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#81
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Low Noise receiving Loop antenna
On Sat, 24 Nov 2007 23:41:19 -0800, Roy Lewallen wrote:
Owen Duffy wrote: . . . If you try something beside a shielded loop, make sure you use an effective means of isolating the feed line from the loop so that pickup on the feedline does not feed the receiver. . . . In my limited experience, it's extremely difficult or impossible to do a good job of isolating the feedline from a small loop. The common mode impedance is just too high. Roy Lewallen, W7EL Hello How much (estimated) would this common mode impedance be for a 3 foot diameter loop for instance 80 meter band? Mario |
#82
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Low Noise receiving Loop antenna
On Nov 25, 12:46 am, Tony Giacometti wrote:
Ok, I want to build a larger loop to get more gain, if possible deep nulls for reducing local noise, I would like to try wire instead of coax. I want to mount it on a PVC pipe frame. Can you give me any ideas as to what shape and size? Whats a solenoid loop? A solenoid loop is like this.. http://home.comcast.net/~nm5k/loop5.jpg A pancake loop is about the same, except that the wires are stacked in the same plane like an electric range burner, vs being side by side like the solenoid loop. All the PVC loops I make are diamonds, fed at the bottom corner. The depth of the nulls is due to balance, not the size of the loop. But... In some cases it's easier to get good balance on a larger loop as it's easier to ensure a symmetrical loop as related to the mounting hardware, feed devices, etc.. But in my case, even my smallest 16 inch loop is very symmetrical and has just as good of balance as my larger loops. So size doesn't mean much. Care in symmetrical construction means much more. I've never had common mode problems to be a big deal. I may have a little, but when I tune the loop, the greatly increased "tuned" level swamps any small common mode signal I may have. So in effect, it's usually not an issue. I do try to keep the feedlines only as long as needed to reach the antenna, and I keep them on the floor. Another thing that I think helps a bit is to use a separate coupling loop to feed the loop. But this may help with balance more than common mode reduction. But I still recommend it none the less. One thing.. make sure there is not metal around the antenna. This can whack things out fairly fast. IE: metal frame chairs, file cabinets, etc... On my loops, I've use no baluns at all so far, but as my recordings prove, I have no lack of nulls, or signal level for that matter. When I null a single point ground wave signal, say bye bye.. It's gone. :/ BTW, you will never have good luck at nulling general "band noise". When you say "band noise", I will assume you mean all the noise received from all directions at once. As has been repeated many times, if the noise source is not from a single source, the loop will not be able to provide a decent null. You will never be able to null random noise. But if say a power line device starts making noise from a single source, you can null almost all of it, and still hear most of the skywave signal you are trying to receive. It's quite possible you are in a location with multiple noise sources, and the small loops may not do the job you want them to. In general, a loop is a loop is a loop.. If the one you have is working properly, and you are having trouble nulling the noise sources, going to a bigger loop may not help much at all. MK |
#83
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Low Noise receiving Loop antenna
mario wrote:
On Sat, 24 Nov 2007 23:41:19 -0800, Roy Lewallen wrote: In my limited experience, it's extremely difficult or impossible to do a good job of isolating the feedline from a small loop. The common mode impedance is just too high. Roy Lewallen, W7EL Hello How much (estimated) would this common mode impedance be for a 3 foot diameter loop for instance 80 meter band? It depends on height above ground, but according to EZNEC it's on the order of 1000 ohms when 6 feet above ground. You'd probably need a receiver or circuit with differential input and very good common mode rejection ratio, and perhaps also an exceptionally good balun, to prevent the feedline response from reducing the null depth unless using a "shielded" loop which by its construction provides good common mode rejection. I'd be interested in hearing how Owen managed to get good nulls from an unshielded loop. Roy Lewallen, W7EL |
#84
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Low Noise receiving Loop antenna
Roy Lewallen wrote in
: .... mode rejection. I'd be interested in hearing how Owen managed to get good nulls from an unshielded loop. Roy, Perhaps some of this goes to the meaning of "good nulls". I have just performed a simple test with the antenna described at http://www.vk1od.net/SmallUntunedSquareLoop/index.htm . I have inserted a step attenuator between the antenna and receiver, have 10m of coax draped over the ground, and measured the depth of null on a nearby broadcast station on 1.2MHz. The depth of the nulls are between 30dB and 40dB, and the two nulls are within a couple of degrees of 180° apart and of similar depth as you would expect from a loop with little feed line contribution. The nulls from this antenna are certainly very sharp for the purposes of direction finding, they are orthogonal to the plane of the loop, and symmetrical, the nulls or of similar depth. In my experience, the smallest contribution of feedline pickup is shown as unequal nulls. Owen |
