Pilot's Guide to Avionics 2017-2018 - 48

PILOT'S GUIDE

Typical VHF comm whip
antennas need replacement
when they show visible wear.

REAL-WORLD AVIONICS:

ANTENNAS & ADS-B
S T O R Y

A

B Y

J O H N

ircraft owners/operators and avionics
technicians spend a lot more time looking at
panel-mounted avionics boxes than they do
antennas. That's understandable - the box in the panel is
what the pilot interacts with, and it's what makes the most
money for the shop owner. But if that box interacts in any
way with other aircraft or ground equipment, it does so
through an antenna and associated cabling, which means
that an avionics device costing thousands of dollars can
have its performance compromised if the antenna isn't
working properly.
Since the VHF/UHF and satellite frequencies used by
aircraft radios and related systems operate on a line-ofsight basis, they are installed outside the hull of metal
aircraft (or the Faraday cage built of foil or embedded
wire on composite aircraft). As such, they're constantly
exposed to the elements both in the air and when parked
outside a hangar. Not surprisingly, this can result in
significant aging that affects performance.
Chris Brand, from Pacific Coast Avionics, told us:
"Old antennas can wear through, which will require
replacement. Even with antennas that are otherwise in
good shape, it's frequently good practice to remove old
antennas and gaskets and bond to bare metal to avoid

D .

R U L E Y

corrosion." He noted that deterioration in comm antennas
is particularly serious because they are used for both
transmission and reception. A poor-performing antenna
will reflect significant power back to the transmitter, which
eventually may result in premature equipment failure. By
contrast, receive-only VHF navigation antennas "will last
forever if they're cleaned up." Brand also noted that while
older antennas had only an on-condition maintenance
requirement, "newer antennas generally will have
recommended inspection intervals."
Russ Myers, senior field service engineer at L3
Aviation Products, added: "Antennas are key to operation
of any radio and they take a lot of abuse." As to
maintenance: "We defer to the antenna manufacturer's
recommendation. In general, check at annual and look
at corrosion, any sign of chipping, and check that the
hardware's tight. Some of the better shops tell me they
recommend replacing antennas at seven to 10 years."
He warned that it's important to check manufacturer
documentation before adding a new antenna: "Our
products specify minimum distances from other antennas,
whether GPS, DME, transponder, etc. In some cases, this
may be more than the typical 3-foot rule from AC 43.132B; in other cases, it may be less."

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Table of Contents for the Digital Edition of Pilot's Guide to Avionics 2017-2018

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Pilot's Guide to Avionics 2017-2018 - Cover3
Pilot's Guide to Avionics 2017-2018 - Cover4
http://www.brightcopy.net/allen/pilotsguide/18-19
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http://www.nxtbook.com/allen/pilotsguide/2014-2015
http://www.nxtbook.com/allen/pilotsguide/2013-2014
http://www.nxtbook.com/allen/pilotsguide/2012-2013
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