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#1
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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. Owen |
#2
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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 |
#3
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Tony Giacometti wrote:
Richard Clark wrote: On Sat, 17 Nov 2007 07:36:28 -1000, Tony Giacometti wrote: Anyone familiar enough with receiving loops to be able to assist me in figuring out whats wrong here? Hi Tony, There is no such thing as a Low Noise antenna (loop or otherwise). Given that what you build is deaf, that should be a reality check of this presumed quality of the antenna. Well, actually, you built it wrong. However, building it right stands only a partial chance of lowering noise, and not because the antenna has some remarkable quality that is not otherwise found in the plug ordinary dipole. In that sense, the reduction of noise would only follow turning the dipole (or loop, same thing) until the source of that noise fell into a null. This is the conventional method of employing a "Low Noise" antenna. Let me guess. It is a shielded loop. You forgot to leave a gap in the shield. 73's Richard Clark, KB7QHC 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. I have corrected my statement! Sorry! |
#4
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On Sat, 17 Nov 2007 12:15:11 -1000, Tony Giacometti
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. I have corrected my statement! Sorry! Hi Tony, No, as Owen pointed out, a poor reading on my part. Sorry. 73's Richard Clark, KB7QHC |
#5
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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 |
#6
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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. |
#7
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Hi Tom, heres the link to the loop I built.
http://www.qsl.net/kc2tx/ I only built the 80 meter loop not both. ============================ Interesting loop(s) Is there any significance in using RG59 (75 Ohms) coax, rather than RG58 (50 Ohms) ? Frank GM0CSZ / KN6WH |
#8
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On Nov 17, 11:36 am, Tony Giacometti wrote:
Anyone familiar enough with receiving loops to be able to assist me in figuring out whats wrong here? TIA Tony I use a lot of solenoid loops. You should not need a preamp with a decent size loop using an R4C in most cases. I'm wondering if you are actually tuning the loop to resonance. Do you notice a sharp, fairly high Q peak of noise and signals at the point where you think it is tuned? There will be no mistake hearing this peak if it's working correctly. 400 pf seems kind of low to me... IE: lets take a regular solenoid loop, being calculated by Reg Edwards loop program rjeloop3.exe.. It's fairly accurate. I set up a one turn loop using a 20 mm wire, each side of the loop 5 ft, or 1525mm. I set 1900kc as the desired frequency, which is the middle of the band. It shows a stray capacitance of appx 9 pf, and requires appx 1350 pf to tune to 1900 kc. See my problem with your meager 400 pf? Of course, you using coax instead of plain wire may be effecting the results.. I'm just not sure if you are actually tuned. I have two solenoid loops that I often use. One is a 16 inch circle, and the other is a diamond loop 44 inches per side. The 16 inch loop uses appx 12 turns. The larger loop, 5 turns. I still use fairly large values of caps to drop down low in frequency. IE: a dual 365pf BC radio cap, with both gangs tied together is usually needed to cover the whole BC band. On my large loop, the various gangs of the cap add up to a good bit more than 730 pf.. More like 1000+ pf or so.. So I get a wider tuning range, and can switch gangs out to get small values for the higher bands. Anyway, when building small loops, you usually must build the loop around the cap at hand, rather than try to match the cap to a set number of wires. You would think a "planned" loop would work though.. Anyway, the first thing I would need to know is do you hear a sharp noise peak when "tuned"? You should. And you really should have enough signal to not need a preamp in most cases. As an example, the loss in comparing the 5 ft per side loop vs a 1/4 wave monopole is appx -21 db according to the program. "1900 kc" That should not be enough to kill you on the lower bands. I run both of my loops straight to my IC-706mk2g most of the time, and need no preamp at all on 160m or BC. And I don't have to enable the radio preamp either. It's possible you could have a problem with the preamp. :/ If you want to try a solenoid loop, here is the program. http://home.comcast.net/~nm5k/Rjeloop3.exe I put it on my server, as I forgot where the archive of all his files are.. A simple way to make a PVC frame is here.. http://home.comcast.net/~nm5k/loop5.jpg The loop you have should work ok once tuned, but there seems to be a problem of some kind. Is your feed line ground shield making a connection to the loop on the other side of the cap from the center pin connection? I use separate coupling loops on mine and the coax feeds that loop. The variable cap is connected in parallel across the main loop connections. I get better balance and cleaner nulls if I use a coupling loop vs feeding directly at the cap/loop connection. But of course, I'm not using a "shielded" loop to aid in balance. But in the end, I get just as clean nulls as the shielded loop. MK |
#9
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On Nov 17, 6:16 pm, wrote:
On Nov 17, 11:36 am, Tony Giacometti wrote: Anyone familiar enough with receiving loops to be able to assist me in figuring out whats wrong here? TIA Tony I use a lot of solenoid loops. You should not need a preamp with a decent size loop using an R4C in most cases. I'm wondering if you are actually tuning the loop to resonance. Do you notice a sharp, fairly high Q peak of noise and signals at the point where you think it is tuned? There will be no mistake hearing this peak if it's working correctly. 400 pf seems kind of low to me... IE: lets take a regular solenoid loop, being calculated by Reg Edwards loop program rjeloop3.exe.. It's fairly accurate. I set up a one turn loop using a 20 mm wire, each side of the loop 5 ft, or 1525mm. I set 1900kc as the desired frequency, which is the middle of the band. It shows a stray capacitance of appx 9 pf, and requires appx 1350 pf to tune to 1900 kc. See my problem with your meager 400 pf? Hummm, I see you say you have only used it on 80m. According to the program, using a 2.5 ft per side loop, you would need appx 850-860 pf to tune 3700 kc. I guess the capacitance of the coax is making up the rest.. ??? IE: I think RG-58 and RG-8 is good for about 28-29 pf per foot or so. Even if you added that say extra 290 pf, you still come up a bit short from the 850+ pf needed for a plain wire loop. I think I would disconnect the preamp, at least for testing, and see if you can determine a noise peak into the receiver. You need to find out of the loop is actually tuning to resonance. Make sure both conductors of the feedline are connected to each connection of the loop, with the variable cap in parallel across those connections. If it's tuning, you should hear a distinct noise peak, and I really doubt you would need the preamp to hear it into a R4C. Once you get that going, it should be downhill from there. I have a R4C, along with an old original R4 too, so I know they should have enough sensitivity to hear the noise peak with no preamp. Well, unless it's broke.. ![]() MK |
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