Avionics News September 2020 - 59

of signal strength is available for
reception, and transmitters are
tolerant (within limits) to slight
losses of efficiency.
When an antenna is used on
the frequency for which it was
designed, it will appear to the
radio as a parallel tuned circuit.
In November 2019, this Theory
& Practice column discussed
the characteristics of parallel
resonance, and how voltage goes
increases and current decreases
at resonance. This characteristic
is good for both transmission and
reception, with lots of voltage to
create the electromagnetic field
in transmission and good voltage
to amplify during reception. As
the frequency is changed away
from that which the antenna was
designed, one of two things will
happen. If the frequency increases,
the antenna will be too long for
the frequency in use and appear
to be inductive. If the frequency
decreases, the antenna will be too
short for the frequency in use and
appear to be capacitive.
One case in particular presents us
with a big compromise, and that's
the high frequency communication
system. Typically, these radios will
work from 3 MHZ (100 m λ) to 30
MHz, (10 m λ). Thus, the antenna
requirements from 3 to 30 MHz
change from 25 meters (82 feet)
to 2.5 meters (8.2 feet). In the old
days, the radio operator would spool
out a long wire behind the airplane
until it reached the appropriate
length for the frequency being used.
Such a technique is not practical
today. Instead, engineers designed
radio couplers for HF systems. The
coupler will sense the inductance
or capacitance of the antenna using
the frequency being transmitted and
then add inductors or capacitors into
the antenna circuit as necessary to
bring the system into resonance.
These couplers work so well that,

in some cases, only a short wire is
connected from the coupler to the
airframe, and the airframe itself
becomes the antenna.
As an avionics technician, you
will work with plenty of antennas.
Virtually all antennas are variations
of the two types described here.
You'll clean corrosion off contacts

in navigation antennas and off the
base of Marconi antennas. You'll
replace antennas that have become
defective. You'll clean grease and
dirt off transponder antennas. You'll
use a variety of test equipment and
deduction to determine which action
to take regarding antennas. You'll
find this antenna theory useful. q

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6/15/2020 4:26:09 PM
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Avionics News September 2020

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