Marker Beacon Antenna Installation - Garmin GMA 347 Installation Manual

For aircraft with two flight crew members, transmit-interlock would probably interfere with
communications. When the pilot or the copilot transmit, no audio is heard on any other receiver. This
means that if the pilot is communicating with ATC while the copilot transmits on another radio, all pilot
reception is cut off during the time of copilot transmission.
If the installation does not have transmit-interlock activated, all the receivers are listening all the time
whether any radio is transmitting or not. Split COM performance varies significantly across installations.
If the transceivers interfere with each other, transmission by one radio produces static or squeal, thus loss
of communication from any other radio.
2.6 Antenna Installation
2.6.1 Location Considerations
The marker beacon antenna should be mounted on a flat surface on the underside of the aircraft. Mount
the antenna so that there is a minimum of structure between it and the ground radio stations. Locate it as
far away as possible from transmitter antennas.
Best results with split COM mode will be obtained when the COM 1 and COM 2 antennas are mounted
on opposite sides of the aircraft (top/bottom).
2.6.2 Marker Beacon Antenna Mounting
Install the marker beacon antenna according to the antenna manufacturer's instructions and FAA
Advisory Circulars AC 43.13-1B and AC 43.13-2A, Chapters 2 and 3, as applicable. If the antenna is
being installed on a composite aircraft, ground planes must be added. Conductive wire mesh, radials or
thin aluminum sheets embedded in the composite material provide the proper ground plane allowing the
antenna pattern (gain) to be maximized for optimum performance.
2.6.3 Marker Beacon Antenna Cable Installation
Use coaxial cable meeting the applicable aviation regulation for the marker beacon antenna. Any cable
meeting specifications is acceptable for the installation. When routing antenna cables, observe the
following precautions:
•
All cable routing should be kept as short and as direct as possible.
•
Avoid sharp bends.
•
Avoid routing cables near power sources (e.g., 400 Hz generators, trim motors, etc.) or near
power for fluorescent lighting.
•
Avoid routing cable near ADF antenna cable (allow at least a 12-inch separation).
GMA 347 Installation Manual
190-00325-01
Page 2-5
Revision A