C-11719 - It's important to understand the control functions. The main function
switch has four positions: Off, Main, Both, ADF. In Both, both the Main Receiver and
the Guard receiver are activated so that the radio operator can communicate as normal
with the main reciever and transmitter while listening to the guard channel for
emergency response. If the aircraft is equipped with a UHF direction finding system,
the ADF position will activate the UHF/DF antenna and actuate relays so that the system
uses a DF antenna instead of the comm antenna. The DF antenna independantly drives
toward the station and sends bearing signals to the appropriate instrumentation.
Meanwhile, the RF signal is still sent to the UHF R/T for demodulation of the audio.
The M-P-G switch has 3 positions: Manual, Preset and Guard (GRD). By selecting manual,
your opertaing frequency will reflect what you have dialed in. In preset, you will use
the selected preset. By putting the switch in Guard, you will be operating at 243 MHz.
The operational difference between this and the Both function, is that in Both, you
want to listen to the guard channel. But if you want to make an emergency transmission,
you want GRD. Selecting GRD tunes the transmitter and main receiver to 243 MHz. If you
leave it in Both, the guard receiver will be disabled so that you do not burn it up
with your your guard transmission (just as the receiver is not receiving when you
transmit for any radio).