Avionics News July 2019 - 56

THEORY & PRACTICE
Continued from page 55

out the approximate location of
various avionics systems, along with
household and aircraft AC, with lines
in red. Note the proximity of marker
beacon, VHF nav, and comm to FM
broadcast. Tune a VHF navigation
receiver to 108 MHz, fly close to a
broadcast station on 107.9 MHz, turn
up the volume, and you might here
the broadcast. Please don't fly close
enough to hear it clearly, because to
do so is likely to violate a Federal
Aviation Regulation or two. Also
nestled in, nearby, in the VHF band of
frequencies are TV channels. I grew
up near a marker-beacon transmitter

56

avionics news

*

july

2019

for runway 23 in Battle Creek,
Michigan. I could tune my TV to
channel 5, hear the marker clearly, and
be mesmerized by the black bars it put
on the TV screen. Note the marking of
microwaves. The textbook definition
of microwaves is those of 1 meter
(300 MHz) or less. This version of the
chart lowers the microwave spectrum
down to 3 meters, or 100 MHz.
Let's return to wavelength.
Radio waves or, for that matter,
any AC electromagnetic field has a
wavelength. The field also has an
amount of power, or amplitude. As a
result, radio waves can be said to exist
in two dimensions. Since we exist
and perceive in three dimensions, we
can't see these waves; however, we

can produce and detect them. Detecting
electromagnetic fields starts with an
antenna, and the size of antennas is
related directly to wavelength. Efficient
antennas will be designed as either
1/2 λ, or 1/4 λ. The size of efficient
antennas is one reason VLF radio is
impractical for most applications. An
efficient "quarter wave" antenna would
be around 7.5 kilometers long. The
next time you see an antenna, note its
length and then study the chart. Doing
so will give you a good idea about the
frequency for which the antenna was
designed. Antenna theory alone is worth
several volumes of books.
Once someone starts thinking of
frequencies and their associated radio
waves as having a size, then some of the
mysterious things we experience will
become clear. For example, you may
have noticed when travelling through
a viaduct, a distant FM station will be
received more clearly. Another time,
passing through a viaduct while listening
to a distant AM station, you may notice
a reduction in clarity. The size of many
viaducts is such that they enhance the
amplitude of wavelengths around 3
meters, the approximate wavelength
of the FM stereo band. Conversely, the
wavelength of AM broadcast is around
300 meters and too large to fit well under
the viaduct. From an avionics standpoint,
long wavelength is one reason why
testing an automatic direction finder
doesn't work well inside a hangar. On the
other hand, short wavelength frequencies
are blocked easily, and for this reason,
among others, GPS is difficult or
impossible to test in a hangar.
Anyone entering the field of
avionics will find knowledge of the
radio spectrum, the map, to be handy.
An avionics technician is expected to
know which frequencies are noisy, and
why; which frequencies are good for
long range communication, and why.
There is more to study, but remember,
it's all AC. q


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