced line I mentioned) is part of a group of hams who is turned to for advice by the others. No guarantee of correctness, perhaps, but if I keep on being told I'm wrong when my stuff is coming as directly as I can get it from others with experience, then as far as I'm concerned I'll do what I think best and get out of the crossfire. A reasonable posture. Specifically, many times I've seen advice that service grounds are not adequate because of common mode noise and local currents, hence the ground rod you vehemently negate. I don't negate its use, I say that it is NOT RF ground. If you tie this ground rod to the service ground, then that wire will probably act more in your behalf than either "ground." There is a world of difference between safety grounds (what those rod-thingies are) and RF grounds (which often don't go into ground at all). Ground is a long and rich story that has been celebrated in this group for years. It deserves respect and attention well beyond these few words. True, I don't doubt that for an instant, but it's also a question of what is practical, and what is recomended by most people I've read words from at times during the last 30 years or more. While I know that CB'ers would just stick a magmount on their car's steel rooftop as often as not, and have read of other schemes that place some small horizontal plate below the antenna, there's a lot of scope between that and a rod driven into salty ocean shoreline. Most people I ever came across asserted the importance of a ground rod local to the antenna to couple with the local water table which is as close as most ever get to the ocean unless they really like getting their feet wet while they sit around at home. The proximity is as close to the point where they want to pick up RF as they're going to get, and means less noise from buildings full of electrical stuff picked up on metal between antenna and whatever other ground might be provided elsewhere. This has been the ONE common factor in pretty much everything I've seen on land-based AM reception. Anything that directly appears to negate that advice makes it hard to know what to trust, and certainly needs to be clearly explained. I can ground to service ground at near end but if the receiver is on batteries, not connected to anything except a transformer coupling RF from the antenna, then the ground only needs to be at the antenna end, according to advice I've seen in several places. To your specific arrangement - quite true. However, many who have claimed to have made every precaution then connect their receiver to an amplifier, computer, what-you-might-call-it and a new path to ground winds its way through interesting environments that are RF rich. I agree. The moment I try to connect to a system that includes a computer, mixer, multiple supply grounds, as mine does, I'll be using a local service ground and improving it the same as I would for audio, though it's currently ok for that, at least. It already uses a star grounding system where possible, as recommended by audio studio designers and others. There's actually a supply ground rod outside the front door too, which presumably helps more than the original wiring 15 years ago which didn't have that. (But note below, where I mention isolation). Even if I do ground to a water pipe or other local ground, all advice I see until now insists on having a ground rod as close to the antenna as possible, no matter what else I do, yet now you urge against this. I urge against mixing grounds. Such things arrive by the most benign and seemingly inconsequential actions. Hence the star network I mentioned, advised for audio setups.. It's kind of why I wonder about what many suggest, grounding a coax at both ends, and even in the middle if you want, and certainly to bury it. More importantly it's why the Dallas Lankford design appeals to me. Isolation baluns that transfer energy rather than use direct contact coupling look like a good way to avoid the ground problems while also avoiding local noise pickup because the twin cable will have good common mode rejection as it passes into the electrically noisy bulding. (Though I can't help wondering if Dallas Lankford also tried balanced microphone cable with a screen grounded at one end, just to see what happened) Such methods have long been used in audio; is RF below 30 MHz really so different in this case? So long as that line doesn't have dire resonances of it's own, isn't attenuation the only big risk? Dallas Lankford certainly thinks it works after working with it for at least 2 years. He says that if you do it as described it will be low noise. (As opposed to 'reducing'). I don't think he's claiming any means of reduction, just saying it's lower relative to inherently noisier systems, if wired as decribed. Based on what I know, the claim seems good. I will stop asking for advice if all I see is vigorous contradiction between people who claim knowledge I do not have. Diverting that disagreement to one with me doesn't alter this, I did not originate the info behind the choices I am considering. Even if all the various contributors come here and duke it out between them it appears I'll be none the wiser. Attention to one detail at a time helps, but a lot of this arrived through responding to the query for antenna port Z. Those adjuncts that massage input/output Z also fold in the discussion of ground. Agreed. But this is why instead of asking more questions whose answers I am probably not prepared for, I described the simplest and apparently best scheme I'd learned of so people see it whole and work from there... Convention has it that you start a new thread for each side-topic that drives you into conniptions. Ah. :) Well, I thought that's exactly what would annoy people most. If something directly arises from discussion in a thread, most people tend to keep it there. I already do start a new one if I'm certain the issue is different, and if I'm originating it. Asking about the facts and foibles of ground would be a good start on a new thread - especially when Art's wet-dreams descend into discussion of particle duality self annihilation driving all participation away from antenna design. For instance "Why are ground rods considered insufficient for RF application?" I am content to respond to either discussion. 73's Richard Clark, KB7QHC Well, sure, if I am asking a direct technical or practical question. But while I'm still slightly reeling from what appears to be a dissention with what otherwise appears to be good advice, I like to keep the discussion in one place, otherwise confusion reigns and spreads to many threads. Trust me, that might annoy people. :) At least in this thread it might be useful to anyone who has that radio. |
Sangean ATS-909 external antenna impedance??
"Mike Kaliski" wrote in
: There is nothing inherently wrong with the receiving system you have decided upon but it will undoubtably overload your receiver with signals and you will be puzzled as to why the reception seems poorer with more noise pickup rather than less. One reason I chose it is that it isn't trying that hard for extreme signal capture. It appeared to be small, easy to use where I have limited space, and include a transformer that I have read in numerous places partially solves one of the main reasons for strongly differing signal strength with frequency. As Richard has stated you need some form of preselection to filter out the unwanted signals before they get into your radio. Basically this is a tuneable filter which only allows through a single band of frequencies at a time. The following site explains the essentials. http://www.dxing.com/tnotes/tnote07.pdf Looks good, I'm not keen on lots of widgets as it happens, fewer and better widgets that co-operate well works better for me. You can buy commercial preselectors but they will probably cost as much as your radio. As they are generally passive devices built from a set of switched coils and a variable capacitor they last forever and old ones do come up from time to time at junk sales and the like. It is possible to make a simple filter to cover just one or two bands that interest you. By all means, try the external antenna system but be prepared to buy a 'better' receiver with front end band pass filters or a preselector. The pre-selection thing isn't a problem, I can see why that helps, and did so much earlier than now. The point that disconcerts me strongly is what appears to be significant difference of opinion between experts, especially when it applies to things as well established as ground rods. Again, this is why I won;t just ask questions. Context is clearly everything, so instead I describe the whole scheme I'm considering. Ultimately it's quicker that way. |
Sangean ATS-909 external antenna impedance??
On Sun, 03 Jan 2010 05:26:32 -0600, Lostgallifreyan
wrote: Most people I ever came across asserted the importance of a ground rod local to the antenna to couple with the local water table which is as close as most ever get to the ocean unless they really like getting their feet wet while they sit around at home. In fact, this almost always NEVER happens. Skin effect defines the layer depth of RF in ground. An 8 foot rod is like a splinter when you are trying to harpoon a Blue Whale. Ground rod engineering has been discussed in this forum to great depth (pun intended, or not). The rods are as well understood as water witching forks. In the HF region, single or several rods have no practical RF use whatever. Above HF, absolutely no one gives them any thought. The proximity is as close to the point where they want to pick up RF as they're going to get, and means less noise from buildings full of electrical stuff picked up on metal between antenna and whatever other ground might be provided elsewhere. This has been the ONE common factor in pretty much everything I've seen on land-based AM reception. Anything that directly appears to negate that advice makes it hard to know what to trust, and certainly needs to be clearly explained. When you can't do anything else that is effective, a ground rod seems like more than enough. It is certainly a need for safety's sake, especially when your vertical could be a lightning magnet. Consider that same antenna: is it directly GROUNDED? Or is it floating? If ground is a panacea, I bet most of your advisors immediately isolate their antenna from it. One has to wonder about faith.... Either design works with equal efficiency. You simply need a coupling system to the grounded antenna design. One method is using a folded monopole. Other methods abound (which are often confined to yagi driven element discussion, but are eminently applicable here). The moment I try to connect to a system that includes a computer, mixer, multiple supply grounds, as mine does, I'll be using a local service ground and improving it the same as I would for audio, though it's currently ok for that, at least. It already uses a star grounding system where possible, as recommended by audio studio designers and others. There's actually a supply ground rod outside the front door too, which presumably helps more than the original wiring 15 years ago which didn't have that. (But note below, where I mention isolation). The Star system is great for exactly as you understand and describe it, but for antenna applications that remote ground could act as a suicide adapter if it does not have its own path to the service ground. Yes, this violates the star, but when path lengths include a lot of resistance and leakage current, voltages can become considerable when you supply a new avenue through your home. This is the story of the classic ground loop. Hence the star network I mentioned, advised for audio setups.. It's kind of why I wonder about what many suggest, grounding a coax at both ends, and even in the middle if you want, and certainly to bury it. More importantly it's why the Dallas Lankford design appeals to me. Isolation baluns that transfer energy rather than use direct contact coupling look like a good way to avoid the ground problems while also avoiding local noise pickup because the twin cable will have good common mode rejection as it passes into the electrically noisy bulding. (Though I can't help wondering if Dallas Lankford also tried balanced microphone cable with a screen grounded at one end, just to see what happened) Such methods have long been used in audio; is RF below 30 MHz really so different in this case? So long as that line doesn't have dire resonances of it's own, isn't attenuation the only big risk? Dallas Lankford certainly thinks it works after working with it for at least 2 years. He says that if you do it as described it will be low noise. (As opposed to 'reducing'). I don't think he's claiming any means of reduction, just saying it's lower relative to inherently noisier systems, if wired as decribed. Based on what I know, the claim seems good. I'm not familiar with Dallas Lankford, but isolation and shielding techniques are topics I have visited professionally throughout the years and they are not simple. Without a concommitant discussion of the noise source, one wrong ground selection can wipe out all pursued benefits. Let's revisit one of your statements above: balanced microphone cable with a screen grounded at one end Which end? Any choice stands an equal chance of being the wrong choice. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
