A-D vs Clearance Delivery

[kc9jmd]

While monitoring the KDLH A-D frequency, I never pick up the airport side of the conversation. I usually hear the aircraft quite clearly. I'm wondering why I seem to pick up the airport side of the conversation on a different frequency - 124.8, which is not listed below but is listed as "Clearance Delivery" for another local airport, KSUW. I hear the airport side quite clearly on 124.8, but never on the below-listed A-D frequency. If I scan both, I hear the whole conversation as it relates to KDLH.

Can someone explain?

 

[andy51edge]

ATC radio transmitter antennas are normally mounted quite low to the ground. The operating assumption being that aircraft are high in the sky or on the ground at the airport, thus line of sight will be maintained either way. Assuming you are monitoring from home, you may have no LOS to the transmitter at DLH but have LOS to the aircraft in the sky.

124.8 is most likely a special case, a Remote Transmitter/Receiver (RTR) at SUW so that aircraft on the ground at SUW can receive their IFR clearances from DLH approach while on the ground at SUW and wouldn't have LOS to DLH.

The final piece of the puzzle is that controllers usually simulcast frequencies. They transmit on all of the frequencies assigned to their sector at the same time and then receive on all of those frequencies. Example being you can have a Delta Connection CRJ on 125.45 and a local ANG F-16 on 233.7. The pilots of both aircraft would hear the same controller simultaneously but wouldn't be able to hear each other.