Crossed Pairs

Author: Edmund A. Laport

Where it is desired to produce two reciprocal wide-angle null zones, there are other means which provide wider nulls than demonstrated in Sec. 2.15.3. We can use two symmetrical pairs of radiators at right angles. Pair A consists of two cophased radiators spaced 270° (0.75λ) and another pair of cophased radiators with its axis normal to the first, and spaced 180° (0.5λ). This is suggested by a study of Appendix IV-A, where we note that the two reciprocal minor lobes for the pair where S = 0.75 and φ = 0° have apparently the same general shape as the pattern for the pair having S = 0.5 and φ = 0°. The pattern amplitudes of the second pair are 0.707 of those in the first, and the two patterns are mutually cophased.

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.

1)	Special attention is directed to the fact that the pattern-field ratios are not necessarily the same as the current ratios between pairs in problems involving the addition of patterns. The current ratios result directly when patterns are synthesized by multiplication.