[RFI] TV RFI

[Tom McDermott]

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 your
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 the
TV antenna input connector. So a large dipole antenna is formed using the shield
of the TV's antenna feedline and the AC power cord, with the TV set stuck right
in the middle of the two with your RF across the TV set antenna/chassis/power
connection. That dipole assures that you couple a lot of RF into the TV set.

The objective is to short out that dipole (at RF) so that your RF bypasses the
TV set.

One approach is to purchase an AC power strip for the TV set that contains 2
F-type connectors on the strip (and has a 3-conductor AC power cord). I've seen
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 to
one of the F-connectors on the power strip, then a short jumper from the other
F-connector on that power strip to your TV set's antenna connector. Plug your
TV's AC power into that  same power strip. This grounds your TV antenna coax
shield to AC ground, encouraging most of the RFI current to bypass the TV set
entirely.

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 signal
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


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