There are three common sources of HF noise:

1. (Propagated) atmospheric noise. There's no magic antenna that can
tell the difference between atmospheric noise and signals coming from
the same direction as the noise. An antenna that attenuates one will
attenuate the other by an equal amount. A directional antenna will
improve the signal to atmospheric noise ratio only if the two are coming
from different directions and the antenna can be oriented to favor the
signal and reject the noise.

2. Static discharge. Rain and snow can carry static charges, creating
noise at the antenna itself. Also, a gathering storm can cause a large
static potential to build up on an ungrounded antenna, even to the point
of corona at the antenna ends and arcing across the feedpoint. I saw
this many times in Denver, but never here in western Oregon. This can be
prevented by making a DC short or near short across the feedline with a
resistor, RF choke, or shorted stub, or by using an antenna that
inherently has a DC short across the feedpoint, and by avoiding any
sharp points like wire ends. A loop of any size has both these features,
although there are other ways to get them.

3. Local noise. One antenna can be much better or worse than another at
rejecting noise that originates close to the antenna. Often, horizontal
polarization is better than vertical for rejecting local noise, or any
noise that's within surface wave range. If the noise source is very
close -- within a fraction of a wavelength -- a small loop might help,
since it's relatively insensitive to electric fields from sources which
are very close by. However, it's difficult to make a small loop that's
efficient, so it's usually a better choice for receiving, with some
other antenna being used for transmitting.

Roy Lewallen, W7EL

Ed wrote:

My thanks to all for responses to my quetion. And my somewhat belated
response to the comment (above): I guess I must be somewhat ambiguous in
reference to the low noise of a loop. I have always heard that proper
loop antennas had a lower noise characteristic than other open ended
antennas. The type of noise in question, I don't know. I suspect at my
locations I have a combination of man made and atmospheric noises being
received at my presesnt dipole.

Since I intend to build a new antenna, and since I am now forced to
consider the construction of a folded dipole in lieu of the more
difficult to erect loop, that was the reason for my original question.

I have two nice 100'+ trees between which it would be quite easy to
install a dipole..... whether it be folded or standard depends on the
comments I receive from you guys on the low noise characteristic I am
questioning.

Ed K7AAT