Military

a. A current flowing in a wire of a length related to the RF produces an electromagnetic field. This field radiates from the wire and is set free in space. The principles of radiation of electromagnetic energy are based on two laws.

b. In space, these two fields will be in-phase and perpendicular to each other at any given moment. Although a conductor is usually considered to be present when a moving electric or magnetic field is mentioned, the laws governing these fields do not say anything about a conductor. Thus, these laws hold true whether a conductor is present or not.

c. The current and voltage distribution on a half-wave Hertz antenna is shown in Figure 1-1. In view A, a piece of wire is cut in half and attached to the terminals of a high frequency (HF), alternating current (AC) generator. The frequency of the generator is set so each half of the wire is one-quarter wavelength of the output. The symbol for wavelength is the Greek letter lambda (D ). The result is the common dipole antenna.

d. At a given moment, the generator's right side is positive and its left side is negative. A law of physics states that like charges repel each other. Consequently, electrons will flow away from the negative terminal as far as possible while the positive terminal will attract electrons. View B of Figure 1-1 shows the direction and distribution of electron flow. The distribution curve shows that most current flows in the center and none flows at the ends. The current distribution over the antenna is always the same, regardless of how much or how little current is flowing. However, current at any given point on the antenna will vary directly with the amount of voltage that the generator develops.

e. One-quarter cycle after the electrons begin to flow, the generator develops it; minimum voltage and the current decreases to zero. At that moment, the condition shown in view C of Figure 1-1 will exist. Although no current is flowing, a minimum number of electrons are at the left end of the line and a minimum number are at the right end. The charge distribution along the wire varies as the voltage of the generator varies (view C).

a. Antenna reciprocity is the ability to use the same antenna for transmitting and receiving. In view A of Figure 1-3, the antenna radiates a minimum amount of energy at right angles to the axis of the antennas. Note the minimum amount of radiation emanating along the axis of the antenna. If the same antenna were used as a receiving antenna (view B), it would receive best in the same directions in which it produced maximum radiation-at right angles to the axis of the antenna.

c. A radiated field is composed of electric and magnetic lines of force which are called fields. A radiated wave's polarization is determined by the direction of the electric field in relation to the Earth. If the lines of electric force are at right angles to the Earth's surface, the wave is vertically polarized (Figure 1-4). If the lines of electric force are parallel to the Earth's surface, the wave is horizontally polarized, as shown in Figure 1-5. When a single-wire antenna is used to extract (receive) energy from a passing radio wave, maximum pickup results when the antenna is oriented so it lies in the same direction as the electric field component. Thus, a vertical antenna is used for efficient reception of vertically polarized waves, and a horizontal antenna is used for horizontally polarized waves. In some cases, the field rotates as the waves travel through space. When this occurs, both the horizontal and vertical components of the field exist, and the wave has elliptical polarization.

d. For ground-wave transmissions, medium and low frequencies are used, and the antennas need to be vertically polarized. This allows the radio wave to travel a considerable distance along the ground surface with minimum absorption by the Earth. Horizontal polarization cannot be used at these frequencies because the electric lines run parallel to and touch the Earth. As a horizontal wave travels across the surface, it is attenuated and is not propagated very far. At high frequencies, either horizontal or vertical polarization may be used with sky wave transmission. This is because the sky wave arrives at the receiving antenna elliptically polarized. This polarization is a result of the wave traveling obliquely through the Earth's magnetic field and striking the ionosphere. Thus, the transmitting and receiving antennas can be either horizontal or vertical. When using frequencies in the ultra high frequency (UHF) or very high frequency (VHF) range, either horizontal or vertical polarization is acceptable. But, you must ensure that the transmitting and receiving antennas have the same polarization.

e. An advantage of vertical polarization is that vertical half-wave and quarter-wave antennas can provide omnidirectional communications. This helps when communicating from a moving vehicle, although it radiates equally to both enemy and friendly forces. Another advantage, when using vertical polarization, is that less inference is picked up from strong VHF and UHF broadcast transmissions (television and FM radio). This is because those systems use horizontal polarization.

f. Horizontally polarized antennas do have certain advantages and are preferred at high frequencies. A horizontal antenna is less likely to pick up man-made interference (such as those produced by automobile ignition systems and electrical appliances), which is usually vertically polarized. A second advantage is that there is less absorption of radiated energy by buildings or wiring when a horizontal antenna is used. In addition, a simple horizontal half-wave antenna is bi-directional which is useful in minimizing interference from certain directions. Finally, horizontally polarized waves suffer lower losses than vertically polarized waves, especially above 100 MHz. Antennas located near dense rests should be horizontally polarized.

