Figure As diagram illustrating the electrical
wave arching from one end of a dipole to the
other implies an orientation that antennas
create for the waves they generate. This effect is
called polarization and it is an important aspect
of antenna design. Antennas have different
polarizations if they are mounted vertically or
horizontally. A receiving antenna operates best
if it has the same orientation as the transmitted
signal its picking up. Other polarization schemes
are used as well. Helical antennas are an
example of a circular polarization design that can
receive both horizontal and vertically polarized
waves. This is especially useful for receiving
signals from satellites (Figure
L
) that are
changing orientation (and therefore polarization)
as they orbit or experiencing an effect called
Faraday rotation as their signal traverses the
ionosphere (Figure
M
).
Most antennas operate by interacting with the
electrical field of an EM wave. But the magnetic
field is always there too, and thats the aspect
used to transmit and receive with most loop
antennas. These antennas can be circular,
hexagonal, square, or any closed geometry.
There may be more than one loop serving the
same purpose as the additional elements on
a beam antenna. Small loop antennas (Figure
N
) can have especially sharp nulls where they
can’t receive. This makes them especially useful
for radio direction finding. By turning the loop
until a signal disappears, you can determine the
direction from which it is coming (Figure
O
).
HOW ELECTROMAGNETIC
WAVES TRAVEL
EM waves can travel, or propagate, in a variety of
ways, though different frequencies have different
abilities in this regard. These propagation types
are: ground waves, direct waves, and sky waves
(Figure
P
). Extremely low-frequency (used for
communication with submarines around the
world) to medium-frequency signals (3Hz–3MHz,
such as AM radio) hug the Earth as they travel
and are called ground waves. High-frequency (HF)
waves in the 3MHz–30MHz range (and medium-
frequency waves) have the ability to refract off the
layers of the ionosphere as sky waves, if they’re at
a shallow enough angle (otherwise they head into
SKILL BUILDER Antenna Spotting
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Wikimedia Commons: Chetvorno, Tim Deagan
M78_112-21_SB_Antennas_F1.indd 116M78_112-21_SB_Antennas_F1.indd 116 7/10/21 3:00 PM7/10/21 3:00 PM
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