space), often multiple times, and reach around
the world. Very high-frequency (VHF) and above
waves (30MHz and up) rely on direct, or line-of-
sight, transmission. These waves are obstructed
by the curvature of the Earth or other obstacles
and tend to have limited range compared to lower
frequencies. Of course, that depends on where
you point them. Aim them at a satellite and even a
handheld radio can contact low Earth orbit!
The higher an antenna is mounted, the
farther its line-of-sight radius is. Tall antenna
towers provide broad coverage for antennas
relying on direct propagation. Cellphone towers,
commercial radio, amateur radio repeaters,
microwave data transmission, emergency
services, and other UHF/VHF/microwave
antennas all tend to be mounted as high as
feasible to increase their range.
Even HF antennas attempting to bounce
signals off the ionosphere benefit from height.
Some part of the signals they output bounce
off the Earth below them. When they are less
than half a wavelength above the ground, these
reflected waves interfere with the transmitted
wave and tend to make most of the signal go
upward at an angle so steep that it either passes
through the ionosphere or bounces straight back
down. This means that an antenna operating
on the 40-meter amateur band would ideally be
at least 20 meters (65 feet) above the ground to
achieve long distance propagation. Figures
Q
and
R
show the comparison of this band being
broadcast at 5 meters high and 20 meters high.
In some cases, if you want to communicate
locally, aiming up and bouncing down is desirable.
This is referred to as near-vertical-incidence
skywave (NVIS) operation.
All these characteristics provide clues when
spotting an antenna in the wild. The length of the
elements relates to the frequency of operation.
The height offers clues to the coverage area. Odd
shapes like horns suggest waveguides which are
most likely used for high-bandwidth microwave
transmissions. Directional antennas are usually
pointed at a target such as a receiver at a given
compass bearing. If they’re pointed at the sky,
that target is probably a satellite.
We spend our lives constantly bathed in the
radio waves propagating around us. Much of our
daily activity relies on radio-enabled cellphones,
tablets, GPS, and Wi-Fi. Recognizing the amazing
proliferation of antennas can be an exciting way to
gain awareness of these tools for manipulation of
the invisible forces underlying our modern world.
And with that, let’s take a look at some of the
more common antennas you’ll see in the wild ...
IONOSPHERE
Sky wave
Direct line of sight wave
Ground reflected
wave
Ground wave
117
make.co
N O
P
Q
R
M78_112-21_SB_Antennas_F1.indd 117M78_112-21_SB_Antennas_F1.indd 117 7/10/21 3:00 PM7/10/21 3:00 PM
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.