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27-4 A small loop antenna
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with a capacitor for adjusting the resonant frequency.
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Large Loop A large loop antenna usually has a circumference of either /2 or (a full wavelength), is circular or square in shape, and lies entirely in a single plane. It can work well for transmitting or receiving. The /2 loop presents a high impedance at the feed point, and maximum radiation/response occurs in the plane of the loop. The loop presents an impedance of about 100 at the feed point, and the maximum radiation/response occurs along the axis (that is, perpendicular to the plane containing the loop). The /2 loop exhibits a slight power loss relative to a /2 dipole in its favored directions. The loop shows a slight gain over a /2 dipole in its favored directions. These properties hold for loops up to several percent larger or smaller than exact /2 or circumferences (as determined for the wavelength in free space). Resonance can be obtained by means of an antenna tuner if the loop is fed with open-wire transmission line. Sometimes, loop antennas measuring several wavelengths in circumference are strung up horizontally among multiple supports. These are technically large loops, but their gain and directional characteristics are hard to predict. If fed with open-wire line and an antenna tuner, and if placed at least /4 above the surface, such an antenna can be exceptionally effective.
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End-fed /4 antennas, such as the ground plane, require low-loss RF ground systems in order to perform efficiently. Center-fed /2 antennas, such as the dipole, do not. However, good grounding is advisable for any antenna system in order to minimize interference and electrical hazards.
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Electrical versus RF Ground Electrical grounding is important for personal safety. It can help protect equipment from damage if lightning strikes in the vicinity. It also minimizes the risk of electromagnetic interference (EMI) to and from radio equipment. In a three-wire electrical system, the ground prong on the plug should never be defeated, because such modification can result in dangerous voltages appearing on exposed metal surfaces. A good RF ground system can help minimize EMI, even if it is not necessary for efficient antenna operation. Figure 27-5 shows a proper RF ground scheme (A) and an improper one (B). In a good RF ground system, each device is connected to a common ground bus, which in turn runs to the earth ground through a single conductor. This conductor should be as short as possible. A poor ground system contains ground loops that can act like loop antennas and increase the risk of EMI. Radials and the Counterpoise With a surface-mounted vertical antenna, there should be as many radials as possible, and they should be as long as possible. They can lie on the surface or be buried a few inches underground. The greater the number of radials of a given length, the better the antenna will work. Also, the longer the radials for a given number, the better. The radials should all converge toward, and be connected to, a ground rod at the feed point. A counterpoise is a means of obtaining an RF ground or ground plane without a direct earthground connection. A grid of wires, a screen, or a metal sheet is placed above the surface and oriented horizontally, to provide capacitive coupling to the earth. This minimizes RF ground loss. Ideally, the radius of a counterpoise should be at least /4 at the lowest operating frequency.
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27-5 At A, the correct method for grounding multiple units. At B, an
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incorrect method creates RF ground loops.
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Gain and Directivity
The power gain of a transmitting antenna is the ratio of the maximum effective radiated power (ERP) to the actual RF power applied at the feed point. Power gain is expressed in decibels (dB). Suppose the ERP, in watts, for a given antenna is PERP, and the applied power, also in watts, is P. Then the following equation holds: Power gain (dB) = 10 log10 (PERP/P) Power gain is always measured in the favored direction or directions of an antenna. These are the directions in which the antenna performs the best. For power gain to be defined, a reference antenna must be chosen with a gain that is defined as 0 dB. This reference antenna is usually a /2 dipole in free space. Power-gain figures taken with respect to a dipole (in its favored directions) are expressed in units called dBd. The reference antenna for power-gain measurements can also be an isotropic antenna, which theoretically radiates and receives equally well in all directions in three dimensions. In this case, units of power gain are called dBi. For any given antenna, the power gains in dBd and dBi are different by approximately 2.15 dB: Power gain (dBi) = 2.15 + Power gain (dBd)
Directivity Plots Antenna radiation and response patterns are represented by plots such as those shown in Fig. 27-6. The location of the antenna is assumed to be at the center (or origin) of a polar coordinate system. The
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