On May 7, 3:05 pm, Larry Benko wrote:
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

Of course Larry's right: any transmitter phase noise will be a
problem. Especially with the density of signals typical on FD, 3dB
loss in a receive filter won't be much of a problem most of the time,
but in a transmit filter it's another issue. 3dB passband attenuation
was about what I got in the filter design I was playing with this
morning, assuming coils with a Q around 400, which should be fairly
easy on 14MHz. That gave better than 25dB attenuation 100kHz away,
which should be a huge help. If you can get the coil Q up enough,
then the filter can be used on both transmit and receive, and you'll
get the same improvement in phase noise output performance that you
get on receive in rejecting the adjacent band. 100 watts will be
incentive to use large enough coils that the Q will indeed be pretty
high--assuming you don't do anything too stupid in the construction.
There are significant advantages at a multi-transmitter FD site in
having a receiver that tolerates strong signals well, and also in
having a transmitter that has low phase noise.

Cheers,
Tom