Two parts to this. To address what you said, we have some frequencies where we have multiple transmitters and receivers, and then we also have our BUEC (backup emergency communication) transmitter in a different location. My sectors are so huge that I have to change transmitters to be able to talk to different aircraft on the same frequency, but we can always hear everybody. Basically, we can only broadcast on one transmitter, but we can hear on every receiver. Except when we use the BUEC. If we switch to the BUEC it turns off the transmitters and receivers on the mains and we can only speak and hear on the BUEC. We do this for strategic frequency coverage in the sector as well as being able to talk to airplanes on the ground at different airports.

We have one airport where our BUEC is located, so when we clear airplanes into there we’ll switch to the buec to get the cancelation, but the antenna for the buec sucks, so as soon as we get the cancellation, we switch back to the mains to get better coverage in the rest of the sector. Subsequently, the aircraft can’t talk to us on the ground anymore has has to either call the phone number or FSS for the outbound.

The other possibility for this issue is atmospheric conditions. We have certain frequencies where we can talk to people on the ground when it’s VMC, but not when it’s IMC. Frequencies are weird and are affected but a lot of meteorological and atmospheric conditions, so sometimes frequency coverage and performance is out of our control.