Antenna

“ The aerial, or antenna, is simply a rod connected to the earth, but it has a certain inductance, and also a certain electrical capacity, and hence any metal rod merely stuck at one end in the earth has a perfectly definite periodic time for the electrical oscillations which can be produced in it. ”

“ 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 effective part of the antenna is a copper wire from which the radio waves are radiated, supported on towers or poles at a height above the ground depending on the wave length used. The wire may be parallel with the earth, or vertical, or arranged at an angle, depending upon the function to be performed. ”

“ The one thing all of these systems have in common is that they lead to and from antennas. Ship's operators who must communicate, navigate, and be ready to fight the ship 24 hours a day depend on you to keep these emitters and sensors operational.
In this volume, we will review wave propagation, antenna characteristics, shore-based and shipboard communications antennas, matching networks, antenna tuning, radar antennas, antenna safety, transmission lines, connector installation and weatherproofing, waveguides, and waveguide couplings. ”

“ The signal which radiates out from the transmitting antenna carries all the little variations in pitch and loudness of the human voice. When this signal reaches the distant antenna it establishes in that antenna circuit a current of high frequency which has just the same variations as did the current in the antenna at the sending station. The human voice isn’t there. ”

“ The antenna system is designed to provide reliable above-ground communication. A variable attenuator is connected to the above-ground antenna coax to limit above-ground RTMN radiation to a minimum of 2,000 feet without interfering with RTMN transmissions at other launch complexes. The below-ground antenna branch is a continuous coaxial slotted cable which is routed from the base station radio equipment bay in the control center throughout the under-ground launch complex. ”

“ Patch asked in amazement.
“That’s the antenna for the transmitter, isn’t it, Mac?” Garry asked.
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Mac nodded. “That long ropelike thing is hollow, and the antenna is in the middle of it. Captain Eaton released a switch that caused the casing to fill with compressed air, and that is what keeps it extended into the sky. That gives us a much better antenna than we could possibly have in here. Also, being as tall as it is, the radio waves leaving it can travel great distances and cross high places which they could not do if it were short. Understand?” ”

“ The length of the antenna must be such that a wave will travel from one end to the other and back again during the period of 1 cycle of the rf voltage.
The distance the wave travels during the period of 1 cycle is known as the wavelength. It is found by dividing the rate of travel by the frequency.
Look at the current and voltage distribution on the antenna in figure 2-4. A maximum movement of electrons is in the center of the antenna at all times; therefore, the center of the antenna is at a low impedance. ”

“ Applicant shall show that there is installed in each aircraft a type certificated radio antenna system, which has for its purpose the collection of radio range signals, weather broadcast and emergency messages transmitted within the frequency range of 200 to 400 kilocycles. The design of this antenna system shall be such as to eliminate insofar as possible, consistent with the advancement of the art, that type of interference commonly known as rain, snow, sleet, or dust static. ”

“ When the transmitting antenna is close to the ground, it should be polarized vertically, because vertically polarized waves produce a greater signal strength along the earth's surface. On the other hand, when the transmitting antenna is high above the ground, it should be horizontally polarized to get the greatest signal strength possible to the earth's surface. ”

“ I didn't even inspect the gleaming maze of apparatus. I merely watched him dully as he plugged in an antenna similar to the one we had left with Imee, and adjusted the things on his head.
is eyes brightened instantly. "She's still wearing her antenna," he said swiftly over his shoulder. "I'll tell her that something's happened; we must leave, but that we will return."
He sat there, frowning intently for a moment, and then dragged the antenna wearily from his head. He touched a switch somewhere, and several softly glowing bulbs turned slowly red and then dark. ”

“ RADIATION RESISTANCE—The resistance, which if inserted in place of an antenna, would consume the same amount of power as that radiated by the antenna.
RADIO FREQUENCIES—Electromagnetic frequencies that fall between 3 kilohertz and 300 gigahertz and are used for radio communications.
RADIO HORIZON—The boundary beyond the natural horizon in which radio waves cannot be propagated over the earth’s surface.
RADIO WAVE— (1) A form of radiant energy that can neither be seen nor felt; (2) An electromagnetic wave generated by a transmitter. ”

“ At one end of the satellite is an entirely separate receiving and transmitting antenna that takes care of all the signals needed for Telstar’s command, tracking and telemetry. The antenna is composed of four metal loops joined in the shape of a helix. It receives the important command signals from the ground that give orders to the satellite’s equipment. ”

“ INCIDENT WAVE— (1) The wave that strikes the surface of a medium; (2) The wave that travels from the sending end to the receiving end of a transmission line.
INDUCTION FIELD—The electromagnetic field produced about an antenna when current and voltage are present on the same antenna.
INDUCTION LOSSES—The losses that occur when the electromagnetic field around a conductor cuts through a nearby metallic object and induces a current into that object. ”

