Avionics News September 2022 - 53

work other than act as a magnet. The magnetic field in a
DC circuit is useful as a magnet, which makes solenoids
and relays possible.
If the current flow is alternating, as in AC, then the
magnetic field will alternate as well. When this magnetic
field passes across wires, current will be induced.
This inductance is useful in transformers. What makes
radio possible is the fact the electric and magnetic
fields will radiate out from the wire. We call this an
electromagnetic field or a radio wave. All AC produces
radio waves; however, not all are useful or practical to
use for communication.
Scientists have defined radio waves by frequency
and by wavelength. In addition, they have divided the
radio frequencies into groups called bands. Each band
has a name, starting with extremely low frequency and
ending with tremendously high frequencies. Please
note the Federal Communications Commission in 2003
defined all frequencies below 30 kilohertz as very low
frequencies and all those above 300 gigahertz as light.
The FCC published the 2016 FCC radio spectrum
allocation chart without band definitions. Those wishing
to earn an RCS endorsement will need to commit to
memory bands from VLF to extremely high frequency.
Please note the names of these frequency bands
were made many years ago. When they were named,
scientists considered frequencies above 30 megahertz
to be theoretically possible but unusable. In those
days, frequencies above 3 MHz were considered to be
extremely high, thus the name high frequencies. Today,
most radio-based avionics operate on frequencies above
30 MHz.
An avionics technician must also be familiar with the
behavior of radio waves within these bands. Radio waves
below 3 MHz, which is the border frequency between
the high frequency band and the medium frequency
band tend, to stay trapped between the ionosphere
and the surface of the Earth. Known as ground waves,
when transmitted with enough power, they have the
potential to follow the curvature of the Earth and travel
completely around it.
The HF band (3 MHz to 30 MHz) is useful for longdistance
communication using sky waves. A sky wave
is a radio wave that bends through the ionosphere and
returns to Earth hundreds or thousands of miles away.
Some call this phenomenon " skip, " because there are
Amplitude or Power
Wavelength
A radio wave has amplitude or power and wavelength.
Courtesy Avotek
gaps between the locations where the radio wave is
transmitted and where it comes back to Earth.
Sky waves are dependent on the ionosphere, which
itself is dependent on radiation from the sun. As a result,
the frequencies useable for sky waves vary constantly.
The pattern is for the useable frequencies to rise during
the day and fall at night.
The very high frequency band (30 to 300 MHz) and
above are not considered useful for sky waves. The
ionosphere will not bend them back to Earth; therefore,
they are called space waves, because they will continue
through the ionosphere into space. Technicians should
take note, during the summer, the sun's radiation may
charge the ionosphere enough for radio waves as high as
150 MHz to act as sky waves for short periods of time
during the day.
Frequency is only one method of defining radio
waves and the other is wavelength. Similar to the
time measurement of an AC waveform, wavelength
is the distance covered by a single wave. The lambda
symbol (λ) is used as an abbreviation for wavelength.
Wavelength is determined by dividing the frequency into
the speed of propagation (speed of light). The result is
the wavelength. The determination is easiest to make in
metric: 300,000,000/Hz=λ. As a shortcut, one can drop a
lot of zeros and simply use 300/MHz=λ.
Wavelength is the reason the HF band is also known
as the short-wave band. Moreover, by definition,
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AVIONICS NEWS * SEPTEMBER 2022 53
Avionics News September 2022

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