[RFI] TV RFI
wa3gin at comcast.net
Sun Jan 27 09:26:53 EST 2008
Reminds me of the old days when our neighborhood didn't have cable or
satellite as an option for TV.
I lived in a row house, 16ft wide. No city restrictions os I had a 15ft
tower with a 2 el tri-band quad. Several feet away was a small VHF antenna
that fed the TVs in the house. After operating on 20m with a KW one
afternoon the wife informed me the reception on the TV had become
degraded...I thought perhaps the antenna connection had gone bad. When I
checked it the little four to one balun used to convert 300ohm to 75ohn coax
apparently had heated up like a toaster element and melted through the
casing. I replaced it with a homebrew balun made from 75ohm coax. Never had
any other issues. The problem was due to proximity of the ham antenna and
the TV antenna...pure overload.
I suspect that is part of the problem you're having. Moving your antenna
and reducing power further may be a faster fix than trying to sheild the TV
appliance from the inflow of unwanted RF through all the various and
numerous paths the HF RF is likely to take.
The RF could be coming in through the phone line drop and coupling to the AC
wiring before it reaches the TV...the possibilities are almost endless.
Trying to solve the problem at the end user side of the scenario could
become another almost full-time hobby.
----- Original Message -----
From: "Tom McDermott" <tom.mcdermott4 at yahoo.com>
To: <rfi at contesting.com>
Sent: Sunday, January 27, 2008 2:31 AM
Subject: Re: [RFI] TV RFI
> There's a lot of possibilities here - low pass filter after the amp, high
> pass filter
> in front of the TV set are the first two easy-to-try things. RF ground on
> transmitter/amplifier is not likely to solve many (or any) TVI problems.
> The basic problem is that TV sets usually have 2-wire AC power cords. This
> causes no end of RFI problems. The chassis in the TV 'floats' up above AC
> ground at RF frequencies.
> Most times people do not provide a good ground (or any ground at all) on
> TV antenna input connector. So a large dipole antenna is formed using the
> of the TV's antenna feedline and the AC power cord, with the TV set stuck
> in the middle of the two with your RF across the TV set
> connection. That dipole assures that you couple a lot of RF into the TV
> The objective is to short out that dipole (at RF) so that your RF bypasses
> TV set.
> One approach is to purchase an AC power strip for the TV set that contains
> F-type connectors on the strip (and has a 3-conductor AC power cord). I've
> them new for less than $20. Internal to the power strip the F-connector
> shells are
> connected right to the AC ground conductor. Run your CATV or antenna feed
> one of the F-connectors on the power strip, then a short jumper from the
> F-connector on that power strip to your TV set's antenna connector. Plug
> TV's AC power into that same power strip. This grounds your TV antenna
> shield to AC ground, encouraging most of the RFI current to bypass the TV
> If the TV set is particularly fussy, you may need an AC-power filter on
> the TV
> set AC power cord. You want the type that has two capacitors on the AC
> power line
> that bypass them to ground. This reduces the chance that you get some RF
> between the ground conductor in the AC cord and the hot and neutral lines
> at RF
> frequencies. Unfortunately you can't just add a couple of capacitors
> yourself because
> they need to be line-rated to handle the 2,000 volt transients that exist
> on AC
> powerlines - you need a UL-listed filter.
> I've seen this approach solve TVI quite a few times.
> -- Tom, N5EG
> Never miss a thing. Make Yahoo your home page.
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