May_June_2022 - 30

WI-F I AND COMMUNICAT IONS
Top, corrosion on an antenna
surface. Below, a good COM antenna
installation with the base sealed with
pro seal to keep moisture out and help
prevent corrosion from setting in.
unstable system operation, " he continued.
The condition of the antennas and
associated cables is critical to the proper
function of a communication system. The
antenna is the last point of contact for the
transmitter and the first for the receiver. If
it is damaged or compromised, it will negatively
impact both the range and quality of
the transmitted and received signal.
" Old RG-58U cable with the black insulation
that has been around for so many
years is no longer considered suitable for
aviation use. Beyond its relative fragility,
vulnerability to temperature and moisture
and penchant for releasing toxic material in
a fire, it will degrade with age. It is strongly
advisable to remove RG-58 cable in favor
of RG-400, which is preferred, or RG-142,
which is low loss, but stiff and it also has
a steel core that can become magnetized, "
said Karpowitz.
Along with the do's and don'ts of antenna
placement, Holland said to make sure that
you follow the guidelines on exactly where
to put this.
" Now, let's just say you don't have any
" Sometimes these can be disturbed during
inspections or other service work. Make
sure the connectors and units are secure
and can pass the tug test. It is possible to
inadvertently switch antennas because the
connectors and cables are used in a variety
of systems, Nav, Com, GPS, etc., " he said.
Antenna Trouble
Antennas can be another leading cause of
communication dysfunction.
" If the antennas on top of the fuselage are
not at a proper distance, they could interfere
with one another. And it's the same thing
too with any Iridium or Satcom system that
you put on board. And you get it too close
to a GPS antenna, which can also impact
the performance of the GPS. I mean, it can
literally cause the GPS in the cockpit basically
to lose coverage, " said Holland.
30 MAY/JUNE 2022
Not only are antennas critical but their
installation and location are vital elements
of proper system operation. Most avionics
installation manuals specify minimum
distances between antennas or between
antennas and potential noise sources or
vulnerable systems. As much as possible,
antenna cables should be routed separately
of signal, digital or power cable bundles for
a variety of reasons, said Karpowitz.
" In an era of so many composites and
other non-metallic aircraft building materials,
VHF com antennas can suffer degraded
performance by not having enough of
a 'ground plane.' Think of a ground plane
as a mirror - the other half of a two-part
antenna. If a typical antenna is mounted on
a non-metallic surface, the lack of ground
plane will severely compromise the efficiency
of the antenna and may even result in
room. There are filters that you can use
to help eliminate that particular problem.
But, if it's transmitting, I mean, and a GPS
antenna's nearby, you're not going to eliminate
that problem unless you give it space, "
Holland continued.
Helicopters are especially problematic in
this regard, added Karpowitz.
" It is important that antennas are not
located near pilots or passengers' heads in
helicopters because the large windows do
not provide any shielding. RF can get back
into the com system through the headset or
headset cable and cause howl and garbled
audio. In general, try and keep the antennas
and the occupants' headsets as separate as
possible, " Karpowitz said.
When testing any of these communication
systems, some com testing requires
calibrated equipment to adequately assess
the health of the system.
AIRCRAFT MAINTENANCE TECHNOLOGY
May_June_2022

Table of Contents for the Digital Edition of May_June_2022
