Brian brings up an excellent point. I don't want to discourage
anyone from using a loop because they do have the advantage of
being easy to feed, but gain is overrated!
Brian sez:
> > Low dipoles are also cloud burners. I suspect they will perform as well
> > as this loop plus you can feed them with coax.
> > After fooling around with the loop for a while I suspect you will be
> > back to the dipoles anyhow. I hope the structures are far enough apart
> > to permit this.
I certainly agree! Low full-wave loops, contrary to myth, do NOT
have gain over a low dipole. As a matter of fact the gain of most
loops is insignificant once the thing is installed over earth.
In order to have gain an antenna must force a null in a direction
where there is significant radiation. Since a low horizontal antenna
already has a null along the horizon, making it a loop (which tries
to force a null where a null already exists) does not produce gain.
You actually obtain much more NVIS gain using a low extended
double zepp than a low loop. The low loop is virtually identical to a
low dipole, as Brian noted.
On the bands where the loop does make gain, it does so by
making a multi-null pattern. The problem is you can't rotate the
loop. That means statistically you are as likely to find random
stations in a null as in a lobe. In practice, effectiveness decreases
because a deep null hurts some directions more than gain helps
other directions.
Gain is only useful when it is at the proper direction and angle, and that may
not happen.
> I failed to mention that I plan to use the loop for more than 160M. One
> of its uses on 80 and 160 will be for in-state communications, so a high
> radiation angle is desirable. My experience with other large horizontal
> loops at this height have been very good.
That's OK then, if you like loops use one. There is nothing wrong
with using an antenna that makes you happy, and there is
something to be said for having one antenna cover all bands.
The advantage of a loop is you can get a low or modest feedpoint
impedance on multiple bands, including even harmonics. That is a
very real advantage.
The disadvantage is you need to figure out how to feed the thing
with coax, or run ladder line above ground.
73, Tom W8JI
W8JI@contesting.com
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