Avionics News April 2015 - 64

GROUND-PLANE ISSUES
Continued from page 63

nearest structure. But while metals
are good conductors, they're not
perfect, which can lead to problems. One common example is

microphone wiring. Shielded cables
should be grounded only at one end,
to the chassis of the audio panel (or
to whatever else they're connected)
with insulating washers used to
"float" the jacks on the instrument
panel. If an installer doesn't real-

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64

avionics news

*

april

2015

ize this and grounds the jacks to
the panel, a "ground loop" can be
created: The two ends of the shield
aren't at exactly the same potential.
This can lead to noise problems in
the audio system. Similar errors
with shielding on other cables can
cause big problems in low-voltage
digital systems.
Now let's consider an antenna
ground-plane - which is easiest to
understand if you imagine a large
vertical mast antenna as used in
AM/FM radio broadcasting. The
radio frequency signal applied to
the mast generates voltage with
respect to the ground potential - in
effect, the physical (and electrical)
ground beneath the tower acts as a
reflector, and if everything is sized
right, most of the power fed to
the antenna will be radiated away.
However, if the connection between
the tower and the ground isn't perfect, there will be a loss of power,
and the antenna will be less effective. That's why broadcast radio
sites go to great lengths to produce
a low-resistance ground, using long
copper rods.
On a smaller scale, the same
thing applies to most aircraft antennas: The antenna requires a good
electrical ground. On aluminum
aircraft, this is accomplished by
bonding the antenna to the aircraft
skin (and if necessary, installing a
doubler to reinforce the structure),
assuring good metal-to-metal contact between the antenna and the
skin. This may involve the use of
chemicals like Alodine and/or Ultra
Seal.
On composite airplanes, there
is no metal skin, though in some
cases the manufacturer may simuContinued on page 66


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Avionics News April 2015

Table of Contents for the Digital Edition of Avionics News April 2015

Avionics News April 2015 - Intro
Avionics News April 2015 - Cover1
Avionics News April 2015 - Cover2
Avionics News April 2015 - 1
Avionics News April 2015 - 2
Avionics News April 2015 - 3
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Avionics News April 2015 - Cover3
Avionics News April 2015 - Cover4
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