[Elecraft] Band pass filters

[Michael Carter]

David,

One need not have a "feeling" about the necessity
of band pass filters.  In addition to the concerns
that Paul cited about isolation between ports
in a triplexer or internal antenna port isolation
in a transceiver, there are analytical methods
to determine susceptibility to receiver damage
and IMD.

A receive-only bandpass filter, whether external
or internal, must suppress the fundamental frequency
of the other TX in a M/2 station sufficiently to prevent overload
and IMD generation.  Do a calculation: TX power (dBW or
dBm) minus band pass filter attenuation in its stop band
at the other TX frequency equals worst case RX input
power due to other TX (obviously RX antenna proximity,
orientation, and sensitivity reduction out of band
also reduce unwanted fundamental TX signal).

The receive-only band pass filter cannot attenuate
the in-band harmonic of the other TX.  One can
estimate the harmonic level, say in a 40 meter/20
meter M/2 setup, using the transceiver and amplifier
manufacturers' specifications.  If that level is at most
equivalent to a strong distant station's received signal
power, then it is likely of no greater concern than
those loud stations.  A 1500 watt amplifier that just
meets the FCC's spurious emission limit at its
2nd harmonic (43 dB or greater attenuation relative
to fundamental) yields approximately +19 dBm
at the TX antenna input.  In-band harmonic level
at the other M/2 receiver again depends on antenna
factors, but those factors combined would need to
provide another ~70 dB of attenuation of the TX
harmonic just to produce an S9+20 dB interferer
from the other M/2 station.

73,
Mike, K8CN