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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 |
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 |
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 |
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. |
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......... |
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. |
Low Noise receiving Loop antenna
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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. |
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 |
Low Noise receiving Loop antenna
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 |
Low Noise receiving Loop antenna
On Sat, 17 Nov 2007 23:14:01 -1000, Tony Giacometti
wrote: 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. how do I calculate the dimensions of the coupling loop? Hi Tony, It has 1/5 the diameter and is a single, shorted loop. Please don't try to make it more elaborate than it should be. One wire, in a loop, connected to the other side of the feed (i.e. the wire runs from the center conductor, 'round the circumference, to the outer conductor). Of course, this demands that the bigger loop be equally, simply described. It too is one piece of wire, turned in a circle, each end connecting to the resonating capacitor. In this case, you want to optimize for the lowest Ohmic connections and materials. This may suggest hardline, but, please, don't think of using the inner conductor for anything at all - that is a waste of time. You could as easily use copper tubing. You can put more effort into it, but it will reward you only in the sense of being the owner of a fine piece of furniture. 73's Richard Clark, KB7QHC |
Low Noise receiving Loop antenna
Tony Giacometti wrote:
"How do I calculate the dimensions of the coupling loop?" You might try to make the radiation pesistance of the loop match the feedline. Arnold B. Bailey 0n page 400 of "TV and Other Receiving Antennas" gives an approximate squation: R = 197 L to the fourth power for L 0,08 wavelengths. Best regards, Richard Harrison, KB5WZI |
Low Noise receiving Loop antenna
On Nov 18, 4:00 am, Tony Giacometti wrote:
I have this feeling I don't know what I am supposed to experience from a working loop. Anyone have a mp3 file which can show what happens using a loop? Here is one I did in 2002 comparing my 16 inch circle loop vs my T vertical on the BC band. ----------------------------------------------------------------------------------------------------- I did a few quick comparisons between a 16 inch loop, and my "T" vertical, which is about 42 ft tall, with a 120 ft long flat top wire. It's pretty hot on MW. The radio was my ic-706mk2g. I'll let the recordings speak for themselves. You can click on the URl for the sound files, and your media player "should" bring them right up and start playing. Hopefully anyway... I did three tests, on three different frequencies, at different times in the evening. I'm in Houston, and used mainly San Antonio as the "target" city. "good 200 miles away" I recorded each test. I did compress the audio greatly to save d/l time, but the audio is still good enough to tell which is best. The files are pretty small and will d/l quickly. They were huge files in the original sample rate and format...I will "narrate" each test, so you will know which antenna was used, and the exact times I switched. You can use the counter in the media player to keep track of the time. Test #1 was at about 7:30 PM on 550 kc. http://home.comcast.net/~nm5k/mwtest1.mpeg "Time in seconds" 0-13 -----loop 13-26 -----wire 26-38 -----loop At 38 seconds I nulled the station, so you can hear the null. 46 -----loop, back pointed to the station 57-69 -----wire 69-end -----loop Test #2 was at about 8:00 PM on 680 kc. http://home.comcast.net/~nm5k/mwtest2.mpeg "Time in seconds" 0-11 -----loop 11-23 -----wire At 37 seconds I nulled the station 46 -----back pointed to the station 55-67 -----wire 67-end -----loop Test #3 was at about 9:00 PM on 570 kc. Multiple stations on this freq... http://home.comcast.net/~nm5k/mwtest3.mpeg "Time in seconds" 0-10 ----loop 10-23 ----wire 23-37 ----loop 37-48 ----wire At 62 seconds, I turn the loop 90 degrees to get a totally different station. At 74, I turned back to the first station. 85 ----turned back to 2nd station again 91 ----back to the first Here is another one on the BC where I turn the loop to null the station. http://home.comcast.net/~nm5k/mwtest4.mpeg At first the station is nulled, and you can hear a Mexican station in the background. At about 12 seconds, I turn to the desired station. At 20 seconds I switch to the T vertical. At 30 seconds I go back to the loop. At 40 seconds, I null the station again. Right now I am hearing very few stations on the loop amd not a one more than a S5, with an S1 noise level. I hear a couple stations in the SS contest from the west coast S5 on the loop no noise to be heard, on the transmitting antenna S6 noise level and the same west coast station S7 I can hear the stations he is working but not well enough to work either on the loop or the transmitting antenna. The stations he is working are in the noise and I know they are there. Does this sound right? Probably so. I haven't found the loops to be that great for pulling out weak stations on 160m. The success will depend on the level of noise you are able to cut, vs the strength of the desired station. This is why I'm playing with beverages a bit out in the country. I'm not sure how this will pan out, as in general changing the size of the loop should keep an equal s/n ratio, but some claim better 160m weak signal use if you use the largest loop you can manage.. I was able to dig up one 160m file, but it's not for nulling comparisons. It's just comparing the general noise received comparing the full size dipole, vs the loop. http://home.comcast.net/~nm5k/dpvloop.mpeg This is pretty much all close NVIS type signals.. Starts on the loop, and switches back and forth. You can tell which is the loop.. Much quieter background noise. It takes my AGC a second to adapt.. It's no good for showing nulls, but it does show the signal level I get from the small 16 inch loop using no preamp at all. None of those clips use a preamp. I've never used a preamp on small loops yet.. I prefer to avoid a preamp if possible.. :/ I was going to record a current file nulling line noise, but I don't have any noise at the moment.. :/ I use the loops for BC band use more than anything. But both do tune 160m, and I use it for that band when I want to kill some noise. MK |
Low Noise receiving Loop antenna
Owen Duffy wrote:
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 I looked at the specs for both cables and the difference is very small. Thats why I used RG-6, I have no source for RG-59 here. |
