Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/46

 Some distribution systems will be more complex. That is, four antennas can be patched to four receivers, or one antenna can be patched to more than one receiver via the multicouplers.

RECEIVING MULTICOUPLER AN/SRA-12

The AN/SRA-12 filter assembly multicoupler provides seven radio frequency channels in the 14-kHz to 32-MHz frequency range. Any of these channels may be used independently of the other channels, or they may operate simultaneously. Connections to the receiver are made by coaxial patch cords, which are short lengths of cable with a plug attached to each end.

ANTENNA COUPLER GROUPS AN/SRA-38, AN/SRA-39, AN/SRA-40, AN/SRA-49, AN/SRA-49A, and AN/SRA-50

These groups are designed to connect up to 20 mf and hf receivers to a single antenna, with a highly selective degree of frequency isolation. Each of the six coupler groups consists of 14 to 20 individual antenna couplers and a single-power supply module, all are slide-mounted in a special electronic equipment rack. An antenna input distribution line termination (dummy load) is also supplied. In addition, there are provisions for patching the outputs from the various antenna couplers to external receivers.

RADAR ANTENNAS

Radar antennas are usually directional antennas that radiate energy in one lobe or beam. The two most important characteristics of directional antennas are directivity and power gain. Most radar systems use parabolic antennas. These antennas use parabolic reflectors in different variations to focus the radiated energy into a desired beam pattern.

While most radar antennas are parabolic, other types such as the comer reflector, the broadside array, and horn radiators may also be used.

PARABOLIC REFLECTORS

To understand why parabolic reﬂectors are used for most radar antennas, you need to understand how radio waves behave. A point source, such as an omnidirectional antenna, produces a spherical radiation pattern, or spherical wavefront. As the sphere expands, the energy contained in a given surface area decreases rapidly. At a relatively short distance from the antenna, the energy level is so small that its reflection from a target would be useless in a radar system.

A solution to this problem is to form the energy into a PLANE wavefront. In a plane wavefront, all of the energy travels in the same direction, thus providing more energy to reflect off of a target. To concentrate the energy even further, a parabolic reflector is used to shape the plane wavefront's energy into a beam of energy. This concentration of energy provides a maximum amount of energy to be reflected off of a target, making detection of the target much more probable.

How does the parabolic reflector focus the radio waves? Radio waves behave much as light waves do. Microwaves travel in straight lines as do light rays. They may be focused or reflected, just as light rays may be. In figure 2-40, a point-radiation source is placed at the focal point F. The field leaves this antenna with a spherical wavefront. As each part of the wavefront moving toward the reflector reaches the reflecting surface, it is shifted 180 degrees in phase and sent outward at angles that cause all parts of the field to travel in parallel paths. Because of the shape of a parabolic surface, all paths from F to the reflector and back to line XY are the same length. Therefore, all parts of the field arrive at line XY at the same time after reflection.

Figure 2-40.—Parabolic reflector radiation.

Energy that is not directed toward the paraboloid (dotted lines in fig. 2-40) has a wide-beam 2-24