Sharply directive antennas can be constructed from two or more simple half-wave dipole elements. They must be positioned so that the fields from the elements add in some directions and cancel in others. Such a set of antenna elements is called an ANTENNA ARRAY. When a reflector is placed behind the dipole array, radiation occurs in one direction with a pattern similar to the one shown in figure 3-13.
Figure 3-13. - Field pattern of an antenna array.
You will encounter two basic types of antenna arrays, PARASITIC and DRIVEN. Both types of antenna arrays were explained in NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas. Only a brief review is presented in this chapter.
The parabolic reflector antennas previously discussed and the antenna shown in figure 3-13 are examples of parasitic arrays. Notice that the reflector in figure 3-13 is not directly connected to the energy source. Driven arrays, in which all the radiating elements are connected to the energy source, have smaller losses than parasitic arrays while retaining some of the narrow-beam characteristics. Parasitic arrays, such as the parabolic reflector, are used primarily as antennas in fire control radars and other installations, such as microwave communication systems, that require very accurate (narrow) beams. Driven arrays are used primarily as search-radar antennas because extremely narrow beams are less critical than low losses.
If you position a number of driven half-wave antenna elements with respect to each other so that energy from the individual elements will add in certain directions and cancel in other directions, then the antenna system is directional.
Signals from a number of different sources may contribute to or subtract from the overall effect. By properly phasing the energy fed to the antenna elements, and by properly locating the elements, you can control the direction of the energy. You can cause the energy to add in the desired direction and to be out of phase (cancel) in the undesired direction.
Driven arrays are usually made up of a number of half-wave dipoles positioned and phased so that the desired directional pattern will be achieved. Figure 3-14, view (A), shows a simple antenna array consisting of two horizontally mounted elements, each a half wavelength long and fed in phase. The resulting radiation pattern is in a direction at right angles to the plane containing the antenna conductor.
Figure 3-14. - Horizontal array field patterns.
Three- and four-element arrays are shown in figure 3-14, views (B) and (C), respectively. The field pattern of each array is shown beneath it. Note that the beam becomes sharper as the number of elements is increased. If a still-narrower beam is desired, you may add additional elements. The field patterns of the antennas in the figure are bidirectional. Unidirectional patterns may be obtained with a parasitic reflector mounted behind the driven antenna elements.
The BEDSPRING ARRAY (figure 3-15), so called because of its resemblance to a bedspring, is an example of a unidirectional antenna. It consists of a stacked dipole array with an untuned reflector. The more dipoles that are used or stacked in one dimension (horizontal, for example), the more narrow the beam of radiated energy becomes in that plane. Consequently, the size of the antenna is not the same for all installations. Antennas such as the bedspring array are commonly used in TWO-DIMENSIONAL SEARCH RADARS that obtain the range and bearing information of a target.
Figure 3-15. - Bedspring array.
The radar antenna in figure 3-16 uses a feed section to drive horizontally stacked array sections which radiate the applied rf pulses. The same array sections receive the target returns. Each array contains slots cut to radiate and receive a particular frequency. Bearing data is obtained by mechanically rotating the antenna 360 degrees. Elevation data is obtained by electronic scanning of the beam in elevation. The radar antenna is frequency sensitive and radiates pulses at an elevation angle determined by the applied frequency. When the frequency is increased, the beam elevation angle decreases. Conversely, when the applied frequency is decreased, the beam elevation angle increases. The beam elevation angle is therefore selected by the application of a frequency corresponding to the desired angle of elevation. The physical length of the antenna feed section, called the SERPENTINE SECTION (figure 3-17), in relation to the wavelength of the applied energy determines the direction of the radiated beam. You may understand this more clearly if you consider how the beam is shifted. The shift occurs with a change in frequency because the positive and negative peaks of the energy arrive at adjacent slotted arrays at different times. The change in the field pattern is such that the angle of departure (angle at which the radiated beam leaves the antenna) of the beam is changed. Note that a change in phase of the applied rf energy would cause the same effect.
Figure 3-16. - Frequency-sensitive antenna.
Figure 3-17. - Serpentine feed.
A SLOT ANTENNA exhibits many of the characteristics of a conventional dipole antenna. When arranged in arrays, a high degree of directivity can be obtained. Also, the beam can be caused to scan a volume of space by changing either the frequency or phase of the energy driving the antenna elements.
Basic Slot Antenna and Its Complementary Dipole
The slot antenna consists of a radiator formed by cutting a narrow slot in a large metal surface. Such an antenna is shown in figure 3-18. The slot length is a half wavelength at the desired frequency and the width is a small fraction of a wavelength. The antenna is frequently compared to a conventional half-wave dipole consisting of two flat metal strips. The physical dimensions of the metal strips are such that they would just fit into the slot cut out of the large metal sheet.
This type of antenna is called the COMPLEMENTARY DIPOLE.
Figure 3-18. - Slot antenna and complementary dipole.
The slot antenna is compared to its complementary dipole to illustrate that the radiation patterns produced by a slot antenna cut into an infinitely large metal sheet and that of the complementary dipole antenna are the same.
Several important differences exist between the slot antenna and its complementary antenna. First, the electric and magnetic fields are interchanged. In the case of the dipole antenna shown in figure 3-18, the electric lines are horizontal while the magnetic lines form loops in the vertical plane. With the slot antenna, the magnetic lines are horizontal and the electric lines are vertical. The electric lines are built up across the narrow dimensions of the slot. As a result, the polarization of the radiation produced by a horizontal slot is vertical. If a vertical slot is used, the polarization is horizontal.
A second difference between the slot antenna and its complementary dipole is that the direction of the lines of electric and magnetic force abruptly reverse from one side of the metal sheet to the other. In the case of the dipole, the electric lines have the same general direction while the magnetic lines form continuous closed loops.
When energy is applied to the slot antenna, currents flow in the metal sheet. These currents are not confined to the edges of the slot but rather spread out over the sheet. Radiation then takes place from both sides of the sheet. In the case of the complementary dipole, however, the currents are more confined; so a much greater magnitude of current is required to produce a given power output using the dipole antenna.
The current distribution of the dipole resembles the voltage distribution of the slot. The edges on the slot have a high voltage concentration and relatively low current distribution; the complementary dipole has a high current concentration and relatively low voltage.
Slot antennas are adaptable for the vhf and uhf ranges. One of their practical advantages is that the feed section which energizes the slot may be placed below the large metal surface in which the slot is cut. Thus, nothing needs to extend from the surface. In addition, the slot itself may be covered by a section of insulating material to provide a seal so that the antenna can be pressurized with dry air. Dry air pressurization reduces moisture in the waveguide and prevents arcing.
Many of the new radar systems reaching the fleet over the next few years will use frequency- or phase-sensitive antennas. Some of the new radars will use antennas that electronically scan the azimuth as well as elevation, eliminating the moving antenna.
Q.15 What is a set of antenna elements called?