Figure 2-6.—Anisotropic radiator.
resonance.
Changing the antenna dimensions
physically is impractical, but changing them electrically
is relatively simple. To change the electrical length
of an antenna, you can insert either an inductor or a
capacitor in series with the antenna. This is shown
in figure 2-8, views A and B. Changing the electrical
length
by
this
method
is
known
as
LUMPED-IMPEDANCE TUNING or LOADING.
If the antenna is too short for the wavelength being
used, it will be resonant at a higher frequency.
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
Figure 2-7.—Major and minor lobes.
antenna is too long for the transmitting frequency, it
will be resonant at a lower frequency and offers an
inductive reactance.
Inductive reactance can be
compensated for by introducing a lumped capacitive
reactance, as shown in view B. An antenna with
normal loading is represented in view C.
Figure 2-8.—Electrical antenna loading.
GROUND EFFECTS
As we discussed earlier, ground losses affect
radiation patterns and cause high signal losses for some
frequencies. Such losses can be greatly reduced if
a good conducting ground is provided in the vicinity
of the antenna. This is the purpose of the GROUND
SCREEN (fig. 2-9, view A) and COUNTERPOISE
(fig. 2-9, view B).
2-5
