Avionics News November 2022 - 56

THE NAVIGATION
ANTENNA BALUN
Advisory Circular AC 43.13-2B includes instructions to build a balun
using coaxial cable. Many small aircraft use this type of balun, which
over time will wear out and need to be replaced. Earlier versions of
AC 43.13 prescribed a particular length, which worked, but favored
the lower end of the VHF navigation band. The current version of the
advisory circular correctly identifies the overall length as being ¼ λ but
does not include instructions on how to calculate wavelength properly.
The basic wavelength equation of 300/MHz= λ in meters needs to be
adjusted for the velocity factor of the coaxial cable.
To adjust the equation, multiply the speed of light by the velocity of
propagation before dividing by the frequency. In addition, make sure to
choose a frequency in the center of the navigation band. The equation
would exactly be as follows: (velocity of propagation x speed of light)/
frequency = λ in meters). Less exact, but perfectly useful, would be the
following (velocity of propagation x 300)/MHz = λ in meters. For RG58AU
cable with a 0.66 velocity of propagation, the formula becomes:
(.66x300)/112.975 or 198/112.975 = 1.7526 m λ.
Dividing 1.7526 by 4 yields .43815 m, or 17.25 inches, which is
almost 2 inches shorter than the length of the balun described in
earlier versions of 43.13. When rebuilding a balun, make sure to check
the velocity of propagation of the coaxial cable you plan to use. Some
coaxial cable have a velocity factor of .95. q
Antenna
Dipoles
AAach
to dipoles
Shield
removed
Protecave outer
coaang
BALUN
Wire wrapped and
soldered to shield
Airframe
ground
Wire wrapped and
soldered to shield
To navigaaon receiver
Center conductor open
Airframe ground
These figures from AC 43.13-2B identify the length as ¼ λ, without identifying the length in inches.
56 avionics news * november 2022
λ//
Twisted Shield
THEORY & PRACTICE
Continued from page 55
In aircraft, we use coaxial cable
to carry the radio energy to or from
an antenna. The coaxial cable we
use has a characteristic impedance
of 52 Ω. This is convenient
since a Marconi antenna using a
counterpoise (the airframe) also
has an impedance of approximately
52 Ω. In contrast, a dipole
antenna will have an impedance
of approximately 72 Ω. Thus,
another function of the balun is
to match the impedance with the
coaxial cable. For aviation use, the
cable must have a characteristic
impedance of 52 Ω, be resistant to
self-corrosion, and have a stranded
center conductor.
To a traveling radio wave,
the coaxial cable appears as a
continuous series of inductors,
capacitors and resistors. In free
space, radio waves travel at
the speed of light. Due to the
appearance of transmission line,
the radio waves slow down.
Coaxial cable will have a published
specification for velocity factor,
expressed as a decimal of the speed
of light. Common velocity factors
for aviation coax are .66 and .95,
indicating radio waves traveling at
66% and 95% of light speed.
The inductance, capacitance
and resistance of transmission line
has an effect on the characteristics
of faults in the cable itself, or the
antenna. In a normally operating
system, current and voltage will
travel along the coaxial cable, from
source to destination. If a mismatch
in impedance occurs, either to a
bad antenna, nick in the coax, cut
coax, or any other problems, this

Avionics News November 2022

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