No Spam wrote:
I'm trying to come up with some ideas for multi-op field day station
isolation. Main problem will be front end desens between Voice and CW
portion of the same band. Main concern is on 20M

I'm thinking simple stub filters. Though they are wide, I would think
that there would be a few DB of isolation 100Khz away. Perhaps several in
parallel to narrow it up a bit.

Would a simple LC resonant circuit with perhaps 20-30Khz of BW work?

Any thing else I should be considering?


Thanks You!


Dear No Spam,

A simple resonant circuit centered on 14.05MHz with a 3dB bandwidth of
20KHz is a circuit of Q = 705 which is tough to make and even then will
not provide enough attenuation at 14.15MHz to be of much value.

I have done exactly what you want to do and you will not accomplish it
with LC filters and stubs alone. Assuming you wish to run 2 stations on
40m or 20m simultaneously and demand a minimum separation of 80KHz which
is only about a 0.5% spread (on 20m) you will need to be more creative.
Transmitter phase noise and receiver blocking dynamic range are the 2
issues and both are about the same magnitude problems. Your friends in
the solution of this problem are the allowed 1000' diameter circle for
FD, the different antenna polarizations, and the antenna patterns.
Expect to need about 60dB isolation between 2 antennas on the same band
if you are using current mid range transceivers such as the IC756PRO3
and maybe about 45 to 50dB isolation between the antennas if using
transceivers such as the K3. With less isolation you will still make
contacts but the noise floor will rise and you may hear receiver
artifacts. If you wish to discuss this further please contact me directly.

73,
Larry, W0QE