Skyhawk 172/Cutlass 172
Flight Crew Flight Instructor; Flight Crew Commercial; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 60; Flight Crew Total 2450; Flight Crew Type 1200
Communication Breakdown
Conflict Airborne Conflict
General None Reported / Taken
Airspace Structure; Procedure; Human Factors
Cleared for takeoff on 21R at PDK with a left turn out for a Northeast departure. We were climbing from 3;000 MSL to 4;000 MSL when we cleared the airport traffic area. The tower called and approved frequency change. I replied and turned down the volume so I could talk to the student.On returning to the airport I was informed the tower was franticly trying to contact me immediately after the approved frequency change. I called the tower after landing and learned the controller had noticed a developing traffic conflict on the radar. A business jet was inbound from the East to land at KRYY descending toward 4;000 MSL. The jet was probably on Atlanta Center or perhaps RYY tower for a visual; so PDK tower had no contact with him. His approach was from our right rear about 4 O'clock and I did not see him. He did see us in time to avoid us. The reported separation was 500 feet over us.  We do not stay on tower frequency any longer than necessary. It is impossible to instruct with the high volume of transmissions at KPDK. Atlanta approach poses the same problem. On cross countries we tune 121.5 and this might be useful. I asked the tower controller if they tried 121.5 however he said PDK tower does not have 121.5 as a frequency?? Why not? I know Atlanta Center does. There should be an established procedure for this the Atlanta area is heavy mixed traffic and not all the flight school planes have traffic.
Upon returning to the airport at the end of the flight; the instructor pilot was informed by the Tower that they had been attempting to contact them on departure due to a traffic conflict with an aircraft not under Tower's control.