ANTENNA SIZE AND SHAPE
Antenna lengths are generally whole-number
multiples or fractions of a target wavelength
(½λ, ¼λ, ⅛λ, 1λ, 2λ, 4λ, etc.), so they often
provides clues about the intended frequency of
operation. Long wavelengths/low frequencies
tend to use long antennas and short wavelengths/
high frequencies tend to use short antennas.
Engineers have created a wealth of tricks to allow
antennas to operate on multiple bands, but there
are almost always limits. Shortwave antennas
designed for wavelengths of tens of meters aren’t
very useful for PCS cell wavelengths in the tens of
centimeters, and vice versa.
If your initial image of an antenna is the wire
sticking out of your car, don’t worry! That single
wire (a monopole instead of a dipole) is doing the
same thing as one arm of the dipole (Figure
D
).
As a substitute for the other arm, monopole
antennas connect that side of the feedline to a
reference source, or ground plane, that’s usually
perpendicular to the monopole (shown below the
radiation pattern in Figure
E
). The ground plane
could be your car, a set of wires, the earth, or in
the case of cell phones, our bodies.
SHAPING ELECTROMAGNETIC
RADIATION
Monopoles and dipoles send their signals out in
a donut-shaped 360° ring around the length of
the wire. This is referred to as an omni-directional
signal. However, electromagnetic (EM) waves
interact with matter in different ways. In some
cases it’s absorbed, in some reflected, and others
refracted. Conductive items that respond to EM
waves may, in turn, re-radiate them out again.
If the secondary radiation interacts with the
initial waves, the two signals may reinforce or
cancel each other out depending on their phase
relationship. This is referred to as constructive
and destructive interference (Figure
F
).
When carefully managed, this effect can
be used to shape and direct the output of an
antenna. Many antenna designs have a driven
element called an emitter (frequently a dipole)
and one or more additional elements (reflectors
and directors) that shape the resulting combined
field. While no antenna can add power to the
initial signal, by using additional elements a
SKILL BUILDER Antenna Spotting
makezine.com114
FREQUENCY AND
WAVELENGTH
Frequency is the number of times the wave
completes a cycle in a given amount of time. The
common unit of measure is the Hertz (Hz), or
cycles per second. Since waves travel through
space at the speed of light, the distance between
peaks, called the wavelength (λ), is directly
related to the frequency. This is generally
measured in meters and derived by dividing the
speed of light by the frequency. A signal with a
frequency of 14.074MHz has a wavelength of
21.03m.
Ranges of frequencies are usually referred
to as bands, such as the US 40m amateur radio
band (7.000MHz–7.300MHz) or the US AM radio
band (525kHz–1705kHz).
D
Tim Deagan, Wikimedia Commons: Psgs123xyz, Chetvorno
M78_112-21_SB_Antennas_F1.indd 114M78_112-21_SB_Antennas_F1.indd 114 7/10/21 2:59 PM7/10/21 2:59 PM
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