Avionics News September 2020 - 56

series
B Y

T H O M A S

I N M A N

Antenna Theory
W

orking with antennas is an important part
of working in avionics. Those who have
earned ASTM International's NCATT
Aircraft Electronics Technician certificate will need to
move on to radio communications systems and dependent
navigation systems endorsements. A good study resource
for the RCS, DNS and other endorsement exams is from
a textbook published by Avotek titled, Avionics: Beyond
the AET. In the textbook, you'll find illustrations and
more variations of the antennas described here. Radio
communication and dependent navigation both rely on
antennas. It's almost a guarantee an avionics technician
will replace or repair antennas multiple times in their
careers. As a result, a technician will find antenna theory
useful.
There are three items to keep in mind as you read
this. First, antennas are dependent on frequency, or
more specifically, wavelength. Second, no antenna is
perfect, which means all antennas compromise efficiency,
bandwidth or some other factor. Third, they follow the rule
of reciprocity, meaning whatever is true for transmit is also
true for receive.
Let's start with wavelength or frequency. You can think
of a radio wave as two dimensional. I suppose if radio
waves were three dimensional we could see them. One
dimension is power or amplitude. The second dimension
is length. The length of the wave has a direct influence on
an antenna. We abbreviate wavelength using the symbol
lambda (λ). Wavelength can be calculated by dividing
the speed of light by the frequency. For a shortcut, I lop
off a bunch of zeros, and calculate 300/MHz= λ, which
56

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september

2020

gives an answer in meters. For example, 300/1.6 MHz
= 187.5 meters. This is the wavelength of the frequency
at the high end of the AM broadcast band. In contrast,
300/117.95 MHz = 2.54 meters, which is the wavelength of
the frequency at the high end of the dependent navigation
(VOR) band. The higher frequencies have shorter
wavelengths and shorter antennas.
For purposes of comparison, physicists have imagined
an antenna called an isotropic radiator, which would be
a perfect antenna. This antenna has no dimensions and
radiates energy equally in all directions. Don't forget the

1/
4

λ

1/4

λ

Cross section

Radiation pattern
Illustration courtesy of Avotek

Figure 2-3-3 : A Hertzian antenna shown on a tower and its radiation pattern.



Avionics News September 2020

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Avionics News September 2020 - Cover2
Avionics News September 2020 - 1
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Avionics News September 2020 - Cover3
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