the resultant voltage and current loops and nodes.
The presence of standing waves describes the condition
of resonance in an antenna. At resonance, the waves
travel back and forth in the antenna, reinforcing each
other, and are transmitted into space at maximum
radiation. When the antenna is not at resonance, the
waves tend to cancel each other and energy is lost
in the form of heat.
RADIATION TYPES AND PATTERNS
A logical assumption is that energy leaving an
antenna radiates equally over 360 degrees. This is
not the case for every antenna.
The energy radiated from an antenna forms a field
having a definite RADIATION PATTERN. The
radiation pattern for any given antenna is determined
by measuring the radiated energy at various angles
at constant distances from the antenna and then plotting
the energy values on a graph. The shape of this pattern
depends on the type of antenna being used.
Some antennas radiate energy equally in all
directions.
Radiation of this type is known as
ISOTROPIC RADIATION. The sun is a good
example of an isotropic radiator. If you were to
measure the amount of radiated energy around the
sun’s circumference, the readings would all be fairly
equal (fig. 2-5).
Most radiators emit (radiate) energy more strongly
in one direction than in another. These radiators are
referred to as ANISOTROPIC radiators. A flashlight
is a good example of an anisotropic radiator (fig. 2-6).
The beam of the flashlight lights only a portion of
the space surrounding it. The area behind the flashlight
remains unlit, while the area in front and to either side
is illuminated.
MAJOR AND MINOR LOBES
The pattern shown in figure 2-7, view B, has
radiation concentrated in two lobes. The radiation
intensity in one lobe is considerably stronger than in
the other. The lobe toward point X is called a MAJOR
LOBE; the other is a MINOR LOBE. Since the
complex radiation patterns associated with antennas
frequently contain several lobes of varying intensity,
Figure 2-5.—Isotropic radiation graphs.
you should learn to use the appropriate terminology,
In general, major lobes are those in which the greatest
amount of radiation occurs. Minor lobes are those
in which the least amount of radiation occurs.
ANTENNA LOADING
There will be times when you may want to use
one antenna system to transmit on several different
frequencies. Since the antenna must always be in
resonance with the applied frequency, you must either
lengthen it or shorten it to produce the required
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