[RFI] Is a low pass filter necessary?
W2RU - Bud Hippisley
W2RU at frontiernet.net
Fri May 11 03:39:23 PDT 2012
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.
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