164Design and practice of small antennas I
wavelength. Equation (7.65) indicates that the spiral radiates with a constant gain of π in free space. However, all three gains have a periodic ripple with period of approximately 2.2 GHz, which is the inverse of time delay td = 0.45 ns that is caused in the radiated field by the wave travelling on the spiral surface. This arises because of the truncation of the spiral, which is 12 cm from the feed. The time delay which causes ripple tends to be approximately R/c, where R is the radius of the spiral and c is velocity of light in free space.
7.2.2.1.4.4 Radiation patterns
Radiation patterns of the spiral antenna shown in Figure 7.114 (εr = 4.2) are illustrated in Figure 7.120 [49], where only upper hemisphere patterns are given, as the lower hemisphere patterns are the same. The patterns are seen to have alternative contraction and expansion along the z-axis that corresponds to the maxima of the gain ripple. In the dielectric case, it becomes further complicated as the frequency becomes higher, because of deflection of radiated power into side lobes.
7.2.2.1.4.5 Unidirectional pattern
In many practical spiral antenna applications, a unidirectional pattern is required. For this purpose, a conducting shallow cavity is placed behind a spiral antenna. However, since simple use of such a cavity deteriorates the inherent wideband property of the spiral antenna, an absorber may be applied to the outer spiral arms to restore the wideband property [50]. The absorber will suppress reflection of waves caused by the truncation of outer spiral arms. Figure 7.121 illustrates an example of an equiangular spiral antenna, to which a ring-shaped absorber (R-ABS) is applied. In the figure, (a) shows an exploded view, (b) the side view, (c) spiral arms, and (d) a magnified view of near the input terminals. Figure 7.122 illustrates the input impedance; (a) Rin (resistance), (b) Xin (reactance) with and without cavity, (c) with R-ABS. The antenna used here is a two-arm equiangular spiral antenna having parameters provided in Table 7.14, which is backed by a conducting cavity of height Hcav (= antenna height), and diameter Dcav. Axial ratios with and without R-ABS are shown in Figure 7.123. As these figures show, fairly wideband characteristics can be seen with use of R-ABS.
The absorber size can be reduced only to cover near the edge of the outer spiral arms so that no or almost no reflection appears at the ends of the outer spiral arms. Figure 7.124 illustrates an example of the use of arc-shaped ABS (A-ABS) [50]. Input impedance Zin (= Rin + jXin) and axial ratio for a case with A-ABS are shown in Figure 7.125(a) and (b), respectively, and radiation patterns are depicted in Figure 7.126, in which (a) and
(b) show with A-ABS, but for the case where ϕarc = 90◦ and 180◦, respectively, and (c) shows without A-ABS (ϕarc = 0◦).
7.2.2.1.4.6 Miniaturization of spiral antenna
Spiral antennas can be miniaturized by lowering phase velocity of the waves travelling on the spiral arms from the feed point. However, since a spiral antenna has an inherently wide bandwidth property, the phase velocity at a single resonance frequency cannot be taken
7.2 Design and practice of ESA |
165 |
|
|
εr = 1 |
εr= 4.2 |
1.78 GHz
2.88 GHz
3.98 GHz
5.18 GHz
6.28 GHz
7.50 GHz
Figure 7.120 Radiation patterns of antenna shown in Figure 7.114 on the upper hemisphere for cases εr = 1 and 4.2 ([49], copyright C 2007 IEEE).
