Pilot's Guide to Avionics 2013-2014 - 45

conductor – air, in this case – that completes its circuit at another antenna plugged into the same circuit.
Yes, at the most-basic level, antennae connect distant circuits to one another.
So a comm antenna connects the transmitter to the receiver
at the other end of an exchange and back again.
The tiny pole-type transponder antenna completes a circuit
connecting the transponder to the ground-based radar pinging
it – and back again.
The GPS antenna is the circuit connection between the
navigator and reference satellites thousands of miles above.
Antennae differ considerably across the range of functions
for one simple reason: they must match up with the frequency
spectrum they service.
As avionics technicians know, they get smaller or shorter as
the frequency ranges increase and the wavelengths shorten.
Aircraft systems fit into one of three frequency ranges: very
high frequency, or VHF; microwave; and long wave.
Long wave radios were aviation’s earliest navigation system: the nondirectional beacon transmitter and the automatic
direction finder, or ADF.
Build in the commercial-radio AM spectrum, and the low-frequency NDB gave pilots a signal to track to and from. Early on,
the stations tracked were predominantly high-power broadcast
radio stations augmented later by stand-alone NDBs commissioned by the Civil Aeronautics Administration – the Federal
Aviation Administration predecessor.
Most of what aviation adapted after the NDB operates in the
shorter-wave VHF spectrum: comm radios, nav radios, glideslope, localizer and our little friends, the marker beacons.
In the shortest-wave microwave spectrum operate our transponders, spherics devices (Strike Finder and Stormscope),
satellite-weather transmitters and receivers, and GPS’ navigation-network satellites – plus cellphones and sundry other
items increasingly common in the 21st century.
On larger turbine aircraft, in-flight entertainment systems
and in-flight Internet access bring their own specific antenna
challenges.
Numbers Add Up
Antennae needs once were modest and minimal – the loop
and wire of an ADF, the whiskers of a four-way radio range or
the similar sensors for receiving VOR broadcasts.
Only a couple of decades ago, aircraft tended to sport at
least one, often two VHF antennae for voice communications;
one each for VHF VOR navigation and another for the localizer
signal, one for the VHF nav’s glideslope function, one for each
transponder installed and a special wire-and-loop antenna
package for the ADF radio.
It sounds like a lot, but spread as they were around the airplane’s exterior, we seldom viewed them as numerous.
About the same time, lightning-detection technology began
to advance requiring yet another dedicated antenna. Adding

(Antennae) serve in roles of vital
importance to the pilot, sometimes
serving as the ears to hear. Other
times they serve as electronic eyes
allowing us to see other traffic and
help feel the way through the sky.
such spherics devices frequently posed the greatest challenge
for an installing tech, given the need to map the skin to find the
area of least interference from other electrical sources.
Not long after the first Stormscopes arrived came the
tandem revolutions in area-navigation technologies, which
brought new and different antennae needs – a long whip
for the Loran C receivers popular from the mid-1980s until the second revolution in space-based navigation from the
Global Positioning System – a receiver that needed a smallfootprint patch antenna.
Following close behind came the advances in collisionavoidance and traffic-alert systems and – yes, you guessed
it – another antenna sprouting on the antenna-farmland of the
aircraft airframe.
More recently, we have seen the emergence of datalink
weather systems, some ground-based, some satellite-based,
and both requiring antenna for the datalink receiver.
The most-recent addition is so new, few are flying. But,there
absolutely will be thousands by 2020: the receivers, transmitters and transceivers of automatic dependent surveillancebroadcast, or ADS-B.
Even within these various roles, we have seen antenna
serving multiple roles, such as the GPS receivers in some
electronic flight instrument systems that serve to round out
the functions and sensors used in some primary flight display systems.
Think of Aspen Avionics Evolution 1000 Pilot PFD and
MFD packages, as well as similar stacks based on Garmin’s
G300.
Without the data of the integral GPS receivers, these two
packages lose some of their functions.
In the cases of these two systems, position and attitude
data depends on input from a functional WAAS-compliant
GPS navigator working hand-in-hand with attitude and airdata sensors to round out the packages’ ability to sense
turns, bank and pitch angles.

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Pilot's Guide to Avionics 2013-2014

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