T
here are easily thousands of different types
of antennas serving different purposes.
Some are simple lengths of wire, others exotic
assemblages of high math and science. Most of
the antennas around us are pretty straightforward
and have a set of identifiable characteristics.
With a few patterns in mind and some rules of
thumb, you can usually make a pretty good guess
at what most antennas are being used for, or at
least the parts of the EM spectrum on which
they’re operating.
MAKING ELECTROMAGNETIC WAVES
Antennas are driven from radios by feedlines
consisting of two conductors (often bundled in a
coax cable with a center conductor and a shield)
which carry the two sides of the alternating
current signal the radio produces. The simplest
antenna, a dipole, takes the two conductors from
the feedline and spreads them out away from
each other. The antenna might appear to be a
break in the circuit since the two arms of the
antennas are separated instead of connected.
What’s actually happening is that the two wires
are acting like a big air-gap capacitor. Instead of
the current being contained within the dielectric
gap of a capacitor, the current is flowing in space
as an electric field arc between the two arms of
the dipole (Figure
A
).
As the oscillating AC signal moves electric
charges back and forth across the antenna arms,
the accelerating charges create a magnetic field
around the antenna, just like an electromagnet.
Without trying to go too deep into Maxwells
equations, rest assured that changing magnetic
fields create electric fields, and changing electric
fields create magnetic fields. The electrical
energy applied to the antenna is thus converted
into electromagnetic waves that self-sustain
(magnetic and electric fields each relentlessly
creating the other) and radiate away from the
antenna (Figures
B
and
C
). Radio!
This only works efficiently if the antenna has
the appropriate length for the frequency of signal
it’s trying to radiate. An antenna that is perfectly
matched to a frequency is called resonant. When
you change the free length of a guitar string by
pressing on frets, you change the note (frequency)
the string resonates at. When you change the
Capacitor Antenna
length of an antenna, you change the frequency
that it resonates at (though resonance has a
number of additional technical meanings when
used in electronics and radio).
Adobe Stock - bitontawan02, Tim Deagan
113
make.co
IN THE WILD
TIM DEAGAN
likes to transform
things from the
digital world into
real life in Austin,
Texas.
A
B
C
M78_112-21_SB_Antennas_F1.indd 113M78_112-21_SB_Antennas_F1.indd 113 7/10/21 2:59 PM7/10/21 2:59 PM
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