To: <towertalk@contesting.com>
> Date: Sat, 02 May 1998 11:02:46 -0500
> I am currently working on a problem with precip static
> on lowband police radio antennas (39 Mhz) for State of SD. It was
>suggested to me by an engineer who is now retired that he used a 1/4
>wavelength'''''stub
Hi Todd,
Let me try to explain this again in different words. I went all
through this stuff for many services over the years, and had
to learn how to cure these problems.
Let's assume the antenna has some dc path to ground, even one through
a high value resistance.
There is a common (but very incorrect) assumption that noise can be
filtered by a selective circuit, like a stub or cavity. That's
untrue, unless the receiver's bandwidth is wider than the
bandwidth of the stub or cavity. Adding one more selective circuit in
the receiver's path, and that is about all the stub would do, would
not greatly change system bandwidth.
But let's assume it did make bandwidth narrower. If that is so, S/N
ratio would improve..... but only if the signal occupied less
bandwidth than the initial system selectivity. That's why a CW
signal gets clearer when a narrower filter is used in the presence of
broadband noise. That's also why narrow band FM receivers produce
better quieting than wideband FM receivers for a given uV input level
(although a narrow band FM filter won't copy a wide band signal, just
like a narrow CW filter doesn't help on SSB).
You see, the stub is an absolute waste of time unless the receiver
has some VERY serious problems with selectivity.
The noise is almost certainly due to corona discharge on the antenna
elements, or very near the antenna. That noise appears to the coax,
antenna, and receiver just like a desired signal. Once in the system,
it can't be filtered out any more by a stub or grounding system than
regular filters in the receiver will do.
Most likely the reason they have this problem is the antenna is the
tallest thing on the support, or near the tallest thing.
Since the noise can't be filtered or trapped out, the only solution
is to stop the noise from getting in to the system. That can be done
by installing a large round smooth conductor above the antenna,
and eliminating sharp points anywhere in the antenna system and
anything near the antenna. That will reduce the voltage gradient near
the antenna, and reduce noise generating corona.
(The last thing you want near the antenna is anything that increases
corona discharge, and that would be anything sharp.)
I corrected a similar problem for a low band system installed on a
tall building by adding a tall 2 inch diameter copper mast with
decoupling skirts (to make it look like a "broken up" conductor at
the operating frequency) with ...don't laugh... a single copper
toilet bowl float mounted on top. Just make sure you use a very
smooth ball at the top, I did that by putting a nice smooth layer of
solder at all sharp seams. Be sure to ground the mast properly, in
case it gets whacked.
A remote base that was totally wiped out with hissing noise during foul
weather became
virtually noise free, with that single change. My first attempt was
to lower the antenna, and while that cured the noise it produced too
much pattern distortion (the antenna was below dozens of other metal
structures).
73, Tom W8JI
w8ji.tom@MCIONE.com
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