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Installation
The antenna should be mounted on a metal surface or a ground plane with a minimum area of
18 x 18 inches. The antenna should be mounted a minimum of two feet away from GPS
antennas.
The comm antenna should also be mounted as far apart as practical from the ELT antenna,
preferably one on top and the other on the bottom of the aircraft fuselage. Some ELTs have
exhibited re-radiation problems generating harmonics that may interfere with GPS signals.
This can happen when the comm (SL40 or any other comm) is transmitting on certain
frequencies such as 121.15 or 121.175MHz, which may cause the ELT output circuit to
oscillate from the signal coming in on the ELT antenna coax.
The antenna coax cable should be made of RG-142B or a comparable quality 50W coax.
Assembly instructions for the rear coax connector are included in Figure 6.
GPS A
NTENNA
The mounting location and cable connections for the GPS antenna are very important. The
antenna should be mounted no closer than two feet from VHF comm transmitter antennas, six
inches from other antennas emitting less than 25 watts, and two feet from higher power
antennas. Special care should be taken to ensure that the GPS antenna is not mounted in close
proximity to antennas that may emit harmonic interference at the L1 frequency of
1575.42MHz. Refer to the antenna installation manual for installation instructions.
The connectors are included in the installation kit, and are intended for use with RG-142B
size coax cable. If using a different diameter coax, alternative connectors may be required.
Assembly instructions for the connectors are included in Figure 6 and Figure 7. RG-142B
cable can be used as long as the length is less than 20 feet. For longer lengths, use a low loss
50W coax.
Suggestion: Temporarily locate the GPS antenna with coax connected to the SL50/60 and
check the GPS performance as described in the GPS Operation and Position test in the Post
Installation Checkout
on page 24. Once a suitable location has been verified, then
permanently mount the antenna.
Note: If using a GPS antenna that was already on the aircraft, or if mounting the antenna
closer than two feet from a comm antenna, conduct the GPS Operation and Position test in
the Post Installation Checkout on page 24. If the SL50/60 passes the test, then moving the
antenna is not necessary.
Once the antenna mounting position has been prepared. route the coax cable from the antenna
to the SL50/60. Proper selection of coax cable and assembly of connectors is critical to GPS
signal performance. The cable loss from the antenna to the SL50/60 should be limited to a
maximum of 4db. Minimize the coax length for optimum performance and DO NOT coil
excess cable. Leave only enough for service loops. The coaxial connectors and adapters, such
as TNC to BNC, add additional loss to the cable and should be considered when computing
the maximum 4db loss. A typical loss of 0.2db can be used for each connection. The typical
cable loss for 20ft of RG-142B coax with a connector on each end is 4db.
During the post-installation checkout, susceptibility to harmonics of VHF comm transmitters
will be evaluated. If problems arise, then better isolation, or distance, may be required
15
Apollo SL50 / SL60 Installation Manual
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