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greater the radiation or reception capability of the antenna in a certain direction, the farther from the center the points on the chart are plotted. A dipole antenna, oriented horizontally so that its conductor runs in a north-south direction, has a horizontal plane (or H plane) pattern similar to that in Fig. 27-6A. The elevation plane (or E plane) pattern depends on the height of the antenna above effective ground at the viewing angle. With the dipole oriented so that its conductor runs perpendicular to the page, and the antenna 1 4 wavelength above effective ground, the E plane antenna pattern for a half-wave dipole resembles the graph shown at B.
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Forward Gain Forward gain is expressed in terms of the ERP in the main lobe (favored direction) of a unidirectional (one-directional) antenna compared with the ERP from a reference antenna, usually a half-wave dipole, in its favored directions. This gain is calculated and defined in dBd at microwave frequencies; large dish antennas can have forward gain upward of 35 dBd. In general, as the wavelength decreases (the frequency gets higher), it becomes easier to obtain high forward gain figures. Front-to-Back Ratio The front-to-back (f/b) ratio of a unidirectional antenna is an expression of the concentration of radiation/response in the main lobe, relative to the direction opposite the center of the main lobe. Figure 27-7 shows a hypothetical directivity plot for a unidirectional antenna pointed north. The outer circle depicts the RF field strength in the direction of the center of the main lobe, and represents 0 dB. The next smaller circle represents a field strength 5 dB down with respect to the main lobe. Continuing inward, circles represent 10 dB down, 15 dB down, and 20 dB down. The origin represents 25 dB down, and also shows the location of the antenna. The f/b ratio is found, in this case, by comparing the signal levels between north (azimuth 0 ) and south (azimuth 180 ).
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27-6 Directivity plots for a dipole antenna. At A, the H plane (horizontal plane) plot;
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at B, the E plane (elevation plane) plot.
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27-7 Directivity plot for a hypothetical antenna. Front-to-back and front-to-side ratios
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can be determined from such a graph. This plot is in the H plane.
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Front-to-Side Ratio The front-to-side (f/s) ratio is another expression of the directivity of an antenna system. The term applies to unidirectional antennas, and also to bidirectional antennas. The f/s ratio is expressed in decibels (dBd), just as is the f/b ratio. The EM field strength in the favored direction is compared with the field strength at right angles to the favored direction. An example is shown in Fig. 27-7. The f/s ratios are found, in this case, by comparing the signal levels between north and east (righthand f/s), or between north and west (left-hand f/s). The right-hand and left-hand f/s ratios are usually the same in theory, although they can differ slightly in practice.
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A phased array uses two or more driven elements (radiators connected directly to the feed line) to produce gain in some directions at the expense of other directions.
End-Fire Array A typical end-fire array consists of two parallel /2 dipoles fed 90 out of phase and spaced /4 apart (Fig. 27-8A). This produces a unidirectional radiation pattern. Or, the two elements can be driven in phase and spaced at a separation of (Fig. 27-8B). This results in a bidirectional radiation pattern. In the phasing system, the branches of the transmission line must be cut to precisely the correct lengths, and the velocity factor of the line must be known and be taken into account.
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27-8 At A, a unidirectional
phased system. At B, a bidirectional phased system.
Longwire A wire antenna measuring or more, and fed at a high-current point or at one end, is a longwire antenna. A longwire antenna offers gain over a half-wave dipole. As the wire is made longer, the main lobes get more nearly in line with the antenna, and their amplitudes increase. The gain is a function of the length of the antenna; the longer the wire, the greater the gain. A longwire antenna must be as straight as possible for proper operation. Broadside Array Figure 27-9 shows the geometric arrangement of a broadside array. The driven elements can each consist of a single radiator, as shown in the figure, or they can consist of more complex antennas with directive properties. In any case, all the elements are identical. If a reflecting screen is placed behind the array of dipoles in Fig. 27-9, the system is known as a billboard antenna. The directional properties depend on the number of elements, whether or not the elements have gain themselves, and on the spacing among the elements. In general, the larger the number of elements, the greater the forward gain, and the greater the f/b and f/s ratios.
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