On Sun, 03 Jan 2010 08:52:46 -0800, Richard Clark
wrote: I'm not familiar with Dallas Lankford I have since visited your suggested page to casually view his works. Interesting set of circuits too (although, some of the phasing systems have been superceded with shift registers - I used to use bucket-brigade chips). I was especially touched to see wide coverage of the R390A. It was the subject of my first class that I taught in the Navy (along with the Collins URC-32). Cadillac equipment. I note in his discussion of stabilizing the BFO, he uses a Rubidium standard for comparison. I calibrated quite a few of those Rubidium standards too with my Cesium Beam whenever a Boomer came along side. An URQ-12 would have worked as easily, but this discussion no doubt exceeds the capacity of your wallet (the Navy provided such a candy store for my Metrology Lab). 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Sun, 03 Jan 2010 05:26:32 -0600, Lostgallifreyan wrote: Most people I ever came across asserted the importance of a ground rod local to the antenna to couple with the local water table which is as close as most ever get to the ocean unless they really like getting their feet wet while they sit around at home. In fact, this almost always NEVER happens. Skin effect defines the layer depth of RF in ground. An 8 foot rod is like a splinter when you are trying to harpoon a Blue Whale. Ground rod engineering has been discussed in this forum to great depth (pun intended, or not). The rods are as well understood as water witching forks. In the HF region, single or several rods have no practical RF use whatever. Above HF, absolutely no one gives them any thought. True. I'm interested in using a single antenna for all of LW up to around 30 MHz as a simple starting point though, so Dallas Lankford's scheme seems to fit the bill. The proximity is as close to the point where they want to pick up RF as they're going to get, and means less noise from buildings full of electrical stuff picked up on metal between antenna and whatever other ground might be provided elsewhere. This has been the ONE common factor in pretty much everything I've seen on land-based AM reception. Anything that directly appears to negate that advice makes it hard to know what to trust, and certainly needs to be clearly explained. When you can't do anything else that is effective, a ground rod seems like more than enough. It is certainly a need for safety's sake, especially when your vertical could be a lightning magnet. Consider that same antenna: is it directly GROUNDED? Or is it floating? If ground is a panacea, I bet most of your advisors immediately isolate their antenna from it. One has to wonder about faith.... Safety is important, even though lightning strike isn't that big a risk here. Actually it's risen because nearly all large trees have been removed in the last two years, and I'd be held reponsible for any damage caused that way. If you look at that PDF you'll see the 15' whip antenna is directly connected to ground through 80 turns of wire on a ferrite toroid. I might add a spark gap in parallel as that wire is not a high current path. So long as it is much more likely to go to ground rather than along the line in to the house, I'll have done what I'm supposed to do. The trouble would only exist (other than unpreventable natural excesses) if it were evident that I had not done this. Either design works with equal efficiency. You simply need a coupling system to the grounded antenna design. One method is using a folded monopole. Other methods abound (which are often confined to yagi driven element discussion, but are eminently applicable here). That PDF shows the coupling in this case. It's similar to other ideas recommended for similar small SW listening setups. Once I have all I need to try it I'm just as happy to try experimenting to see what happens as to try any plan. There are some limits though, the location is too built up to expect much from anything intended to be directional. I'm just intending to look around, not looking out there for something specific. The moment I try to connect to a system that includes a computer, mixer, multiple supply grounds, as mine does, I'll be using a local service ground and improving it the same as I would for audio, though it's currently ok for that, at least. It already uses a star grounding system where possible, as recommended by audio studio designers and others. There's actually a supply ground rod outside the front door too, which presumably helps more than the original wiring 15 years ago which didn't have that. (But note below, where I mention isolation). The Star system is great for exactly as you understand and describe it, but for antenna applications that remote ground could act as a suicide adapter if it does not have its own path to the service ground. Yes, this violates the star, but when path lengths include a lot of resistance and leakage current, voltages can become considerable when you supply a new avenue through your home. This is the story of the classic ground loop. Well, a ground rod isn't going to cost much, and making and breaking connections to it is one of the easiest and cheapest things I'll be able to do, so I'll test that empirically when I'm ready. I won't try to predict it now. Whenever I find some new ground noise problem in anything I do here, I usually manage to isolate it and solve it acceptably within an hour or less, so I'll trust my chances. Usually the purpose hasn't been for RF, but quite often the sources did involve RF too so my instincts might help me more than my knowledge. Hence the star network I mentioned, advised for audio setups.. It's kind of why I wonder about what many suggest, grounding a coax at both ends, and even in the middle if you want, and certainly to bury it. More importantly it's why the Dallas Lankford design appeals to me. Isolation baluns that transfer energy rather than use direct contact coupling look like a good way to avoid the ground problems while also avoiding local noise pickup because the twin cable will have good common mode rejection as it passes into the electrically noisy bulding. (Though I can't help wondering if Dallas Lankford also tried balanced microphone cable with a screen grounded at one end, just to see what happened) Such methods have long been used in audio; is RF below 30 MHz really so different in this case? So long as that line doesn't have dire resonances of it's own, isn't attenuation the only big risk? Dallas Lankford certainly thinks it works after working with it for at least 2 years. He says that if you do it as described it will be low noise. (As opposed to 'reducing'). I don't think he's claiming any means of reduction, just saying it's lower relative to inherently noisier systems, if wired as decribed. Based on what I know, the claim seems good. I'm not familiar with Dallas Lankford, but isolation and shielding techniques are topics I have visited professionally throughout the years and they are not simple. Without a concommitant discussion of the noise source, one wrong ground selection can wipe out all pursued benefits. Let's revisit one of your statements above: balanced microphone cable with a screen grounded at one end Which end? Any choice stands an equal chance of being the wrong choice. Well, I did think of that. :) And I didn't state it because I didn't know for sure. As I imagine that local RF couplings from various digital devices might place small currents on the local ground, I imagine that grounding a shield at the remote ground makes sense. Doesn't matter to me though. It's far easier and faster to experiment than to try to predict because there are only two ways to try. Dallas Lankford directly states that no shield is even required, and I doubt he'd have said that if he couldn't demonstrate it, and as that line is a two-wire loop that has no direct contact with anything, it should reject any common mode noise that hits it. Even in audio this matters because the same method is used to reject RF pickup on audio lines. I think some people persist in baluns instead of op-amp common mode rejection specs for this reason, despite the chances of modest distortion in audio bands from the transformers used. Not entirely relevant but it illustrates how people can find themselves choosing between two less-than-ideal circumstances for best effect. I understand that noise context matters for a real attempt to plan for it, but that's far more difficult that presenting the basic antenna scheme. |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Sun, 03 Jan 2010 08:52:46 -0800, Richard Clark wrote: I'm not familiar with Dallas Lankford I have since visited your suggested page to casually view his works. Interesting set of circuits too (although, some of the phasing systems have been superceded with shift registers - I used to use bucket-brigade chips). I was especially touched to see wide coverage of the R390A. It was the subject of my first class that I taught in the Navy (along with the Collins URC-32). Cadillac equipment. I note in his discussion of stabilizing the BFO, he uses a Rubidium standard for comparison. I calibrated quite a few of those Rubidium standards too with my Cesium Beam whenever a Boomer came along side. An URQ-12 would have worked as easily, but this discussion no doubt exceeds the capacity of your wallet (the Navy provided such a candy store for my Metrology Lab). 73's Richard Clark, KB7QHC Not just my wallet. :) This is cool though, I have encountered both bucket brigade IC's and shift registers. Offtopic question: Did the bucket brigade lead directly to the switched capacitor filter? I ask because they seem related, and the BB seems to have become obsolete, but the SCF, anything but. |
Sangean ATS-909 external antenna impedance??
On Sun, 03 Jan 2010 13:10:17 -0600, Lostgallifreyan
wrote: Did the bucket brigade lead directly to the switched capacitor filter? Same thing if I read you correctly. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Lostgallifreyan wrote in
: It was the subject of my first class that I taught in the Navy Curious... You might have experiences that give useful insight into something I'm discussing in another group, some off-the-wall topic I entered into during a quiet new-year's moment in alt.lasers... I can accept that you might not want to get into that at all once you see it but if you do I welcome the input because as I state there, one issue is to find helpful signals rather than the noise such discussions usually provoke. But it might be a lengthy misdirection if you're busy so I won't push the point. But I will say now that my dad was a naval officer when he was younger and he SAW that thing I described right there with me, and he had no clue either. I just keep thinking that some people in the navy might be better able to explain it than most, especially if they have a strong science or engineering background. |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Sun, 03 Jan 2010 13:10:17 -0600, Lostgallifreyan wrote: Did the bucket brigade lead directly to the switched capacitor filter? Same thing if I read you correctly. 73's Richard Clark, KB7QHC Interesting. I guess fabrication accuracy and timing accuracy improvements led to its sharp cutoff and improved ability. I'm using one in some project I have on hold, a way to get pitched sounds to control MIDI signals for musical instrument control. I'll stop that line of thought there though, I'm not ready to go into that project for a while. |
Sangean ATS-909 external antenna impedance??