#85
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Low Noise receiving Loop antenna
On Nov 25, 5:39 pm, Roy Lewallen wrote:
How much (estimated) would this common mode impedance be for a 3 foot diameter loop for instance 80 meter band? It depends on height above ground, but according to EZNEC it's on the order of 1000 ohms when 6 feet above ground. You'd probably need a receiver or circuit with differential input and very good common mode rejection ratio, and perhaps also an exceptionally good balun, to prevent the feedline response from reducing the null depth unless using a "shielded" loop which by its construction provides good common mode rejection. I'd be interested in hearing how Owen managed to get good nulls from an unshielded loop. Roy Lewallen, W7EL Hummm.. I knew that the shielded loop helped with balance, but I had never really given much thought to it reducing common mode currents when feeding with coax. But I guess it should. I compared using both a plain wire and a shielded coax loop as the coupling loop on my 16 inch version, and saw little if any difference in performance, or perceived common mode problems. But I decided to use the shielded coupling loop just to be on the safe side and help ensure balance even though the actual performance of the two were nearly identical. Maybe I should consider using a shielded coupling loop on my larger loop just to give it that extra advantage. I might have already tried one, but I forgot... It's been a while since I built those. It does have very deep nulls with just the plain wire coupling loop though. Both of my loops seem about the same as far as null depth. I just get more level from the bigger one, and maybe just a tiny bit better weak signal performance. I'm usually using an Icom 706mk2g as the receiver, so it's nothing special thats for sure. BTW, I just remembered something.. When comparing a loop fed directly , you often see a skewing of the pattern slightly off where it's actually pointing. But I found when using a coupling loop, even if plain wire, this skewing of the pattern is greatly reduced. So this leads me to believe using even a plain wire coupling loop will reduce common mode problems vs feeding directly. But using the shielded loop for coupling should be a pretty good "brute force" method. MK |
#87
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in : Owen Duffy wrote: ... You have already been told there is no such thing as a low noise antenna. The loop has the advantage of being able to null out signals from two opposite directions, which can help if the interference is predominantly from one (or both) of those directions. You can improve the depth of those nulls, but pattern or directivity is unrelated to the power transfer problem. did I misunderstand you, I thought the noise was nullable but at 75 meters the signals were mostly omnidirectional? Or is this one of the unlearned things? Keep in mind that noise from very close sources can be much stronger than the aggregated noise from more distant source. If you live in a street where everyone has the same noisy plasma TV, the noise from your own one is likely to be much stronger than the aggregate of the others... think through the propagation mechanism and especially the effects where the source is in the near field (say within a quarter wave of your antenna). You can't do much to null the noise from a hundred TVs spread around the neighborhood, but you can null the noise from a close by dominant source like your own, or immediate neighbor's. Of course, in nulling the noise in that direction you also null signals from the same direction. Fortunately, lots of signals come via an ionospheric path (a higher angle), and the null of the loop is normally oriented horizontally so it doesn't knock the ionospheric signals down as much. If you try something beside a shielded loop, make sure you use an effective means of isolating the feed line from the loop so that pickup on the feedline does not feed the receiver. Perhaps by solenoid loop you mean a multi turn loop. A multiturn loop is another way of increasing induced voltage, but it increases the source impedance and you have to solve the impedance matching issue. some type of balun maybe? ... Ok, I want to build a larger loop to get more gain, if possible deep nulls for reducing local noise, I would like to try wire instead of coax. I want to mount it on a PVC pipe frame. Can you give me any ideas as to what shape and size? Whats a solenoid loop? You started of with a loop that was too insensitive, so you have improved that with better matching solving that problem. You can increase the gain by further improvement of the matching... but you don't need more gain than sufficient to have the band noise swamp the receiver internal noise. at this point I din't think it does. When I orient the antenna for lowest noise the S meter never reads above S3, its really quiet. I guess the question is this, DX signals would be skywave, a loop is omni directional to skywave signals I believe. Is this correct? I know you are convinced that matching also degraded the pattern. If the matching network was entirely inside an effectively shielded box, it should not affect the nulls. Make sure that you retained the shielded loop properly. I didn't use a shielded box, I have a plastic box. The instructions for the loop I built said either was OK. Why do you still think that a larger loop has deeper nulls. If I implied that I didn't intend to do so, what I mean is to "keep" the deep nulls I now have. Owen |
