I have put together a block diagram for a combiner system for 4 channels of semi duplex radio. Am I heading in the right direction?
Sort of. The RX side is good. On the TX side, the hybrid coupler is wrong. Hybrid couplers are 2 x 2 (4 port) device; two inputs, two outputs. (There are actually other mult-iport designs, but not relevant here.)
You need a total of three hybrid couplers: The first HC's input ports are connected to TX1 & TX2. The second HC's input ports are connected to TX3 & TX4. One output from each HC is terminated with a load => the total RF power of two transmitters. Each of the other output ports of each HC connects to one of the two inputs of the third hybrid. On the third hybrid's output ports, one port connects to the antenna feed line. The other port has a load capable of => the total RF power of the combined transmitters. (see page 40 of
http://www.emrcorp.com/documents/Technical%20Information/hybrid-filter_combiners%2836-44%29.pdf)
The other considerations are:
- RF gain structure on the TX side to make sure the passive losses and any antenna gain ultimately provides sufficient ERP.
- Determining the correct amount of TX to TX isolation, and using the correct isolator(s).
- If the RX frequencies or other adjacent channel users are too close, or the RF noise floor is rather high, you may want to put a very narrowband, high Q bandpass cavity filters between the splitter outputs and the receiver inputs.