I have ideas, but no hard schematics to post. You'll need to make up a good-sized switch box, metal, and install 6 SO239's in it. One will be for your antenna, and the others will be for your radios. If you want to automatically switch to the radio thats transmitting, you'll need to design yourself 5 RF sensors, and wire them into 5 relays. The radio thats transmitting will fire the relay that connects the antenna to itself. One radio will need to be connected to the antenna by default, or you won't be receiving on any radio. If you want to be able to switch the radios for receive as well, just install a toggle to activate the proper relay, but be sure you don't key another radio while one of the toggles is active.
You can go further by tying the RF sensors to a logic circuit, and use the logic circuit to determine which relay is fired. the logic can also be wired to sound a warning if more than one radio keys at once, or automatically shut your power supply down if that happens.
