ANTENNA CHARACTERISTICS

As you will learn in this section, all antennas exhibit common characteristics. The study of antennas involves the following terms with which you must become familiar:

Antenna Reciprocity

The ability of an antenna to both transmit and receive electromagnetic energy is known as its reciprocity. Antenna reciprocity is possible because antenna characteristics are essentially the same for sending and receiving electromagnetic energy.

Even though an antenna can be used to transmit or receive, it cannot be used for both functions at the same time. The antenna must be connected to either a transmitter or a receiver.

Antenna Feed Point

Feed point is the point on an antenna where the RF cable is attached. If the RF transmission line is attached to the base of an antenna, the antenna is end-fed. If the RF transmission line is connected at the center of an antenna, the antenna is mid-fed or center-fed.

Directivity

The directivity of an antenna refers to the width of the radiation beam pattern. A directional antenna concentrates its radiation in a relatively narrow beam. If the beam is narrow in either the horizontal or vertical plane, the antenna will have a high degree of directivity in that plane. An antenna can be highly directive in one plane only or in both planes, depending upon its use.

In general, we use three terms to describe the type of directional qualities associated with an antenna: omnidirectional, bidirectional, and unidirectional. Omnidirectional antennas radiate and receive equally well in all directions, except off the ends. Bidirectional antennas radiate or receive efficiently in only two directions. Unidirectional antennas radiate or receive efficiently in only one direction.

Most antennas used in naval communications are either omnidirectional or unidirectional. Bidirectional antennas are rarely used. Omnidirectional antennas are used to transmit fleet broadcasts and are used aboard ship for medium-to-high frequencies. A parabolic, or dish, antenna (figure 2-14) is an example of a unidirectional antenna. As you can see in the figure, an

Figure 2-14.-Principle of parabolic reflection.

antenna (normally a half wave) is placed at the "focal" point and radiates the signal back into a large reflecting surface (the dish). The effect is to transmit a very narrow beam of energy that is essentially unidirectional. Figure 2-15 shows a large, unidirectional parabolic antenna. Directional antennas are commonly used at shore installations.

Wave Polarization

Polarization of a radio wave is a major consideration in the efficient transmission and reception of radio signals. If a single-wire antenna is

Figure 2-15.-Unidirectional parabolic antenna.

used to extract energy from a passing radio wave, maximum signal pickup results when the antenna is placed physically in the same direction as the electric field component. For this reason, a vertical antenna is used to receive vertically polarized waves, and a horizontal antenna is used to receive horizontally polarized waves.

At lower frequencies, wave polarization remains fairly constant as it travels through space. At higher frequencies, the polarization usually varies, sometimes quite rapidly. This is because the wave front splits into several components, and these components follow different propagation paths.

When antennas are close to the ground, vertically polarized radio waves yield a stronger signal close to the Earth than do those that are horizontally polarized. When the transmitting and receiving antennas are at least one wavelength above the surface, the two types of polarization are approximately the same in field

intensity near the surface of the Earth. When the transmitting antenna is several wavelengths above the surface, horizontally polarized waves result in a stronger signal close to the Earth than is possible with vertical polarization.

Most shipboard communication antennas are vertically polarized. This type of polarization allows the antenna configuration to be more easily accommodated in the limited space allocated to shipboard communications installations. Vertical antenna installations often make use of the topside structure to support the antenna elements. In some cases, to obtain the required impedance match between the antenna base terminal and transmission line, the structure acts as part of the antenna.

VHF and UHF antennas used for ship-to-aircraft communications use both vertical and circular polarization. Because aircraft maneuvers cause cross-polarization effects, circularly polarized shipboard antennas frequently offer considerable signal improvements over vertically polarized antennas.

Circularly polarized antennas are also used for ship-to-satellite communications because these antenntas offer the same improvement as VHF/UHF ship-to-aircraft communications operations. Except for the higher altitudes, satellite antenna problems are similar to those experienced with aircraft antenna operations.