Page 2-2
GDR 66 Installation Manual
Revision A
190-00303-24
2.3 Installation Considerations
Fabrication of a wiring harness is required. Sound mechanical and electrical methods and practices are
required for installation of the GDR 66.
2.3.1 Antenna Considerations
Antenna installations on pressurized cabin aircraft require FAA approved installation design and
engineering substantiation data whenever such antenna installations incorporate alteration (penetration) of
the cabin pressure vessel by connector holes and/or mounting arrangements. For needed engineering
support pertaining to the design and approval of such pressurized aircraft antenna installations, it is
recommended that the installer proceed according to any of the following listed alternatives:
1. Obtain approved antenna installation design data from the aircraft manufacturer.
2. Obtain an FAA approved STC, pertaining to, and valid for the antenna installation.
3. Contact the FAA Aircraft Certification Office in the appropriate Region and request identification
of FAA Designated Engineering Representatives (DERs) who are authorized to prepare and
approve the required antenna installation engineering data.
4. Obtain FAA Advisory Circular AC-183C and select (and contact) a DER from the roster of
individuals listed in it.
5. Contact an aviation industry organization such as the Aircraft Electronics Association for
assistance.
2.3.2 Com Antenna Location
The GDR 66 COM antenna should be well removed from all projections, engines and propellers. The
ground plane surface directly below the antenna should be a flat plane over as large an area as possible (18
inches square, minimum). The antenna should be mounted a minimum of three feet from any DME
antennas, three feet from any GPS antennas, and as far as practical from the ELT antenna. Some ELTs
have exhibited re-radiation problems generating harmonics which may interfere with other signals.
In addition, the COM antenna must have at least 16 dB of isolation from other COM antennas to prevent
damage to the GDR 66 COM receiver. For COM antennas mounted on the same side of the fuselage, 16
dB of isolation can be achieved by a physical separation of approximately 3 feet (0.9 meters).
If simultaneous use of two or more COM transceivers is desired the COM antennas must be spaced for
maximum isolation. For a two COM installation, one COM antenna should be mounted on the top of the
fuselage and the other antenna should be mounted on the bottom of the fuselage. For installations with
three COM’s, one COM antenna should be mounted on the top of the fuselage and the other two antennas
should be mounted on the bottom of the fuselage and physically separated from each other as much as
possible.
The recommended minimum isolation between COM antennas for simultaneous use of two or more
COM’s is 40 dB. Separating the COM antennas between the top and bottom of the fuselage typically
provides 35 – 45 dB of isolation for metal skin aircraft. For COM antennas mounted on the same side of
the fuselage, 40 dB of isolation can be achieved by a physical separation of approximately 60 feet (18.3
meters). Receiver sensitivity could be significantly reduced during the transmission of a co-located COM
for installations with less than 40 dB of isolation between the COM antennas. For installations with less
than 23 dB of isolation between COM antennas, cross modulation (bleed-through) could also be
experienced during the transmission of a co-located COM. For COM antennas mounted on the same side
of the fuselage, 23 dB of isolation can be achieved by a physical separation of approximately 9 feet (2.7
meters).
Simultaneous COM performance varies significantly across installations and is affected by both the
isolation between the COM antennas and the separation of the tuned frequencies. Each installation should
be individually examined to determine the expected performance of simultaneous COM.
DRAFT
