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#21
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Low Noise receiving Loop antenna
Tony Giacometti wrote in
news:bcSdnVU4bZ7W86LanZ2dnUVZ_vShnZ2d@hawaiiantel. net: I am using a heavily modified Drake R-4c receiver. Sherwood mods. And I at this point I only built the 80 meter loop as I wanted to make sure it worked on 80 before I went and built one for 160. I obviously misunderstood you when you provided a link to a two band antenna... or did you forget to tell us that? I will let you rework the calcs for your scenario. I don't think ITU-R P.372-8 had my neighborhood in mind when it was composed. S-9 on this receiver is about the minimum on 160 and S-7 is the minimum on 75. S meter readings are pretty meaningless, but using the convention of 50uV for S 9, 6dB/SPoint, ITU-R P.372-8 does indeed suggest median noise at ~S7 for a 2kHz bandwidth and Residential locale. As to whether your receiver indicates S7 accurately, that is another matter. People who brag about S1 noise level on 80m have a lossy antenna and/or (and) an innaccurate S meter. Much of the time its higher and a royal pain in the arse. Something many of us experience. Owen |
#22
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in news:suednRqSYJd58aLanZ2dnUVZ_uDinZ2d@hawaiiantel. net: also, if these loops don't work then why do these designs exist? Tony, A few questions: 1. With a 50 load on the preamp input, when tuned to a quiet spot in the band, RF gain full, preamps on, telephony bandwidth, does the S meter deflect at all? not noticeable 2. If no s meter deflection, note the receiver audio output voltage. 125mv 3. With the loop connected on the preamp input, when tuned to a quiet spot in the band, RF gain full, preamps on, telephony bandwidth, does the S meter deflect at all? not noticeable 4. If no s meter deflection, note the receiver audio output voltage. 150mv 5. What is the ratio of the voltage at 4 to the voltage at 3? If it is more than about 3:1, you have achieved nearly as good a S/N ratio as is possible. Owen |
#23
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in news:bcSdnVU4bZ7W86LanZ2dnUVZ_vShnZ2d@hawaiiantel. net: I am using a heavily modified Drake R-4c receiver. Sherwood mods. And I at this point I only built the 80 meter loop as I wanted to make sure it worked on 80 before I went and built one for 160. I obviously misunderstood you when you provided a link to a two band antenna... or did you forget to tell us that? I used the design for the 80 meter loop - I would think they are mostly independent of each other. I will let you rework the calcs for your scenario. ???? I don't think ITU-R P.372-8 had my neighborhood in mind when it was composed. S-9 on this receiver is about the minimum on 160 and S-7 is the minimum on 75. S meter readings are pretty meaningless, but using the convention of 50uV for S 9, 6dB/SPoint, ITU-R P.372-8 does indeed suggest median noise at ~S7 for a 2kHz bandwidth and Residential locale. As to whether your receiver indicates S7 accurately, that is another matter. S-9 is 50 uv on this receiver on 40meters - can't be sure its exactly the same on 80, but the calibration voltage for S9 is very very close on 80. People who brag about S1 noise level on 80m have a lossy antenna and/or (and) an innaccurate S meter. or they are lying........ Much of the time its higher and a royal pain in the arse. Something many of us experience. Owen |
#24
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Low Noise receiving Loop antenna
Tony Giacometti wrote in
news:A5SdnQdv8vz466LanZ2dnUVZ_t2inZ2d@hawaiiantel. net: Owen Duffy wrote: Tony Giacometti wrote in news:suednRqSYJd58aLanZ2dnUVZ_uDinZ2d@hawaiiantel. net: also, if these loops don't work then why do these designs exist? Tony, A few questions: 1. With a 50 load on the preamp input, when tuned to a quiet spot in the band, RF gain full, preamps on, telephony bandwidth, does the S meter deflect at all? not noticeable 2. If no s meter deflection, note the receiver audio output voltage. 125mv 3. With the loop connected on the preamp input, when tuned to a quiet spot in the band, RF gain full, preamps on, telephony bandwidth, does the S meter deflect at all? not noticeable 4. If no s meter deflection, note the receiver audio output voltage. 150mv 5. What is the ratio of the voltage at 4 to the voltage at 3? If it is more than about 3:1, you have achieved nearly as good a S/N ratio as is possible. So, the ratio is 1.2. Two thirds (actually 1/1.2^2) of your total noise is from the receiver internal noise. That is not a good situation, S/N on signals will be degraded by relatively excessive contribution from the receiver, actually caused by inadequate antenna gain. 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. Things aren't working like they should, so you need to localise the problem. I have no idea of the sensitivity or bandwidth of your receiver, but a good (not outstanding, just good) (bare) receiver with a noise floor of - 130dBm should see a large increase in noise moving from dummy load to the loop. (Ambient at -97dBm would be equivalent to S5 if your S meter was accurate.) Just for verification, I performed the same test you did, but with a 600mm a side untuned loop and an ICR20 receiver, and I got a 10 fold increase in noise from the loop compared to the dummy load. Your loop is larger and tuned, so it should be a 10dB more sensitive. BTW, I didn't state it, but those noise measurements MUST be made in SSB mode. Owen |
