Pilot's Guide to Avionics 2017-2018 - 49
One major development with respect to antennas is
the Federal Aviation Administration's ADS-B Out mandate,
requiring operators who fly in most controlled airspace
to either update their transponder, or - for aircraft that
operate only on domestic flights at low altitudes - retain
an existing transponder and add a universal access
transceiver. In many cases, this will require both a new
antenna and cable.
"ADS-B typically requires a particular TSO, and an
existing antenna may not meet the spec," Brand said.
He added that even if an older antenna would work,
you'd likely need to change the coax connecting it to the
transponder, since virtually all ADS-B transponders specify
low-loss, double-shield cable, and in some cases the
connectors aren't compatible.
Myers commented on the same topic: "Connecting a
brand new piece of equipment to old antennas and cabling
may not deliver the best response. We recommend a new
antenna for L3's Lynx ADS-B installations. Older L-band
antennas were cut for 1020-1060 MHz transponders.
We're also using it to receive 978 MHz UAT transmissions,
which creates a bandwidth issue. Using an old antenna
may work, but won't deliver as effective a signal -
probably with reduced range."
There was considerable discussion about antenna
diversity when the ADS-B mandate was under discussion.
Ultimately, the mandate only requires a bottom-mount
antenna. Brand said that in his experience, coverage
is equivalent. "We only recommend a top antenna on
VHF navigation antennas can be used almost indefinitely,
but note the deteriorated insulators on this one.
business-class aircraft," he said. "We haven't seen any
performance difference in compliance testing. The only
situation where it might make a difference would be
operations at low altitude in high terrain. We experimented
with a Cessna 210 that had an ADS-B installation that
could be configured to use either single or dual antennas
and could see no difference." He added that at least one
major manufacturer doesn't offer a diversity option in their
most popular ADS-B transponder.
Customers who opt to retain an existing Mode-C
transponder and add a UAT will need to add a second
L-band blade antenna. According to Brand, "Generally
you should do a separate antenna - you'll see splitters
on some experimentals, but for certified airplanes where
you must comply with TSO specs, you need separate
An increasingly common option with ADS-B installations
is a dedicated GPS receiver built into the transponder
or UAT - which raises another question: Is a separate
antenna required? Not necessarily, according to Myers.
"If the antenna you have meets at least TSO-C144A,
you can use it with a splitter," Myers said. "We prefer
antennas that meet TSO-C190. We incorporate both
splitter and coupler in our STC, to provide dealers with
options." However, less than perfect consistency in
regulatory guidance from the FAA can be a problem.
"Different FSDOs may or may not accept the same setup.
Continued on following page...
The belly of piston singles is an especially harsh environment.
Coating antennas with carbon blow-by and oil is bound to reduce
performance, and with a visible crack, this one is due for replacement.
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