CHAPTER 2
ANTENNAS
As an Electronics Technician, you are responsible
for maintaining systems that both radiate and receive
electromagnetic energy. Each of these systems requires
some type of antenna to make use of this electromag-
netic energy. In this chapter we will discuss antenna
characteristics, different antenna types, antenna tuning,
and antenna safety.
ANTENNA CHARACTERISTICS
An antenna may be defined as a conductor or group
of conductors used either for radiating electromagnetic
energy into space or for collecting it from space.
Electrical energy from the transmitter is converted
into electromagnetic energy by the antenna and radiated
into space. On the receiving end, electromagnetic
energy is converted into electrical energy by the
antenna and fed into the receiver.
The electromagnetic radiation from an antenna
is made up of two components, the E field and the
H field. The total energy in the radiated wave remains
constant in space except for some absorption of energy
by the earth. However, as the wave advances, the
energy spreads out over a greater area. This causes
the amount of energy in a given area to decrease as
distance from the source increases.
The design of the antenna system is very important
in a transmitting station. The antenna must be able
to radiate efficiently so the power supplied by the
transmitter is not wasted. An efficient transmitting
antenna must have exact dimensions, determined by
the frequency being transmitted. The dimensions of
the receiving antenna are not critical for relatively low
frequencies, but their importance increases drastically
as the transmitted frequency increases.
Most practical transmitting antennas are divided
into two basic classifications, HERTZ ANTENNAS
(half-wave) and MARCONI (quarter-wave) ANTEN-
NAS. Hertz antennas are generally installed some
distance above the ground and are positioned to radiate
either vertically or horizontally. Marconi antennas
operate with one end grounded and are mounted
perpendicular to the earth or a surface acting as a
ground.
The Hertz antenna, also referred to as a
dipole, is the basis for some of the more complex
antenna systems used today. Hertz antennas are
generally used for operating frequencies of 2 MHz
and above, while Marconi antennas are used for
operating frequencies below 2 MHz.
All antennas, regardless of their shape or size, have
four basic characteristics: reciprocity, directivity, gain,
and polarization.
RECIPROCITY
RECIPROCITY is the ability to use the same
antenna for both transmitting and receiving. The
electrical characteristics of an antenna apply equally,
regardless of whether you use the antenna for
transmitting or receiving. The more efficient an
antenna is for transmitting a certain frequency, the
more efficient it will be as a receiving antenna for
the same frequency. This is illustrated by figure 2-1,
view A. When the antenna is used for transmitting,
maximum radiation occurs at right angles to its axis.
When the same antenna is used for receiving (view
B), its best reception is along the same path; that is,
at right angles to the axis of the antenna.
DIRECTIVITY
The DIRECTIVITY of an antenna or array is a
measure of the antenna’s ability to focus the energy
in one or more specific directions. You can determine
an antenna’s directivity by looking at its radiation
pattern. In an array propagating a given amount of
energy, more radiation takes place in certain directions
than in others. The elements in the array can be
arranged so they change the pattern and distribute the
energy more evenly in all directions. The opposite
is also possible. The elements can be arranged so the
radiated energy is focused in one direction. The
2-1
