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>From k3lr Sun Apr 5 14:24:39 1998
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From: wrt@eskimo.com (Bill Turner, W7TI)
To: martyt@pobox.com (Marty Tippin)
Cc: rfi@contesting.com
Subject: Re: [RFI] HF Interference from TV
To: <rfi@contesting.com>
Date: Sun, 05 Apr 1998 18:24:44 GMT
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On Fri, 03 Apr 1998 15:24:37 GMT, martyt@pobox.com (Marty Tippin)
wrote:
>My dad has an odd problem with interference on 17m apparently coming
>from his television set - it took several months to figure out what
>the cause was, becuase the TV is almost always on.
>
>While the TV is on, he has about an S5 to S7 noise level (sounds like
>a buzzsaw or something, worse at night then in the daytime) on 17m;
>15m and 20m are both clear - drops to nothing when the TV is off.
>Noise is present throughout entire 17m band.
>
>Apparently is coming in through his HF antennas as there's no noise
>when the antenna is unplugged. Dad operates almost exclusively on 17m
>so this is a real problem for him.
>
>He lives in a rural area and has Primestar satellite and an
>over-the-air TV antenna; he disconnected both of those from the
>television but still the noise remains. The TV is a 35" Sears model,
>he says it was made by RCA for Sears and is about 5 years old.
>
>My guess is that there's RF being generated from the TV into the house
>wiring; he's checked all the plugs on that AC circuit and everything
>is grounded properly.
>
>Any suggestions as to how he might clean up this noise? I suggested a
>brute-force AC filter at the TV, but he may have a hard time coming up
>with one of them because of where he lives. Somehow I don't think
>ferrites on the TV power cord are giong to do much.
>
>Thanks for any help! Replies via direct e-mail as well as the
>reflector would be appreciated.
>
>-Marty KI0LO
> martyt@pobox.com
_______________________________________________________________
Whether or not the AC plugs are grounded correctly has little to do
with RFI. You can not "ground" 17 meter RF energy by running a ground
wire. All you will do is add another radiator to the incidental
antenna system.
What you need to do is prevent the 17 meter interference from flowing
into a radiating element in the first place. The TV has at least two
"antennas" connected to it: The cable or twin-lead to the TV antenna
jack, and the AC line cord. Either one is capable of radiating 17
meter energy, and from your description of non-interference on other
bands, I'd say at least one of them is doing it very well.
The best way to cure the problem is to prevent RF current from flowing
by the use of ferrite chokes. Radio Shack sells two different types,
a clamshell type and one that comes apart so you can wrap multiple
turns around it. Either one can be effective, and you may need more
than one to cure the problem. I've seen them advertised by other
companies and those are probably ok too, but all I've used is RS.
The above filters are common-mode filters, but you also have a
differential-mode filter in the antenna line. This is the type where
you insert it in the line.
And last but not least, get a copy of the ARRL interference handbook.
There is a lot of good information there. Good luck.
73, Bill W7TI
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