On Sun, 03 Jan 2010 13:07:26 -0600, Lostgallifreyan
wrote: If you look at that PDF you'll see the 15' whip antenna is directly connected to ground through 80 turns of wire on a ferrite toroid. I might add a spark gap in parallel as that wire is not a high current path. So long as it is much more likely to go to ground rather than along the line in to the house, I'll have done what I'm supposed to do. The trouble would only exist (other than unpreventable natural excesses) if it were evident that I had not done this. Yes, it is a grounded design. The folded monopole is simpler, however and you can deal with matching identically. 80 turns of wire on a ferrite toroid is going to test the limits of self-resonance, poor Q, and efficiency across so large a span of frequency. Well, a ground rod isn't going to cost much, and making and breaking connections to it is one of the easiest and cheapest things I'll be able to do, so I'll test that empirically when I'm ready. I won't try to predict it now. Whenever I find some new ground noise problem in anything I do here, I usually manage to isolate it and solve it acceptably within an hour or less, so I'll trust my chances. Usually the purpose hasn't been for RF, but quite often the sources did involve RF too so my instincts might help me more than my knowledge. Sounds like a lot of faith and work that will eventually require more investment in faith and work. A shallow buried radial system would puncture these superstitions. Let's revisit one of your statements above: balanced microphone cable with a screen grounded at one end Which end? Any choice stands an equal chance of being the wrong choice. Well, I did think of that. :) And I didn't state it because I didn't know for sure. That's why I tossed that hand grenade into the mix. As I imagine that local RF couplings from various digital devices might place small currents on the local ground, I imagine that grounding a shield at the remote ground makes sense. It does, but that isn't the complete solution if you don't choke the feedline. Again, ground is not found in the rod you drive into the earth (which, by the way, will take years to "cure" to the ground resistance you hope to achieve). Doesn't matter to me though. It's far easier and faster to experiment than to try to predict because there are only two ways to try. This is about experience. You will find (and I have found) damn little reference to grounding by connection to the earth. It has taken me years to accumulate these rare references. They have been topics of discussion here (use google to search the archives). Dallas Lankford directly states that no shield is even required, and I doubt he'd have said that if he couldn't demonstrate it, and as that line is a two-wire loop that has no direct contact with anything, it should reject any common mode noise that hits it. Many people make direct statements (hard not to in this environment that relies on textual postings). You need to find a better source of study material as it relates to Common Mode. Twin line suffers it equally. Again, all such discussion arrives through where the source and load are, not in the line between. Even in audio this matters because the same method is used to reject RF pickup on audio lines. I think some people persist in baluns instead of op-amp common mode rejection specs for this reason, despite the chances of modest distortion in audio bands from the transformers used. Not entirely relevant but it illustrates how people can find themselves choosing between two less-than-ideal circumstances for best effect. You are confusing topics here. BalUns and what are properly chokes are not always the same, although their discussion is often co-mingled to considerable misunderstanding. BalUns are NOT transformers as you might imagine from the point of view of AF. It is regrettable that BalUns are called transformers, as their full nomenclature is Transmission Line Transformer - meaning the transform of Z by transmission lines that have their ends isolated through choking action. Lest that sound too obtuse: The best BalUns do not operate through magnetic flux linkage. You are not in Kansas anymore. I understand that noise context matters for a real attempt to plan for it, but that's far more difficult that presenting the basic antenna scheme. Hence the novelty of individual threads. Noise arrives in the same manner as RF - it is indistinguishable until you put on your headphones. Noise is what arrives between your ears. In regards to this last epithet, I noticed that Lankford wrote a piece about quad detectors. I first designed one 40 years ago and the critical component missing in Lankford's discussion (and probably from many such discussions surrounding this method of detection) is that the two channel output of a phase quadrature detector is meant to drive STEREO headphones so that the last step of detection is found in the brain's capacity to differentiate noise from signal. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
On Sun, 03 Jan 2010 13:25:30 -0600, Lostgallifreyan
wrote: Lostgallifreyan wrote in : It was the subject of my first class that I taught in the Navy Curious... You might have experiences that give useful insight into something I'm discussing in another group, some off-the-wall topic I entered into during a quiet new-year's moment in alt.lasers... I can accept that you might not want to get into that at all once you see it but if you do I welcome the input because as I state there, one issue is to find helpful signals rather than the noise such discussions usually provoke. But it might be a lengthy misdirection if you're busy so I won't push the point. But I will say now that my dad was a naval officer when he was younger and he SAW that thing I described right there with me, and he had no clue either. I just keep thinking that some people in the navy might be better able to explain it than most, especially if they have a strong science or engineering background. Well, I'm not sure what you are trying to bring up here. As far as I'm concerned, any topic is open for discussion. If others fume and fulminate about this being an antenna group, I can easily turn the discussion into one that is antenna specific. What did your Dad see? (We used to joke that after the Pueblo incident they put Marine guards outside of the radio shack for security. If the ship was in danger of being boarded they were to step into the shack and shoot the operators. Then I went into the submarine navy, aboard a tender, the USS Holland, AS-32. We had an escort sub that followed us at sea - to sink us if the Ruskies got too close. Ah, the humor of those days.) 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Sun, 03 Jan 2010 13:07:26 -0600, Lostgallifreyan wrote: If you look at that PDF you'll see the 15' whip antenna is directly connected to ground through 80 turns of wire on a ferrite toroid. I might add a spark gap in parallel as that wire is not a high current path. So long as it is much more likely to go to ground rather than along the line in to the house, I'll have done what I'm supposed to do. The trouble would only exist (other than unpreventable natural excesses) if it were evident that I had not done this. Yes, it is a grounded design. The folded monopole is simpler, however and you can deal with matching identically. 80 turns of wire on a ferrite toroid is going to test the limits of self-resonance, poor Q, and efficiency across so large a span of frequency. Ok, I'll definitely extend reaserch to cover folded monopoles. (Another curious term given that there has to be some kind of polarity there, but never mind..) I'll be making by own transformers and housings so I don't mind plenty of messing around with those. Sounds like a lot of faith and work that will eventually require more investment in faith and work. A shallow buried radial system would puncture these superstitions. The yard out there is partially covered by most of the trunk of a large felled lime tree, along with lots of branch logs, as part of an effort to retain nutrients on land that is part of a limited strategy to preserve wildlife in an urban district. Burying anything out there is harder work than sinking a ground rod. :) I think I'm going to have to take my chances. Let's revisit one of your statements above: balanced microphone cable with a screen grounded at one end Which end? Any choice stands an equal chance of being the wrong choice. Well, I did think of that. :) And I didn't state it because I didn't know for sure. That's why I tossed that hand grenade into the mix. As I imagine that local RF couplings from various digital devices might place small currents on the local ground, I imagine that grounding a shield at the remote ground makes sense. It does, but that isn't the complete solution if you don't choke the feedline. Again, ground is not found in the rod you drive into the earth (which, by the way, will take years to "cure" to the ground resistance you hope to achieve). Even if I add a little salt water or dilute acid to accelerate that? This is something I've been considering.. By choking the feedline, do you mean placing ferrite slugs round it like those used on VDU cables? That's something else that will be cheap and easy to test empirically. Doesn't matter to me though. It's far easier and faster to experiment than to try to predict because there are only two ways to try. This is about experience. You will find (and I have found) damn little reference to grounding by connection to the earth. It has taken me years to accumulate these rare references. They have been topics of discussion here (use google to search the archives). I'll do that. I gravited to Usenet for exactly this reason, I already collected a few rare sources of info this way in other matters. A lot of the best stuff is in web archives of usenet posts. (Although for audio I found a single magazine called Sound On Sound to deal with it efficiently. Not RF, but it's not exactly far removed technology). I think a lot of good references just never reached the net, except via postings by people who have read them. or if they have, they're in university archives I can't reach anyway. Dallas Lankford directly states that no shield is even required, and I doubt he'd have said that if he couldn't demonstrate it, and as that line is a two-wire loop that has no direct contact with anything, it should reject any common mode noise that hits it. Many people make direct statements (hard not to in this environment that relies on textual postings). You need to find a better source of study material as it relates to Common Mode. Twin line suffers it equally. Again, all such discussion arrives through where the source and load are, not in the line between. There looks like one difference. Any signal hitting a coax screen if used in this scheme will have a corresponding return current in the core wire, but if there is any frequency dependent effect based on the nature of the coax braid's dimensions or in the difference between that and those of the core wire, then complexites beyond my ken might result (and maybe just as likely be insignificant). The equality of nature in each half of a twin wire appeals to me, so long as it actually works. Should be cheap and easy to test that one... Even in audio this matters because the same method is used to reject RF pickup on audio lines. I think some people persist in baluns instead of op-amp common mode rejection specs for this reason, despite the chances of modest distortion in audio bands from the transformers used. Not entirely relevant but it illustrates how people can find themselves choosing between two less-than-ideal circumstances for best effect. You are confusing topics here. BalUns and what are properly chokes are not always the same, although their discussion is often co-mingled to considerable misunderstanding. BalUns are NOT transformers as you might imagine from the point of view of AF. It is regrettable that BalUns are called transformers, as their full nomenclature is Transmission Line Transformer - meaning the transform of Z by transmission lines that have their ends isolated through choking action. Very likely I am missing plenty. Thing is, I see an unbalanced antenna-to- ground on one winding and an isolated and