Key word here is "commercial"... I have seen a filter design by N6CA that
is narrow enough at 10 Ghz to allow for a 10 meter band IF... and he
homebrewed a
10 Ghz transverter based on his filter design for that IF!
Sooo.... Should be possible to build filters that would knock the LO down
to a dull roar at closer frequencies... likely out of waveguide, or lots
of pipecaps! Getting the shape factor to knock down or kill off the LO
radiation
on transmit is the key here... and, generally this isn't done because of the
higher losses that may be present in such a "tight" filter design...
requiring more gain somewhere... but then, microwave frequency MMIC's are
getting cheap
now, or at least cheaper than they were... so designs can be adjusted
accordingly...
Eric
KB7DQH
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