Low Noise receiving Loop antenna
Richard Clark wrote:
On Sat, 17 Nov 2007 23:14:01 -1000, Tony Giacometti wrote: 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. how do I calculate the dimensions of the coupling loop? Hi Tony, It has 1/5 the diameter and is a single, shorted loop. Please don't try to make it more elaborate than it should be. One wire, in a loop, connected to the other side of the feed (i.e. the wire runs from the center conductor, 'round the circumference, to the outer conductor). Of course, this demands that the bigger loop be equally, simply described. It too is one piece of wire, turned in a circle, each end connecting to the resonating capacitor. In this case, you want to optimize for the lowest Ohmic connections and materials. This may suggest hardline, but, please, don't think of using the inner conductor for anything at all - that is a waste of time. You could as easily use copper tubing. You can put more effort into it, but it will reward you only in the sense of being the owner of a fine piece of furniture. 73's Richard Clark, KB7QHC And to think I was just trying to build a simple low noise receiving loop :) Lets try this question..... if I were to start from scratch....... and I wanted to build a low noise receiving loop...... what style of low noise loop would you suggest I build? OR is there another solution to my situation on my 1/3 of an acre lot? |
Low Noise receiving Loop antenna
Tony Giacometti wrote in
: Owen Duffy wrote: 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 I looked at the specs for both cables and the difference is very small. Thats why I used RG-6, I have no source for RG-59 here. Tony, You might regard the difference as small, but in the models that I created the difference in transmission loss, velocity factor, and shield diameter are relevant. Anyway, I have built and calibrated a model of the 80m loop described in the ARRL Antenna Book (19th ed), and I am still checking the model. Initial results are that it does exhibit a peak in gain at about 300pF of tuning C, and that peak in gain is just a little less than an untuned unshielded loop of the same size. It looks like the gain is about -53dBi (not including feedline). If the expected noise from a 0dBi antenna in 2kHz was -83dBm, this would give around -136dBm which is in the region of the quiet noise floor on a good HF receiver. Of course, if you were in a very noise place, then this might be sufficient gain to achieve close to max S/N. I have also modelled the tuned loop with the LHS shield bonded to the inner conductor at the gap, which removes the lossy s/c stub from the picture, increasing gain a little. Now this connection will not appeal to the people with misconceptions about how a shielded loop works, but the s/c stub formed by the coax on one side of the loop is just another loss element that can be avoided. Made of RG6, the gain is about a half dB higher. I will look at it later in the day and put some notes together. Owen |
Low Noise receiving Loop antenna
On Nov 18, 3:14 am, Tony Giacometti wrote:
how do I calculate the dimensions of the coupling loop? It's really not that critical. But there is a "sweet spot" where you get the maximum coupling. The best way is to just test it and see where you get the strongest level, and nail it down. I'm looking at my 44 inch per side loop, and the coupling loop is about 5 inches inside of the main loop windings. It's a plain single wire. On my 16 inch loop, it's a circle, and is built on a plastic humidifier blower housing. The motor mount acts as the boom to attach to the mast. When it's on the stand, it almost gives the impression of some kind of perverted microwave dish at first look... :/ It's coupling loop is a three turn coax shielded loop that is appx three inches away from the main coil. It's also slightly less across, and is about a 12 inch loop inside the 16 inch loop. Yes, it has a gap in the shield at the top, which is the center of the total length of the coax. I tested various coupling loops on that one, and got the best level using the three turns, and the use of a "shielded" coax loop just aids in better balance. The main coil is plain insulated #12 wire. As you can tell in the recordings, my nulls are deep. In most cases with groundwave path signals, I can make an unwanted station vanish if I want. That applies to local single source noise too.. :) My usual culprit is line noise due south or north or me. But I can kill 95% of it. I'm lucky it seems to come from one location. In general, the nulls are not near as good with actual skywave signals late at night. But they can still help a bit, as they tend to reject a bit of unwanted crud vs a larger wire antenna. If I'm listening to the BC band, I'm always on the loop, no matter what the time of day. I prefer it even with skywave signals. But due to the generally lower signal levels on the weaker 160m band, it's a bit different animal. You might have better luck on 160 using say a set of phased short verticals. There was a thread talking about those just the other day. I think W8JI uses those quite a bit, and I know he often works stuff I don't even hear on the transmit vertical, or one of my loops. On 160, I think just being vertical is an advantage for long haul DX. Either that or use beverages, which I'm gonna assume are probably not possible at your QTH... :( BTW, one time I hooked the 16 inch loop to a AC/Delco car radio in a truck.. Worked great. :) MK |
Low Noise receiving Loop antenna
Tony Giacometti wrote:
And to think I was just trying to build a simple low noise receiving loop :) Lets try this question..... if I were to start from scratch....... and I wanted to build a low noise receiving loop...... what style of low noise loop would you suggest I build? OR is there another solution to my situation on my 1/3 of an acre lot? What you should do is realize that there is no such thing as a "low noise receiving loop". No antenna has any way to tell the difference between signals and noise, so a "low noise receiving loop" is also just as much a "low signal receiving loop". All a loop can do that might help with a noise problem is provide a sharp null in one direction (actually, two, but it's very unlikely that both will be useful at the same time). If the noise is coming from a single narrow direction, you can turn the loop to reduce it. If the noise isn't coming from one narrow direction, a loop won't be "low noise" and, as I pointed out earlier, might well be worse than an outside antenna if the noise is being radiated by house wiring. From all the descriptions, it sounds like your main problem with the loop is that the overall gain of your loop + receiver is simply too low for comfortable listening. What you need in order to provide the necessary additional gain is either a better impedance match between the loop and receiver (for which there have been multiple suggestions), an amplifier either ahead of the receiver (a preamp) or after the receiver (an audio amplifier), or some combination of those. If you can null out the noise by turning the loop, then the additional gain will get you what you're trying to achieve. If not, then a loop isn't the solution, and the only solution might be to find and eliminate the source of the noise. Roy Lewallen, W7EL |
