Gps Coaxial Cable Requirements (Gtx 335 With Sbas/Gps); Table 3-12 Gps Db Loss Allowance - Garmin GTX 335 w GPS Installation Guidance

3.12 GPS Coaxial Cable Requirements (GTX 335 with SBAS/GPS)

This section supplies information on the GPS antenna cable requirements. The location of the GTX 335 unit
GPS Antenna compared to other COM transceivers and antennas is very important. During the post-
installation checkout, susceptibility to harmonics of VHF COM transmitters is analyzed. If problems occur,
better isolation or distance can be required between the GPS and COM antennas. A 1575.42 MHz notch
filter can be installed in series with the antenna coaxial of the VHF COM transceiver to decrease or
eliminate the harmonic interference. A notch filter for this use (P/N 330-00067-00) is available from
Garmin.

Table 3-12 GPS dB Loss Allowance

Unit Minimum Maximum
GTX 3X5 Internal GPS 1.5 dB 6.5 dB
To reduce or stop signal interference, the GPS antenna cable assembly loss must be in accordance with
table 3-12. Think about additional loss from coaxial connectors and adapters (e.g., TNC to BNC) when
cable loss is calculated. A typical loss of 0.2 dB can be used for each connection. To keep integrity of the
WAAS signal, the GPS Antenna coaxial cable must have a minimum of two shields (e.g., RG-400 or
RG-142B).
GPS antennas listed in Table 3-11 require a cable loss between 1.5 dB and 6.5 dB. If RG-142B or RG-400
is used, then 1.5 dB is equal to a length of approximately 6.5 feet of cable with a connector on each end.
RG-142B or RG-400 cable can be used if the length is less than 35 feet. For longer lengths, use low-loss,
double- or triple-shielded, 50Ω coaxial cable.
For very short cables where the loss is less than 1.5 dB, add more cable to increase the loss to within 1.5 dB
and 6.5 dB. This cable can be coiled, taking into account the minimum bend radius of the cable.
When the antenna position is selected, make sure that the routing of the coaxial cable from the antenna to
the GTX 335 Internal GPS is correct. Correct selection of coaxial cable and installation of connectors is
very important to GPS signal performance.
If a VHF COM transmitter causes problems with the GPS on the selected frequencies as listed in the post
installation checkout, the problem can be because of the ELT. To find out if the ELT is a problem,
disconnect the ELT antenna coaxial at the ELT unit. If the ELT is found to cause the problem, contact the
ELT manufacturer.
190-00734-17 GTX 335 w/GPS Installation Guidance
Rev. 1 Page 3-12