United States. Department of Defense

“ Sky-wave propagation makes the selection of an antenna more complex. The first step is to find the distance between radio stations so that the required takeoff angle can be determined. The takeoff angle versus the distance tables gives approximate takeoff angles for day and night sky-wave propagation. If the circuit distance is 966 kilometers (600 miles) during the day, the required takeoff angle is about 25 degrees. At night, it is 40 degrees. Therefore, the operator selects an antenna that has high gain from 25 to 40 degrees. ”

“ A directional antenna has a single large lobe of radio energy in one direction (Figure D-8) . It is much like a bidirectional antenna with one of its lobes cut off. Several bidirectional antennas (long-wire, sloping-Vee) are made directional by adding a terminating resistor that absorbs the second main lobe. A terminating resistor matches the antenna. A terminating resistor must be able to absorb one-half the power output of the connected transmitter and provide 400 to 600 ohms of resistance. A directional antenna concentrates almost all the radio signal in one specific direction ”

“ Most radio sets come with a ground rod that should provide enough ground if used properly in good soil. The radio operator ensures the ground rod is free from oil or corrosion. He ensures the rod is driven into the ground so that the top of the rod is below surface. ”

“ Antenna construction is limited only by imagination. There are many types and configurations. However, the operator must be careful not to construct an antenna that has a high standing wave ratio, which can damage radio equipment. Standing wave ratio meters should be used when testing or using unfamiliar antennas. In selecting an antenna for an HF circuit, the operator must know the type of propagation.
(1) Ground-wave propagation requires low takeoff angles and vertically polarized antennas. The whip antenna provides good omnidirectional ground-wave radiation. ”

“ Many expedient antennas require supports to hold the antenna above the ground. The most common supports are strong trees that can survive heavy windstorms. However, even the largest trees sway enough in the wind to break wire antennas. To keep the antenna taut and to prevent it from breaking or stretching as the trees sway, the operator attaches a spring or piece of old inner tube to one end of the antenna. If a small pulley is available, he attaches the pulley to the tree and passes a rope through the pulley. ”

“ Depending on the antenna design, the main lobe of a directional antenna can cover 60 degrees or more or be a narrow pencil beam. Directional antennas are used on long-range point-to-point circuits where the concentrated radio energy is needed for circuit reliability. Directional antennas are difficult for the enemy to find the direction of transmission. ”

“ If a water system uses metal pipe, a good ground can be established by clamping the ground strap to a water pipe. Underground pipes, tanks, and metal building foundations also work. (2) In dry soil, electrical grounds can be improved by adding water and chemicals to the soil. ”

“ Because the radio signal travels in the air, any obstruction (such as a mountain) between the antennas can block or reduce the signal. For an antenna 10 feet above the ground, 8 kilometers (5 miles) is the maximum line-of-sight distance. (2) The reflected wave reflects off the earth in going from the transmitting antenna to the receiving antenna. Together, the reflected wave and the direct wave are called the space wave. (3) The surface wave travels along the surface of the earth. It is the usual means of ground-wave communication. ”

“ The radio operator makes a solution of one pound of chemical to one gallon of water. He slowly pours the solution in a hole dug around the ground rod. Water should be added periodically to keep the area damp. If water is not available, urine can be used. (3) Multiple ground rods can also be used to improve electrical grounds. If enough rods are available, a "star ground" can be built. A single rod is driven in the center of about a 20-foot circle. Ground rods are driven along the outside of the circle. ”

“ An LRS team that can see everything and report nothing is useless. LRSUs normally use high-frequent radios to report information and receive instructions. Because of the complex nature of using the HF radio spectrum, a LRSU radio operator must have an in-depth knowledge of radios, antennas, and radio wave propagation.
D-1. HIGH-FREQUENCY RADIO FUNDAMENTALS For successful communication, HF radio performance depends on the type of emission (voice or burst device) , transmitter power output, and type of antenna. The challenge facing HF radio operators is tremendous. ”

“ Along-wire antenna is made directional by placing a terminating resistor at the distant station end of the antenna. The terminating resistor should be a 600-ohm noninductive resistor capable of absorbing at least one-half of the transmitter power. Terminating resistors are part of some radio sets. They can also be locally made, using supply system parts (National Stock Number 5905-00-764-5573, 100-watt, 106-ohm resistor) . (2) Building a long-wire antenna only requires wire, support poles, insulators, and a terminating resistor (if directionality is desired) . ”

“ All the construction factors for a dipole also apply for the inverted Vee. The inverted Vee has less gain than a dipole, but the use of only a single support could make this the preferred antenna in some tactical situations. (See Figure D-10) .
h. Long-Wire Antenna. A long-wire antenna is one that is long compared to a wavelength. A minimum length is 1/2 wavelength; however, antennas that are at least several wavelengths long are needed to obtain good gain and directional characteristics. The construction of long-wire antennas is simple and straight forward. ”

“ If communication cannot be established or maintained between the teams and the COB or DOB, the AOB is moved forward or rearward (mission dictated) to establish communication with the deployed teams and the COB or DOB. When the AOB is used as the primary reporting link, it must maintain a constant communication path with the COB or DOB, while the COB or DOB moves with the corps or division main. ”

“ This allows the tree to sway without straining the antenna.
d. Terminating Resistors. Terminating resistors have been a continual problem for the field communicator. Resistors for low-power (man-pack) HF radios are readily available from commercial radio supply stores. Carbon resistors that can dissipate more than 5 watts are hard to find. However, the 5-watt resistors can be connected in parallel to make a terminator to handle greater power. For example, eight 5-watt, 4,000-ohm resistors connected in parallel results in a 500-ohm, 40-watt terminator. ”