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#41
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Low Noise receiving Loop antenna
On Nov 17, 7:51 pm, Tony Giacometti wrote:
This link was the guide I used to build the loop, I am only using the 80 meter loop. http://www.qsl.net/kc2tx/ Without the preamp I do get a noise spike when I tune the cap. Its very noticeable also. I would have thought that the signals I have heard would be much louder especially using the preamp. They will be down, but the overall s/n ratio is what really counts. I have 2 different types of preamps and they both behave the same way. For what its worth, I have never considered just plain wire for the loop. I do use coax RG-6 - its all I can get, no RG-59 around here. Another ham mentioned to me that using 75 ohm hardline would be the best. None of that stuff here either. Type of coax should matter little if any.. Hardline would be a waste of time I would think. I am beginning to think my feedline could be a problem. I can replace that stuff rather easily. Not unless you have a common mode/ noise pickup problem. And changing to another run of coax alone is unlikely to help that. I like your idea of a separate coupling loop. Any idea what the loop would need to be electrically and physically? Do I need to change my tuning cap if I change to a coupling loop? No. The main loop will still be tuned to the same frequency, no matter if direct fed, or with a coupling loop. Many people build BC loops and use the ferrite bar antennas in the portable radios themselves to couple to the loop. But my radios are not portables, so I use a coupling loop fed with coax. MK |
#42
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Low Noise receiving Loop antenna
Tony Giacometti wrote in
news:sO2dnUZbkeShCaLanZ2dnUVZ_v2pnZ2d@hawaiiantel. net: Owen Duffy wrote: Owen Duffy wrote in : .... It is questionable whether the parallel tuned circuit is an efficient coupling method for a low Z receiver. I am able to get a noise peak tuning the capacitor and the preamps I use are supposed to be a match from approx 25 ohms to about 125 ohms. Tony, I have written a program to solve for transmission loss from the source being the emf induced into the electrically small loop through to the 50 ohm receiver load on the output. The program models the transmission line stub on one side of the loop gap, and the transmission line on the other side to the tuning capacitor and 50 ohm load. I need to do some more checking, but the program results support my proposition that parallel "tuning" is not an efficient coupling. The only way to ensure a near 50 ohm receiver for test purposes is to put a 10dB 50ohm attenuator in front of it. If you are able to do such, does the receiver noise peak with the loop tuning capacitor at minimum C? Owen |
#43
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Low Noise receiving Loop antenna
Owen Duffy wrote in
: .... Expected ambient noise level from a lossless antenna in 2kHz at 3.6MHz should be around -82.9+33dBm or -49.9dBm. The 80m loop gain is about - 47dBi, so expected receive level would be -97dBm which is some 40dB above your receiver noise floor. That is wrong, the 82.9dBm was for 2kHz bandwidth, and I should not have added the 33dB bandwidth factor... it is a double count. So the receive level would be -82.9 -47 or -129dBm which would be just 10dB above the receiver noise floor... in the ballpark of what you measured Tony. Sorry for the bum steer. I am still working on a more detailed model, and it will drive the gain figure downwards a bit. Owen |
#45
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Low Noise receiving Loop antenna
Roy Lewallen wrote:
Tony Giacometti wrote: Roy, you scared the daylights out of me for a minute, By the way, any idea why this loop might be under performing? Well, first of all, I think the problem might be your expectations. maybe I just don't know what to expect with this loop. Having never heard one perform I have no idea if this loop is doing what its supposed to be doing. A small loop has a very broad pattern, with a couple of very narrow and deep nulls. If you have noise coming from a very narrow angular region, you can use a loop to null it out. But if it's coming from the wiring in a neighbor's house, is getting on the power lines, or otherwise comes from a range of angles, the loop won't help. If the noise is getting into your house via the mains wiring, then the loop will probably make things worse compared to an outside antenna, since it's closer to at least one source of the noise. I powered my receiver and a few other items on a battery backup, switched all the power off in my home and nothing changed. The noise was still there. Funny though, everynite between 8:30 and 9:30pm the noise reduces by half. I keep watching my neighbors homes to see if I can tell if someone is turning something off but no clues yet. And this does seem to be the case. Although you didn't say in so many words, it sounds like the signal/noise ratio is worse when using the loop than when using the outside antenna. If so, then the last couple of sentences in the above paragraph apply. no, the noise is worse on the transmitting antenna. the noise on the loop is very low, and if I rotate it I can sometimes make the noise increase as it points along the plane of the loop at something in the neighborhood. The loop does reduce the noise but the signals are very weak and sometimes difficult to copy. In a recent posting you say the noise level comes up substantially when you connect the loop, so you can quit worrying about your receiver noise figure in my opinion -- and with it, the AGC operation, S-meter calibration, and so forth. It means that external noise is considerably louder than receiver noise. You can also quit worrying about how many turns. A preamp, or even an audio amplifier connected