Despite the fact that most of the TVI cases I've personally investigated over
the years were the result of either front-end overload or direct ingress to the
TV set's IF circuitry (which used to be around 15 meters!), I've kept a
kilowatt low-pass filter on the output of my HF rig and amplifier for decades
on the theory that "it can't hurt".
This year, "encouraged" by the loss of semi-local analog broadcast TV channels
2 and 3 in our area, I finally put my home station on 6 meters for the first
time. Imagine my surprise when my beautiful new beam appeared to have a very
high SWR as measured back at the 100-watt rig.
Guess what turned out to be causing the high SWR....
Perhaps new low-pass filters have a higher corner frequency to accommodate the
rash of HF rigs (and now the Elecraft KPA500 amplifier) that include 6-meter
coverage but, if they do, they certainly will be ineffective on the Low VHF TV
channels.
These days I expect I get most of my external harmonic filtering from the
natural selectivity of the HF antennas I use.
Bud, W2RU
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