![]() ![]() A virtual doublet is achieved by placing a single spiral in a fight angle trough. A doublet fed from a ring network can be employed as a polarization diversity circuit. An array of these doublets can be made to scan by rotation of the several spiral elements an eight-doublet array which was made to scan over an 83deg sector with small amplitude variation is discussed. ![]() Theoretical results are confirmed using results from experimental work.Ī pair of equally excited but oppositely sensed Archimedean two-wire spirals situated close to one another in the same plane-a doublet-is used to generate a linearly polarized field in which the direction of polarization and phase are controlled or varied independently of each other by rotation of the spiral radiators. It is shown that when the arc-angle is chosen to be greater than degARC=90=, the antenna characteristics obtained for =ARC=180= are reproduced. Subsequently, the R-ABS is divided into two arc-shaped strip absorbers and the volume of each is reduced by decreasing the arc-angle ARC from 180deg (corresponding to an R-ABS) to 0deg (corresponding to a cavity without an absorber). It is found that the wideband antenna characteristics are successfully restored with the use of the R-ABS. To restore the wideband characteristics inherent to the ARSP, a ring-shaped strip absorber (R-ABS) is placed under the spiral arms. It is revealed that the presence of the cavity causes noticeable variation in the antenna characteristics at low frequencies. The characteristics of an Archimedean spiral (ARSP) antenna backed by an extremely shallow cavity (0.07 wavelength at the lower operating design frequency of 3 GHz) are analysed. The analysis results are verified by the measured results of the fabricated antenna. For comparison, the AR for the spiral antenna backed by a conventional perfect electric conducting reflector is also presented. It is revealed that the CirHISRĬontributes to a constant input impedance and a small axial ratio (AR) across the design frequency range of 3–10 GHz (the fractional bandwidth is approximately 108%). To the spiral antenna is very small: 0.09 wavelength at a lowest design frequency of 3 GHz. The total antenna height from the bottom of the CirHISR ) fan-shaped patch elements is investigated. Note that, for comparison, the antenna characteristics for an EAS isolated in free space and an EAS backed by a perfect electric conductor are also presented.Ī low-profile spiral antenna backed by a circular high-impedance surface (HIS) reflector composed of homogeneous (CirHISR This equiangular spiral with a modified EBG reflector shows wideband characteristics with respect to the axial ratio, input impedance and gain within the design frequency band (4-9 GHz). Based on this finding, next, the EBG reflector is modified by gradually removing the patch elements from the center region of the reflector, thereby satisfying the required constructive relationship between the two field components. The examination reveals that the amplitudes and phases of these two field components do not satisfy the constructive relationship necessary for circularly polarized radiation. The deterioration in the axial ratio is examined by decomposing the total radiation field into two field components: one component from the equiangular spiral and the other from the EBG reflector. The analysis shows that the EAS backed by the EBG reflector does not reproduce the inherent wideband axial ratio characteristic observed when the EAS is isolated in free space. The antenna height, measured from the upper surface of the EBG reflector to the spiral arms, is chosen to be extremely small to realize a low-profile antenna: 0.07 wavelength at the lowest analysis frequency of 3 GHz. The bi-directional beam from an equiangular spiral antenna (EAS) is changed to a unidirectional beam using an electromagnetic band gap (EBG) reflector. ![]()
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