Low Noise receiving Loop antenna
Owen Duffy wrote in
: I just went and re-examined the article, and it is not series tuned as I first thought, it is parallel tuned. My calcs of your loop were all on the basis of series tuned loop, and are not applicable to the parallel tuned circuit. It is questionable whether the parallel tuned circuit is an efficient coupling method for a low Z receiver. Owen |
Low Noise receiving Loop antenna
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 |
Low Noise receiving Loop antenna
Tony Giacometti wrote in
: I did the measurements in the SSB mode. This receiver is very quiet with no antenna connected. I know it is popular to make measurements with no antenna connected (meaning the antenna jack is left o/c). The measurements are meaningless. Measurements with a matched load are meaningful, the noise output power of the receiver is due to the equivalent receiver noise power + the noise in a matched load. You will also see discussion of whether receiver noise increases or decreases when a matched load is disconnected... it varies from receiver to receiver and it highlights the useless nature of the o/c measurment. When I tune the preselector for a peak in noise the noise jumps up noticeably. What about the AGC setting? To make a meaningful comparison, the receiver gain must not change between measurements, so no change in AGC, no change in RF or AF gain, safest if there is no S meter reading for both measurements. Turning AGC off doesn't necessarily extend the range of the SSB receiver where audio output is linearly related to RF input, don't depend on it unless you have measured its performance. Owen |
Low Noise receiving Loop antenna
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 |
Low Noise receiving Loop antenna
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. 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. 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. 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 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 |
Low Noise receiving Loop antenna
Owen Duffy wrote:
Owen Duffy wrote in : I just went and re-examined the article, and it is not series tuned as I first thought, it is parallel tuned. My calcs of your loop were all on the basis of series tuned loop, and are not applicable to the parallel tuned circuit. It is questionable whether the parallel tuned circuit is an efficient coupling method for a low Z receiver. Owen 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. I believe this should work, but for some reason not like I thought it would. |
Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in : I did the measurements in the SSB mode. This receiver is very quiet with no antenna connected. I know it is popular to make measurements with no antenna connected (meaning the antenna jack is left o/c). The measurements are meaningless. I did put a matched load on the receiver for these tests. Measurements with a matched load are meaningful, the noise output power of the receiver is due to the equivalent receiver noise power + the noise in a matched load. You will also see discussion of whether receiver noise increases or decreases when a matched load is disconnected... it varies from receiver to receiver and it highlights the useless nature of the o/c measurment. When I tune the preselector for a peak in noise the noise jumps up noticeably. What about the AGC setting? To make a meaningful comparison, the receiver gain must not change between measurements, so no change in AGC, no change in RF or AF gain, safest if there is no S meter reading for both measurements. Turning AGC off doesn't necessarily extend the range of the SSB receiver where audio output is linearly related to RF input, don't depend on it unless you have measured its performance. Owen I am thinking I should take another look at the receiver and make sure it does'nt have something strange going on. I do appreciate your input. |
Low Noise receiving Loop antenna
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 |
Low Noise receiving Loop antenna
On Nov 17, 7:31 pm, Tony Giacometti wrote:
I am thinking I should take another look at the receiver and make sure it does'nt have something strange going on. I do appreciate your input. You are getting a peak so the loop is tuned.. I doubt the receiver is the problem, or you wouldn't hear the noise peak. So it's down to pretty much what Roy just said at this point. You need to check the nulls. And if the noise is multiple sources, it can only do one at a time, unless two are in the same exact direction, or exactly 180 apart. So it might, or might not do what you want. The null should be very sharp. Just a slight movement can cause quite a change in the depth of the null on a local noise source, so the aim has to be near perfect for best results. MK |
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