I've tried combining signals onto a single antenna( albeit in the same band at 5 GHZ) using what is called a circulator. A ciculator is sort of like a diplexor, but only allows energy to flow in one direction. They have three ports, an input or transmitter port, an antenna port, and an output or receiver port. The TX signals go into the tx port and energy would only flow OUT to the antenna port. Signals coming IN on the antenna port would only flow to the output or receiver port. In theory it sounds great and I bought some high dollar circulators to try it out. I was getting great received signal levels on both ends of the link, but virtually no throughput (was getting 20+Mbps with seperate antennas on different polarities and the same boards/radios, but only about 300kbps with the circulator and a single antenna). I put the cirulator on the spectrum analyzer and found that I was only getting about 18 dB of isolation between ports. Obviously, more is needed, but I can't find a ciculator that has more. The other theory I have is that since enrgy only flows in 1 direction, I wasn't getting the reponse from the other side on both radios. Even though one radio is TX and one is RX, I'm assuming there is still some messaging going on between the TX-RX individual links. It's possible that the combiner was functioning too well and not allowing the return messages to arrive. For example if you just consider one half of the link, the two radios must handshake and acknolege eachother. It's possible that once the TX radio sent out handshake signal to the RX on the other side, the combiner sent all the incoming signal coming back from that receiver to the RX card instead of back to the TX card. This seems a more likely explanation. Any ideas?