I'm basing those isolation figures on a calculator I found online that asked for the gain of the antennas and the separation (horizontal or vertical)
Re splitting the simplex: a circulator with the radio hooked to the input, the transmit chain on the standard output, and the receive chain feeding the load port is what I was thinking - a relay would do the job just as well, but would require changes to the PC based Packet Engine software to support flipping the relay before and after transmitting. I don't think that's a built in feature, and source code is not available. Re 4/5 ports: I have 3 transmit frequencies. 144.39, 145.05, and 145.25, and 3 receive frequencies. 144.39, 144.65, and 145.05 - if I keep transmit and receive on separate chains, I only need a 4 port splitter/combiner on each chain. if I go full out with all BP cavities, combined transmit/receive chain for the simplex rigs, and no circulators/isolators, I need a 5 port. If I understand the products correctly, a standard star coupler is just resistance on each port to balance the impedance presented, and there is no port to port isolation. I feel I would be better off with a Wilkinson at that point because it would give me some additional port to port isolation, and If I'm reading it right, for about the same insertion loss. I have approximately 18"x18"x60" without moving cans to the cargo bay. perhaps a little less - I'll have to measure - say enough for 9 5-6" cans, or as many as 18 of the smaller cans that I have - the cans I have are a mix from 5" to 8", with the idea that the larger cans would be used where I needed sharper skirts, on the close spaced frequencies. The key here is the height of the cans. if they are short enough (less than 30" total including tuning rod), I can do 2 banks, facing different sides of the trailer.. doubling the number of cans I can pack in. Only one of the cans I have so far exceeds that spec, and only by a little.. once its tuned it may be less than the 30" max, or I can trade it off with a shorter can on the other side to make up the difference. I would prefer to keep all the cans on one side if possible, but it is looking more and more like it will not be, so I'm willing to give up some space in the power electronics bay to make space for more cans. _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Jeff DePolo Sent: Tuesday, October 23, 2007 6:43 PM To: Repeater-Builder@yahoogroups.com Subject: RE: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise budget > 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
