> If I do two antennas, the best I can do is about 30db
> isolation (30ft separation, 6db multi-bay folded dipole
> antenna on bottom, 9db 2m/440 base station antenna on top),
If you can get 30 feet of separation, you'll get more than 30 dB of
isolation. More like 50 dB on VHF, 60 dB or more on UHF as a guess.
> Going to split the simplex radios with a circulator on each..
Maybe I'm missing something. I was talking about splitting the transmitter
and receiver apart so you could combine the transmitters separate from the
receivers if you were going to use hybrids as the primary means of
combining.
> I've done some checking around for "stars".. haven't found
> any - I'm combining three transmitters - so 4 ports ??
You have four frequencies (144.39, 144.65, 145.05, 145.25), plus an antenna.
Five ports.
> got
> some vendors or links I can look at
Try Delta Electronics, Pasternak, maybe Kings. A 4-port "cross" is easy to
find. It's easy to build stars with more ports in a small die-cast box (the
ideal-sized box would make all of the center pins of the connectors
coincident).
> If I can use the star
> to eliminate hybrid couplers, that would be great :-) that
> would leave me with a 2 stage isolator and one or more cans
> per transmitter.
Split antennas is, by far, the best way to go. Your biggest problem is the
145.05 Tx/Rx versus 145.250 Tx. I'd be inclined to start doing the analysis
assuming 145.05 is on its own antenna, with the remaining frequencies
three-wayed on another antenna using conventional cavity-ferrite combining.
> I don't have t-pass cavities, but since I'm still acquiring
> cavities, I can get them if warranted. I've got 4 regular
> band pass cans right now, 2 more on ebay I'm trying to get,
> and a 6 can helical BR/BR duplexer that I can use for a
> really deep notch if I need it somewhere (or will become part
> of the receiver filters if I decide to stack BR filters for
> receive, as per my previous post)
A helical pass/reject duplexer isn't going to help with the close spacings
involved. You might get lucky and be able to it to get some filtering
between the extremes (144.39 vs 145.25), but otherwise, the notches aren't
going to be sharp enough to avoid degrading the frequencies in between.
Have you swept it to really see what its performance is like (both
transmission and reflection)?
How much room do you have?
--- Jeff