Low Noise receiving Loop antenna
wrote:
On Nov 18, 4:00 am, Tony Giacometti wrote: I have this feeling I don't know what I am supposed to experience from a working loop. Anyone have a mp3 file which can show what happens using a loop? Here is one I did in 2002 comparing my 16 inch circle loop vs my T vertical on the BC band. ----------------------------------------------------------------------------------------------------- I did a few quick comparisons between a 16 inch loop, and my "T" vertical, which is about 42 ft tall, with a 120 ft long flat top wire. It's pretty hot on MW. The radio was my ic-706mk2g. I'll let the recordings speak for themselves. You can click on the URl for the sound files, and your media player "should" bring them right up and start playing. Hopefully anyway... I did three tests, on three different frequencies, at different times in the evening. I'm in Houston, and used mainly San Antonio as the "target" city. "good 200 miles away" I recorded each test. I did compress the audio greatly to save d/l time, but the audio is still good enough to tell which is best. The files are pretty small and will d/l quickly. They were huge files in the original sample rate and format...I will "narrate" each test, so you will know which antenna was used, and the exact times I switched. You can use the counter in the media player to keep track of the time. Test #1 was at about 7:30 PM on 550 kc. http://home.comcast.net/~nm5k/mwtest1.mpeg "Time in seconds" 0-13 -----loop 13-26 -----wire 26-38 -----loop At 38 seconds I nulled the station, so you can hear the null. 46 -----loop, back pointed to the station 57-69 -----wire 69-end -----loop Test #2 was at about 8:00 PM on 680 kc. http://home.comcast.net/~nm5k/mwtest2.mpeg "Time in seconds" 0-11 -----loop 11-23 -----wire At 37 seconds I nulled the station 46 -----back pointed to the station 55-67 -----wire 67-end -----loop Test #3 was at about 9:00 PM on 570 kc. Multiple stations on this freq... http://home.comcast.net/~nm5k/mwtest3.mpeg "Time in seconds" 0-10 ----loop 10-23 ----wire 23-37 ----loop 37-48 ----wire At 62 seconds, I turn the loop 90 degrees to get a totally different station. At 74, I turned back to the first station. 85 ----turned back to 2nd station again 91 ----back to the first Here is another one on the BC where I turn the loop to null the station. http://home.comcast.net/~nm5k/mwtest4.mpeg At first the station is nulled, and you can hear a Mexican station in the background. At about 12 seconds, I turn to the desired station. At 20 seconds I switch to the T vertical. At 30 seconds I go back to the loop. At 40 seconds, I null the station again. Right now I am hearing very few stations on the loop amd not a one more than a S5, with an S1 noise level. I hear a couple stations in the SS contest from the west coast S5 on the loop no noise to be heard, on the transmitting antenna S6 noise level and the same west coast station S7 I can hear the stations he is working but not well enough to work either on the loop or the transmitting antenna. The stations he is working are in the noise and I know they are there. Does this sound right? Probably so. I haven't found the loops to be that great for pulling out weak stations on 160m. The success will depend on the level of noise you are able to cut, vs the strength of the desired station. This is why I'm playing with beverages a bit out in the country. I'm not sure how this will pan out, as in general changing the size of the loop should keep an equal s/n ratio, but some claim better 160m weak signal use if you use the largest loop you can manage.. I was able to dig up one 160m file, but it's not for nulling comparisons. It's just comparing the general noise received comparing the full size dipole, vs the loop. http://home.comcast.net/~nm5k/dpvloop.mpeg This is pretty much all close NVIS type signals.. Starts on the loop, and switches back and forth. You can tell which is the loop.. Much quieter background noise. It takes my AGC a second to adapt.. It's no good for showing nulls, but it does show the signal level I get from the small 16 inch loop using no preamp at all. None of those clips use a preamp. I've never used a preamp on small loops yet.. I prefer to avoid a preamp if possible.. :/ I was going to record a current file nulling line noise, but I don't have any noise at the moment.. :/ I use the loops for BC band use more than anything. But both do tune 160m, and I use it for that band when I want to kill some noise. MK Thank you for the audio clips, the loop is really working for you. No preamp!!! WOW!!! Truly Amazing! |
Low Noise receiving Loop antenna
Tony, you have received a crash course on small loop antennas minus
the calculus... Let me make a few comments as a country boy engineer who cannot do fancy math... My thought for you to test the loop+Drake is for you to put out a temporary Beverage as a wire on the ground... Go out to the curb in the wee hours of night and roll out 500 foot of insulated wire on the grass between the curb and sidewalk, match it to the Drake with a 9:1 transformer and a ground stake - a terminating resistor and a ground stake at the far end is a good idea, but can be dispensed with in a pinch...' By flipping back and forth between the wire and the loop you will get an idea of how well, or poorly, the loop is performing... By having everything ready to go to roll it out and then roll it back up you should be able to do this test without the neighbors being any wiser in the morning... A variation of the poor boy Beverage, is a 100' untuned dipole running along the perimeter of your estate, waist high no higher which will be omni directional... It might even prove to be a full time installation given your city constraints... Even those of us in the rural areas have noise problems.. Electric fences, etc.. I have a nasty broad band up through 40 meters noise source somewhere to my NorthEast, which essentially wipes out weak dx from europe for me... It is miles from me and I have yet to locate it... denny / k8do |
Low Noise receiving Loop antenna
Tony,