#25
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Low Noise receiving Loop antenna
On Nov 17, 9:36 am, Tony Giacometti wrote:
My neighborhood is loaded with some kind of electrical noise which makes working 160 & 75 meters difficult. I have tried to locate the source but have come up empty at this point. It appears to be something other than power line noise which may mean its coming from someones house. I was hoping the solution to the problem was to build a low noise receiving loop antenna. Well, the noise is mostly gone..........but so are the signals.....even when I use a preamp. I have tried an ICE 75 meter preamp and a KD9SV 160/75 meter preamp. Still the noise is gone but the signals are really weak, if I hear anything at all. Not sure what if anything I did is wrong or if I am expecting too much from this loop or the preamps don't have enough gain to make this work. Anyone familiar enough with receiving loops to be able to assist me in figuring out whats wrong here? TIA Tony Hi Tony, I haven't bothered to wade through ALL the responses so far, but I am left wondering just what you did build. Could you explain it in more detail? Diameter, number of turns, how you're feeding it, how it's "shielded," exactly where the gap is, ... All the details. A loop can be effective in decreasing noise in two ways. If the noise is electromagnetic radiation it can only work if that radiation is coming from one direction, and in that case, you orient the loop to reject radiation from that direction. You better not want to be receiving a signal from the same direction, of course. The second way it can help is by rejecting locally generated electric field noise-- where you are in the near field of the source, and the electric field is considerably stronger in relationship to the magnetic field than it is in electromagnetic radiation. But the electric and magnetic fields fall off with distance rather quickly, so this only works if the noise source is on the order of a wavelength or less away. In order to build a loop that's effective in not responding to an electric-only field, it must be small compared with a wavelength. I'd think you'd want something around a foot in diameter for 80 meters, possibly slightly larger. You won't pick up much signal, but more importantly, it will be a high Q loop that you need to tune, and the resulting bandwidth will not be great. I have a suspicion that's where your problem lies. Reg Edwards supplied us with a very decent loop analysis program that might give you some insights. Look for rjeloop3.exe under http://www.we0h.us/G4FGQ-index.html. Cheers, Tom |
#26
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in news:A5SdnQdv8vz466LanZ2dnUVZ_t2inZ2d@hawaiiantel. net: Owen Duffy wrote: Tony Giacometti wrote in news:suednRqSYJd58aLanZ2dnUVZ_uDinZ2d@hawaiiantel. net: also, if these loops don't work then why do these designs exist? Tony, A few questions: 1. With a 50 load on the preamp input, when tuned to a quiet spot in the band, RF gain full, preamps on, telephony bandwidth, does the S meter deflect at all? not noticeable 2. If no s meter deflection, note the receiver audio output voltage. 125mv 3. With the loop connected on the preamp input, when tuned to a quiet spot in the band, RF gain full, preamps on, telephony bandwidth, does the S meter deflect at all? not noticeable 4. If no s meter deflection, note the receiver audio output voltage. 150mv 5. What is the ratio of the voltage at 4 to the voltage at 3? If it is more than about 3:1, you have achieved nearly as good a S/N ratio as is possible. So, the ratio is 1.2. Two thirds (actually 1/1.2^2) of your total noise is from the receiver internal noise. That is not a good situation, S/N on signals will be degraded by relatively excessive contribution from the receiver, actually caused by inadequate antenna gain. 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. Things aren't working like they should, so you need to localise the problem. I have no idea of the sensitivity or bandwidth of your receiver, but a good (not outstanding, just good) (bare) receiver with a noise floor of - 130dBm should see a large increase in noise moving from dummy load to the loop. (Ambient at -97dBm would be equivalent to S5 if your S meter was accurate.) Just for verification, I performed the same test you did, but with a 600mm a side untuned loop and an ICR20 receiver, and I got a 10 fold increase in noise from the loop compared to the dummy load. Your loop is larger and tuned, so it should be a 10dB more sensitive. BTW, I didn't state it, but those noise measurements MUST be made in SSB mode. Owen I did the measurements in the SSB mode. This receiver is very quiet with no antenna connected. When I tune the preselector for a peak in noise the noise jumps up noticeably. What about the AGC setting? |
#27
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Low Noise receiving Loop antenna
Tony Giacometti wrote:
