Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/26

 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 ANIS OTROPIC 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

Figure 2-5.—Isotropic radiation graphs.

intensity, 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