The key to improving the loop is to -increase the induced voltage; and / or to -improve its efficiency. Increasing the induced voltage means increasing the size of the loop. With a small loop, doubling the size of the side will win nearly 6dB of induced voltage, but it also increases the inductance of the loop which might degrade impedance matching and defeat most of the increased induced voltage. Improving the efficiency means addressing conductor / transmission line losses and better impedance matching. Most of the 50 some dB of loss is attributable to impedance matching. I expressed surprise at the shunt tuning capacitor in an earlier posting. I know it is a popular circuit, and it features in the ARRL Antenna Handbook, but that doesn't make it a good circuit. Try the variable capacitor in series with the coax inner conductor, you should improve the gain by around 20dB. Then try a shunt capacitor on the receiver side of the variable capacitor, start with 1000pF, you should see further improvement in gain but with a narrower bandwidth. This is not a new circuit, you will find it in books, certainly at least where the tuning / matching network is right at the loop gap. The relocation of the capacitors by a length of transmission line does change things a little, and it is more complicated to solve, but behaviour is soemwhat similar. Should you try this, your findings would be interesting. The mathematically based approach might not be popular, and I am no mathemetician, but the approach does reveal why the antenna is inadequate, and suggests what can be done to improve it. Owen |
Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony, The key to improving the loop is to -increase the induced voltage; and / or to -improve its efficiency. I agree, but how is the question Increasing the induced voltage means increasing the size of the loop. With a small loop, doubling the size of the side will win nearly 6dB of induced voltage, but it also increases the inductance of the loop which might degrade impedance matching and defeat most of the increased induced voltage. Improving the efficiency means addressing conductor / transmission line losses and better impedance matching. Most of the 50 some dB of loss is attributable to impedance matching. I expressed surprise at the shunt tuning capacitor in an earlier posting. I know it is a popular circuit, and it features in the ARRL Antenna Handbook, and alot of other places also. but that doesn't make it a good circuit. Try the variable capacitor in series with the coax inner conductor, you should improve the gain by around 20dB. this one I will try, does this not make the loop a closed loop? Then try a shunt capacitor on the receiver side of the variable capacitor, start with 1000pF, you should see further improvement in gain but with a narrower bandwidth. Don't have anything like this on hand, this is a variable capacitor I would assume. This is not a new circuit, you will find it in books, certainly at least where the tuning / matching network is right at the loop gap. The relocation of the capacitors by a length of transmission line does change things a little, and it is more complicated to solve, but behaviour is soemwhat similar. Should you try this, your findings would be interesting. I might be able to get to this in a day or two and I will let you know what happened. The mathematically based approach might not be popular, and I am no mathemetician, but the approach does reveal why the antenna is inadequate, and suggests what can be done to improve it. Owen I was thinking that maybe I didn't have enough capture area with the loop I currently use. My next step was to figure out how to increase the size of the loop so it captures more signal. Since I havn't looked for any info on a loop like this I was going to ask you if you had anything on a larger loop but one which isn't too large and can be rotated either by hand or a rotator. I think you answered my question(s) at the top of this post. Doubling the size would still make it small enough to rotate by hand or with a small tv rotator, matching though might be the other issue as you mentioned. |
Low Noise receiving Loop antenna
Denny wrote:
Tony, you have received a crash course on small loop antennas minus the calculus... Let me make a few comments as a country boy engineer who cannot do fancy math... My thought for you to test the loop+Drake is for you to put out a temporary Beverage as a wire on the ground... Go out to the curb in the wee hours of night and roll out 500 foot of insulated wire on the grass between the curb and sidewalk, match it to the Drake with a 9:1 transformer and a ground stake - a terminating resistor and a ground stake at the far end is a good idea, but can be dispensed with in a pinch...' By flipping back and forth between the wire and the loop you will get an idea of how well, or poorly, the loop is performing... By having everything ready to go to roll it out and then roll it back up you should be able to do this test without the neighbors being any wiser in the morning... A variation of the poor boy Beverage, is a 100' untuned dipole running along the perimeter of your estate, waist high no higher which will be omni directional... It might even prove to be a full time installation given your city constraints... Even those of us in the rural areas have noise problems.. Electric fences, etc.. I have a nasty broad band up through 40 meters noise source somewhere to my NorthEast, which essentially wipes out weak dx from europe for me... It is miles from me and I have yet to locate it... denny / k8do this is a good idea, except I can't go 500 ft in any direction without getting into my neighbors property. Best I can do is about 175ft and that would be into my neighbors property who has some dogs which will at any hour of the day or nite bark until the neighbor comes out to quiet them down. I am in the middle of a cul-de-sac that about 200 ft long. All my neighbors on the other side of the street have dogs. The 2 north and south of me wouldn't give me enough room to put out enough wire to make this test worthwhile and the neighbors behind me are anti amateur radio types. I was here first and had antennas up before they even built their houses but yet I am the bad dude in the neighborhood. FORTUNATELY, I have never been accused of any interference even running 1500 watts. Oh this is a "rural" subdivision so to speak, no curbs no sidewalks. But the lots are 1/3 acre or slightly more. Damn, you give me a good idea and I can't use it. |
Low Noise receiving Loop antenna
Tony Giacometti wrote in
