On Tue, 19 Jun 2012 12:59:43 -0500, 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

One thing must be mentioned:
The noise cancelling effect of loops is best when the loop is small
compared to the wavelength.
As the loop gets greater, the effect reduces to nil.

Imagine an electric field
------------------------------------
------------------------------------
------------------------------------
------------------------------------
------------------------------------

now imagine the loop, I can only draw a square
in fixed pitch font:

upper conductor
| |
| |
| |
| |
| |
lower conductor

It is immediately clear that the upper part
signal will be cancelled by the lower part signal
and the same applies to the left and right parts
of the loop and of course this applies also to
a circular loop in the electric field, while the signal
from a magnetic field perpendicular to the computer screen
where you are watching this, ahemm..., the magnetically
induced signal adds up around the whole circumference
of the loop.

I hope the SECRET of the loop is finally lifted
and everybody says: yeah, I knew it anyway,
you don't have to tell me, it is so simple.

Large loops do not cancel out the electric signal completely
as the el. field is not in phase on all sides of the loop
and therefore such a loop should be shielded.
When acting at 20 meters, a meter diameter loop is small
compared to the wavelength, at 150 MHz a 40cm diam. loop
is no longer small compared to the wavelength.
This is where shielding helps to minimize electrical
field pickup.

w.