balanced line that loops back on itself carrying induced current from the other. It seems to me that balun and transformer are terms that apply equally there. In other words, it appears that the distinctions are neatly avoided while I am directed to exploit the common nature of the thing being used. Once I see if it works my interest will grow and I'll explore it further. Lest that sound too obtuse: The best BalUns do not operate through magnetic flux linkage. You are not in Kansas anymore. I understand that noise context matters for a real attempt to plan for it, but that's far more difficult that presenting the basic antenna scheme. Hence the novelty of individual threads. Noise arrives in the same manner as RF - it is indistinguishable until you put on your headphones. Noise is what arrives between your ears. That's actually the single best argument for empirical testing. :) Something else that audio work has told me many times... In regards to this last epithet, I noticed that Lankford wrote a piece about quad detectors. I first designed one 40 years ago and the critical component missing in Lankford's discussion (and probably from many such discussions surrounding this method of detection) is that the two channel output of a phase quadrature detector is meant to drive STEREO headphones so that the last step of detection is found in the brain's capacity to differentiate noise from signal. That's interesting. I've considered exploiting that idea before now. Not in much detail, just in principle because I've heard sounds that needed this to resolve them. |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Sun, 03 Jan 2010 13:25:30 -0600, Lostgallifreyan wrote: Lostgallifreyan wrote in 5: It was the subject of my first class that I taught in the Navy Curious... You might have experiences that give useful insight into something I'm discussing in another group, some off-the-wall topic I entered into during a quiet new-year's moment in alt.lasers... I can accept that you might not want to get into that at all once you see it but if you do I welcome the input because as I state there, one issue is to find helpful signals rather than the noise such discussions usually provoke. But it might be a lengthy misdirection if you're busy so I won't push the point. But I will say now that my dad was a naval officer when he was younger and he SAW that thing I described right there with me, and he had no clue either. I just keep thinking that some people in the navy might be better able to explain it than most, especially if they have a strong science or engineering background. Well, I'm not sure what you are trying to bring up here. As far as I'm concerned, any topic is open for discussion. If others fume and fulminate about this being an antenna group, I can easily turn the discussion into one that is antenna specific. What did your Dad see? (We used to joke that after the Pueblo incident they put Marine guards outside of the radio shack for security. If the ship was in danger of being boarded they were to step into the shack and shoot the operators. Then I went into the submarine navy, aboard a tender, the USS Holland, AS-32. We had an escort sub that followed us at sea - to sink us if the Ruskies got too close. Ah, the humor of those days.) 73's Richard Clark, KB7QHC Antennas? In this case you'd have your work cut out for you. :) Wasn't just him who saw it. I saw it first, for an hour and a half, before I decided it would stick around long enough to justify getting my parents to look at it too. I'm a bit embarrssed to mention it here other than to point to it there, but it was two crossed 'beams' in the sky. Trust me, it's hell of a misdirection if you're not up for it, you'd have to go to alt.lasers and take a shufti at the posts there.. But I agree, opening it to discussion is good. I spend about 99.999999% of the time keeping it to myself but I'm getting too old to want to leave it as unresolved as it is now. If it has to be, ok, but I ought to try.. Submarines? Those scare me. I never had to go in one, I don't know if claustrophobia would have beaten me. I have found the sound of water passing all round me as I waited for sleep in a crew cabin below decks in a ship's bow to be one of the most comforting sounds I ever knew, so maybe I wouldn't have. |
Sangean ATS-909 external antenna impedance??
On Sun, 03 Jan 2010 14:19:05 -0600, Lostgallifreyan
wrote: Burying anything out there is harder work than sinking a ground rod. :) I think I'm going to have to take my chances. Consider the significance of "shallow" inches, not feet; and sometimes barely beneath the surface if you have to. If you don't have to (no trip hazards to worry about) on the surface is equally suitable. You don't need bare wire, but you can use it - it doesn't matter. Radials need only be as long as your vertical is high. 16 to a couple of dozen are sufficient. Even if I add a little salt water or dilute acid to accelerate that? This is something I've been considering.. Don't go there. Curing takes years and is an issue of soil compaction. By choking the feedline, do you mean placing ferrite slugs round it like those used on VDU cables? That's something else that will be cheap and easy to test empirically. That is exactly one very good solution. You need to research the appropriate ferrite mix which is frequency specific when we are talking about huge swaths of LF to HF coverage. There looks like one difference. Any signal hitting a coax screen if used in this scheme will have a corresponding return current in the core wire, This one statement exposes a very large problem in understanding about the physics of coaxial cable. The equality of nature in each half of a twin wire appeals to me, so long as it actually works. Should be cheap and easy to test that one... Actually, it is harder than you might imagine at first glance. Yes, the methods are simple, but getting past preconceived notions is the single greatest hurdle. Many engineers are ill suited to the task. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Sun, 03 Jan 2010 14:19:05 -0600, Lostgallifreyan wrote: Burying anything out there is harder work than sinking a ground rod. :) I think I'm going to have to take my chances. Consider the significance of "shallow" inches, not feet; and sometimes barely beneath the surface if you have to. If you don't have to (no trip hazards to worry about) on the surface is equally suitable. You don't need bare wire, but you can use it - it doesn't matter. Radials need only be as long as your vertical is high. 16 to a couple of dozen are sufficient. I won't have scope for that much but I can certainly run a few longer ones on unpaved ground alongside walls if that helps. I've thought about doing that anyway. Even if I add a little salt water or dilute acid to accelerate that? This is something I've been considering.. Don't go there. Curing takes years and is an issue of soil compaction. Ok. I intend to use one of those drills that have a hammer action without runing, to push the rod in. If I find a rod with a slight taper that would help. By choking the feedline, do you mean placing ferrite slugs round it like those used on VDU cables? That's something else that will be cheap and easy to test empirically. That is exactly one very good solution. You need to research the appropriate ferrite mix which is frequency specific when we are talking about huge swaths of LF to HF coverage. I will, I've already been looking into that so I don't have to blindly hunt for some toroid by make and model number.. I notice the US has much easier access to high permeability materials than the UK does, but no idea why this is so. I saved a couple of PDF's with tables of frequency ranges vs materials used. There looks like one difference. Any signal hitting a coax screen if used in this scheme will have a corresponding return current in the core wire, This one statement exposes a very large problem in understanding about the physics of coaxial cable. The equality of nature in each half of a twin wire appeals to me, so long as it actually works. Should be cheap and easy to test that one... Actually, it is harder than you might imagine at first glance. Yes, the methods are simple, but getting past preconceived notions is the single greatest hurdle. Many engineers are ill suited to the task. Try me. All it needs is a clear statement that I can relate to something I've already experienced. I'm well used to being cautious about what I learn. Unless it's as tough as atomic physics, it should be digestible, even if slowly. And I won't be able to taste it till tomorrow... got to sleep soon. Thanks for this, it encourages me to take the time when others do. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
On Sun, 03 Jan 2010 15:35:54 -0600, Lostgallifreyan
wrote: Actually, it is harder than you might imagine at first glance. Yes, the methods are simple, but getting past preconceived notions is the single greatest hurdle. Many engineers are ill suited to the task. Try me. All it needs is a clear statement that I can relate to something I've already experienced. I'm well used to being cautious about what I learn. Unless it's as tough as atomic physics, it should be digestible, even if slowly. And I won't be able to taste it till tomorrow... got to sleep soon. Thanks for this, it encourages me to take the time when others do. We shall proceed with a object lesson found in Fig. 5 of: http://www.kongsfjord.no/dl/Amplifie...Amplifiers.pdf Consider that this is described in the right side of Fig. 2 with all the expectations of being "balanced." It is not. Even on viewing the rough block diagram of Fig. 2 it is apparent in this lack of balance. Fig. 5 and photos do nothing but support this shortfall. At first glance, it seems as though these materials are classic text book stuff that begs to be accepted as "balanced;" reality intrudes in that RF (far more so than AF) finds interwinding capacitance is not distributed equally for T2 and T3 primaries to secondaries. The input/output windings are easily as unbalanced as one could find. "Of Course! a protest might start. T2 is meant to support an unbalanced input." However, a closer glance would admit that the capacitive coupling from the top of T2 input to the top of T2 output is NOT the same as the capacitive coupling from the bottom of T2 input to the bottom of T2 output (irrespective of the implied ground connecting, or not, to the middle of the T2 output winding). The top Basic Amp is thus presented with a higher potential than the lower Basic Amp. One might protest that this does not matter as T3 uncouples the two to present a new output through it to the 50 Ohm output. My rejoinder would be "What practical advantage is then found in the symmetry of upper/lower Basic Amps?" I would then point out that the Common Modality present in the input, is amplified in the upper half and finds its way through the system. The solution involves complex winding taps with capacitors tied from them to common to enforce a balance. This is not practiced (obviously) in the object lesson presented at the page referenced above. A protest might erupt that this example's capacitive asymmetry only admits of a very small imbalance. I would respond that Common Modalities may easily support considerable currents/voltages that when reduced through divider action are easily in excess of small signals of interest. The Basic Amp input Z is 50 Ohms; an imbalance in the interwinding coupling capacitance amounting to 50 KOhms will add 1000 microvolts for any 1V CM signal on the left. At 1MHz, this 50 KOhms would be a capacitance imbalance of 3 pF. Looking at the wire dressing of the coils in the photo would suggest 3 times this easily (and no attention has been paid to this at all). Now extrapolate to the 31 Meter band.... Could your AGC tolerate the local AM station's 10,000 microvolt leakage sitting on top of Radio Burundi's signal? Grounding and shielding is not a simple topic although it is deceptive in appearance. