Avionics News July 2019 - 54

series
B Y

T H O M A S

I N M A N

The Spectrum
L

ast month in this column, we discussed
alternating current. In most textbooks, the
next subjects to examine are inductors
and capacitors. When I think back to my electronics
education, I specifically remember not thinking about
AC and radio being different aspects of the same
idea. After I started teaching, I began to think more
inclusively about the spectrum of AC. In the format of a
monthly essay, we have the luxury of addressing AC in a
more inclusive manner.
AC starts at one cycle per second, or 1 hertz.
Worldwide, power companies use 50 Hz or 60 Hz to
supply power to homes and businesses. At times, I've
wondered why radio astronomers looking for intelligent
life elsewhere in the universe don't listen for signals in
the 40 to 70 Hz range. The habitable areas of our planet

are practically covered with wires transferring energy at
50 Hz and 60 Hz. Some of that energy must be detectible
from space. It seems to me that advanced civilizations on
other planets would use the same type of energy.
Frequencies up to 20 Hz are considered infrasonic
frequencies. Frequencies from 20 Hz to 20,000 Hz, or
20 kilohertz, can be heard when connected to a suitable
speaker. These would include the 400 Hz used for
AC power on aircraft. Frequencies above 3 kHz are
considered radio frequencies.
As stated above, in some ways, these are all the same. At
any frequency, the voltage and current in a circuit build to a
peak, return to zero, and build to a peak in the other polarity.
Along with the voltage and current, the magnetic field will
be doing the same thing, with the lines of magnetic flux
going in one direction and then reversing. The changing

A modified portion of the frequency allocation chart published by the United States Department of Commerce National
Telecommunications and Information Administration Office of Spectrum Management.
54

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Avionics News July 2019

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