Gps Antennai - Garmin GNS 480 Installation Manaul

3b. The GPS antenna should be mounted no closer than two feet from any antennas emitting
more than 25 watts of power. An aircraft EMC check can verify the degradation of GPS in
the presence of interference signals.
3c. To minimize the effects of shadowing at 5° elevation angles, the GPS antenna should be
mounted no closer than 6 inches (edge to edge) from other antennas, including passive
antennas such as another GPS antenna or XM antenna.
4. To maintain a constant gain pattern and limit degradation by the windscreen, avoid
mounting the antenna closer than 3 inches from the windscreen.
5. For multiple GPS installations, the antennas should not be mounted in a straight line from
the front to the rear of the fuselage. Also varying the mounting location will help minimize
any aircraft shading by the wings or tail section (in a particular azimuth, when one antenna
is blocked the other antenna may have a clear view).
Figure 2-11 shows the recommended placement of antennas.
The internal GNS 480 COM does not interfere with its own GPS receiver. However,
placement of the GNS 480 GPS antenna relative to other COM transceivers and antennas
(including the GNS 480 COM antenna) may be critical.
The connectors are included in the installation kit, and are intended for use with RG-142B or RG-400 size coax
cable. If using a different diameter coax, alternative connectors may be required. Assembly instructions for the
connectors are included in Figure 2-12. RG-142B or RG-400 cable can be used as long as the length is less
than 35 feet. For longer lengths, use low-loss 50Ω coax.
Suggestion: Temporarily locate the GPS antenna with coax connected to the GNS 480 and check the GPS
performance as described in the GPS Operation and Position test in the Post Installation Checkout in
section 3.3.2.1.2. Once a suitable location has been verified, then permanently mount the antenna.
If using a GPS antenna that was already on the aircraft, or if mounting the antenna closer
than two feet from a COM antenna, conduct the GPS Operation and Position test in the
Post Installation Checkout on page 3-32. If the GNS 480 passes the test, then further
measures are not necessary.
Once the antenna mounting position has been prepared, route the coax cable from the antenna to the GNS
480. Proper selection of coax cable and assembly of connectors is critical to GPS signal performance. The
cable loss from the GPS antenna (excluding antennas in Table 1-2) shall be between 1.5 dB and 6.5 dB in
order to maintain proper rejection to interference signals.
GPS antenna in Table 1-2 require a cable loss between 3 dB and 7 dB.
The coaxial connectors and adapters, such as TNC to BNC, add additional loss to the cable and should be
considered when computing the cable loss. A typical loss of 0.2 dB can be used for each connection. To
maintain integrity of the WAAS signal, the GPS antenna coaxial cable must have a minimum of two
shields (e.g. RG-400 or RG-142B). The typical cable loss for 20 feet of RG-142B or RG-400 coax with a
connector on each end is 4 dB. For very short runs, where the loss is less than 3dB, additional cable should
be used to increase the loss to within 3 dB to 7 dB. This additional cable may be coiled, taking into
account the minimum bend radius of the cable.
During the post-installation checkout, susceptibility to harmonics of VHF COM transmitters will be
evaluated. If problems arise, then better isolation, or distance, may be required between the GPS and COM
antennas, or a 1575.42 MHz notch filter may be installed in series with the antenna coax of the VHF COM
transceiver to reduce or eliminate the harmonic interference. A notch filter for this use (part #162-1059) is
available from Garmin AT.
GNS 480 (CNX80) Installation Manual
NOTE
NOTE
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560-0982-01 Rev L
Installation Procedures
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