Pilot's Guide to Avionics 2013-2014 - 46

AIRCRAFT AS ANTENNA FARM
Continued from page 45

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Now, create an image of all the
receivers and the antennae crowded
onto the skin of a fuselage or other
surface.
It looks and is crowded, or it would
be if not for advances in antenna
technology that let some serve combined functions.
Even with those advances, the
antenna crop puts more pressure
than ever on the care required to
determine where on the aircraft to
mount some of them.
The issue could become less
complicated in time as technology
solves more of the problems associated with mounting decisions.
In the meantime, careful attention
to the role of each antenna, manufacturers’ instructions and multiplerole aspects of some demand careful
planning and execution when moving or adding an antenna.
Challenges of Proliferation:
Like Real Estate, Location
is Everything
While aircraft and avionics provide new services and functions not
thought of just a few years ago, including satellite navigation, satellite weather, ground-based weather and ADSB, the proliferation of antenna needs
presents the same age-old challenges
of the past: mounting location.
Mounting locations must be chosen with the avoidance of external
interference in mind.
Some systems require greater care
than others, in large part because of
their sensitivity to interference and the
low signal-to-noise ratio of the transmission.
Spherics devices, in particular, are
sensitive to electronic noise sources
external to the system. To hear lightning strikes with sufficient signal
strength for the computer to calculate
direction and distance, the antenna
must be mounted on a spot with minimal electronic noise from other devic-

- 46 -

es. The cable should be routed away
from other wiring – even other coaxial
cables – so the faint cry of a distant
lightning strike gets heard loudly and
clearly.
Technicians have long worked to
map the aircraft skin to find the quietest spot for the spherics antenna.
Separation standards also must
be followed between VHF and microwave antennae – think nav/comm and
transponder – to avoid the prospect of
a transmission bleeding through to a
GPS receiver.
It’s the issue of interference of
GPS navigators traced to an adjacent microwave-spectrum source that
brought down Lightsquared’s ambitious plan to launch an independent
4-G smartphone service to resell
to cellphone companies. Too much
power from even miles away simply
drowned out the relatively weak signal
of the navigation satellites.
Avoiding the same problem on a
single-airplane scale drives the location decisions for mounting everything, from the GPS antenna to the
transponder and CAS antennae, VHF
nav/comm antennae and datalinkreceiver antennae.
The issue also strikes at the heart
of some avionics equipment that uses
GPS as one of several references
used to determine attitude, heading
and air data provided by some of the
newest primary flight displays.
But some of these issues have
changed, thanks to some very clever
engineers who undertook the design
challenge of new-technology antennae and help rectify the issues of
cross- and intra-spectrum interference, while reducing the crop in the
antenna farm.
Multitasking Comes
to the Antenna Farm
Multitasking has come to aviation radio antennae, and not too
soon.
“Just when you think this is getting too complicated to complete,
someone comes along with a solu-

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Pilot's Guide to Avionics 2013-2014

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