g. Directionality is the characteristic of an antenna that allows it to transmit and receive in a specific direction. Other stations operating on the same or nearby frequencies may interfere with the desired signal and make reception difficult or impossible. Reception of a desired signal can be improved by using directional antennas. Horizontal half-wave antennas accept radio signals from all directions, with the strongest reception being received in a line perpendicular to the antenna (broadside), and the weakest reception being received from the direction of the antenna's ends. Changing the antenna's axis so either end points directly toward the interfering source may eliminate or reduce interference. Directional transmitting antennas concentrate radiation in a given direction and minimize radiation in other directions. A directional antenna may be used to lessen enemy interception and interference with friendly stations.

h. Since antennas are erected over the Earth and not out in free space, except for those on satellites, the ground's presence alters the free space radiation patterns of antennas. The ground also affects some of the electrical characteristics of an antenna. It has the greatest effect on those antennas that must be mounted relatively close to the ground, in terms of wavelength. For example, medium and high frequency antennas, elevated above the ground by only a fraction of a wavelength, will have radiation patterns that are quite different from the free-space patterns.

i. Grounded antenna theory. The ground, a good conductor for medium and low frequencies, acts as a large mirror for the radiated energy. This results in the ground reflecting a large amount of energy that is radiated downward from an antenna mounted over it. Using this characteristic of the ground, an antenna only a quarter-wavelength long can be made into the equivalent of a half-wave antenna. A quarter-wave antenna erected vertically, with it's lower end connected electrically to the ground, as shown in Figure 1-6, behaves like a half-wave antenna. Under these conditions, the ground takes the place of the missing quarter-wavelength, and the reflections supply that part of the radiated energy that normally would be supplied by the lower half of an ungrounded half-wave antenna.

j. There are several types of grounds.

k. When an actual ground connection cannot be used because of the high resistance of the soil or because a large buried ground system is not practical, a counterpoise can be used to replace the usual direct ground connection.

l. Depending on the type of antenna used, radio energy radiated by an antenna forms an electromagnetic field that has a definite pattern. A radiation pattern shows the directional characteristics of an antenna. A vertical antenna (Figure 1-7) or a whip antenna on a vehicle radiates energy equally in all directions (omnidirectionally). A horizontal antenna, like those used with RATT systems, is bidirectional. Unidirectional antennas radiate energy in only one direction. These radiation patterns do not provide perfectly symmetrical coverage. There are distortions in the patterns (back lobes and side lobes) which are attributed to the antenna's physical characteristics and nearby obstructions or terrain features. Three common radiation patterns are shown in Figure 1-8. The upper pattern is that of a quarter-wave vertical antenna. Note its omnidirectional pattern. The center pattern shows a half-wave horizontal pattern, located one-half wavelength above the ground. RATT rigs use these long-wire or doublet antennas. The bottom pattern is that of a vertical half-rhombic antenna that radiates in one direction.

As an example, a radio operator must design a long-wire (doublet) antenna for use with an AM radio, tuned to 12 MHz. Using the formula:

The operator must measure the antenna 39 feet in length to operate at a frequency of 12 MHz. The AN/GRA-50 antenna group is the most common antenna assembly used to erect a long-wire antenna. Figure 1-9 shows the AN/GRA-50's component parts. A very useful component is the tape measure, which is graduated in both feet and meters. An operator lays out the antenna wire and measures the required length. The electrical length of an antenna must be compatible with the frequency used. When you change the frequency on the radio, the antenna matching unit mounted on the vehicle will automatically change the electrical length of the whip antenna.

a. Current flows in the antenna with an amplitude that varies with the generator voltage.

b. Two fields are established at an antenna (the induction field and the radiation field). The radiation field is made up of an electric component and a magnetic component (electromagnetic).

c. Output from a transmitter is released at the antenna in the form of electromagnetic energy. At the receiving antenna, electromagnetic energy is collected and converted into electrical energy, where it is fed to the radio receiver.

d. Antenna reciprocity is the feature that allows an antenna to both transmit and receive radio signals.

e. Antenna gain is the ratio between the amount of energy propagated in certain directions, compared to the energy that would be propagated if the antenna were not directional. Antenna gain also gives directional antennas greater transmission range than omnidirectional antennas for an equal signal.

f. The orientation of a radio wave's electric field in relation to the Earth is known as its polarization. A radiated wave can be vertically, horizontally, or elliptically polarized.

g. Directionality is the characteristic of an antenna that allows it to transmit and receive in a specific direction.

h. The earth (ground) can be used as a conductor for radio frequencies.

i. A counterpoise is a conductor or system of conductors used as a substitute for a ground in an antenna system.

j. Radiation patterns can be omnidirectional (all directions), bidirectional (two directions), or unidirectional (one direction).

k. To calculate the physical length of a half-wave long-wire antenna, use the following formulas: