Duplexers used as combiner?

[RESCUE161]

I have access to one antenna on a commercial site and would like to put two repeaters on this one antenna. We use combiners at work, but that may be overkill for my application as the four frequencies are at least 5 MHz apart.

444.675/449.675
462.625/467.625

Here is a drawing I made up. Do you guys think this will work? It looks good to me on paper.

[RFguy]

I would use 4 of the Sinclair C3034 multicouplers.,

Or 2 of the C3034 multicouplers and one duplexer on the feedthrough.

Isolators on the Transmitters would be a good idea.

[Bill_G]

You probably don't need duplexer #3 depending on how good duplexers #1 & 2 are. That's how it used to be done - tie the duplexers together at the antenna port through a tee, and cross your fingers.

[RESCUE161]

Yeah, I was adding the notch duplexer as extra protection. I'm going to give it a shot and see how my loss I have. I'm getting about 15 Watts out of the #2 duplexer and will tune the #1 duplexer when it gets here today. I guess I'll order one of those mobile type notch filters to try it all out.

I just can't justify the cost of a combiner as the frequencies are not close at all.

[motorola_otaku]

I know a group of guys who are trying to do this same exact thing: 70cm and GMRS on one antenna. I can tell you the method they were given absolutely does not work: install a tee on the antenna port of Duplexer A, tie one side to the antenna port on Duplexer B with a "magic length" 8'' RG142 cable, and run the other side to the antenna itself. The repeater on Duplexer B behaves normally but the repeater on Duplexer A will always experience severe desense whenever the Duplexer B repeater is keyed. Swapping places with GMRS and 440 does not solve the issue. It was interesting sweeping the entire network with a spec-an - you could actually see the notches on the opposite duplexer, albeit -10 to -20dB lower (higher, I should say.. not as deep. You get the point.)

I like your idea of using a mobile notch duplexer as a psuedo-isolator, but I worry about IMD. One thing you absolutely do not want to do is use any kind of braided coax on the full-duplex side anywhere in the network - use 1/4'' Heliax at the minimum and preferably FSJ4 Ultraflex if you can get it.

[FMROB]

Otaku,

We are trying to do the exact same thing, and have experience exactly what you describe. We are attempting to use 2 526 duplexers phased into one antenna for a 451 and 461 pair. We have thought that the 526 is not creating deep enough notches to allow for ample rejection, so were are experimenting with those old motorola cans with the brass skirt knobs ontop. They seem to produce deeper response. I dont believe that you would have an issue if you were using the this method for transmit only, it is just trying to keep the RX happy.

Rob

[RESCUE161]

Well, the notch filter I bought does not go up to 462, so I'm going to have to wait a bit longer to try this out. I'm having to replace the antenna as there is intermittent VSWR issues. What a mess. So I have the two duplexers tuned, but there is not enoughisolation on each respective duplexer to notch out each other's TX signal, so the third notch filter will be required. I still don't know if it'll work, but we'll see.