The antenna system routes the pulse from the transmitter, radiates it in a directional beam, picks up the returning echo, and passes it to the receiver with a minimum of loss. The antenna system includes the antenna; transmission lines and waveguide from the transmitter to the antenna; and transmission lines and waveguide from the antenna to the receiver.

Before we discuss some types of antennas used in fire control, we need to review the basic principles of electromagnetic wave radiation and reflectors.

The radar energy that forms the target-tracking and illumination beams is transmitted by an antenna at the control point. Radiated energy tends to spread out equally in all directions, as shown in figure 1-6. Figure 1-6 compares the radiation from a radio antenna with that from a lamp. Both light waves and radio waves are electromagnetic radiation; the two are believed to be identical, except in frequency of vibration. From both sources, energy spreads out in spherical waves. Unless they meet some obstruction, these waves will travel outward indefinitely at the speed of light.

The energy at any given point decreases with range since the wave, and therefore the energy, is spreading out to cover a larger area. Because of its much higher frequency, light has a much shorter wavelength than a radio wave. This is suggested in figure 1-6 but it cannot be shown accurately to scale. The wavelength of radar transmission maybe measured in centimeters, whereas the wavelength of light varies from about three to seven ten-thousandths of a millimeter. We mentioned earlier

Figure 1-6.-Radiation waves from a radio antenna and a lamp.

that radio wave energy must be concentrated to be useful. We can concentrate this energy by mounting a suitable reflector behind the antenna, to form a large part of the radiated energy into a relatively narrow beam. The following paragraphs discuss the more commonly used reflectors.

PARABOLIC REFLECTORS.-You should be familiar with the use of polished reflectors to form beams of light. An automobile headlight uses a parabolic reflector to produce a fairly wide beam. A spotlight uses a slightly differently shaped parabolic reflector to produce a more narrow beam.

A type of reflector generally used in missile fire-control radars is the parabolic dish. It is similar in appearance to the reflector used in an automobile headlight. Since radar operates in the microwave region of the electromagnetic spectrum, its waves have properties and characteristics similar to those of light. This permits radar antennas to be designed using well-known optical design techniques.

A basic principle of optics is that a light ray striking a reflecting surface at a given angle will reflect from that surface at the same angle. Now refer to figure 1-7. Think of the circular wavefronts generated by source F as consisting of an infinite number of rays. The antenna's parabolic reflecting surface is designed, using the reflection principle, so that as the circular wavefronts strike the reflector, they are reflected as straight wavefronts. This action concentrates them into a narrow circular beam of energy.

HORN RADIATORS.-Horn radiators (fig. 1-8), like parabolic reflectors, may be used to create concentrated electromagnetic waves. Horn radiators are readily adaptable for use with waveguides because they serve both as an impedance-matching device and

Figure 1-7.-Principles of the parabolic reflector.

Figure 1-8.-Horn radiators.

as a directional radiator. Horn radiators may be fed by coaxial or other types of lines.

Horns are constructed in a variety of shapes, as illustrated in figure 1-8. The shape of the horn, along with the dimensions of the length and mouth, largely determines the beam's shape. The ratio of the horn's length to mouth opening size determines the beamwidth and thus the directivity. In general, the larger the opening of the horn, the more directive is the resulting field pattern.

FEEDHORNS.-A waveguide horn may be used to feed into a parabolic dish. The directivity of this horn, or feedhorn, is then added to that of the parabolic dish. The resulting pattern (fig. 1-9, view A) is a very narrow and concentrated beam. Such an arrangement is ideally suited for fire control use. In most radars, the feedhorn is covered with a window of polystyrene fiberglass to prevent moisture and dirt from entering the open end of the waveguide.

One problem associated with feedhorns is the shadow introduced by the feedhorn if it is in the path of

Figure 1-9.-Reflector with feedhorn.

the beam. (The shadow is a dead spot directly in front of the feedhorn.) To solve this problem the feedhorn can be offset from center (fig. 1-9, view B). This takes it out of the path of the RF beam, thus eliminating the shadow.

LENS ANTENNA.-Another antenna that can change spherical waves into flat plane waves is the lens antenna. This antenna uses a microwave lens, which is similar to an optical lens to straighten the spherical wavefronts. Since this type of antenna uses a lens to straighten the wavefronts, its design is based on the laws of refraction, rather than reflection.

Two types of lenses have been developed to provide a plane-wavefront narrow beam for tracking radars, while avoiding the problems associated with the feedhorn shadow. These are the conducting (acceleration) type and the dielectric (delay) type.

The lens of an antenna is substantially transparent to microwave energy that passes through it. It will, however, cause the waves of energy to be either converged or diverged as they exit the lens. Consider the action of the two types of lenses.

The conducting type of lens is illustrated in figure 1-10, view A. This type of lens consists of flat metal strips placed parallel to the electric field of the wave and spaced slightly in excess of one-half of a wavelength. To the wave these strips look like parallel waveguides. The velocity of phase propagation of a wave is greater in a waveguide than in air. Thus, since the lens is concave, the outer portions of the transmitted spherical waves are accelerated for a longer interval of time than the inner portion. The

Figure 1-10.-Antenna lenses: A. Conducting (acceleration) type of microwave lens; B. Dielectric (delay) type of microwave lens.

spherical waves emerge at the exit side of the conducting lens (lens aperture) as flat-fronted parallel waves. This type of lens is frequency sensitive.

The dielectric type of lens, shown in figure 1-10, view B, slows down the phase propagation as the wave passes through it. This lens is convex and consists of dielectric material. Focusing action results from the difference between the velocity of propagation inside the dielectric and the velocity of propagation in the air. The result is an apparent bending, or refracting, of the waves. The amount of delay is determined by the dielectric constant of the material. In most cases, artificial dielectrics, consisting of conducting rods or spheres that are small compared to the wavelength, are used. In this case, the inner portions of the transmitted waves are decelerated for a longer interval of time than the outer portions.

In a lens antenna, the exit side of the lens can be regarded as an aperture across which there is a field distribution. This field acts as a source of radiation, just as do fields across the mouth of a reflector or horn. For a returning echo, the same process takes place in the lens.

ARRAY ANTENNAS.-An array type of antenna is just what the name implies-an array or regular grouping of individual radiating elements. These elements may be dipoles, waveguide slots, or horns. The most common form of array is the planar array, which consists of elements linearly aligned in two dimensions-horizontal and vertical-to form a plane (fig. 1-11).

Unlike the lens or parabolic reflector, the array applies the proper phase relationship to make the

Figure 1-11.-Planar array antenna.

wavefront flat before it is radiated by the source feed. The relative phase between elements determines the position of the beam; hence the often used term, phased array. This phase relationship is what allows the beam to be rotated or steered without moving the antenna. This characteristic of array antennas makes it ideal for electronic scanning or tracking. (We will discuss scanning shortly.)