“ 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. ”

“ We opened the lid, however, and I hooked the knife and the light into their proper places outside my armor. Then, with the antenna for the girl, so that we could establish connections with her, and through her, with the villagers, I moved off.
This antenna was entirely different from the one used in previous experiments. The four cross-members that clasped the head were finer, and at their junction was a flat black circular box, from which rose a black rod some six inches in height, and topped by a black sphere half the size of my fist. ”

“ The energy radiated from an antenna forms a field having a definite RADIATION PATTERN. The radiation pattern for any given antenna is determined by measuring the radiated energy at various angles at constant distances from the antenna and then plotting the energy values on a graph. ”

“ As we discussed earlier, ground losses affect radiation patterns and cause high signal losses for some frequencies. Such losses can be greatly reduced if a good conducting ground is provided in the vicinity of the antenna. ”

“ The azimuth drive can rotate the antenna in either 360 degrees or in limited-sector modes of operation in the horizontal plane.
Figure 2-50.—AN/SPN-35A aircraft control approach radar.
The elevation antenna, AS—1669/SPN-35, is a truncated paraboloid-type reflector with a dual-channel feedhorn and a polarizer assembly providing monopulse-type radiation and reception of X-band rf pulses. The horizontal shape of the laminated fiberglass reflector is cosecanted. The dual-channel feedhorn and polarizer are fixed in circular polarization by an external grid device. ”

“ This concentration of energy provides a maximum amount of energy to be reflected off of a target, making detection of the target much more probable.
How does the parabolic reflector focus the radio waves? Radio waves behave much as light waves do. Microwaves travel in straight lines as do light rays. They may be focused or reflected, just as light rays may be. In figure 2-40, a point-radiation source is placed at the focal point F. The field leaves this antenna with a spherical wavefront. ”

“ In the surveillance mode the azimuth antenna rotates through the full 360-degree search pattern at 16 rpm
while the elevation antenna scans a 10-degree sector. In the simultaneous mode, the azimuth antenna rotates through the full 360-degrees search pattern in 60-degree increments while the elevation antenna scans a 10-degree sector. The data rate in this mode is approximately 16 azimuth sweeps and 24 elevation sweeps every 60 seconds. ”

“ Each antenna consists of two curtains with a total of 8 horizontal dipoles. The dipoles are all fed by open wire feeders which can be remotely switched to enable radiation in two directions 180 degrees apart. There are also three curtains for the 11 metre band (26 MHz) which may be put into service during sunspot cycle 22 if the M.U.F. allows it. ”

“ 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. ”

“ And in our loop aerial at Ocean Point you noticed that the receptivity of the tube was modified when we touched it with our fingers.”
“Of course, in theory,” observed Bob thoughtfully, “the human body possesses inductance as well as capacity, and so might serve as an antenna. But I never thought of demonstrating it in practice.”
“So Bob is an aerial,” grinned Herb. “I always knew he was a ‘live wire,’ but I never figured him out as an antenna.”
“And don’t forget that if Bob is an aerial we’re amplifiers,” put in Jimmy.
“There’s glory enough for all,” laughed Joe. ”

“ Various factors in the antenna circuit affect the radiation of electromagnetic energy. In figure 2-3, for example, if an alternating current is applied to the A end of wire antenna AB, the wave will travel along the wire until it reaches the B end. Since the B end is free, an open circuit exists and the wave cannot travel further. This is a point of high impedance. ”

“ 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. ”

“ The mast is one inch in diameter on all spacecraft. Two connectors, supported by the antenna body, provide a means of applying power and connecting the antenna to the rf connector on the hf voice transmitter/receiver. The recovery hf whip antenna weighs approximately 9.0 pounds. The 16-foot version of the adapter hf whip antenna weighs approximately 7.5 pounds and the 13-foot version 6.0 pounds. The main supporting structure of the antenna mechanism housing is the antenna body consisting of a thin fiberglass shell. ”

“ This antenna is intended for use with the AN/W SC-5 (V) transceiver at major shore stations. The antenna is oriented manually and can be locked in position to receive maximum signal strength upon capture of the satellite signal. Hemispherical coverage is 0 to 110 degrees above the horizon. Polarization is right-hand circular in both transmit and receive. The antenna's operating frequency range is 240 to 318 MHz. With its mount,
Figure 2-30.—A82815/SSR-1 fleet broadcast satellite receiving antenna. ”

“ For lower frequencies the radiation of a radio signal takes place by interaction between the antenna and the ground. For a vertical antenna, efficiency increases with greater length of the antenna. For a horizontal antenna, efficiency increases with greater distance between antenna and ground. Near-maximum efficiency is attained when this distance is one-half wavelength. This is the reason for elevating low frequency antennas to great heights. ”