On 6/15/2012 4:06 PM, NM5K wrote:
On 6/15/2012 2:23 PM, Boomer wrote:


I have used a 75 meter loop antenna here where I live for the past 5
years. It works very well. I live right in town on a lot surrounded by
other homes. I started with a dipole but was advised that a loop would
hear less noise. It turned out to be quite true. I am now a convert to
the loop antenna. Have no idea of the physics of how it works, but it
sure does work well on bands between 75 and 20 meters. It actually seems
to work best on 40 meters.

It is not true. Only in the cases of corona buildup, etc, on the
elements would that be the case.
If you hear less noise with the loop, vs the dipole, it's due to
the change in pattern. Not due to any qualities of the loop itself.
Noise is RF the same as any other signal, and follows all the same
rules. It's no different than an actual signal.
If what you/they say is true, and the loop received less noise, it would
also receive less "desired" signals. Or in other words, everything
would be down vs the dipole.
The most likely explanation is the change in pattern less favored
the direction the noise is coming from. Either that, or the noise is
local to your shack, and for some reason the loop's feed line is better
decoupled than the one feeding the dipole.
If I had to bet, I'd say it's the change in pattern.
There are no magical anti noise properties with loops.




Here is quote from N4JA's informative antenna book.

......XIV. ONE-WAVELENGTH SINGLE LOOP ANTENNAS


1. The Horizontally Oriented Loop


To calculate the length in feet of any one-wavelength loop, divide 1005
by the frequency in MHz. Horizontally oriented one-wavelength loop
antennas have become very popular on 160, 80, and 40 meters and it is
one type of NVIS antenna. (NVIS stands for "near vertical incidence
skywave" because of its high angle radiation pattern.) It is claimed by
its users that the loop antenna is quieter than other antennas. This is
because it doesnt pick up the noise from power lines, thunderstorms,
etc., coming in at low angles. These antennas radiate on their
fundamental frequencies with a broad pattern straight up to put a strong
signal for nearby contacts. Recently published articles on this type of
antenna have called them "cloud warmers." There are other types of
antennas called NVIS antennas other than loops. They are dipoles at low
heights or dipoles with parasitic reflectors placed under them to cause
the signal to radiate mostly straight up. The NVIS antennas have an
advantage in working nearby stations because you dont get the static
noise and interference from far distances. They are definitely not DX
antennas. An article on NVIS antennas appears in the December 2005 QST......