: I expressed surprise at the shunt tuning capacitor in an earlier posting. I know it is a popular circuit, and it features in the ARRL Antenna Handbook, and alot of other places also. Ok, but they are all flawed. but that doesn't make it a good circuit. Try the variable capacitor in series with the coax inner conductor, you should improve the gain by around 20dB. this one I will try, does this not make the loop a closed loop? I don't understand what you mean. Just reconfigure the tuning box so that the capacitor is in series with the inner conductor of the feedline to the radio. Is that ambiguous? Then try a shunt capacitor on the receiver side of the variable capacitor, start with 1000pF, you should see further improvement in gain but with a narrower bandwidth. Don't have anything like this on hand, this is a variable capacitor I would assume. No, just try a fixed capacitor. You could also try 2200pF, it will have a little more gain but narrower bandwidth. This is not a new circuit, you will find it in books, certainly at least where the tuning / matching network is right at the loop gap. The relocation of the capacitors by a length of transmission line does change things a little, and it is more complicated to solve, but behaviour is soemwhat similar. Should you try this, your findings would be interesting. I might be able to get to this in a day or two and I will let you know what happened. Thanks. I was thinking that maybe I didn't have enough capture area with the loop I currently use. My next step was to figure out how to increase the size of the loop so it captures more signal. Since I havn't looked for any info on a loop like this I was going to ask you if you had anything on a larger loop but one which isn't too large and can be rotated either by hand or a rotator. I think you answered my question(s) at the top of this post. Doubling the size would still make it small enough to rotate by hand or with a small tv rotator, matching though might be the other issue as you mentioned. In the simple case of an untuned loop loaded with 50 ohms, when the loop inductive reactance becomes more than becomes more than about 150 ohms, doubling the size of the loop doubles the induced voltage and doubles the reactance, so the loop current hardly increases and not much more voltage is developed in the 50 ohm load. Do you follow? A lossless loop of the size you use has a source impedance of 0.0002 ohms. For maximum power transfer, you have to do something better than deliver a load of the order of a hundred ohms. It is perhaps a difficult concept to grasp. But it explains why small antennas don't work so well, it is just so hard to deliver or extract power because the radiation resistance is so small. Owen |
Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in : I expressed surprise at the shunt tuning capacitor in an earlier posting. I know it is a popular circuit, and it features in the ARRL Antenna Handbook, and alot of other places also. Ok, but they are all flawed. but that doesn't make it a good circuit. Try the variable capacitor in series with the coax inner conductor, you should improve the gain by around 20dB. this one I will try, does this not make the loop a closed loop? I don't understand what you mean. Just reconfigure the tuning box so that the capacitor is in series with the inner conductor of the feedline to the radio. Is that ambiguous? I guess I am unsure as to how the loop gets connected. since the loop and cap are currently parallel, removing the cap leaves me with 2 ends of the loop. Putting the cap in series with the center of the coax feedline allows me to have only one connection to the loop. Did I miss something? Then try a shunt capacitor on the receiver side of the variable capacitor, start with 1000pF, you should see further improvement in gain but with a narrower bandwidth. Don't have anything like this on hand, this is a variable capacitor I would assume. No, just try a fixed capacitor. You could also try 2200pF, it will have a little more gain but narrower bandwidth. I have caps in that range, and since its receive only, the voltage can be low. This is not a new circuit, you will find it in books, certainly at least where the tuning / matching network is right at the loop gap. The relocation of the capacitors by a length of transmission line does change things a little, and it is more complicated to solve, but behaviour is soemwhat similar. Should you try this, your findings would be interesting. I might be able to get to this in a day or two and I will let you know what happened. Thanks. As soon as I figure out how it needs to be wired. :) I was thinking that maybe I didn't have enough capture area with the loop I currently use. My next step was to figure out how to increase the size of the loop so it captures more signal. Since I havn't looked for any info on a loop like this I was going to ask you if you had anything on a larger loop but one which isn't too large and can be rotated either by hand or a rotator. I think you answered my question(s) at the top of this post. Doubling the size would still make it small enough to rotate by hand or with a small tv rotator, matching though might be the other issue as you mentioned. In the simple case of an untuned loop loaded with 50 ohms, when the loop inductive reactance becomes more than becomes more than about 150 ohms, doubling the size of the loop doubles the induced voltage and doubles the reactance, so the loop current hardly increases and not much more voltage is developed in the 50 ohm load. Do you follow? yes, wellbrook has a large capture area loop, I wonder how they get that one to work? A lossless loop of the size you use has a source impedance of 0.0002 ohms. For maximum power transfer, you have to do something better than deliver a load of the order of a hundred ohms. It is perhaps a difficult concept to grasp. But it explains why small antennas don't work so well, it is just so hard to deliver or extract power because the radiation resistance is so small. Owen I never though this was going to be this complicated, I thought all I needed do was to build the loop and problem solved.......so I thought......:) |
Low Noise receiving Loop antenna
On Mon, 19 Nov 2007 21:07:30 -1000, Tony Giacometti
wrote: I never though this was going to be this complicated, I thought all I needed do was to build the loop and problem solved.......so I thought......:) At the risk of repetition still not resolving what is simple: On Sat, 17 Nov 2007 23:14:01 -1000, Tony Giacometti wrote: 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. how do I calculate the dimensions of the coupling loop? Hi Tony, It has 1/5 the diameter and is a single, shorted loop. Please don't try to make it more elaborate than it should be. One wire, in a loop, connected to the other side of the feed (i.e. the wire runs from the center conductor, 'round the circumference, to the outer conductor). Of course, this demands that the bigger loop be equally, simply described. It too is one piece of wire, turned in a circle, each end connecting to the resonating capacitor. In this case, you want to optimize for the lowest Ohmic connections and materials. This may suggest hardline, but, please, don't think of using the inner conductor for anything at all - that is a waste of time. You could as easily use copper tubing. You can put more effort into it, but it will reward you only in the sense of being the owner of a fine piece of furniture. 73's Richard Clark, KB7QHC 73's Richard Clark, KB7QHC |
