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. '

I was referring more to small loops used for receiving. Not large
loops.. It's mostly in the case of small loops that many claim a
shielded loop is "quieter" than an unshielded loop. I still consider
that claim as pure hogwash...

This is not the same as the full size horizontal loops you are using
on 75m. But.. Even in this case, the loop is not any "quieter" than
the dipole. Like I said before, and you've had verified by the other
guy, it's the pattern that changes, and may receive less signal at
certain heights or angles. There is no "immunity" to noise. It's just
that the change in pattern has less gain at the lower angles than the
usual dipole at the same height.
As you have noted with your conversions with the other guy...
If the noise were from overhead, as in lightning 200 miles away, the
noise of which goes up, and reflected right back down like any other
signal, the loop would be the noisiest, by about a 1/2 db.

If you model a 80m dipole at 30 feet, the gain at 5 degrees is -10.49
dbi according to the plot I'm looking at in eznec.
If you model an 80m full size horizontal loop at 30 feet, the gain at
the same 5 degrees is -12.72 dbi. So.. about 2 db or so, which happens
to be the appx gain the loop has over a dipole in free space.

So, if you have a noise source on the horizon at 5 degrees, you can
expect it to be a good 2 db down on the loop. But even 2 db is not
all that much. Usually less than an S meter on most receivers, but
could vary. So on paper, the signals at low elevations should be down
on the loop, but not hugely so.
The only difference in the pattern between the two antennas is the
dipoles pattern is a little more squat than the one from the loop.
The loop has a tighter beamwidth than the dipole.

At 90 degrees, "straight up" there is only about .4 db difference
between the two antennas. "8.82 dbi vs 8.41 dbi" That's hardly
enough difference to see on a meter. And one reason why I came to
the conclusion the loops were not worth the extra work and wire..
But I'm not trying to reduce signals from low elevations..

If you raised both antennas to 50 feet, the gain at 5 degrees
with the dipole would slightly increase.
With the horizontal loop at 50 ft, the gain at 5 degrees will be
even less than at 30 ft. I wasn't really expecting that, but
that's what the plots are showing.

Both are very good at NVIS paths. BTW, both are using the same
"medium ground" qualities in eznec.
The best way to see the difference in pattern is modeling them.
This is the elevation plot of the 80m dipole at 30 ft.
http://home.comcast.net/~nm5k/80dipole.jpg

This is the 80m horizontal loop at 30 ft.
http://home.comcast.net/~nm5k/80horloop.jpg

Both were made using the eznec demo.
http://www.eznec.com/demoinfo.htm

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 problem is calling it noise immunity. When one says a
loop is immune to noise, it *is* a myth.
If an antenna were immune to noise, it would be immune to all
signals. Would be a fairly useless piece of hardware.
If you point a loop towards noise, it will pick it up just as well as
any other antenna, if the gain at a certain angle were equal or greater
than another antenna.
Saying it has less gain at a certain lower angle is more accurate in
your case. And that gain at low angles will increase as you raise the
antenna off the ground.

BTW, 300 miles is about average for 80 in the day, dipole or
loop.. And your loop couldn't be too awful bad at the somewhat
lower angles, or your contacts to CA wouldn't pan out too well.
That is one handy thing about the eznec program. You can use the
mouse and place the green angle indicator at any angle, and see
what the gain will be. You can see on both of those plots I
set it for 5 degrees. The "DBmax" is the level compared to the
gain at it's maximum angle, which in both of these cases, is straight
up.

I'm in the city of Houston, not out in the country. And I never
noticed the horizontal loop to receive much less local noise
than the dipoles or turnstiles I have used.
But generally, I don't have bad noise that would overpower
the usual atmospheric noise on 80m at night.