Hi folks This is a long and interesting thread and I would like to add some of my observations from the last 5 years using loaded loops on 160m in a suburban lot.
Jim wrote; >>The transition from near field to far field is gradual, and for A POINT SOURCE, begins at roughly one-sixth wavelength. For larger and more complex sources, the transition may be at a far greater distance. Doing the math is instructive. At 160M, 1/6 wavelength is 350 ft. Many noise sources are NOT point sources.<< That is absolute true and most of the power line sources are farther then we may expect. Here is South Florida the power company has a very effective way to detect power line noise. The sniffer engineer has a radio where I plug my dual Flag antenna and rotate to the direction of the buzz, and then we take a signature of the noise. With the signature it is possible to spot the right noise. There are hundreds of power line noises around me "BUT" on 160m I can see only few of those. Based on the last 10 power line sources we removed I can tell by looking at my s-meter how far the source actually is. One source 1.3 miles (~2 km) can measure S9 on my s-meter. 1.7 miles (~ 2.7 Km) S7 and a source that produces S2 can be up to 3 miles away or 5 Km. If you sniff on any band different then 160m, you will find several other sources that is NOT causing any noise on 160m, but will be very strong on 116 MHz 400 MHz ., etc. If you need to find a QRN source on 160m you need to sniff with a 160m radio. A single vertical loop can null one QRN source or two if they are in the same line. The only way to null all directions is to use a polarization filter. One horizontal loop can null QRN from all directions, BUT for 160, the loop must be very high above ground. I recommend above 70 ft., but then the feed line becomes an issue itself. The most important thing is to eliminate common mode noise. It is necessary to hear signal only from the loop and not from the deed line. Most results from single loops differ right here, on the ability of the builder to eliminate common mode noise. It is common to hear that the loop works better near the ground and this is very far from the real problem. Any antenna works better high above ground the issue is the noise pick up by the feed line. When you elevate a loop 49 ft., you need to remove the noise from the feed line. You can find excellent chokes on Jim's web page, A Ham's Guide to RFI, Ferrites, Balun's, and Audio Interfacing by Jim Broun K9YC. http://www.audiosystemsgroup.com/RFI-Ham.pdf If you are using any RX antenna and you don't care about reducing the common node noise, chances you are listening with your feed line. Regards Jose Carlos N4IS _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
