Topband: Pixel Technologies Magnetic RX Loop

n4is n4is at comcast.net
Tue May 10 11:40:09 PDT 2011

```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

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