Hi Doug,
> So when I installed the INRAD 455/ 125hz IF filter, it was not
> surprising to find that it rings excessively on 80/160m. That was
Generally very selective filters have group delay problems, where
the time for a signal to pass through the filter is different for different
frequencies. The effect of that is "ringing" or stretching of the signal
you hear, because you perhaps hear the upper passband noise
before the lower passband noise as the same broad noise pulse
hits the filter. The effect of that is the noise peak amplitude is
reduced but the duration of a noise pulse becomes stretched.
That's why mechanical filters, although they have poorer skirts, are
often better than crystal filters when digging weak signals out of
"rough" noise.
The second effect is signals have to change level slower when
selectivity is increased. All filters show that effect, there is no way
around it.
That's why I use 250Hz filters when the band is quiet with only
white noise, and 600Hz filters when there is QRN or "rough" noise.
> problem, and I was lucky to find this. Now if I can just get these
> experimental, floating shield phase lines to quit receiving noise on
> the flag, I will be in business. Steelwool and Moxon linear RF
chokes
> on the floating coax shield looks promising........mebbe even
> permanent magnet rings........
I hear that rumor from time-time, but it isn't the least true! Even
some of our Handbooks get that wrong, but eventually most correct
the mistake.
Steel wool has no effect at RF at all as a choke, because the
strands of even the finest steel wool are much too large. You'll
notice the ARRL Handbook removed the "steel wool balun" that
never did a thing!
It is also electrically and physically impossible to "shield" an
antenna for noise and still have it receive signals, because the
noise is at the same frequency as the radio signals. The noise is,
in effect, a signal exactly like the one from any intentional
transmitter or radiator. If you surround a conductor with a shield
and place an effective choking material over that shield, and you
have what you might call an "antenna" inside the shield, the
antenna will go dead.
When we add a shield to a conductor and the shield has a gap or
is open at one or both ends, the shield itself simply becomes the
antenna! That's why it is electrically impossible to use a hunk of
coax to shield a ground lead, use only an earth ground and shield
the drop lead of a Beverage, or shield a loop antenna.
That is also why when you short or close the gap in a "shielded-
loop", it goes dead. The only thing the shield can actually do is
change the electrical balance of the antenna, and it can make it
either better or worse depending on how you build the antenna.
73, Tom W8JI
W8JI@contesting.com
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