As always, Tom has done a great job educating us on this topic. I
completely agree with everything he said about antenna selection for
effective noise canceling. Unfortunately it isn't always possible to
achieve the ultimate. In some cases it may be possible to achieve
some improvement with much less than ideal antennas.
I have a noise that I have been trying to locate for years. Despite
many hours (better measured in days or weeks by now) out with DF
equipment I have not been able to identify the source. In part due
to the fact my 160 meter transmit antenna does a wonderful job of
re-radiating this noise, I hear it to varying degrees in all of my 8
Beverage directions.
Due to my lack of skill and/or adequate instrumentation to find the
source or stop the tower from re-radiating, some other approach must
be tried. It isn't possible to put up any more Beverages.
Yesterday I erected a short vertical somewhat near my transmit
antenna. It is a simple 24 foot whip with a single short ground rod.
I fed it with a 900:50 ohm matching transformer and did not attempt
to do any tuning or matching. Using this as the AUX or NOISE antenna
with my MFJ-1026 noise canceler I am able to achieve a good null for
all 8 Beverage directions. This isn't perfect but it does help. I
will describe the limitations.
In some directions my Beverages are in reverse mode and signal level
is quite low due to loss in the WD-1A wire in transmission line
mode. The noise is relatively low in these directions, so I am able
to run the MAIN antenna gain at maximum and the AUX antenna gain at
about 50 per cent of maximum. There seems to be little or no signal
to noise degradation from the AUX antenna being omnidirectional.
However, the noise floor of the MFJ-1026 is enough to raise the
system noise floor and reduce signal to noise a small amount. This
could probably be solved by using a preamp ahead of the MFJ-1026. I
am not able to try that right now.
In other directions, where the noise level on Beverages is very
strong, I have to run the AUX antenna gain at maximum and reduce the
MAIN antenna gain to get a good null. In this configuration, the AUX
antenna does add considerable noise to the system.
However, even with the described limitations, the setup allows
improved signal to noise in each of the 8 directions, compared to
using a Beverage alone with no ability to null the offending noise.
This very simple setup is clearly better than nothing. It is often
enough to make the difference between no copy and 100 per cent copy.
My point is... If possible, follow Tom's advice about antennas! If
not, don't give up. Try what you can. You might get lucky.
73,
Paul N1BUG
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Topband Reflector
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