Avionics News July 2019 - 55

current flow induces the magnetic field. In the space around
The legend calls out band designations. The frequencies
the wire, there is an electric field and a magnetic field.
have been grouped. All frequencies below 30 kHz are
The AC current in one wire will induce current in another
called very low frequencies, or VLF. Next are medium
wire oriented parallel with the first (think antenna here).
frequencies, followed by high frequencies. When these
The electric and magnetic fields travel away from the wire
frequency bands were first designated, scientists considered
at the speed of light. Both fields together are called the
30 MHz to exist in theory, but unusable in any practical
electromagnetic field, which is a radio wave by definition.
way. A relatively large set of frequencies, just below 30
The United States Department of Commerce National
MHz (10 meters λ), was given over to amateur radio and
Telecommunications and Information Administration
citizen's band radio. Back when these frequencies were
Office of Spectrum Management publishes a chart starting
first allocated, these were truly "high" frequencies. The
at 1 Hz and ending with cosmic
next two bands are familiar to many
rays. Indeed, AC reaches from 1
people, VHF and UHF, or very
Hz all the way to 1,024 Hz. The
high frequencies and ultra-high
radio spectrum starts at 3 kHz
frequencies. Engineers kept pushing
ANYONE ENTERING THE
and ends at the start of infrared,
the upper limit of usable frequencies
FIELD OF AVIONICS WILL
or 300,000,000,000 Hz, or 3
higher, and we were running out of
FIND KNOWLEDGE OF THE
gigahertz. Google United States
names. Starting at 3 GHz, we have
RADIO SPECTRUM, THE
frequency allocations and you will
super high frequencies followed by
MAP, TO BE HANDY.
find this chart in PDF or jpg form.
extremely high frequencies. There
AN
AVIONICS
TECHNICIAN
The PDF form works best because
is a short bit of spectrum above
IS EXPECTED TO KNOW
you will be able to see detail, and
the EHF band ending point of 300
WHICH FREQUENCIES
there is a lot of detail. We have
GHz, which has been referred
ARE NOISY, AND WHY;
included a small portion of this
to as millimeter waves. I would
WHICH
FREQUENCIES
chart, which has been modified to
propose changing the millimeter
ARE GOOD FOR LONG
show the approximate locations of
wave designation to ludicrously
various avionics systems.
high frequencies, but no one would
RANGE COMMUNICATION,
For those who like to experiment,
take me seriously. Those wishing
AND WHY.
note the chart lists frequencies
to earn an aircraft electronics
below 9 kHz are not allocated, and
technician certificate with a radio
a small band of frequencies from 59
communications endorsement from
to 64 GHz is designated for unlicensed devices. All of the other
ASTM's NCATT must know the frequency limits of each
frequencies up to 300 GHz are assigned. A good way to think
band.
of this chart is as a map.
Those wishing to earn the radio communications
The far left side of the chart excerpt has a legend
endorsement will also want to know the details of the radio
showing wavelength, band designations, activities, and
propagation characteristics of each band. In general, the
at the bottom, frequency. Last month, we discussed the
LF, MF and HF bands are subject to a fair amount of radio
time between repetitions of an AC waveform. As the
noise created by thunderstorms; however, radio waves on
electromagnetic field travels away from the radiating
those bands can bend through the ionosphere and return to
wire, this time becomes length, or wavelength, which is
earth, far beyond the horizon. The bands VHF and above
usually abbreviated with the Greek character lambda (λ).
are quieter from a noise standpoint, but normally, these
Wavelength is easy to calculate. The formula is simply the
radio waves will not bend through the ionosphere enough
speed of light divided by frequency. In practice, we can
to return to earth. As a result, these frequencies are not
round the speed of light up slightly to 300,000,000 meters
useable for communication over long distances unless a
per second. We can simplify further by lopping off a bunch
relay station is used.
of zeros, and simply saying λ=300/MHz (megahertz). For
The legend also indicates activities, which are shown in
example, a frequency within the FM broadcast band is 100
the center portion of the graphic. Our modification calls
MHz: 300/100=3, or 3 meters. The wavelength of the FM
Continued on following page
broadcast band is around 3 meters.
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july

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Avionics News July 2019

Table of Contents for the Digital Edition of Avionics News July 2019

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Avionics News July 2019 - Intro
Avionics News July 2019 - No label
Avionics News July 2019 - Cover2
Avionics News July 2019 - 1
Avionics News July 2019 - 2
Avionics News July 2019 - 3
Avionics News July 2019 - 4
Avionics News July 2019 - 5
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Avionics News July 2019 - Cover3
Avionics News July 2019 - Cover4
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