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: At 1MHz, this 50 KOhms would be a capacitance imbalance of 3 pF. Looking at the wire dressing of the coils in the photo would suggest 3 times this easily (and no attention has been paid to this at all). That's the bit that seems most important, and I can also relate to it. It doesn't take much to cause a few pF difference. A few points occur to me though... First, to get it out of the way.. if this was causing serious bother, which it surely might if it is that bad at the lowest end of the SW range, what on earth induces a designer to persist in thinking it's working? Either it isn't, and he's deluded; or it is, so why? Second, I imagined balance to pretty much relate to symmetry. I hadn't seen that file yet (even though I'd actually grabbed it with intent to), and the looseness of the coil wiring isn't lost on me. My idea was to omit the amps and just wind the toroids with neat symmetry to reduce obvious causes of imbalance and take it from there. If it works, I use it, if not, I try something else. Given that I have more than once been told that I might overdrive the input on the ATS-909 radio with a 18' whip in the back yard, I decided that I might as well omit the amps as they only boost a few dB, and instead rely initially on the built in attenuator for first efforts to see what's out there, then consider building (or getting lucky with on eBay) a preselecting filter on the input. Third, the amount of effect a few pF has on a circuit would depend also on the inductance, or resistance, or any delay in the circuit. I hadn't looked closely at that (I'd want to see what happened with a simple test first), but I am guessing that the simpler idea of a twin wire with transformers and no amps would have a smaller risk of imbalanced signals, so a better common mode rejection. Why not do the amplification after the second transformer where balance is clearly irrelevant, if it has to be done at all? I may still be missing something other than a grasp of quantities, but whatever I do, it has to be something that aims to do what that scheme was said to be able to do. I don't have a lot of space, and that radio ideally needs a single antenna to cover all of its AM range, at least for initial efforts. Ideally some scheme that can be improved rather than thrown out when I need something better. |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: At 1MHz, this 50 KOhms would be a capacitance imbalance of 3 pF. Looking at the wire dressing of the coils in the photo would suggest 3 times this easily (and no attention has been paid to this at all). That's the bit that seems most important, and I can also relate to it. It doesn't take much to cause a few pF difference. A few points occur to me though... First, to get it out of the way.. if this was causing serious bother, which it surely might if it is that bad at the lowest end of the SW range, what on earth induces a designer to persist in thinking it's working? Either it isn't, and he's deluded; or it is, so why? Second, I imagined balance to pretty much relate to symmetry. I hadn't seen that file yet (even though I'd actually grabbed it with intent to), and the looseness of the coil wiring isn't lost on me. My idea was to omit the amps and just wind the toroids with neat symmetry to reduce obvious causes of imbalance and take it from there. If it works, I use it, if not, I try something else. Given that I have more than once been told that I might overdrive the input on the ATS-909 radio with a 18' whip in the back yard, I decided that I might as well omit the amps as they only boost a few dB, and instead rely initially on the built in attenuator for first efforts to see what's out there, then consider building (or getting lucky with on eBay) a preselecting filter on the input. Third, the amount of effect a few pF has on a circuit would depend also on the inductance, or resistance, or any delay in the circuit. I hadn't looked closely at that (I'd want to see what happened with a simple test first), but I am guessing that the simpler idea of a twin wire with transformers and no amps would have a smaller risk of imbalanced signals, so a better common mode rejection. Why not do the amplification after the second transformer where balance is clearly irrelevant, if it has to be done at all? I may still be missing something other than a grasp of quantities, but whatever I do, it has to be something that aims to do what that scheme was said to be able to do. I don't have a lot of space, and that radio ideally needs a single antenna to cover all of its AM range, at least for initial efforts. Ideally some scheme that can be improved rather than thrown out when I need something better. |
Sangean ATS-909 external antenna impedance??
Lostgallifreyan wrote:
. . . Second, I imagined balance to pretty much relate to symmetry. . . You might find this interesting: http://eznec.com/Amateur/Articles/Baluns.pdf. Roy Lewallen, W7EL |
Sangean ATS-909 external antenna impedance??
On Mon, 04 Jan 2010 11:57:33 -0600, Lostgallifreyan
wrote: Richard Clark wrote in : At 1MHz, this 50 KOhms would be a capacitance imbalance of 3 pF. Looking at the wire dressing of the coils in the photo would suggest 3 times this easily (and no attention has been paid to this at all). That's the bit that seems most important, and I can also relate to it. It doesn't take much to cause a few pF difference. A few points occur to me though... First, to get it out of the way.. if this was causing serious bother, which it surely might if it is that bad at the lowest end of the SW range, what on earth induces a designer to persist in thinking it's working? Either it isn't, and he's deluded; or it is, so why? That is answered rather simply: isolated experience. Having solved a problem for single application does not make it a universal solution - it is a good start however. For instance (as to this issue of isolation), these designs were originally intended for lowfers where this coupling would have been decimated (its impact reduced by 10). Spinning out the same "successful" design to 10MHz without consideration for this one issue I've pointed out would bring the user to their knees: "I've tried everything and I still get this interference!" Well, the user has not tried everything; it was simply the right design in the wrong application which with a bit of effort could be improved dramatically. Many of the layout and design considerations for the lowfers are commendable and easily applicable into the VHF. A VHF designer would have chosen other input topologies is all. In a sense, the VHF designer is also suspect for having an isolated experience. Practical solutions are not always scaleable. As I pointed out, very few changes are necessary and there is even a simpler solution. You can shield the primaries from the secondaries (kills the capacitive link). Unfortunately this brings a new problem: where do you tie the new shield to? The wrong choice will actually inflate the problem. As I said, this is not a simple field where there are nesting layers of shields and broken shields with non-contacting overlaps. Which one overlaps the other can bring success or misery. Second, I imagined balance to pretty much relate to symmetry. I hadn't seen that file yet (even though I'd actually grabbed it with intent to), and the looseness of the coil wiring isn't lost on me. My idea was to omit the amps and just wind the toroids with neat symmetry to reduce obvious causes of imbalance and take it from there. If it works, I use it, if not, I try something else. Well, here the topology you describe will carry the common mode directly through (with some attenuation) as it is inherently out of balance when the input primary's capacitive coupling is out of whack. This is why I harp on "first principles." The amps have nothing to do with the problem, they simply help illustrate the imbalance. As to you "giving it a try" returns us to the heavily qualified "success" of isolation. You could easily connect your antenna barefoot to your receiver and experience no problems at all! This does not constitute your "solution" as being universal. When you move and try your "solution" again, it could easily fail for not observing first principles. However, this is not to impede you from simply getting on with it. The hallmark of becoming successful is failing as many times as you can. Given that I have more than once been told that I might overdrive the input on the ATS-909 radio with a 18' whip in the back yard, I decided that I might as well omit the amps as they only boost a few dB, and instead rely initially on the built in attenuator for first efforts to see what's out there, then consider building (or getting lucky with on eBay) a preselecting filter on the input. Ham radio was invented on a bread board. You need one coil (selectable) and two variable capacitors that can be scavenged from very, very old table top radios. You would spend more in shipping for a "preselector" than what you would pay at the nearest junk store for these three items. Someone who had to pinch pennies would do it with one cap and one coil and a lot of alligator jumpers. Even this solution can be made elegant and occupy a space no larger than a pocket sized notebook. If you robbed these componets from very, very old transistor radios, you could build it in a mint tin. Third, the amount of effect a few pF has on a circuit would depend also on the inductance, or resistance, or any delay in the circuit. I hadn't looked closely at that (I'd want to see what happened with a simple test first), but I am guessing that the simpler idea of a twin wire with transformers and no amps would have a smaller risk of imbalanced signals, so a better common mode rejection. Why not do the amplification after the second transformer where balance is clearly irrelevant, if it has to be done at all? As I said, the problem is in the topology, not the amplifiers. I may still be missing something other than a grasp of quantities, but whatever I do, it has to be something that aims to do what that scheme was said to be able to do. I don't have a lot of space, and that radio ideally needs a single antenna to cover all of its AM range, at least for initial efforts. Ideally some scheme that can be improved rather than thrown out when I need something better. You have barely nicked the surface of possibility in discussion here. Loop antennas (which can be truly balanced and passed through coax) are a natural answer for a lot of the spectrum you want to listen to. In fact, "shielded loops" are explicitly coaxial and extremely simple to construct (although many web sources describe them incorrectly). They exhibit very sharp nulls (when that matters), and you can lay them over 90 degrees to make them vertically polarized (and then use them in phased configurations for beam steering). 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Trying to keep my post size down...
Ok, I see that isolated experience can be a problem. As far as I can see there will be a level of complexity best averted by empirical testing, so long as I don't miss something really fundamental. Which I am trying not to do.. I found a USMC guide to RF and antenna selection, a 5MB PDF file, that has a Marine's habit of cutting to the chase, it looks like I found a good guide there. http://www.armymars.net/ArmyMARS/Ant...antenna-hb.pdf The topolgy I describe WILL carry imbalance through, I know. :) I thought it might at least be a start if it didn't add any of its own. I realise you were using the amps as an illustration of how imbalance can arise between those toroids, I was thinking that fewer parts means less to go wrong. Precise symmetry in topology might be a lot easier if I didn't have to use those. Again, I wonder why they're even there. Isn't it easier to put a gain stage after the second toroid? Point taken on the costs of the selector. It was a long time ago, but I think I saw something intended to do this task, a design that built it from wire wound on a bit of plastic water pipe or something. I started Googling for designs earlier but got waylaid. One bit of waylaying involved a remark that tuning might offer more problems that a fixed narrowband filter, but so little context was offered that I don't know how to judge the remark. If I put up conspicuous loops here I might get people bothering me about planning permission or some other means of negative compulsion. :) If I can do this with a vertical whip it will be much less awkward. |
Sangean ATS-909 external antenna impedance??
