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Copyright 2006, 2002, 1997, 1993 by The McGraw-Hill Companies, Inc. Click here for terms of use.
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ANTENNAS CAN BE CATEGORIZED INTO TWO MAJOR CLASSES: RECEIVING TYPES AND TRANSMITTING
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types. Most transmitting antennas can also function effectively for reception. Some receiving antennas can efficiently transmit EM signals; others cannot.
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When RF current flows in an electrical conductor, some EM energy is radiated into space. If a resistor is substituted for the antenna, in combination with a capacitor or inductor to mimic any inherent reactance in the antenna, the transmitter behaves in the same manner as when connected to the actual antenna. For any antenna operating at a specific frequency, there exists a specific resistance, in ohms, for which this can be done. This is known as the radiation resistance (RR ) of the antenna at the frequency in question. Radiation resistance is specified in ohms. This phenomenon was introduced in Chap. 17. Now you ll see why it s important!
Determining Factors Suppose a thin, straight, lossless vertical antenna is placed over perfectly conducting ground. Then RR is a function of the vertical-antenna conductor height in wavelengths (Fig. 27-1A). Suppose a thin, straight, lossless wire is placed in free space and fed at the center. Then RR is a function of the overall conductor length in wavelengths (Fig. 27-1B). Antenna Efficiency Efficiency is rarely crucial to the performance of a receiving antenna system, but it is important in any transmitting antenna system. Radiation resistance always appears in series with loss resistance (RL ). The antenna efficiency, Eff, is given by:
Eff = RR/(RR + RL )
Copyright 2006, 2002, 1997, 1993 by The McGraw-Hill Companies, Inc. Click here for terms of use.
472 Antennas
27-1 Approximate values of radiation resistance for vertical antennas over perfectly
conducting ground (A) and for center-fed antennas in free space (B).
which is the ratio of the radiation resistance to the total antenna system resistance. As a percentage, Eff% = 100 RR/(RR + RL) High efficiency in a transmitting antenna is obtained when the radiation resistance is much larger than the loss resistance. Then most of the power goes into useful EM radiation, and relatively little is dissipated as heat in the earth and in objects surrounding the antenna. When the opposite situation exists radiation resistance comparable to, or smaller than, the loss resistance, a transmitting antenna is inefficient. This is often the case for extremely short radiators, because they always have low radiation resistance. To some extent this can be overcome by careful antenna design and location to minimize loss resistance, but even the most concerted efforts rarely reduce the loss resistance to less than a few ohms. If an antenna system has a high loss resistance, it can nevertheless work efficiently if its radiation resistance is high enough. When an antenna radiator is just the right length at a given frequency, and if it is constructed of wire or thin tubing, its radiation resistance can be in excess of 1000 . This makes it easy to construct an efficient antenna even when the loss resistance is fairly high.
Half-Wave Antennas 473
Half-Wave Antennas
A half wavelength in free space is given by the equation: Lft = 492/fo where Lft is the linear distance in feet, and fo is the fundamental frequency, in megahertz, at which the antenna exhibits resonance. A half wavelength in meters, Lm, is given by: Lm = 150/fo For ordinary wire, the results as obtained above should be multiplied by a velocity factor, v, of 0.95 (95 percent). For tubing or large-diameter wire, v can range down to about 0.90 (90 percent).
Open Dipole An open dipole or doublet is a half-wavelength ( /2) radiator fed at the center (Fig. 27-2A). Each leg of the antenna is a quarter wavelength ( /4) long. For a straight wire radiator, the length Lft, in feet, at a design frequency fo, in megahertz, for a center-fed, /2 dipole is approximately:
Lft = 468/fo The length in meters is approximately: Lm = 143/fo
27-2 Basic half-wave
antennas. At A, dipole antenna. At B, foldeddipole antenna. At C, zepp antenna.
474 Antennas
These values assume v = 0.95. In free space, the impedance at the feed point is a pure resistance of approximately 73 . This represents the radiation resistance alone, in the absence of reactance at the resonant frequency.
Folded Dipole A folded dipole antenna is a /2, center-fed antenna constructed of two parallel wires with their ends connected together (Fig. 27-2B). The feed-point impedance of the folded dipole is a pure resistance of approximately 290 . This makes the folded dipole ideal for use with high-impedance, parallelwire transmission lines in applications where gain and directivity are not especially important. Half-Wave Vertical A radiator measuring /2 can be stood up, fed at the base (the bottom end) against an earth ground with a transmatch or antenna tuner designed for high impedances, and connected to a radio by a coaxial cable feed line. This type of antenna is an efficient radiator even in the presence of considerable loss resistance, because the radiation resistance is extremely high. Zepp A zeppelin antenna, also called a zepp, is a /2 radiator, fed at one end with a /4 section of openwire line (Fig. 27-2C). The impedance at the feed point is an extremely high, pure resistance. At the transmitter end of the line, the impedance is a low, pure resistance. A zeppelin antenna can operate well at all harmonics of the design frequency. If an antenna tuner, also called a transmatch, is available to tune out reactance, the feed line can be of any length. Feed-line radiation can be minimized by carefully cutting the radiator to /2 at the fundamental frequency, and by using the antenna only at this frequency or one of its harmonics. Zepp antennas are rarely used at frequencies above 30 MHz, except when modified to form a J pole. J Pole A zepp can be oriented vertically, and the feed line placed so it lies in the same line as the radiating element. This antenna, called a J pole, radiates equally well in all horizontal directions. The J pole is used as a low-budget antenna at frequencies from approximately 10 MHz up through 300 MHz. It is, in effect, a /2 vertical fed with an impedance matching section consisting of a length of transmission line measuring /4. It does not require any radials, and this makes it convenient in locations where space is at a premium. Some radio amateurs hang long J poles, cut for 3.5 MHz or 1.8 MHz, from kites or heliumfilled balloons. Such antennas work well, but they are dangerous if they are not properly tethered to prevent them from breaking off and flying away, or if they are placed where they might fall on power lines. They can develop considerable electrostatic charge, even in clear weather. They are deadly if flown in or near thunderstorms.
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