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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. |
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