> OK - Here are my requirements for the transmit chain. minimal
> physical space and minimal insertion loss :-) (ok - too
> bloody obvious) Tuning simplicity is also a factor. I'm
> combining 3 transmitters at 144.39, 145.05 +/- 0.04 and 145.25
Before we get to what hardware to use, we still need to quantify how much
isolation you really need between each transmitter and receiver. I'll throw
out real rough numbers but they really need to be determined ahead of time:
144.39 Tx noise supression at 145.05 (660 kHz) - 80 dB
144.39 Tx noise supression at 144.65 (260 kHz) - 105 dB
145.05 Tx noise supression at 144.39 (660 kHz) - 80 dB
145.05 Tx noise supression at 144.65 (400 kHz) - 95 dB
145.25 Tx noise supression at 144.39 (860 kHz) - 70 dB
145.25 Tx noise supression at 145.05 (200 kHz) - 110 dB
145.25 Tx noise supression at 144.65 (600 kHz) - 85 dB
144.39 carrier supression at 145.05 Rx (660 kHz) - 80 dB
144.39 carrier supression at 144.65 Rx (260 kHz) - 85 dB
145.05 carrier supression at 144.39 Rx (660 kHz) - 80 dB
145.05 carrier supression at 144.65 Rx (400 kHz) - 80 dB
145.25 carrier supression at 144.39 Rx (860 kHz) - 80 dB
145.25 carrier supression at 145.05 Rx (200 kHz) - 85 dB
145.25 carrier supression at 144.65 Rx (600 kHz) - 80 dB
144.39 Tx to/from 145.05 Tx - 60 dB
144.39 Tx to/from 145.25 Tx - 60 dB
145.05 Tx to/from 145.25 Tx - 60 dB
>From lowest to highest, it looks like this:
144.39 Tx/Rx --260 khz-- 144.65 Rx --400 kHz-- 145.05 Tx/Rx ---200 kHz--
145.25 Tx
Question 1: Are you locked in to a single antenna, or are two antennas a
possibility?
Question 2: If two antennas are a possibility, how much isolation can you
reasonably expect to get between them?
Question 3: Did you decide how you're going to "split" the simplex
transmitters and receivers (digis)?
> Right now by best bet for minimal space is the hybrid coupler
> approach, but I pay in insertion loss.
As a real rough estimate, you'd be looking at 10 dB or more insertion loss
for two of the transmitters and 7 dB or more for the third once you factor
in filter losses. If you're willing to take a 7 dB hit on one transmitter
alone, you'd be better off putting up two half-height antennas (3 dB gain
reduction) which will buy you 20-30 dB of isolation right there, and you'd
be almost guaranteed to come out better in ERP and sensitivity even
including the 3 dB antenna gain hit.
> loss is the T-Pass, but the T-Pass is starting to cut heavily
> into my available space --- I'm already looking at 6-9 cans
> on the receive side and would prefer something with NO cans
> on the transmit side.
No cans on the transmit site - forget it. You've got 100 dB+ of noise
supression to make up somehow.
> Despite the space issues I'm still considering the T-Pass
> because of the improved spurious signal suppression. Getting
> the cans is another issue - could I use a regular band pass
> can with a coax T rather than an actual T-Pass can ??
You could do a five-port star. If you don't already have cavities with
T-pass style loops in them, there's no reason to try to build using that
design.
> Can you list out some of the other options that I might be
> able to squeeze onto my trailer ??
What are the space limitations?
--- Jeff
