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), or I can get close to the same isolation with a circulator and one antenna.. so I don't see a difference there and one antenna really simplifies things. (plus gets more gain and height compared to the dipole array mounted low on the tower).. there is another issue, a 2nd antenna is at best a month away before I can slip it into the budget - I have the base antenna available now.
Going to split the simplex radios with a circulator on each.. anything that leaks through from transmit to receive will be 30db down and have to get through the receive splitter and receive filters on the other receivers before it can be a problem. Also, non-mechanical so removes a potential point of failure !! I've done some checking around for "stars".. haven't found any - I'm combining three transmitters - so 4 ports ?? got some vendors or links I can look at ?? 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. 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) _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Jeff DePolo Sent: Tuesday, October 23, 2007 4:37 PM To: Repeater-Builder@yahoogroups.com Subject: RE: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise budget > 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
