On May 6, 11:29 pm, 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!

In general, you can make a better filter in a given volume by using
lumped LC filters at HF frequencies, than by using transmission line
stubs. The reverse is true when you get into the hundreds of MHz
region.

You need high Q elements to get good isolation and not incur too much
loss when frequencies are spaced that closely. A saving grace is that
you probably don't need a whole lot of attenuation, assuming the
receiver front end you're trying to protect isn't too bad. It's not
like you're trying to completely kill the offending signal, just get
it down to where it doesn't cause trouble. Intermod (third order
distortion) generally drops around 3dB for every dB you drop the
signals causing it. Desense should similarly go away fairly rapidly
as you drop the big signal levels.

Looks to me like a design with five coils and five capacitors and what
at least I consider practical values should give you 25dB differential
between the passband and the stopband, with a 100kHz separation
between the two, and a loss of about 5dB in the passband if you use
Q=500 coils. Air core coils about an inch and a half in diameter
should give you Q that high. I haven't tried optimizing the design,
and may be able to do a bit better than that with the same number of
parts. I can imagine building it "on the cheap" in a string of tin
cans soldered together, or else by using pieces of copper-clad
soldered together. (I've built several somewhat similar filters using
copper clad for the shielding, and they work quite well.) Would a
suggested design be helpful?

Another option (or something to consider in addition) is a small loop
receiving antenna that you can orient to null the other transmitter.
I believe N6RK just gave a paper on doing this, though with emphasis
on lower bands. He may have a PDF of the paper he'd be willing to
share.

Cheers,
Tom