Craig Davidson wrote:
GeorgeF wrote:


David Casey wrote:

Do aircraft and folks on the ground talk to each other on 2 different
frequencies?


Here is what you're hearing which probably gave the impression
of ATC and the aircraft using two different freq.

This is common at night time where there is less
traffic and less controllers on duty. Since you didn't mention any
frequencies I'll use this as an example, this was an actually example of
someone with the same question who lives in Boston, MA. Lets say
you're hearing the ground controller (ATC) on 124.4 but you're hearing
the aircraft answering on 118.25. One controller is probably
controlling three different sectors. The controller in our example is
keying up
the mike and his voice is being broadcast on 120.6, 118.25, 133.0, and
124.4 at the same time. The antenna site for 124.4 is probably near you
as that is the only one you are hearing. The other three antenna sites
are located near the center of the North sector, West sector, and South
sector, too far away from you to hear.


Excellent answer! I have also heard the same controller's voice being
transmitted on civilian and military frequencies simultaneously.

Yes that is normal as well. One reason for that is lets say a Military
aircraft is coming up on an ATC freq, the military aircraft can hear the
controller talking to a civilian plane so the military aircraft will
wait for him to stop talking. Then he knows the freqs are clear. If
ATC didn't do that a military plane could call up on freq however the
same controler would be on the civialian freq talking at the same time
causing ATC to miss the military's call.

George
http://www.MilAirComms.com