Originally Posted by
MarcK
But will the radio let you tune "121.500"? If you wanted 122.500 the radio wants to display 122.505. Does it know something special about 121.500? As I said, I don't have or need an 8.33 radio, so I haven't actually twiddled the knobs.
Yes, the radios are intelligent. If you select 122.500 on the radio, it knows it's a 25 kHz separated frequency. So it'll transmit on 122.500 but on receive will use a 25 kHz wide filter. On the other hand, if you select 122.505 it knows it's an 8.33 kHz separated channel. So it'll again transmit on 122.500 but on receive will use the 8.33 kHz wide filter.
All this is done to make the system fairly idiot-proof. You don't have to worry about 25 kHz or 8.33 kHz in reality. All you need to do is dial-in the frequency ("channel") that's been given to you, and the radio will take care of the rest. Also the 8.33 "channels" that are now being used cannot be tuned on a 25 kHz-only radio anyway, so that would hopefully give a clue to the pilot that he's doing something wrong.
(Unless it's an Icom A220, apparently.)