Antenna Considerations; Table 3-8 Maximum Db Loss Allowed From Transponder To Antenna; Transponder Antenna; Transponder Antenna Location - Garmin GTX 335 w GPS Installation Guidance

3.11 Antenna Considerations

This section contains general installation guidance to make sure the installed antennas meet the GTX
transponder and ADS-B requirements. The approval of antenna installations is outside the scope of this
STC.

3.11.1 Transponder Antenna

This STC does not install the transponder antenna. The transponder antenna(s) is existing equipment.If an
existing GTX transponder is already installed, no changes to the antenna or coax are required.
When upgrading from a non-Garmin transponder, the existing approved transponder antenna should be
verified to meet these requirements:
• Be approved to TSO-C66( ) or C74( ), and
• The aircraft ground plane should be electrically bonded to the antenna baseplate, which should
achieve direct current (DC) resistance less than or equal to 2.5 milliohms.
The installer should determine if the aircraft's current antenna and installation meets the above
requirements. If the antenna does not meet these requirements, the installer is responsible for obtaining FAA
approval for the installation of compliant equipment.

3.11.2 Transponder Antenna Location

For the most optimum transponder antenna location installation:
The transponder antenna should be located away from major protrusions (i.e., engine(s),
propeller(s), and antenna masts).
◦ It should be as far as practical from landing gear doors, access doors, or other openings that
can affect its radiation pattern.
The transponder antenna must be attached vertically on the bottom of the aircraft.
The transponder antenna must be installed at least 20 inches from any FADEC (Full Authority
Digital Engine Control).
The transponder antenna must be installed at least 20 inches from any transponder, TAS/TCAS,
DME, or other L-Band antenna.

Table 3-8 Maximum dB loss allowed From Transponder to Antenna

3.11.3 Transponder Antenna Installation

NOTE
Installation approval for the transponder antenna is not supplied through this STC. Refer
to the antenna manufacturer's installation guidance for the particular model antenna for
minimum performance specifications.
Refer to the aircraft manufacturer's data and the antenna manufacturer's installation instructions for
attaching the transponder antenna. The installer can use other FAA approved data to gain a separate antenna
installation approval.
Refer to table 3-9 for examples of the recommended antenna cable vendors and the type of cable used for
specific lengths of cable. Any cable that meets the minimum specifications is satisfactory for the
installation.
190-00734-17
Rev. 1
GTX 335
2.0 dB
GTX 335 w/GPS Installation Guidance
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