On Mon, 04 Jan 2010 14:44:31 -0600, Lostgallifreyan
wrote: Again, I wonder why they're even there. Isn't it easier to put a gain stage after the second toroid? A circuit serves more purposes than gain. Of course, the simple circuitry found in the file we are discussing has limited offerings. Of what is offered is controlling input and output Z which is not a trivial matter. My preference is found in using Operational Amplifiers instead of discrete transistors. There is more design flexibility and more purposes may be served. OpAmps will control input and output Z with far more rigidity (it is very difficult for externalities to shift these parameters making for a rock solid design). OpAmps will also preserve fidelity (faithful phase, magnitude) and not introduce any distortion, and will drive out noise not already in the signal. Other advantages can be obtained, but this is enough. If I put up conspicuous loops here I might get people bothering me about planning permission or some other means of negative compulsion. :) If I can do this with a vertical whip it will be much less awkward. Practicality needs to be served too. Loops can be useful indoors as well, and they needn't fill a room. They will test the limits of balance with the nearby clutter - an opportunity to turn your environment into an RF lab. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Lostgallifreyan wrote:
. . . Note that the idea that has caught my attention is to use transformers.. there would be no direct current contact between either the line and the antenna, or the line and the receiver. This isn't my own idea, it's just a wheeze that looks most interesting because it looks like it ought to work, and if it does it certainly pretects the receiver input rather well. It's beginning to look as if choosing coax or twin line isn't so important as knowing if that transformer scheme is valid, so long as I do something to block common mode currents with a choke balun on the line. An ideal transformer will effect perfect current balance. What I don't know, and have never seen any theoretical or experimental work on, is how effective a real physical transformer can be made to be in that regard. There will be capacitive coupling between windings which could be a source of common mode current (current imbalance), and it might be necessary to take some or a lot of care in the winding to maintain good balance. And of course the system will have the windings' impedance between the conductors, necessitating care in transformer design for that reason. A fundamental advantage of the balun type connection is that the balance improves as the winding coupling becomes more intimate, while that tends to work against a conventional transformer. It would be an interesting study -- I regret I don't have the time to dig into it more. Roy Lewallen, W7EL |
Sangean ATS-909 external antenna impedance??
Roy Lewallen wrote in
: Lostgallifreyan wrote: . . . Note that the idea that has caught my attention is to use transformers.. there would be no direct current contact between either the line and the antenna, or the line and the receiver. This isn't my own idea, it's just a wheeze that looks most interesting because it looks like it ought to work, and if it does it certainly pretects the receiver input rather well. It's beginning to look as if choosing coax or twin line isn't so important as knowing if that transformer scheme is valid, so long as I do something to block common mode currents with a choke balun on the line. An ideal transformer will effect perfect current balance. What I don't know, and have never seen any theoretical or experimental work on, is how effective a real physical transformer can be made to be in that regard. There will be capacitive coupling between windings which could be a source of common mode current (current imbalance), and it might be necessary to take some or a lot of care in the winding to maintain good balance. And of course the system will have the windings' impedance between the conductors, necessitating care in transformer design for that reason. A fundamental advantage of the balun type connection is that the balance improves as the winding coupling becomes more intimate, while that tends to work against a conventional transformer. It would be an interesting study -- I regret I don't have the time to dig into it more. Roy Lewallen, W7EL I'll give it a go, though I don't know if what I try will have enough rigour to give useful results back. I just sent a cheque for some FT114-43 toroids which are big enough to consider a screen between windings, though as Richard Clarke said, it might be awkward finding the best point to tie that screen to. Re impedance of windings, I have no idea or plan except to aim for close spacing (within each coil) and neat symmetry, and glue to keep them in place so that whatever results it remains steady. |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Mon, 04 Jan 2010 14:44:31 -0600, Lostgallifreyan wrote: Again, I wonder why they're even there. Isn't it easier to put a gain stage after the second toroid? A circuit serves more purposes than gain. Of course, the simple circuitry found in the file we are discussing has limited offerings. Of what is offered is controlling input and output Z which is not a trivial matter. My preference is found in using Operational Amplifiers instead of discrete transistors. There is more design flexibility and more purposes may be served. OpAmps will control input and output Z with far more rigidity (it is very difficult for externalities to shift these parameters making for a rock solid design). OpAmps will also preserve fidelity (faithful phase, magnitude) and not introduce any distortion, and will drive out noise not already in the signal. Other advantages can be obtained, but this is enough. I really like op-amps too, they have often made my life easier. Not used them in RF though, just audio and modest DC instrumentation designs of my own.. About those amps in that scheme, I think I didn't grasp what they were doing, other than gain, because I assumed the idea of balancing implied by the design would be central whether they were used or not. It still seems to me that if the line worked without them, then a single stage could be applied after the signal passed to the unbalanced input after the second toroid. If not (as in not possible as opposed to merely awkward), then I'm still missing something. If I put up conspicuous loops here I might get people bothering me about planning permission or some other means of negative compulsion. :) If I can do this with a vertical whip it will be much less awkward. Practicality needs to be served too. Loops can be useful indoors as well, and they needn't fill a room. They will test the limits of balance with the nearby clutter - an opportunity to turn your environment into an RF lab. 73's Richard Clark, KB7QHC I'll definitely read up on loops. (Been reading the first of two USMC radio handbooks today, second is an update of the one I found yesterday. No loops mentioned in first, but the second is specific to antennas. Both guides are quickly filling forgotten gaps in what I knew, plus showing me plenty I didn't). Btw, how critical is the resistance of wire in a few ground radials? I have some thin stainless steel wire that would be strong and enduring out there but at around 1.5 ohms or more per 6 inches I can't help thinking that's too much. I like the idea though, because clamping ends of it very firmly between copper washers could be fast and easy for good and reliable contact. |
Sangean ATS-909 external antenna impedance??
On Tue, 05 Jan 2010 11:54:23 -0600, Lostgallifreyan
wrote: Btw, how critical is the resistance of wire in a few ground radials? I have some thin stainless steel wire that would be strong and enduring out there but at around 1.5 ohms or more per 6 inches I can't help thinking that's too much. I like the idea though, because clamping ends of it very firmly between copper washers could be fast and easy for good and reliable contact. The stainless steel is a non-starter. Use more radials of wire-wrap wire if you are concerned about visibility vs. thickness. Vegetation/grass will quickly bury most wire when Spring comes (and possibly before). Grass will be so tenacious that even mowing the lawn will not bring it up (unless you have a thatching attachment). 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Tue, 05 Jan 2010 11:54:23 -0600, Lostgallifreyan wrote: Btw, how critical is the resistance of wire in a few ground radials? I have some thin stainless steel wire that would be strong and enduring out there but at around 1.5 ohms or more per 6 inches I can't help thinking that's too much. I like the idea though, because clamping ends of it very firmly between copper washers could be fast and easy for good and reliable contact. The stainless steel is a non-starter. Use more radials of wire-wrap wire if you are concerned about visibility vs. thickness. Vegetation/grass will quickly bury most wire when Spring comes (and possibly before). Grass will be so tenacious that even mowing the lawn will not bring it up (unless you have a thatching attachment). 73's Richard Clark, KB7QHC Ok, copper's no problem, was just wondering about stuff I had plenty of at hand.. I wish I had a lawn. :) I was thinking that it would be an ideal method. My main difficulty (apart from a large amount of heavy logs (and most of a tree trunk) is that the best place to mount the antenna is in a far coner of a plot so I can't lay radials all round it. I can probably get the permission of one neighbour to run a ground wire along the far end of his garden along a low wall, but that same wall is a high wall on the other side, there's a drop of several feet as well as no chance of permission to lay wires there. This is why I'll want a ground rod, as a tree used to grow there, the rotted roots might be my best chance of anything like a conductive network that is close to the surface, in addition to a few ground wires. I'm a tenant, I don't own the land, and can't do much except work round what is there. |
Sangean ATS-909 external antenna impedance??
Lostgallifreyan wrote in
: Richard Clark wrote in : On Tue, 05 Jan 2010 11:54:23 -0600, Lostgallifreyan wrote: Btw, how critical is the resistance of wire in a few ground radials? I have some thin stainless steel wire that would be strong and enduring out there but at around 1.5 ohms or more per 6 inches I can't help thinking that's too much. I like the idea though, because clamping ends of it very firmly between copper washers could be fast and easy for good and reliable contact. The stainless steel is a non-starter. Use more radials of wire-wrap wire if you are concerned about visibility vs. thickness. Vegetation/grass will quickly bury most wire when Spring comes (and possibly before). Grass will be so tenacious that even mowing the lawn will not bring it up (unless you have a thatching attachment). 73's Richard Clark, KB7QHC Ok, copper's no problem, was just wondering about stuff I had plenty of at hand.. I wish I had a lawn. :) I was thinking that it would be an ideal method. My main difficulty (apart from a large amount of heavy logs (and most of a tree trunk) is that the best place to mount the antenna is in a far coner of a plot so I can't lay radials all round it. I can probably get the permission of one neighbour to run a ground wire along the far end of his garden along a low wall, but that same wall is a high wall on the other side, there's a drop of several feet as well as no chance of permission to lay wires there. This is why I'll want a ground rod, as a tree used to grow there, the rotted roots might be my best chance of anything like a conductive network that is close to the surface, in addition to a few ground wires. I'm a tenant, I don't own the land, and can't do much except work round what is there. Thinking about what I read recently, it seems that if the whip is not vertical but slightly leaning back over the plot of land toward the houses, it will have a better chance of using the sky waves, but what I don't know is whether that demands ground radials to be biased (if biased at all) to favour coverage on the side the antenna is leaning over, or the other side. My guess is the side it's leaning over... Is this true? If so, it will help a lot to make the best of that space. |
Sangean ATS-909 external antenna impedance??
