Garmin APOLLO GX SERIES, Service Manual

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Garmin AT Rev --
5-2 GX Service Manual © 2004 Garmin AT
also include extra time in installation quotes to allow time to identify and isolate the interference
source(s).
What are the sources of VHF interference?
The main sources of VHF interference fall into one of the categories listed below.
1. VHF harmonic interference broadcast on the 12
th
and/or 13
th
harmonic of the fundamental VHF
frequency by Comm and NAV/Comm transceivers.
2. A clean VHF signal can produce interference when it is picked up by an ELT antenna. Non-linear
components in the ELT antenna generate interfering harmonics which are re-broadcast and
interfere with the GPS.
3. A clean VHF signal can produce interference when it is picked up by a direction finding antenna
(used mainly in search and rescue aircraft). Non-linear components in the antenna system or
front end generate harmonics which are re-radiated from the direction finding antenna.
4. VHF harmonic interference emitted from the NAV/Comm radio case and front panel.
5. Inter-coupling between multiple Comms and NAV/Comms. Usually, both Comm antennas are
mounted on the top side in close proximity. In these cases, we’ve seen the Comm 1 transmitter
couple to the Comm 2 receiver or vice versa causing the re-broadcast of L1 interference through
the case and display of Comm 2. For example, let’s say a GPS/Comm is designated as Comm 1
and a NAV/Comm is designated as Comm 2. When the Comm 1 transmitter is keyed on one of
the interfering VHF frequencies, the RF transmit energy is coupled to the Comm 2 receiver and
interference on the L1 GPS frequency can be re-emitted from the case and display of Comm 2.
More than one of these VHF interference problems can occur on the same aircraft at the same time.
The real problem is being able to identify and isolate the interference.
Isolating the sources of VHF Interference
The following steps are to be taken only if you have verified L1 interference during the pre-installation
checkout. Install the GPS unit in the panel according to the manufacturer’s instructions. Run the
antenna coax (RG-142B) and leave some extra lead length, but do no mount the antenna. We
suggest you make up a test coax harness for use during GPS installations. Using the test coax
harness, connect it to the GPS antenna on top as close to the desired location as possible.
Remove Comm 2 and the ELT from the aircraft. Test Comm 1 on all the known interfering
frequencies listed in the AC or the Apollo Installation manual.
If there is GPS signal degradation, re-test Comm 1 on a dummy load to determine if the VHF
harmonic interference is being transmitted out the antenna or radiated from the NAV/Comm unit. If
the VHF interference occurs only during transmitting, install a GPS notch filter in line with the
antenna. The GPS notch filters will attenuate the interference by 40 dB, which will suppress the 12
th
and 13
th
harmonic interference.
If the problem exists only under dummy load, then the interference is radiating from the case and
front panel of Comm 2. Try to re-position the GPS antenna to minimize the effects of the interference.
Additional shielding around the case of the NAV/Comm has been effective in suppressing
interference. Wrap the entire unit in aluminum foil for testing. A metal case with appropriate grounding
may be needed as a permanent fix.
We have experienced interference on a number of installations involving an existing GPS receiver.
Try installing GPS filters and/or appropriate metal shielding, such as 0.020" aluminum, around the
case to suppress the L1 interference. The L1 interference on the GPS receiver emits from the sheet
metal seams in the case. It is important to completely enclose the GPS receiver chassis in
appropriate metal shielding. We have also seen some extreme cases where the interference emits
from the front panel and display area of the GPS receiver. Re-positioning the antenna can usually
correct the problem. If not contact the manufacturer of the GPS receiver for additional help. It’s