#88
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Low Noise receiving Loop antenna
Roy Lewallen wrote:
mario wrote: On Sat, 24 Nov 2007 23:41:19 -0800, Roy Lewallen wrote: In my limited experience, it's extremely difficult or impossible to do a good job of isolating the feedline from a small loop. The common mode impedance is just too high. Roy Lewallen, W7EL Hello How much (estimated) would this common mode impedance be for a 3 foot diameter loop for instance 80 meter band? It depends on height above ground, but according to EZNEC it's on the order of 1000 ohms when 6 feet above ground. You'd probably need a receiver or circuit with differential input and very good common mode rejection ratio, and perhaps also an exceptionally good balun, to prevent the feedline response from reducing the null depth unless using a "shielded" loop which by its construction provides good common mode rejection. I'd be interested in hearing how Owen managed to get good nulls from an unshielded loop. Roy Lewallen, W7EL I ran across a small loop with some interesting add ons to solve the common mode problem while looking for info on the loop here on the web. I will try and find it. |
#89
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Roy Lewallen wrote in : Owen Duffy wrote: . . . If you try something beside a shielded loop, make sure you use an effective means of isolating the feed line from the loop so that pickup on the feedline does not feed the receiver. . . . In my limited experience, it's extremely difficult or impossible to do a good job of isolating the feedline from a small loop. The common mode impedance is just too high. Roy Lewallen, W7EL Hello Roy, I have a small loop with inexpesive voltage balun, and it gives a quite deep null (though I can't put dB figures on it off hand), and very sharp null. I wonder if the reason that Tony perceives that the nulls are shallow is that he is assessing it on band noise. If so Tony, you should assess the depth of the nulls on a local (ie low elevation) point source that is much stronger than band noise. The test signal should dominate the receiver. If an antenna with deep nulls doesn't reduce band noise much, it suggests that the noise is not mainly from a single direction... as discussed earlier in this thread. Owen the band has been devoid of strong noise the past few days. I think I know who isn't home in my neighborhood and where the noise may be coming from. All I need do is have the noise return when she gets home from her trip. The loop points at her home and the one behind me when the noise is up. Maybe we got the culprit. I had recorded an hour and a half of a pileup awhile back and in lstening to the recording again there was almost no noise. this was 3-4 months ago. As I had posted previously the noise showed up a little over a month ago. Oh, one more thing, I will be replacing the coax on the loop over the next day or so. My gut tells me to try this. Just because you purchase a new run of coax it doesn't mean it doesn't have a defect. Its the only thing I haven't looked at. |
#90
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Low Noise receiving Loop antenna
Tony Giacometti wrote in
news:aeydnehFvJBYatHanZ2dnUVZ_o2vnZ2d@hawaiiantel. net: I guess the question is this, DX signals would be skywave, a loop is omni directional to skywave signals I believe. Is this correct? In free spacem the nulls are orthogoal to the plane of the loop. If you make the plane of the loop vertical and mount it near ground, the nulls will be modified somewhat, but the gain at high elevations (eg ionospheric propagation from nearby) would be pretty much unaffacted by loop orientation. That cannot be said for very low angle signals. .... I didn't use a shielded box, I have a plastic box. The instructions for the loop I built said either was OK. Well, the instructions had you tuning the loop in a manner that wasn't very effective too didn't they. Are they credible? I am not saying a shielded box is necessary, but I wouldn't use a plastic box without verifying it works as well as a shielded box... and the way you do that is to build the shielded box version. Why do you still think that a larger loop has deeper nulls. If I implied that I didn't intend to do so, what I mean is to "keep" the deep nulls I now have. I pointed out in an earlier post that you might well compromise symmetry with a larger loop and so compromise the depth of the nulls. You only need enough gain to swamp receiver internal noise under the best band conditions and loop orientation. Of course, when the loop becomes large enough, its directivity does change, and the nulls are a little broader. Proximity to ground modifies large loops significantly. Owen |
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