Good question John. Modern rigs have to meet a tighter level of
suppression of off frequency signal in transmit than the rigs of the day
when Low Pass filters were in demand to help with the TVI problems. At
one time, old TV sets had an IF right in the 15 meter band, to really
make things hard!
Also, in those days of the 50's, more rigs were home built and might
have minimal TVI suppression. Today, most TV is digital, and on
frequencies up in UHF well removed from harmonics, which aren't there
since the transmitters are better. Thus, I too have questioned leaving
a LPF in line if I am trying to get maximum clean output.
I would say the definitive answer might be to measure the effect of the
LPF on any harmonics you can find in the rig output, and measure the
loss of the signal going in to the signal going out of the filter, (the
desired frequency).
Also, be cautious about flea market/ swap meet filters, even commercial
ones, if they had easy to access adjustments. They may have drifted off
with age, or worse, someone may have twisted the screwdriver adjustments
"just to see what happens". Sweep frequency testing is the best way to
line up a filter. Most labor intensive is to do a point by point
frequency chart with a graph made at the end to show the filter
response. Of course, both kinds of tests should be done into a well
shielded dummy load.
-Stuart Rohre
K5KVH
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