> I am assuming that you place band pass filters (what ever
> ilk) in line with BOTH radios. That being the case (if it
> is) then using bandpass filters on 40 you would be
> knocking down (but not eliminating) the harmonics on 20
> meters. In addition if you have a 20 meter filter on your
> second radio, that do would have attenuation on the 7 Mhz
> energy. So if the transmit filter on 40 is...say....40 db
> down and the receive filter on the 20 meters station is
> another 40 db down....would not those figures combine?
> That is a question! So, you are protecting the 20 meter
> radio from the 40 meter radio.
Radios are already, only with a very few exceptions, very
good for harmonics on the output. I wouldn't waste a filter
there. Typically I find them 60 dB down or more. The real
problems on TX are the amplifiers. They typically are only
50dB down. The biggest issue is protecting the receivers and
filtering amps. Personally I think 25-30dB is more than
enough unless you have some really nasty gear, or some very
closely coupled antennas (like a 15 and 40 stacked in close
proximity).
We get by (so far) without any filters here using FT-1000MP
MKV and modified AL1200's, but the antenna layout is pretty
much spread out and nothing points at anything else.
I'm planning to build some amps using surplus MRI tubes
operating at low voltage and high current. That way we can
switch the tanks with reasonable size vacuum relays. I'm
going to use two-stage L networks followed by low pass
filters so the amps are broadband no-tune. For now though
just AL1200's.
73 Tom
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