On Tue, 05 Jan 2010 12:49:38 -0600, Lostgallifreyan
wrote: Ok, copper's no problem, was just wondering about stuff I had plenty of at hand.. I wish I had a lawn. :) I was thinking that it would be an ideal method. My main difficulty (apart from a large amount of heavy logs (and most of a tree trunk) is that the best place to mount the antenna is in a far coner of a plot so I can't lay radials all round it. Running a fan of 90 degrees is fine, there's nothing exact about this except for those who imagine they will suffer the dB of out-of-symmetry. I can probably get the permission of one neighbour to run a ground wire along the far end of his garden along a low wall, but that same wall is a high wall on the other side, there's a drop of several feet as well as no chance of permission to lay wires there. Don't bother. It isn't worth anyone's effort or intrusion. This is why I'll want a ground rod, as a tree used to grow there, the rotted roots might be my best chance of anything like a conductive network that is close to the surface, in addition to a few ground wires. A wire mesh or mat (like chicken coop wire) over the surface of that area would serve far better. That doesn't sound like an option so the matter of pursuing conductivity of rotted roots is an illusion. I'm a tenant, I don't own the land, and can't do much except work round what is there. You'll be able to do enough without much impact. Thinking about what I read recently, it seems that if the whip is not vertical but slightly leaning back over the plot of land toward the houses, it will have a better chance of using the sky waves, but what I don't know is whether that demands ground radials to be biased (if biased at all) to favour coverage on the side the antenna is leaning over, or the other side. My guess is the side it's leaning over... Is this true? If so, it will help a lot to make the best of that space. Your gain/loss advantage will be in the direction from the antenna base out along of the middle radial in a 90 degree fan. Leaning won't significantly alter things for a very short antenna (in terms of wavelength). Now as to these advantages and disadvantages. Once you get to a minimun set of radials (call it four), the addition of more wire won't budge your S-Meter more than a needle width (and that is being generous). The addition of more radials concerns establishing a firm reference of ground for Z. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Tue, 05 Jan 2010 12:49:38 -0600, Lostgallifreyan wrote: Ok, copper's no problem, was just wondering about stuff I had plenty of at hand.. I wish I had a lawn. :) I was thinking that it would be an ideal method. My main difficulty (apart from a large amount of heavy logs (and most of a tree trunk) is that the best place to mount the antenna is in a far coner of a plot so I can't lay radials all round it. Running a fan of 90 degrees is fine, there's nothing exact about this except for those who imagine they will suffer the dB of out-of-symmetry. I can probably get the permission of one neighbour to run a ground wire along the far end of his garden along a low wall, but that same wall is a high wall on the other side, there's a drop of several feet as well as no chance of permission to lay wires there. Don't bother. It isn't worth anyone's effort or intrusion. This is why I'll want a ground rod, as a tree used to grow there, the rotted roots might be my best chance of anything like a conductive network that is close to the surface, in addition to a few ground wires. A wire mesh or mat (like chicken coop wire) over the surface of that area would serve far better. That doesn't sound like an option so the matter of pursuing conductivity of rotted roots is an illusion. I'm a tenant, I don't own the land, and can't do much except work round what is there. You'll be able to do enough without much impact. Thinking about what I read recently, it seems that if the whip is not vertical but slightly leaning back over the plot of land toward the houses, it will have a better chance of using the sky waves, but what I don't know is whether that demands ground radials to be biased (if biased at all) to favour coverage on the side the antenna is leaning over, or the other side. My guess is the side it's leaning over... Is this true? If so, it will help a lot to make the best of that space. Your gain/loss advantage will be in the direction from the antenna base out along of the middle radial in a 90 degree fan. Leaning won't significantly alter things for a very short antenna (in terms of wavelength). Now as to these advantages and disadvantages. Once you get to a minimun set of radials (call it four), the addition of more wire won't budge your S-Meter more than a needle width (and that is being generous). The addition of more radials concerns establishing a firm reference of ground for Z. 73's Richard Clark, KB7QHC Thankyou. This is good, it sounds like the basic plan will work then, and I might be able to get some chicken wire to cover at least part of it. One possible complication I didn't mention is that the intended mounting point is at a T junction of three wire mesh fences of equal height, about 6'. They don't have very reliable conductivity between each zigzag strand (oriented vertical) as at least one fence has a green plastic coating on its wires. I intend mounting the whip on a concrete post at the junction of these fences. I imagine the fences will raise (and make diffuse) the precise physical level of the RF ground, but I don't know whether they'll be a serious problem, or maybe even be helpful. I can try grounding them a bit better, but otherwise there's not a lot I can do about them. One other thought... In that USMC antenna manual there is a mention of something similar, a 15' whip tilted and also tied back so the upper part is almost horizonatal, it's intended as a way to use short(ish) distances for skywave propagation. It looks useful given the context of trees and buildings within 100m of my best mounting point. What I'm not sure of is whether the curvature of their tied antenna is relevant, or a straight tilted whip would have no significant differences. |
Sangean ATS-909 external antenna impedance??
Lostgallifreyan wrote:
Thankyou. This is good, it sounds like the basic plan will work then, and I might be able to get some chicken wire to cover at least part of it. Chicken wire rusts out pretty fast.. You'd probably be better off just scrounging some AWG20 copper wire and improvising a little grid or randomly laying it out. One One other thought... In that USMC antenna manual there is a mention of something similar, a 15' whip tilted and also tied back so the upper part is almost horizonatal, it's intended as a way to use short(ish) distances for skywave propagation. It looks useful given the context of trees and buildings within 100m of my best mounting point. What I'm not sure of is whether the curvature of their tied antenna is relevant, or a straight tilted whip would have no significant differences. Curvature isn't "significant", and for a lot of cases, the tilt isn't either. But, tying the whip back does keep it from whacking too hard on low hanging branches. |
Sangean ATS-909 external antenna impedance??
On Wed, 06 Jan 2010 04:18:10 -0600, Lostgallifreyan
wrote: Thankyou. This is good, it sounds like the basic plan will work then, and I might be able to get some chicken wire to cover at least part of it. There is a down-side to this and what you have revealed below: One possible complication I didn't mention is that the intended mounting point is at a T junction of three wire mesh fences of equal height, about 6'. They don't have very reliable conductivity between each zigzag strand (oriented vertical) as at least one fence has a green plastic coating on its wires. Both the mesh of chicken wire, and the fence crosspoints may suffer from cross-modulation products due to corrosion at the joints AND if the metal is galvanized. This is evidenced in a nearby transmitter (and nearby is relative measure) exciting the wire, and rectifying at the corroded crosspoints. This rectification creates harmonics and you are off to the races in terms of spurious frequency generation across a wide bandwidth. Oh Brave New World of common mode. I intend mounting the whip on a concrete post at the junction of these fences. I imagine the fences will raise (and make diffuse) the precise physical level of the RF ground, but I don't know whether they'll be a serious problem, or maybe even be helpful. I can try grounding them a bit better, but otherwise there's not a lot I can do about them. As you describe at least one fence having insulated wire (which is good from the cross-mod point of view) this makes no difference RF ground-wise; and being elevated only slightly shifts things. One exception is found in proximity in that this elevated ground will indirectly short out the lower section of your vertical. This is more a matter of Z than sensitivity. The solution is to elevate your vertical's feedpoint to the height of the fence top. One other thought... In that USMC antenna manual there is a mention of something similar, a 15' whip tilted and also tied back so the upper part is almost horizonatal, it's intended as a way to use short(ish) distances for skywave propagation. It looks useful given the context of trees and buildings within 100m of my best mounting point. What I'm not sure of is whether the curvature of their tied antenna is relevant, or a straight tilted whip would have no significant differences. Well, what they (or you) call sky wave is properly NVIS (near vertical incident _____ - I forget the last part) which is meant for local communications, which is more what the ground forces are interested in. The Marines in Afghanistan are not going to DX headquarters back at Pennsylvania Avenue in DC. When I taught VHF/UHF comm in the Navy, our equipement easily lost 10dB of transmitted/received signal levels just getting from the shack to the antenna. We didn't care. Push more power if necessary, as for reception, line of sight was all that was necessary, and that was to the horizon (no more than 8 or 10 miles). I don't think the government has bought any QRP rigs since WWII. In fact, that tilting's mission has also been satisfied with end loaded dipoles place directly on the ground (which was largely sand). I and my buddy used one for field day. Another antenna is an unterminated coax laid across the ground. Both suffer mightily in efficiency, but they offer ease of construction and purport to enjoy less noise problems. To this last, most local noise arrives by vertical polarization, and signals in the sky arrive by elliptical (both vertical and horizontal by varying degree) polarization. For a quick and dirty test, I doubt any other test could be quicker to do. You might want to add a short pig-tail to the unterminated coax. Further experimentation would be to add 8 to 10 inches of ferrite beads to the coax, half way back on it toward your shack. This would snub your home's injection of noise into your receiver (conducting out from the house on the coax outer shield and folding back at the far end). The next experiment (if this first proved useful) would be to add a local ground at the same point and tie it to the shield (after penetrating the jacket, of course). Yes, this violates some of my other advice about mixing grounds, but for experiment's sake, it will add to your repertoire of learning the complexities and benefits of ground. The purpose of this new ground is to discharge that choked noise into ground. I have successfully done this to quiet my home's noise generation as detected in my receivers. The ultimate proof of this concept is being able to throw the master breaker on your home and noting any change in the noise floor. Throwing that breaker is probably at the extreme of your family's tolerance of your hobby. To this end you want to plan to do it once. This means doing a noise floor survey at hourly intervals for all bands and keeping notes for a week or two. Then, one day when there's the least possibility of disrupting domestic tranquility, throw the main and do a quick survey again. I have managed to quiet my receivers by 5 to 10 dB through tests like these. Buried cable, or ground level run cable can snub local noise induction too (but it is still a good idea to choke at the feed point if no where else). This last observation is to inform you that metallic connection to earth is not always necessary. You should be equally informed that the proximity of earth can also negatively affect what positive gains you are seeking too. 73's Richard Clark, KB7QHC |
Sangean ATS-909 external antenna impedance??
