Voyager's antenna is aimed at the sun (because at its distance, earth and sun have close angular separation). So this means the command signal has to overcome

Voyager's antenna is aimed at the sun (because at its distance, earth and sun have close angular separation). So this means the command signal has to overcome microwave noise from the sun. So this is the same problem as SETI. I know for SETI they reduce stellar noise by assuming that aliens are transmitting in very narrow bandwidths.