To further emphasize the previous posters, you have to have the coupler/repeater that mounts in the breaker panel. It transfers the PLC signal from one leg to the 240 to the other. Without it, devices on the opposite leg are going to get very little signal, and are likely to not work correctly.
I note that you need to double-pole breaker to mount the repeater, as it (obviously) connects to both legs. I believe that either 15 or 20 amps is OK (consult the instructions, of course).