Jim Lux wrote in
: Lostgallifreyan wrote: Thankyou. This is good, it sounds like the basic plan will work then, and I might be able to get some chicken wire to cover at least part of it. Chicken wire rusts out pretty fast.. You'd probably be better off just scrounging some AWG20 copper wire and improvising a little grid or randomly laying it out. True. Was just thinking that it's not that hard to find something meshlike so even if it did it could be replaced for free. Given the price of scrap copper, that rarely comes free these days. :) One other thought... In that USMC antenna manual there is a mention of something similar, a 15' whip tilted and also tied back so the upper part is almost horizonatal, it's intended as a way to use short(ish) distances for skywave propagation. It looks useful given the context of trees and buildings within 100m of my best mounting point. What I'm not sure of is whether the curvature of their tied antenna is relevant, or a straight tilted whip would have no significant differences. Curvature isn't "significant", and for a lot of cases, the tilt isn't either. But, tying the whip back does keep it from whacking too hard on low hanging branches. In the case of a 15' whip, tilt might be. At least, if it isn't I have to wonder why that USMC antenna manual was suggesting one for short-range skywave use. Document is "MCRP 3-40.3c" (PDF) 'Figure 4-34. AN/MRC-138 with NVIS Antenna'. (About 4/7 of the way through that file, as gauged by scrollbar). Where I am it seems the best chance due to surrounding buildings, trees, fences, etc, is to use a slightly less exaggeratedly steep 'view' of the sky than is implied by that description and drawing. If you look at that drawing you can see that it isn't done to tie the antenna out of the way. :) There seems to be specific reason for doing what they do there. The curvature made me wonder, based on a relector, in an indirect way... I read that a longer straight isolated rod placed directly behind a whip, separated by a quarter wavelength, could be a reflector. While I guess any curvature in either that rod or the whip might be insignificant at low HF, I inferred that it had to be straight to work efficiently if the whip was straight. There seemed to be implications that straightness, or lack of it, mattered. That made me wonder if curvature in a whip (forget the reflector, I have...) leaning toward a building might very slightly favour pickup on the convex side and attenuation on the concave side, or have any other mildly directional benefits. If so it might be a useful part of a strategy to reduce noise from nearby buildings. Of course it might be useless trying, but it looks like a nice idea... |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: Well, what they (or you) call sky wave is properly NVIS (near vertical incident _____ - I forget the last part) Skywave. :) At least, according to the document I just mentioned in previous post. Sorry, I'll get to the detail in your post now.. |
Sangean ATS-909 external antenna impedance??
Richard Clark wrote in
: On Wed, 06 Jan 2010 04:18:10 -0600, Lostgallifreyan wrote: Thankyou. This is good, it sounds like the basic plan will work then, and I might be able to get some chicken wire to cover at least part of it. There is a down-side to this and what you have revealed below: One possible complication I didn't mention is that the intended mounting point is at a T junction of three wire mesh fences of equal height, about 6'. They don't have very reliable conductivity between each zigzag strand (oriented vertical) as at least one fence has a green plastic coating on its wires. Both the mesh of chicken wire, and the fence crosspoints may suffer from cross-modulation products due to corrosion at the joints AND if the metal is galvanized. This is evidenced in a nearby transmitter (and nearby is relative measure) exciting the wire, and rectifying at the corroded crosspoints. This rectification creates harmonics and you are off to the races in terms of spurious frequency generation across a wide bandwidth. Oh Brave New World of common mode. Hmm. :) Just going to have to chance that, the bigger fence (main road in T junction metaphor) is a galvanised type, with plenty of weathering. If I have patience I might weave in a grounded length of copper wire, but I suspect more weathering will nullify any useful initial results of that move so I probably won't. I intend mounting the whip on a concrete post at the junction of these fences. I imagine the fences will raise (and make diffuse) the precise physical level of the RF ground, but I don't know whether they'll be a serious problem, or maybe even be helpful. I can try grounding them a bit better, but otherwise there's not a lot I can do about them. As you describe at least one fence having insulated wire (which is good from the cross-mod point of view) this makes no difference RF ground-wise; and being elevated only slightly shifts things. One exception is found in proximity in that this elevated ground will indirectly short out the lower section of your vertical. This is more a matter of Z than sensitivity. The solution is to elevate your vertical's feedpoint to the height of the fence top. Oh, it will be. A tad higher, if anything. Just a few inches though, between bottom end of whip and top of fence. One other thought... In that USMC antenna manual there is a mention of something similar, a 15' whip tilted and also tied back so the upper part is almost horizonatal, it's intended as a way to use short(ish) distances for skywave propagation. It looks useful given the context of trees and buildings within 100m of my best mounting point. What I'm not sure of is whether the curvature of their tied antenna is relevant, or a straight tilted whip would have no significant differences. Well, what they (or you) call sky wave is properly NVIS (near vertical incident _____ - I forget the last part) which is meant for local communications, which is more what the ground forces are interested in. The Marines in Afghanistan are not going to DX headquarters back at Pennsylvania Avenue in DC. When I taught VHF/UHF comm in the Navy, our equipement easily lost 10dB of transmitted/received signal levels just getting from the shack to the antenna. We didn't care. Push more power if necessary, as for reception, line of sight was all that was necessary, and that was to the horizon (no more than 8 or 10 miles). I don't think the government has bought any QRP rigs since WWII. I don't have a lot of line of sight.. While I'm not living in a well, that might be a closer analogy than the terrain most SWL'ers assume they'll find. I do get a fair chunk of south and western sky though. I figured a scheme intended to transmit to such a space should receive ok from it. If I try any other angle I might as well try to hear the local neighbourhood noises, but getting a sense of what lies beyond all that is why I'm doing this. Maybe after some initial tries I might have to consider something much more directional anyway, but hopefully not. In fact, that tilting's mission has also been satisfied with end loaded dipoles place directly on the ground (which was largely sand). I and my buddy used one for field day. Another antenna is an unterminated coax laid across the ground. Both suffer mightily in efficiency, but they offer ease of construction and purport to enjoy less noise problems. To this last, most local noise arrives by vertical polarization, and signals in the sky arrive by elliptical (both vertical and horizontal by varying degree) polarization. For a quick and dirty test, I doubt any other test could be quicker to do. You might want to add a short pig-tail to the unterminated coax. An odd idea, but I like it. It might be that in my location some drastic shifts from convention, to get best SNR never mind losses, then add gain later, might be best. If something like that worked I'd leave it working. Further experimentation would be to add 8 to 10 inches of ferrite beads to the coax, half way back on it toward your shack. This would snub your home's injection of noise into your receiver (conducting out from the house on the coax outer shield and folding back at the far end). The next experiment (if this first proved useful) would be to add a local ground at the same point and tie it to the shield (after penetrating the jacket, of course). Yes, this violates some of my other advice about mixing grounds, but for experiment's sake, it will add to your repertoire of learning the complexities and benefits of ground. The purpose of this new ground is to discharge that choked noise into ground. I have successfully done this to quiet my home's noise generation as detected in my receivers. The ultimate proof of this concept is being able to throw the master breaker on your home and noting any change in the noise floor. Throwing that breaker is probably at the extreme of your family's tolerance of your hobby. To this end you want to plan to do it once. This means doing a noise floor survey at hourly intervals for all bands and keeping notes for a week or two. Then, one day when there's the least possibility of disrupting domestic tranquility, throw the main and do a quick survey again. I have managed to quiet my receivers by 5 to 10 dB through tests like these. Buried cable, or ground level run cable can snub local noise induction too (but it is still a good idea to choke at the feed point if no where else). This last observation is to inform you that metallic connection to earth is not always necessary. You should be equally informed that the proximity of earth can also negatively affect what positive gains you are seeking too. 73's Richard Clark, KB7QHC Ok, I'll be keeping the feed line as mobile as I can, because I already suspected that position and number of ferrite slugs might be something I want to change a lot to test. Ultimately I hope I can run it along the top of one of the chainlink fences. Circuit breakers will be no problem, I live alone in a basement flat. :) |
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