4-18
Except for trailing-wire antennas used in aircraft installations (which may be lengthened or
shortened), physically lengthening the antenna is not very practical. But you can achieve the same result
by changing the electrical length of the antenna. To change the electrical length, you can insert either an
inductor or a capacitor in series with the antenna. This is shown in figure 4-13, views A and B. Changing
the electrical length by this method is known as LUMPED-IMPEDANCE TUNING, or LOADING. The
electrical length of any antenna wire can be increased or decreased by loading. If the antenna is too short
for the wavelength being used, it is resonant at a higher frequency than that at which it is being excited.
Therefore, it offers a capacitive reactance at the excitation frequency. This capacitive reactance can be
compensated for by introducing a lumped-inductive reactance, as shown in view A. Similarly, if the
antenna is too long for the transmitting frequency, it offers an inductive reactance. Inductive reactance
can be compensated for by introducing a lumped-capacitive reactance, as shown in view B. An antenna
without loading is represented in view C.
Figure 4-13.—Electrically equal antenna.
BASIC ANTENNAS
Before you look at the various types of antennas, consider the relationship between the wavelength at
which the antenna is being operated and the actual length of the antenna. An antenna does not necessarily
radiate or receive more energy when it is made longer. Specific dimensions must be used for efficient
antenna operation.
Nearly all antennas have been developed from two basic types, the Hertz and the Marconi. The basic
Hertz antenna is 1/2 wavelength long at the operating frequency and is insulated from ground. It is often
called a DIPOLE or a DOUBLET. The basic Marconi antenna is 1/4 wavelength long and is either
grounded at one end or connected to a network of wires called a COUNTERPOISE. The ground or
counterpoise provides the equivalent of an additional 1/4 wavelength, which is required for the antenna to
resonate.
HALF-WAVE ANTENNAS
A half-wave antenna (referred to as a dipole, Hertz, or doublet) consists of two lengths of wire rod,
or tubing, each 1/4 wavelength long at a certain frequency. It is the basic unit from which many complex
antennas are constructed. The half-wave antenna operates independently of ground; therefore, it may be
installed far above the surface of the Earth or other absorbing bodies. For a dipole, the current is
