Avionics News September 2020 - 58

THEORY & PRACTICE
Continued from page 57

MHz, and efficiency will drop off
as the user selects channels farther
away from that center frequency.
Fortunately, the efficiency does
not drop off enough to cause any
problems navigating.
Another factor with VOR/
LOC antennas is the antenna is
usually shared with two VOR/
LOC receivers. In addition, in
most cases, the antenna is shared
with the glideslope receiver, or
receivers. A direct connection is
not possible. For power to transfer
efficiently, the impedance of the
input and output must be the same.
Direct connections would lower
the impedance (think resistance
in parallel) of the radio side of

the connection. So, a transformer
is necessary. These transformers
are known by multiple names,
but a common one is diplexer
for the connection of two VOR/
LOC receivers, triplexer for two
VOR/LOC receivers and a single
glideslope, and quadraplexer for
two VOR/LOC receivers and two
glideslope receivers. This device
will match the impedances so
maximum power transfer will occur.
Glideslope receivers function from
329.15 MHz to 335 MHz, so the
triplexer and quadraplexer also
match the antenna, which is too long
for those frequencies, to the input of
the glideslope receivers.
Marconi type antennas are used
for most all other radio systems on
aircraft. A Marconi antenna is not
balanced, so it's possible to connect

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58

avionics news

*

september

2020

DESIGNED AND
MANUFACTURED IN

it directly to coaxial cable. Figure
2-3-4 from Avionics: Beyond the
AET shows a Marconi antenna and
its radiation and reception pattern.
The great majority of Marconi
antennas are configured vertically.
Thus, a common name for these
types of antennas is vertical. The
Marconi antenna works best when
used next to the ground. The shield
of the coaxial cable is grounded,
and the earth makes up the other
half of the antenna. Of course, this
isn't possible on an aircraft or on a
tower. As a result, a counterpoise is
used on a tower as shown in Figure
2-3-5 from Avionics: Beyond the
AET. On an airplane, the airframe
itself makes the counterpoise.
So, as surprising as it may seem,
the aircraft is the other half of a
Marconi antenna.
Like the VOR/LOC antenna,
communication radio antennas are a
compromise. They must work from
118 MHz to 136.99 MHz. These
antennas will be most efficient on
one of those frequencies near the
middle of the band and will lose
efficiency in either direction away
from that center frequency. Since
the airframe makes up the other half
of a Marconi antenna, it's important
to make sure there is a good
connection between the antenna
base and the airframe. Corrosion
will reduce antenna efficiency. On
a wood or fabric-covered aircraft,
some sort of counterpoise will need
to be fabricated and placed below
the wood or fabric surface.
Antennas are affected by anything
within a few wavelengths of the
frequency for which they are
designed. Their efficiency will be
reduced, and their radiation pattern
will be distorted. Aircraft have
limited space availability. In many
cases, pattern distortion and a loss
of efficiency are things with which
we live. Fortunately, the overall
system is designed so that plenty
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