On 6/19/2012 12:59 PM, Boomer wrote:
There is the knowledge amongst most hams who work low bands that a loop
antenna has much less noise than a dipole at the same height. This is
especially true in a suburban area. The ability to hear the other
station in a qso is profoundly effected by the noise level heard in your
receiver.

I just had a very long conversation with a fellow ham who has been
experimenting with antennas for over 50 years. He has lived in the same
rural place all that time. He has no homes anywhere near his own. His
noise level in his receiver is not much reduced by using a horizontal
loop. He does like loops and his most used antenna is a 160 meter quad
loop vertically mounted with its top at 200 feet.

Anyway I was trying to understand why my loop antenna reduced my noise
level in my receiver by so many S units. After at least an hour of
conversation we got to the fact that I had built a portable loop antenna
for 2 meters as a direction finder. He pointed out how sharp the null
was when this antenna was pointed (edge on) toward the source station.
He then said to take this antenna and then turn it horizontal and see if
I could hear anything. Voila. Nothing at all was heard. I finally
understood the reason for the remarkable noise immunity of the loop
antenna. The loop simply nulls all noise and signals from its edge on
plane. So, any signals from nearby homes, power lines, and industry are
received by me at this very low angle of radiation. They are nulled out.
I receive only high angle radiation. This is also true of transmitted
signal and thus the large amount of gain it has as an NVIS antenna. This
is not magic, just physics.

My 75 meter transmitted signal in the day stops at about a 300 mile
radius. This is fine for me. It may not be fine for others. I regularly
talk to California from Michigan during the night on 75. This same
antenna works DX on 40. I do not understand why at this point. I have
received an S9 signal report from Barcelona Spain on 40. It also works
other bands, but its performance on 40 meters is better than any other
band. In any case my interest in this subject was piqued when someone
here said that it was just mythology that loop antennas had any more
noise immunity than a dipole. I knew this was not true from my own
experience but wanted to understand why. '

So in finishing up, a horizontal loop probably offers little noise
immunity when operated in the countryside where you have no nearby
sources of interference. It offers a lot of noise suppression when
operated in the city or suburban areas.

Michael


As I said above, I am not talking about atmospheric noise. I am
suppressing the local noise generated in my community by various
electrical and electronic devices. These are close enough to arrive
where I live from an extremely low angle. This low angle is exactly what
my loop "magically" nulls out.

I understand now that when I use it at multiple wavelengths that it no
longer behaves as it does where it is a single wavelength on 75 meters.

All the contrarians keep citing atmospheric noise cannot be suppressed
with a loop. Please read my post above more carefully. I do not make
that claim. If you read carefully you will see that I am trying to
suppress ".... So, any signals from nearby homes, power lines, and
industry are received by me at this very low angle of radiation....."

Now if you are claiming this does not work, then say so rather than
talking about atmospheric noise.

Those of us who live in a suburban area bombarded with local noise know
very well which antenna will be more quiet. At least those who have
tried a loop. I suppose there are some suburban areas without noise. I
have not lived in one.

I always prefer empirical evidence over theoretical postulates. Those
who prefer to defer to a theory over evidence are welcome to do so. I
like to use what actually works.

There appears to be an anti-loop antenna feeling among some people. I
have no such innate prejudice against most antennas. I have tried
various types of antennas and use what works best where I live. I found
the random wire antenna and the G5RV to be the worst performing of all
antennas I have tried. If I had no room for a loop, I would use a folded
dipole. After that, I would use a simple 1/2 wave dipole. I have tried
the double bazooka but found it only marginally wider in bandwidth than
a dipole and heavier and more difficult to support.

I do admit that I find the wide spread acceptance of the G5RV by new
hams to be a mistake. I certainly understand why they might originally
chose this antenna. It claims operation from 75 meters to 10 with just a
102 foot length. This is an attractive specification, but comes with a
serious performance cost.

Michael