to the receiver output, will make both signals and noise louder, in the same ratio, if they're not loud enough to hear. So the only thing which can be wrong with the loop that you can't fix with a little amplification is that maybe it's poorly balanced so the nulls aren't what they should be. The nulls are well defined considering what I am using. I have the loop about as far away as I can get it from my home and 2 others right now. when I rotate it where it is now vs. where it was 2 weeks ago its not very noisy. The only way I know of to test for this is to rotate the loop when listening to a distant station or a small battery powered signal source -- something coming from only one direction. You should be able to null it out pretty effectively. If you can't, the problem might be loop construction or it might be proximity of other conductors warping the pattern. If you can successfully null out point-source signals, then the loop is performing as it should. And if that's not good enough, then a loop isn't the solution to your problem. Roy Lewallen, W7EL The loop does reduce the noise, but the signals I want to hear are not very loud even with the preamp. This is what the problem is. |
#46
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Owen Duffy wrote in : ... Expected ambient noise level from a lossless antenna in 2kHz at 3.6MHz should be around -82.9+33dBm or -49.9dBm. The 80m loop gain is about - 47dBi, so expected receive level would be -97dBm which is some 40dB above your receiver noise floor. That is wrong, the 82.9dBm was for 2kHz bandwidth, and I should not have added the 33dB bandwidth factor... it is a double count. So the receive level would be -82.9 -47 or -129dBm which would be just 10dB above the receiver noise floor... in the ballpark of what you measured Tony. Sorry for the bum steer. I am still working on a more detailed model, and it will drive the gain figure downwards a bit. Owen you had me worried......... |
#47
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in news:sO2dnUZbkeShCaLanZ2dnUVZ_v2pnZ2d@hawaiiantel. net: Owen Duffy wrote: Owen Duffy wrote in : ... It is questionable whether the parallel tuned circuit is an efficient coupling method for a low Z receiver. I am able to get a noise peak tuning the capacitor and the preamps I use are supposed to be a match from approx 25 ohms to about 125 ohms. Tony, I have written a program to solve for transmission loss from the source being the emf induced into the electrically small loop through to the 50 ohm receiver load on the output. The program models the transmission line stub on one side of the loop gap, and the transmission line on the other side to the tuning capacitor and 50 ohm load. I need to do some more checking, but the program results support my proposition that parallel "tuning" is not an efficient coupling. The only way to ensure a near 50 ohm receiver for test purposes is to put a 10dB 50ohm attenuator in front of it. If you are able to do such, does the receiver noise peak with the loop tuning capacitor at minimum C? Owen I dont have access to an attenuator or anything that would work as one. Right now the loop gets max noise with about 2/3 max capacitance. |
#48
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Low Noise receiving Loop antenna
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#49
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in news:sO2dnUZbkeShCaLanZ2dnUVZ_v2pnZ2d@hawaiiantel. net: I believe this should work, but for some reason not like I thought it would. Tony, I have described a simple untuned loop for field strength measurement. The article is at http://www.vk1od.net/SmallUntunedSquareLoop/index.htm . The sensitivity of the loop is sufficient that external noise on 3.6MHz is much greater than the receiver internal noise, ie S/N of signals on the band will be about as good as they can be, a higher gain antenna will increase the S meter reading, but not improve S/N ignoring the effects of noise blankers and noise reduction. The predicted performance has been confirmed by comparison to a calibrated EMC measurement loop. The purpose of tuning a loop is preselection and / or better impedance matching to improve gain (by reducing loss). The purpose of shielding a loop is for better balance to achieve deeper nulls, but shielding isn't the only way, nor the best way necessarily. Roy mentioned that. Try a simple untuned loop, the balun is REAL important (for deep nulls), see how it works then see if you can get the improved version to work. It is questionable whether the shielded loop construction is a real improvement, it brings some loss elements (the s/c stub loss, the line loss in the other half the loop) to the design, losses that be worse than a balun. Owen Its late here, 11:37pm - I will look this over in the morning. Thank You Just for your info, the loop ends when pointed east-west I get S meter reading of 3.5 when pointed north-south I get S1 I believe this tell me the noise is either east or west of the antenna location. When I had the loop on the other side of the property I got an S5 noise reading in the same direction. I believe either one of 3 houses might be the culprit, maybe they will go away for the holiday and I will be able to make a better determination as to who the bad guy is. |
#50
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Low Noise receiving Loop antenna
Tony Giacometti wrote in
news:NNydnfODM_iOB6LanZ2dnUVZ_r6rnZ2d@hawaiiantel. net: .... For what its worth, I have never considered just plain wire for the loop. I do use coax RG-6 - its all I can get, no RG-59 around here. Did I miss something. I did see you refer us to a web page that described the loop using RG59... and now you tell us you used RG6. You might not yet know it, they are different, and the difference is relevant. Owen |
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