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.



I understand that my experience contravenes your theories about how
antennas should work.

I had several local hams suggest that I use a loop after I kept
complaining about noise. I finally put it up. It is more difficult to
erect than a simple dipole. It is at the same height as was my dipole 35
feet.

The receiver noise level dropped dramatically. I was so glad I went to
the trouble to do it. I finally took down my dipole after switching back
and forth for a year just to be sure the dipole was not better in some
circumstance.

The other effect was an immediate increase in the signal received by my
friends who live within about 200 miles. We operate at 3913. I could not
figure this out until I consulted a pdf file about antennas. The 75
meter loop has a 9 dbi gain at the optimum height of 25 feet. Mine is a
bit high but still gets some gain. All my friends reported on this
increase in my output signal. This information can be found at
http://www.hamuniverse.com/n4jaantennabook.html

This configuration is basically a NVIS antenna. It works so much better
than did my dipole. It gets pretty good dx on 40 and 20. I use an Imax
2000 for 15 and 10.

So, I and my friends know that this antenna works better than a dipole
by actually using it for several years. One of the hams in our group has
been using a loop hung from 2 200 foot towers in a vertical position. He
has done extensive experimentation over the last 45 years.

And yes, a 75 meter loop at 35 feet above ground has a different pattern
than a dipole. I don't care why it works better at all. I and others
just know that it works better and with less noise than a dipole.

And no, a loop antenna is not magic. It is different than a dipole.
Different types of antennas behave differently. The worst antenna I hear
on the air comes from people using G5RVs. Their signal is just totally
lame when they use the recommended 80 foot dipole on 75 meters. If they
would just extend that same antenna to 120 feet they would do so much
better.

Sorry my loop works so well.

Michael