[TowerTalk] more - precip noise
Dale Jones K5MM
Sun, 3 May 1998 10:03:28 -0700 (PDT)
Tom, et al:
Many years ago, while living in Arizona, I used a Folded Dipole, made
out of TV twinlead, and fed with the same stuff, for 160 meters.
Separately, I had a regular-old 80 meter dipole (not folded) on
a separate tower. Both were up about 100 feet.
During hissing-static times, the 80 meter antenna was terrible, and
the 160 mtr folded dipole was perfectly quite. I have no explanation
for why this was the case. I just liked folded dipoles (even though
they required a tuner near the transmitter) and I had one for 160
already built up.
Now, for a commercial installation like Todd has for his local
public utility, it would be fun to experiment with a Crossed-Dipole
made up with two Folded Dipoles. Can't prove it, but I'd bet it
would be much better than what he has.
At 01:34 AM 5/3/98 +0000, you wrote:
>> 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
>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
>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
>73, Tom W8JI
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