Avionics News October 57-10 - 53

When referring to coaxial cable, this impedance is called
surge impedance. The surge impedance of coax for aviation,
and most other communication services, is 50 Ω. In
contrast, coaxial cable used for cable television is 75 Ω.
As the radio wave travels down the coax, it must
charge and discharge the infinite series of capacitors and
inductors. In addition, there is a small amount of resistance
in the cable. As a result, the wave slows down due to
the charging and discharging action. In free space, radio
waves travel at the speed of light. Inside transmission
line, it slows down to a fraction of the speed of light.
This fraction, expressed as a decimal, is referred to as the
velocity of propagation. For example, RG58/U coax has
a propagation velocity of 0.66, which means the radio
waves travel along the cable at approximately two-thirds
the speed of light. In general, the more air contained in
the dielectric, the faster the wave travels. Some air-spaced
cables have a velocity factor up to 0.9 or more.
Where you have impedance, you have loss. So another
factor to consider with coaxial cable is the reduction
of energy as the wave travels. For example, 100 feet of
RG400/U will reduce a 100-megahertz signal by 4.4 dB,
which means the signal will be reduced to less than half of
its value at the input. Increase the frequency to 1 gigahertz,
and that same 100-foot length will reduce the signal by
14.7 dB, which cuts the signal level in half, almost five
times. At 1.09 GHz, the transmit frequency of an air traffic
control transponder, a 200-watt output, forced through 100
feet of RG400/U would be cut to a little more than 3 watts
at the antenna.
This brings up the first rule for installing coaxial cable.
Keep the distance between the antenna and the radio as
short as practical and safe. Cables for GPS, transponders,
and distance measuring equipment need to be short, and
preferably of high quality, meaning low loss. Avionics
installation manuals will have recommendations for
acceptable cable and length requirements.
The second rule for installing coax is to make sure it
isn't going to be squeezed or chaffed. The RF electrical
characteristics of coaxial cable are determined in part
by its diameter. If the cable is clamped tight enough to
reduce its diameter, then the electrical characteristics
will change, which usually will appear to the radio
wave as a partial short circuit. Chaffing will wear

through the outer jacket and eventually through the
shielding, which will result in a partial open.
Related to the second rule is the third rule, which
is to keep the bending radiuses large, at least 10 times
the outside diameter. A kinked cable has the same bad
effect as a squeezed cable.
The rule of reciprocity applying to antennas applies
to coaxial cables. What is true for transmit is true for
receive. Our perception and that of the pilot does not
follow the rule of reciprocity. Pilots and technicians
are slow to recognize coaxial cable problems when the

Figure 3: A simplified look at incident, reflected and
standing waves on a transmission line.

coax is connected to a radio receiver. This is especially
true if the problem developed slowly. When a
transmitter is part of the system, pilots and technicians
tend to notice the problem quickly. When energy
between 5 watts for a low-powered communication
radio up to 200 watts for an ATC transponder are
involved, cable problems become obvious.
Thanks to velocity of propagation, along with the
capacitive, inductive and resistive properties of coaxial
cable, problems can be rather confusing. In order to
begin understanding these tricky problems, we must
discuss how the energy travels within the coaxial cable.
Using a transmitter as an example, the radio wave
traveling from the transmitter toward the antenna is
called the incident wave. In a perfect system, there
would be no other waves present. If the antenna rejects
a portion of the energy, a reflected wave will travel in
the wrong direction back toward the transmitter. The
Continued on following page
AVIONICS NEWS

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OCTOBER

2020

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Avionics News October 57-10

Table of Contents for the Digital Edition of Avionics News October 57-10

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Avionics News October 57-10 - Intro
Avionics News October 57-10 - No label
Avionics News October 57-10 - Cover2
Avionics News October 57-10 - 1
Avionics News October 57-10 - 2
Avionics News October 57-10 - 3
Avionics News October 57-10 - 4
Avionics News October 57-10 - 5
Avionics News October 57-10 - 6
Avionics News October 57-10 - 7
Avionics News October 57-10 - 8
Avionics News October 57-10 - 9
Avionics News October 57-10 - 10
Avionics News October 57-10 - 11
Avionics News October 57-10 - 12
Avionics News October 57-10 - 13
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Avionics News October 57-10 - 15
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Avionics News October 57-10 - 17
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Avionics News October 57-10 - 20
Avionics News October 57-10 - 21
Avionics News October 57-10 - 22
Avionics News October 57-10 - 23
Avionics News October 57-10 - 24
Avionics News October 57-10 - 25
Avionics News October 57-10 - 26
Avionics News October 57-10 - 27
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Avionics News October 57-10 - 30
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Avionics News October 57-10 - 53
Avionics News October 57-10 - 54
Avionics News October 57-10 - 55
Avionics News October 57-10 - 56
Avionics News October 57-10 - 57
Avionics News October 57-10 - 58
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Avionics News October 57-10 - 61
Avionics News October 57-10 - 62
Avionics News October 57-10 - 63
Avionics News October 57-10 - 64
Avionics News October 57-10 - Cover3
Avionics News October 57-10 - Cover4
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