Low Noise receiving Loop antenna
Tony Giacometti wrote in
news:yKednQUgeNMRGt_anZ2dnUVZ_oaonZ2d@hawaiiantel. net: .... I guess I am unsure as to how the loop gets connected. since the loop and cap are currently parallel, removing the cap leaves me with 2 ends of the loop. Putting the cap in series with the center of the coax feedline allows me to have only one connection to the loop. Did I miss something? Someone else will probably have to transate the following for you (I must have an upside down view of the world from down under): Disconnect both wires from the capacitor. Now cut the centre wire from the loop the the feed line coax in the tee box... there is only one wire that fits this description. Insert the tuning capacitor (two wires) where you cut the centre conductor , ie connect the capacitor wires to each side of where you cut the centre conductor The capacitor is now in series with the centre conductor of the feedline connection in the tuning / matching box. Now connect a 100pF fixed capacitor between the feedline centre conductor and shield in the tuning box. Now adjust the capacitor for maximum rx noise. Owen |
Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in news:yKednQUgeNMRGt_anZ2dnUVZ_oaonZ2d@hawaiiantel. net: ... I guess I am unsure as to how the loop gets connected. since the loop and cap are currently parallel, removing the cap leaves me with 2 ends of the loop. Putting the cap in series with the center of the coax feedline allows me to have only one connection to the loop. Did I miss something? Someone else will probably have to transate the following for you (I must have an upside down view of the world from down under): Disconnect both wires from the capacitor. sorry, I'm not trying to be an ass here but the diagram I have shows 3 connections to the cap. 2 for the loop and one for the center conductor of the feedline. Is my diagram whats confusing the issue? Now cut the centre wire from the loop the the feed line coax in the tee box... there is only one wire that fits this description. Insert the tuning capacitor (two wires) where you cut the centre conductor , ie connect the capacitor wires to each side of where you cut the centre conductor The capacitor is now in series with the centre conductor of the feedline connection in the tuning / matching box. Now connect a 100pF fixed capacitor between the feedline centre conductor and shield in the tuning box. Now adjust the capacitor for maximum rx noise. Owen |
Low Noise receiving Loop antenna
Tony Giacometti wrote in
news:bMednQ1HY8JTVN7anZ2dnUVZ_gGdnZ2d@hawaiiantel. net: .... sorry, I'm not trying to be an ass here but the diagram I have shows 3 connections to the cap. 2 for the loop and one for the center conductor of the feedline. Is my diagram whats confusing the issue? Here is a diagram of what I suggested: http://www.vk1od.net/SmallTunedSquareLoop/index.2.gif . The variable capacitor needs to be 0-400pF. The fixed capacitor should be at least 1000pF. If you couldn't arrive at this circuit from the word description, and you aren't being silly, then it questions whether your intial construction was based on misinterpretation of the original article. Here is an even better idea: http://www.vk1od.net/SmallTunedSquareLoop/index.3.gif . There is only one change here, the inner conductor is bonded to the outer conductor at the left hand side of the shield gap. Ask yourself why that improves things... some will suggest it just halved the loop size, but it hasn't. Owen |
Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in news:bMednQ1HY8JTVN7anZ2dnUVZ_gGdnZ2d@hawaiiantel. net: ... sorry, I'm not trying to be an ass here but the diagram I have shows 3 connections to the cap. 2 for the loop and one for the center conductor of the feedline. Is my diagram whats confusing the issue? Here is a diagram of what I suggested: http://www.vk1od.net/SmallTunedSquareLoop/index.2.gif . The variable capacitor needs to be 0-400pF. The fixed capacitor should be at least 1000pF. If you couldn't arrive at this circuit from the word description, and you aren't being silly, then it questions whether your intial construction was based on misinterpretation of the original article. Here is an even better idea: http://www.vk1od.net/SmallTunedSquareLoop/index.3.gif . There is only one change here, the inner conductor is bonded to the outer conductor at the left hand side of the shield gap. Ask yourself why that improves things... some will suggest it just halved the loop size, but it hasn't. Owen I understand now, seeing your drawing clears it up for me. No, I wasn't being difficult or silly, I kept thinking of how I have the unit wired and I couldn't picture in my mind (senior moment?) the changes. Anyway now I see what you are asking me to try and I will give this a try. Hopefully tonite after work. I will let you know the results. Thank You. |
Low Noise receiving Loop antenna
Tony Giacometti wrote in
news:geadnZCVBcuhxdnanZ2dnUVZ_ojinZ2d@hawaiiantel. net: Owen Duffy wrote: Tony Giacometti wrote in news:bMednQ1HY8JTVN7anZ2dnUVZ_gGdnZ2d@hawaiiantel. net: ... sorry, I'm not trying to be an ass here but the diagram I have shows 3 connections to the cap. 2 for the loop and one for the center conductor of the feedline. Is my diagram whats confusing the issue? Here is a diagram of what I suggested: http://www.vk1od.net/SmallTunedSquareLoop/index.2.gif . The variable capacitor needs to be 0-400pF. The fixed capacitor should be at least 1000pF. If you couldn't arrive at this circuit from the word description, and you aren't being silly, then it questions whether your intial construction was based on misinterpretation of the original article. Here is an even better idea: http://www.vk1od.net/SmallTunedSquareLoop/index.3.gif . There is only one change here, the inner conductor is bonded to the outer conductor at the left hand side of the shield gap. Ask yourself why that improves things... some will suggest it just halved the loop size, but it hasn't. Owen I understand now, seeing your drawing clears it up for me. No, I wasn't being difficult or silly, I kept thinking of how I have the unit wired and I couldn't picture in my mind (senior moment?) the changes. Anyway now I see what you are asking me to try and I will give this a try. Hopefully tonite after work. I will let you know the results. Thank You. Tony, Keep in mind this scheme improves the impedance match (ie reduces the mismatch "losses", but it doesn't approach an MPT match unless the fixed capacitor is very large, and in that case bandwidth would probably be too narrow for your use. The trick is to improve the system gain enough to be usable without making the system unusable because it is too narrow. The suggested fixed capacitor of 1000pF should win you enough system gain, if it doesn't try larger values (eg 4,700pF). I will be offline for a few days, I look forward to hearing of the outcome. Owen |
