Radio Antenna Engineering is a free introductory textbook on radio antennas and their applications. See the editorial for more information.... |
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Crossed PairsAuthor: Edmund A. Laport
Since both pairs have a common center point, their resulting patterns can be added algebraically angle by angle.1 These patterns are symmetrical about 0° and 90° axes and are tabulated in Table 2.13.
The pattern F( β) is plotted in Fig. 2.46, which shows that, for a field strength less than 2 percent of maximum, the null width is ± 25° on each side of the pattern, or 50° total. This example suggests a further experimentation with the same idea to see whether or not even broader nulls can be obtained. If the spacing of the first pair is increased to 0.875λ (315°) and the spacing of the second pair reduced to 160°, a further broadening of the null region is realized. Computing the pattern for the condition where the maximum field strength of the second pair just cancels the maximum of the negative lobe of the first pair, the normalized pattern is
This pattern is superimposed on that for the preceding example (Fig. 2.46) for comparison, and we find that the field strength can be held at or below 2 percent of maximum over an angle of ±36°, or 72° total, on each side of the pattern.
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