Garmin GPS 400W, Installation Manual

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Page 2-4 GNS 400W Series Installation Manual
Rev. G 190-00356-08
2.4 Antenna Considerations
This section contains mounting location considerations for the antennas required for the 400W Series
units.
The GPS 400W requires a GPS/WAAS antenna, the GNC 420W/420AW requires a GPS/WAAS and
COM antenna, and the GNS 430W and GNS 430AW require four antennas: a GPS/WAAS antenna, a
COM antenna, a NAV antenna, and a Glideslope antenna.
2.4.1 GPS Antenna Location
The GPS antenna is a key element in the overall system performance and integrity for a GPS/WAAS
navigation system. The mounting location, geometry, and surroundings of the antenna can affect the
system performance and/or availability. The following guidance provides information to aid the installer
in ensuring that the most optimum location is selected for the installation of the GPS antenna. The
installation guidelines presented here meet the intent of AC 20-138C section 12. The greater the variance
from these guidelines, the greater the chance of decreased availability. Approach procedures with vertical
guidance are the most sensitive to these effects. LNAV only approaches, terminal operations, and en
route operations may also be affected. Because meeting all of these installations guidelines may not be
possible on all aircraft, these guidelines are listed in order of importance to achieve optimum
performance. Items 3 below are of equal importance and their significance may depend on the aircraft
installation. The installer should use their best judgment to balance the installation guidelines.
1. Mount the antenna as close to level as possible with respect to the normal cruise flight attitude of
the aircraft. If the normal flight attitude is not known, substitute the waterline, which is typically
referenced as level while performing a weight and balance check.
2. The GPS antenna should be mounted in a location to minimize the effects of airframe shadowing
during typical maneuvers. Typically mounting farther away from the tail section reduces signal
blockage seen by the GPS antenna.
3a. The GPS antenna should be mounted no closer than two feet from any VHF COM antenna or any
other antenna which may emit harmonic interference at the L1 frequency of 1575.42 MHz. An
aircraft EMC check (reference VHF COM interference check in Post Installation Checkout
procedures) can verify the degradation of GPS in the presence of interference signals. If an EMC
check reveals unacceptable interference, insert a GPS notch filter in line with the offending VHF
COM or the (re-radiating) ELT transmitter.
Note: When mounting a combination antenna (ex. GPS and COM, GPS and XM), the
recommended distance of two feet or more is not applicable to the distance between the
antenna elements provided the combination antenna is TSO authorized and has been
tested to meet Garmin‘s minimum performance standards.
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