Roy Lewallen wrote: Tony Giacometti wrote: it is a shielded loop and no the gap is there, thats why I am wondering why its not doing what I thought it would do. Roy, I don't fully understand why you made this response to me. You mentionI built a truly shielded loop. How so? I did exactly what the directions called for including having a gap. My apology. I too-quickly and incorrectly read your "and no the gap is there" as "no gap is there". My statement about your creating a truly shielded loop was wrong. The loop with the gap should work as intended. If these loops are a folk tale, other than using a beverage which I don't have the room for, how am I going to reduce the noise enough to be able to woprk 160 and 75 meters effectively? "Shielded" loops aren't a folk tale, they work fine. They have a broad pattern with, if constructed properly, narrow and deep nulls. It's a mistaken idea about how they work that's the folk tale. Roy Lewallen, W7EL |
#28
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Roy Lewallen wrote in : Tony Giacometti wrote: it is a shielded loop and no the gap is there, thats why I am wondering why its not doing what I thought it would do. What you've done is to build a truly shielded loop. The notion that a I think you guys are reading this the wrong way. Perhaps it should have been written "it is a shielded loop and no, the gap is there,..." You know people asking questions don't take much care in expression. I'm sorry to say, all too often I don't take proper care in reading postings. Although lack of punctuation does tend to exacerbate misunderstanding, the fault was mine. I'm trying to do better, but still slip up as I did this time. Roy Lewallen, W7EL |
#29
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Low Noise receiving Loop antenna
Roy Lewallen wrote:
Tony Giacometti wrote: Roy Lewallen wrote: Tony Giacometti wrote: it is a shielded loop and no the gap is there, thats why I am wondering why its not doing what I thought it would do. Roy, I don't fully understand why you made this response to me. You mentionI built a truly shielded loop. How so? I did exactly what the directions called for including having a gap. My apology. I too-quickly and incorrectly read your "and no the gap is there" as "no gap is there". My statement about your creating a truly shielded loop was wrong. The loop with the gap should work as intended. If these loops are a folk tale, other than using a beverage which I don't have the room for, how am I going to reduce the noise enough to be able to woprk 160 and 75 meters effectively? "Shielded" loops aren't a folk tale, they work fine. They have a broad pattern with, if constructed properly, narrow and deep nulls. It's a mistaken idea about how they work that's the folk tale. Roy Lewallen, W7EL Roy, you scared the daylights out of me for a minute, By the way, any idea why this loop might be under performing? |
#30
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Low Noise receiving Loop antenna
K7ITM wrote:
On Nov 17, 9:36 am, Tony Giacometti wrote: My neighborhood is loaded with some kind of electrical noise which makes working 160 & 75 meters difficult. I have tried to locate the source but have come up empty at this point. It appears to be something other than power line noise which may mean its coming from someones house. I was hoping the solution to the problem was to build a low noise receiving loop antenna. Well, the noise is mostly gone..........but so are the signals.....even when I use a preamp. I have tried an ICE 75 meter preamp and a KD9SV 160/75 meter preamp. Still the noise is gone but the signals are really weak, if I hear anything at all. Not sure what if anything I did is wrong or if I am expecting too much from this loop or the preamps don't have enough gain to make this work. Anyone familiar enough with receiving loops to be able to assist me in figuring out whats wrong here? TIA Tony Hi Tony, I haven't bothered to wade through ALL the responses so far, but I am left wondering just what you did build. Could you explain it in more detail? Diameter, number of turns, how you're feeding it, how it's "shielded," exactly where the gap is, ... All the details. A loop can be effective in decreasing noise in two ways. If the noise is electromagnetic radiation it can only work if that radiation is coming from one direction, and in that case, you orient the loop to reject radiation from that direction. You better not want to be receiving a signal from the same direction, of course. The second way it can help is by rejecting locally generated electric field noise-- where you are in the near field of the source, and the electric field is considerably stronger in relationship to the magnetic field than it is in electromagnetic radiation. But the electric and magnetic fields fall off with distance rather quickly, so this only works if the noise source is on the order of a wavelength or less away. In order to build a loop that's effective in not responding to an electric-only field, it must be small compared with a wavelength. I'd think you'd want something around a foot in diameter for 80 meters, possibly slightly larger. You won't pick up much signal, but more importantly, it will be a high Q loop that you need to tune, and the resulting bandwidth will not be great. I have a suspicion that's where your problem lies. Reg Edwards supplied us with a very decent loop analysis program that might give you some insights. Look for rjeloop3.exe under http://www.we0h.us/G4FGQ-index.html. Cheers, Tom Hi Tom, heres the link to the loop I built. http://www.qsl.net/kc2tx/ I only built the 80 meter loop not both. |
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