Low Noise receiving Loop antenna
On Nov 21, 1:10 pm, Owen Duffy wrote:
I understand now, seeing your drawing clears it up for me. No, I wasn't being difficult or silly, I kept thinking of how I have the unit wired and I couldn't picture in my mind (senior moment?) the changes. Anyway now I see what you are asking me to try and I will give this a try. Hopefully tonite after work. I will let you know the results. Thank You. Tony, Keep in mind this scheme improves the impedance match (ie reduces the mismatch "losses", but it doesn't approach an MPT match unless the fixed capacitor is very large, and in that case bandwidth would probably be too narrow for your use. The trick is to improve the system gain enough to be usable without making the system unusable because it is too narrow. The suggested fixed capacitor of 1000pF should win you enough system gain, if it doesn't try larger values (eg 4,700pF). I will be offline for a few days, I look forward to hearing of the outcome. Owen I guess it's worth a shot if the level is really that low. But something seems strange that he would have that problem. Using even my small 16 inch loop, fed using the usual cap in parallel method, I have more signal level than I actually need, even at the upper end of its range. In general if the background noise level peaks, and there is a definite difference between the antenna connected, and unconnected, seems that should be enough signal level. On the recordings I made, I was comparing the 16 inch loop vs full size wire antennas, and the drop in level is really not that drastic. IE: the stations still sound "loud" vs the large wire antennas, and I have pretty decent S meter levels on the loops. I'm tuned to 740 kc AM-BC on my big loop right now and the S meter is reading 40 over 9, and the radio preamp is off. I didn't really notice any level problems on the shielded loops I've tried. But I'm starting to wonder if maybe he might get a higher level just from using a regular solenoid loop. It's also possible the radio could use a tweak.. Which is easy to do on the drake receivers. All the trimmers are on top of the chassis. Don't even need any fancy gear. He can tune for the middle of each band appx, and use the xtal calibrator as the signal to tweak all the trimmers per each band. Of course, you would peak each trimmer for max S meter reading. On the drakes, he should show a pretty healthy S meter reading on the calibrator signal if it's working right. MK |
Low Noise receiving Loop antenna
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Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in news:geadnZCVBcuhxdnanZ2dnUVZ_ojinZ2d@hawaiiantel. net: Owen Duffy wrote: Tony Giacometti wrote in news:bMednQ1HY8JTVN7anZ2dnUVZ_gGdnZ2d@hawaiiantel. net: ... sorry, I'm not trying to be an ass here but the diagram I have shows 3 connections to the cap. 2 for the loop and one for the center conductor of the feedline. Is my diagram whats confusing the issue? Here is a diagram of what I suggested: http://www.vk1od.net/SmallTunedSquareLoop/index.2.gif . The variable capacitor needs to be 0-400pF. The fixed capacitor should be at least 1000pF. If you couldn't arrive at this circuit from the word description, and you aren't being silly, then it questions whether your intial construction was based on misinterpretation of the original article. Here is an even better idea: http://www.vk1od.net/SmallTunedSquareLoop/index.3.gif . There is only one change here, the inner conductor is bonded to the outer conductor at the left hand side of the shield gap. Ask yourself why that improves things... some will suggest it just halved the loop size, but it hasn't. Owen I understand now, seeing your drawing clears it up for me. No, I wasn't being difficult or silly, I kept thinking of how I have the unit wired and I couldn't picture in my mind (senior moment?) the changes. Anyway now I see what you are asking me to try and I will give this a try. Hopefully tonite after work. I will let you know the results. Thank You. Tony, Keep in mind this scheme improves the impedance match (ie reduces the mismatch "losses", but it doesn't approach an MPT match unless the fixed capacitor is very large, and in that case bandwidth would probably be too narrow for your use. The trick is to improve the system gain enough to be usable without making the system unusable because it is too narrow. The suggested fixed capacitor of 1000pF should win you enough system gain, if it doesn't try larger values (eg 4,700pF). I will be offline for a few days, I look forward to hearing of the outcome. Owen I tried both configurations, both raised the noise level which I am thinking must be related to the gain you said I would get, I didn't need the preamp at all. I didn't get the nulls like I get from the original loop - that was a mild surprise - there was only a 2-3 S unit difference between noise peak and noise null, The original loop normally has S1-2 99% of the time when nulled. The current loop has a difference between noise peak and noise null of 7-9 S units. Now don't misunderstand me, but I like the low noise level. What concerns me with the other 2 configurations is the gain makes the noise increase so much that I am not so sure I would be able to hear the weaker ones. I think this project as simple as I thought it would be ...... just isn't. What I would like to accomplish with this loop idea is to have the low noise and the ability to null out the noise from my neighbors houses. But at the same time have enough gain to be able to hear the weak signals. Now I don't have preference for using the preamp or not. Whatever works is what I want to go with. Maybe changing the type of loop to a different style of loop would be the way to achive what I would like to obtain. Obviously, there is a miriad of designs and configurations. Since I need the loop only for 75 meters and have a limited amount of real estate this cuts the pile down to a lesser amount. What to do...................well, maybe I should start over - why not try a different type/style of loop. A larger loop might be better for gain but how big before its not able to null out noise cause I can't rotate it due to its size? Does the loop have to be coax? can it be some other type of wire? Your input is much appreciated. |
Low Noise receiving Loop antenna
Tony Giacometti wrote in
