[TowerTalk] more - precip noise

[Dale Jones K5MM]

 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.

 73
 Dale  K5MM

 


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 
>>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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>


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