Avionics News September 2020 - 57

rule of reciprocity here, for the rule means the isotropic
radiator also receives energy equally from all directions.
Awareness of the isotropic radiator is surprisingly useful,
since promotional material for many antennas advertise
gain, in dB; however, the gain is compared to the isotropic
radiator, which doesn't really exist.
Practical antennas on aircraft take one of two forms. One
form, used for VOR, LOC and glideslope is the Hertzian,
or dipole antenna. The other, which is used for nearly
everything else, is the Marconi type antenna.
Hertzian antennas are 1/2 λ in length, from end to end,
and are connected to the radio in the middle. Another
important point about dipole antennas is they are balanced.
In other words, neither side of the antenna is grounded. The
radiation and reception pattern for dipole antennas takes
the form of a rolling donut. The antenna does not receive
or transmit well from the ends. The dipole used on aircraft
is polarized horizontally. In other words, in normal, level
flight, the antenna is level with the ground. Figure 2-3-3
from Avionics: Beyond the AET shows a Hertzian antenna
mounted on a tower along with an illustration of the
radiation pattern.

Figure 2-3-5: A Marconi
antenna using a counterpoise.
Courtesy of Avotek

On small aircraft, the VOR/LOC antenna is close to a
classic dipole. When viewed from the top or bottom, the
antenna takes a V shape, which you may be surprised to
learn is not all for aerodynamics. Instead, the broad side of
one antenna element covers the lack of coverage off the tip
of the other antenna element. In this way, the antenna has
a more circular reception pattern. Larger aircraft and some
small aircraft will use a form of dipole called a balanced
loop. This antenna will look like a set of blades or a towel
bar. Loop antennas are a form of dipole.
As mentioned above, Hertzian antennas are balanced.
As a result, they cannot be connected directly to coaxial
cable, which is an unbalanced radio transmission line.
VOR/LOC and glideslope antennas require a transformer
called a balun, which couples the energy from the antenna
to the coaxial cable. In many antenna systems, the balun
is built into the antenna assembly. In a large number
of small aircraft, the balun is constructed from coaxial
cable. Instructions are available in Federal Aviation
Administration Advisory Circular 43.13-2b. The FAA
specifies the length of the coaxial cable balun assembly
to be 1/4 λ. If you are constructing one of these, keep in
mind the wavelength formula needs to be adjusted for the
velocity factor of the coaxial cable in use.
The calculation of a quarter wave for the navigation
antenna balun points out one of the ways antennas are
always a compromise. The VOR/LOC band of frequencies
range from 108 MHz to 117.95 MHz. It makes sense
to pick a frequency in the middle of this band for
balun construction. By my calculation, 112.975 MHz is
exactly in the middle of the navigation band. Engineers
will design the antennas and baluns for this frequency.
The antenna and the balun would work best at 112.975

Illustration courtesy of Avotek

Figure 2-3-4: A Marconi antenna and its radiation pattern.

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Avionics News September 2020

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