: Owen Duffy wrote: .... I will be offline for a few days, I look forward to hearing of the outcome. Owen I tried both configurations, both raised the noise level which I am thinking must be related to the gain you said I would get, I didn't I take it that 'raise the noise level' actually means you got more noise out of the receiver / higher S meter reading on band noise. need the preamp at all. I didn't get the nulls like I get from the original loop - that was a mild surprise - there was only a 2-3 S unit Neither the preamp, nor the tuning changes you made should change the directivity (ie the pattern, the depth of the nulls) of the antenna. difference between noise peak and noise null, The original loop normally has S1-2 99% of the time when nulled. The current loop has a difference between noise peak and noise null of 7-9 S units. Now don't I have already told you those measurements don't mean anything to me. misunderstand me, but I like the low noise level. What concerns me with the other 2 configurations is the gain makes the noise increase so much that I am not so sure I would be able to hear the weaker ones. If you want to improve the depth of the nulls, you have to concentrate on the balance of the antenna, the symmetry of the loop, its feedline, its environment etc. I think Roy might have said that earlier. I think this project as simple as I thought it would be ...... just isn't. What I would like to accomplish with this loop idea is to have the low noise and the ability to null out the noise from my neighbors houses. But at the same time have enough gain to be able to hear the weak signals. Now I don't have preference for using the preamp or not. Whatever works is what I want to go with. It is fairly simple, you might have needed to learn a bit, but you probably also had to unlearn some stuff. Maybe changing the type of loop to a different style of loop would be the way to achive what I would like to obtain. 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. Obviously, there is a miriad of designs and configurations. Since I need the loop only for 75 meters and have a limited amount of real estate this cuts the pile down to a lesser amount. What to do...................well, maybe I should start over - why not try a different type/style of loop. A larger loop might be better for gain but how big before its not able to null out noise cause I can't rotate it due to its size? Does the loop have to be coax? can it be some other type of wire? Remember that symmetry is key to the depth of the null. If it is harder to build a larger loop with equally good symmetry (including to the environment), you have more gain, but poorer nulls... and your main reason for selecting the loop is for rejection of interference using the nulls. You need adequate gain, and the best balance rather than the other way round. Your input is much appreciated. Thanks 73 Owen |
Low Noise receiving Loop antenna
Owen Duffy wrote:
Tony Giacometti wrote in : Owen Duffy wrote: ... I will be offline for a few days, I look forward to hearing of the outcome. Owen I tried both configurations, both raised the noise level which I am thinking must be related to the gain you said I would get, I didn't I take it that 'raise the noise level' actually means you got more noise out of the receiver / higher S meter reading on band noise. yes! need the preamp at all. I didn't get the nulls like I get from the original loop - that was a mild surprise - there was only a 2-3 S unit Neither the preamp, nor the tuning changes you made should change the directivity (ie the pattern, the depth of the nulls) of the antenna. I didn't use the preamp for these tests, but the noise level did change and yes, the nulls were not that deep. As I wrote this was a mild surprise and I did recheck all connections and redid the test with the same result. difference between noise peak and noise null, The original loop normally has S1-2 99% of the time when nulled. The current loop has a difference between noise peak and noise null of 7-9 S units. Now don't I have already told you those measurements don't mean anything to me. misunderstand me, but I like the low noise level. What concerns me with the other 2 configurations is the gain makes the noise increase so much that I am not so sure I would be able to hear the weaker ones. If you want to improve the depth of the nulls, you have to concentrate on the balance of the antenna, the symmetry of the loop, its feedline, its environment etc. I think Roy might have said that earlier. yes he did, and I am not sure why the depth of the nulls changed. I think this project as simple as I thought it would be ...... just isn't. What I would like to accomplish with this loop idea is to have the low noise and the ability to null out the noise from my neighbors houses. But at the same time have enough gain to be able to hear the weak signals. Now I don't have preference for using the preamp or not. Whatever works is what I want to go with. It is fairly simple, you might have needed to learn a bit, but you probably also had to unlearn some stuff. true, I had to unlearn some things that most likely floated around for years. Maybe changing the type of loop to a different style of loop would be the way to achive what I would like to obtain. 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? Obviously, there is a miriad of designs and configurations. Since I need the loop only for 75 meters and have a limited amount of real estate this cuts the pile down to a lesser amount. What to do...................well, maybe I should start over - why not try a different type/style of loop. A larger loop might be better for gain but how big before its not able to null out noise cause I can't rotate it due to its size? Does the loop have to be coax? can it be some other type of wire? Remember that symmetry is key to the depth of the null. If it is harder to build a larger loop with equally good symmetry (including to the environment), you have more gain, but poorer nulls... and your main reason for selecting the loop is for rejection of interference using the nulls. You need adequate gain, and the best balance rather than the other way round. Your input is much appreciated. Thanks 73 Owen 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? |
Low Noise receiving Loop antenna
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. .... 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. 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. Why do you still think that a larger loop has deeper nulls. Owen |
Low Noise receiving Loop antenna
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 |
Low Noise receiving Loop antenna
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 |
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