: : Subject: [RFI] Follow up to TV RFI
:
: Jon,
:
I finally looked in a more recent RS catalog as was surprised to find
a coax type 54-900 MHz HP filter. I will do some tests on it today.
Although I can sweep it only up to about 400 MHz, I can plot what
it does between 5-70 to get an idea. I put it on the TV and saw no
degradation on any of my normal channels from 2-58.
I only mentioned external factors previously, but here I am back to
illustrate another possibility. There is problem know as the "Rusty
Nail effect", where corrosion or galvanic action between 2 pieces
of metal causes rectification of a strong RF signal much as a
semiconductor would. This thread having peaked my interest, I
decided to an inspection. Sure enough, the radio barefoot into the
2 section Drake filter showed more then 70 dB down on any spur I
could see at all. Even so, with HP filter and an extreme common mode
choke into the VCR and TV showed extreme channel 2 herringbone
with as little as 40 watts on 14.040 MHz! There was not a whimper on
10 meters at all. On an outside antenna to the spectrum analyzer (a SW
receiver would have found it) there was sure enough lots (now only
-30 dB from the fundamental) of 4th harmonic energy from the 20 m
radio! This is unacceptable. I keyed the radio at 40 watts and went
around the room and noticed some changes, but when I reached out
the door of the shack and shook one of my poles supporting several
VHF antennas, the TV picture would go from unreadable to perfect!
Bingo. This weekend I will be throwing away the entire support and
mounting brackets (rusty and loose) and using antenna grease on all
contact points. If I hold the mast a certain way, there is no trouble
on any channel, so what I thought was the carport was actually the
mast that was (sort of) bolted to it. The length of the mast is close
to a quarter wave and must have built up enough current at the flaky
mounting end to breed harmonic energy and re-radiate into the TV
antenna 2 feet away. This is often the culprit when a local repeater
acts up with intermod too.
It is imperative that all mounting and tower hardware and any other
metal in the vicinity of the antennas be clean, tight and corrosion free.
Antenna grease on mating and mounting surfaces will prevent
galvanic action. Use only a very thin coating. Just dab a little on and
spread it all around the contact points with a rag.
As for the 75-300 ohm baluns back to back, They do the same thing as
wrapping a large torroid with several turns of coax, to prevent common
mode RF from reaching the set, but they also introduce anywhere from
3-10 dB loss (depending on quality of the baluns and their bandwidth)
and also unshield your coax, allowing RF in and out at that point. If
you are a cable subscriber, there shouldn't be anything at all to cause
you trouble IN the cable, so it has to be either common mode (the
cable acts as an antenna connected to your chassis) or there is enough
RF very close to the set to cause trouble directly into the circuits.
Good luck some more,
Jim, ac6tk
http://www.cybertime.net/~ac6tk
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