> Transmit and receive paths will be separate with 30db of
> isolation (either split antennas, or a 30db isolation
> circulator just before the feed to the antenna)
I posted a followup to one of your previous messages regarding using
isolators as a makeshift way of getting more Tx to Rx isolation, using the
reject port to feed to a receiver, and the problems therein. For some
reason, that post never showed up.
Anyway, the 30 dB of theoretical isolation that you refer to is never going
to be realized in your application. The isolator's spec of 30 dB that
you're quoting is from antenna to transmitter, assuming an ideally-matched
reject load. You're using the isolator differently; that spec doesn't
apply. In your configuration, the Tx to "Rx" (reject port) isolation is
only as good as the return loss of the antenna; consider that a 1.5:1 VSWR
is only 14 dB return loss. Maybe you'll do a little better than that, but
you'll probably never get the full 30 dB.
And there are a whole slew of other issues related to IM and harmonic
generation within the isolator.
> That brings the transmit power down to +17dbm (55mw) before
> any other splitting and filtering to get to the 3 receivers
What about noise supression? At close T/R spacings, the noise supression
requirement is likely going to be higher than the carrier supression
required.
> Am I blowing smoke ?? Is there a way to figure from the usual
> receiver specs just how much signal can be handled before
> de-sense occurs ??
A typical 50 watt solid state 2m repeater typically needs 80 dB or more of
noise supression at 600 kHz split, and about 80 dB of carrier supression
(both figures plus or minus maybe 10 dB depending on the radios used). If
your Tx is closer than 600 kHz to your Rx, you're going to need
proportionally more noise supression; 100 dB at 400 kHz spacing would
probably be a good guess. Carrier attenuation likely won't need to increase
substantially until you get appreciably close to the Rx frequency, like
maybe within 200 kHz as a rough ballpark guess (it will likely vary quite a
bit depending on the quality of IF filtering in the radio).
Instead of guessing at all of these numbers, why not quantify them if you
already have the equipment you're planning on using.
--- Jeff
