Bob Close writes...
> a band pass filter is nothing but a low-pass combined with a high-pass
> filter.
Sure. But a low-pass filter is designed to attenuate broadband noise
well outside the range of desired frequencies a station might want to
use. It's designed to attenuate spurious harmonics, not IMD resulting
from mixing products. Looking at the ARRL report for the Omni VII (as
an example), spurs were evaluated over a 1-MHz range from the carrier
frequency and found to be below -100 dB, and below -120 dB more than
about 100 kHz away from the carrier frequencyh. But most tests just
look at IMD spurs from a two-tone test, and those are what I call
close-in spurs. Those are typically -40 dB or better, but they are
much too close to the transmitted frequency to be affected at all by a
low-pass filter used to prevent RFI.
In any case, spurious harmonics from modern transceivers seem to be
well below 100 MHz, and in the range where a RFI low-pass filter might
be effective, lower than that. A spur at -120 dB from a 100-watt
transmitter is a tiny fraction of a microwatt. It's still a tiny
fraction of a microwatt when amplified by 10. If the MDS of a
television is -60 dBm (guessing), then maybe noise might result, and a
general-purpose low-pass filter would eliminate it. But I suspect the
majority of TVI is caused by the fundamental frequency of the radio
transmitted signal (which is more like +60-70 dBm when amplified
through a gain antenna) coupling with some poorly shielded part of the
TV's antenna system or internals. That won't be fixed by a low-pass
filter, it seems to me. But it might be minimized by a high-pass
filter at the TV, which can help keep those RF signals out of the TV
in the first place.
> Most 75 mtr conversations I hear are two hams located ten miles apart
> running "texas kilowats" into humongus antennas telling each other: "well
> Fred, ol buddy, you are the loowwdest durn signals ah can heah, 10-4" "But
> , crank it up a bit, 'cause someone is complaining up the band about
> splatter or some such thang, and botherin' me with their bit@hin'" .
> "Roger, Jackson, I added a bit mo' power theah to hep you out--mah pair of
> 4-10,000s is runnin a dite warm, but ah think they is gonna hold ok" "Oh,
> dat's good now--howz your gout?" "Still bothern me like every day last
> month" . ...... .......... ......... .......
I listen (far more than talk) a good bit on 75 phone late at night
while I'm soldering stuff. I've heard some pretty silly things, but
I've never heard anything like this (except for the last sentence). I
sure see it written about a lot, though.
There are times when I run power on 75 phone to talk to someone across
town. They are too close to reach by sky wave, and with ground wave
line of sight counts and I have a ridge between me and many of those
to whom I talk locally. My antenna is low enough with respect to 75
meters to be a cloud warmer, and if it was at the proper height for
the best sky-wave propagation, I might have to run even more power for
the local ground wave contacts. I can usually hear folks 1500 miles
out better than the guys in my club 25-50 miles away, unless they are
running a little power.
Of course, running too much power and producing splatter (which is
equally as likely to result from overmodulating too little power) are
two different things.
Rick, KR9D
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