Open Communication
by Louis E. Frenzel
The Latest in Networking and Wireless Technologies
Open Communication
The Magic of Antennas
If you really want to know what
makes any wireless application
work, it is the antenna. Most peo-
ple working with wireless — radio to
those of you who prefer that term —
tend to take antennas for granted. It
is just something you have to add on
to a wireless application at the last
minute. Well, boy, do I have news for
you. Without a good antenna, radio
just doesn’t work too well. In this age
of store/online-bought shortwave
receivers, scanners, and amateur
radio transceivers, your main job in
getting your money’s worth out of
these high-ticket purchases is to
invest a little bit more and put up a
really good antenna. In this column,
I want to summarize some of the
most common types and make you
aware of what an antenna really is
and how it works.
Transducer to the
Ether
In every wireless application,
there is a transmitter and a receiver.
They communicate via free space or
what is often called the ether. At the
transmitter, a radio signal is devel-
oped and then amplified to a specif-
ic power level. Then it is connected
Figure 1. A dipole is a pair of conductors that, together, are
a half-wavelength long at the operating frequency.
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to an antenna. The antenna is the
physical “thing” that converts the
voltage from the transmitter into a
radio signal. The radio signal is
launched from the antenna toward
the receiver.
A radio signal is the combination
of a magnetic field and an electric
field. Recall that a magnetic field is
generated any time a current flows in
a conductor. It is that invisible force
field that can attract metal objects
and cause compass needles to
move. An electric field is another
type of invisible force field that
appears between conductors across
which a voltage is applied. You have
experienced an electric field if you
have ever built up a charge by shuf-
fling your feet across a carpet then
touching something metal ... zaaapp.
A charged capacitor encloses an
electric field between its plates.
Anyway, a radio wave is just a
combination of the electric and mag-
netic fields at a right angle to one
another. We call this an electromag-
netic wave. This is what the antenna
produces. It translates the voltage of
the signal to be transmitted into
these fields. The pair of fields are
launched into space by the antenna,
at which time they propagate at the
speed of light through
space (300,000,000
meters per second or
about 186,000 miles
per second). The two
fields hang together
and in effect, support
and regenerate one
another along the way.
This strange rela-
tionship was proved
mathematically by
physicist James Clerk
8
Maxwell in the late 19th century.
Therefore, we call these Maxwell’s
equations and they are at the heart
of all wireless systems. The radio
wave then passes by the receiving
antenna and induces a voltage into it
that contains all of the originally
transmitted signal. The receiver then
amplifies this voltage and recovers
the information.
So, in summary, an antenna is a
transducer for converting a voltage
into a radio wave and then converting
the radio wave back into the voltage.
Any antenna will work just as well at
sending as it does at receiving, and
you will hear this referred to as anten-
na reciprocity. Also, let’s get this
straight right now: If you do not have
an antenna, you won’t have any wire-
less communications. Even a bad
antenna is better than none at all.
Those who really appreciate
what antennas do are those people
who had to fool around with TV
antennas back before cable ( BC). No
antenna, no TV. If you were lucky
enough to be near a TV station, you
could use the epitome of all anten-
nas — rabbit ears. Don’t you remem-
ber how you had to rotate them,
adjust their length, and even attach
aluminum foil to them to get a good
picture? It was even more of a big
deal if you had to have an outside
antenna. You had to adjust the direc-
tion of the antenna to get the best
reception. You discovered the basic
rule of antennas used for high fre-
quencies: the taller, the better.
If you are a ham, I won’t belabor
the point. You know darn well that
most of your experimentation as a
ham has probably been with anten-
nas. You have to have one even if your
equipment is totally store-bought or
APRIL 2005
