Abstract-A monofilar spiral antenna is analyzed in the presence of a conducting plane reflector, using the method of moments. The circumference of the spiral antenna is chosen to be 2.3 wavelengths. A tilted beam of circular polarization is realized by superposing the fields from two active regions. The gain of the tilted beam is approximately 8 dB. The frequency bandwidths for 1-dB gain drop and 3-dB axial ratio criterions are 12% and 23%, respectively. An array consisting of the four monofilar spiral antennas is also analyzed, where the array element spacing is chosen to be 0.8 wavelength at a design frequency f 0 . The input impedances of the four spirals are almost the same as the impedance of the single monofilar spiral antenna at f 0 . The array radiates a tilted fan beam with a gain increase of approximately 6 dB from the gain of the single spiral antenna at f0. The frequency bandwidth for a 3-dB axial ratio criterion is almost the same as that of the single spiral antenna.
Two types of spiral array antennas that form circularly polarized fan beams are numerically analyzed. One is composed of two-arm elliptical spiral elements, and the other is composed of single-arm round spiral elements. It is found that the elliptical spiral array antenna can radiate a wider fan beam than a conventional round spiral array antenna. It is also found that the single-arm spiral array antenna can radiate a fan beam off-normal to the antenna plane.ARRAY OF TWO-ARM ELLIPTICAL SPIRAL ELEMENTS Fig. l(a) illustrates the configuration and coordinate system of an array antenna composed of two-arm elliptical spiral elements. Four spiral elements are arrayed with spacing d in the X direction. The four elements are located above a conducting plane reflector at height h. The elliptical shape of each spiral element is defined by ellipticity b/a shown in Fig. l(b). Note that an ellipticity of b/a = 1 gives a conventional round spiral element shown in Fig. l(c), where the two arms are characterized by the Archimedean spiral function [l], and the outer circumference is designated as C.The outer circumference is chosen to be C = 1.5 hg (h6 is the free-space wavelength at a test frequency of 6 GHz), and the other spiral configuration parameters are the same as those in Reference [I]. The element spacing and height of the array antenna are d = 0.8 h6 and h = h6/4, respectively. Note that the elliptical spiral elements are excited with the same phase and uniform amplitude. The current distributions of the elliptical spiral elements are determined using the method of moments. T h e radiation characteristics of the array antenna are evaluated on the basis of the determined current distributions.The axial ratio of the array antenna as a function of the ellipticity b/a is shown in Fig. 2. The array antenna can radiate a circularly polarized wave (CPW) with an axial ratio of less than 3 dB over an ellipticity range from 1 to 0.4. A minimum axial ratio of 1.0 dB is obtained for b/a = 0.7. The radiation patterns for b/a = 0.7 are shown in Fig. 3, where the electric field is decomposed into the right-hand (ER) and left-hand (EL) CPW components. It is observed that a CPW fan beam is obtained in the direction normal to the antenna plane. The half-power beamwidth (HPBW) is calculated to be 90" in the 41 = 90" plane,The HPBW in the I$ = 90" plane vs. the ellipticity is shown in Fig. 4. It should be noted that the HPBW increases as the ellipticity decreases. The HPBW for b/a = 0.4 is 97", which is wider than that for b/a = 1 by 12".Further calculations indicate that the HPBW in the Q = 0" plane remains unchanged irrespective of the variation in the ellipticity.The frequency responses of the axial ratios for b/a = 0.4, 0.7 and 1.0 are presented in Fig. 5. It is found that the array antennas can radiate a CPW over a wide frequency range. The bandwidth for a 3-dB axial ratio criterion is calculated to be 31 % for b/a = 0.4, and 40 % for b/a = 0.7.ARRAY OF SINGLE-ARM ROUND SPIRAL ELEMENTS We analyze another array antenna composed of sin...
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