Abstract-The bi-directional beam from an equiangular spiral antenna (EAS) is changed to a unidirectional beam using an electromagnetic band gap (EBG) reflector. 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 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 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 examination reveals that the amplitudes and phases of these two field components do not satisfy the constructive relationship necessary for circularly polarized radiation. 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. 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). 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.Index Terms-Electromagnetic band gap (EBG) reflector, equiangular spiral, extremely low-profile structure, wideband operation.

X-ray diffraction studies have been performed up to 38 T in conjunction with synchrotron x-ray beams and a split-pair pulse magnet at the beamline BL19LXU of SPring-8. The magnet consists of coaxial two-coils which are made by winding CuAg wire up to 18 layers. The magnet has four windows with wide aperture angles and the height of 3 mm for passage of the x-ray beam. A pixel array detector was used to measure the diffraction patterns during an exposure of 5 millisecond around the peak field. The details of the instrumentations are presented.

Abstract-When a conducting shallow cavity is placed behind an equiangular spiral to obtain a unidirectional beam, the inherent wideband characteristics of the spiral deteriorate. To restore the wideband characteristics, a ring-shaped absorbing strip (R-ABS) is placed under the spiral arms. Analysis of the equiangular spiral with the R-ABS is performed using the finitedifference time-domain method. It is found that the R-ABS successfully restores the wideband characteristics. Subsequently, the R-ABS is divided into two absorbing strips, each specified by an arc-angle . Analysis reveals that the radiation characteristics obtained using the R-ABS are reproduced when the arc-angle is greater than = 90 . Throughout this paper an extremely small cavity depth is selected for the analysis: 0.07 wavelength at the lower operating design frequency of 3 GHz.

An equiangular spiral antenna consisting of equiangular spiral slots cut in a finite-sized round conducting sheet (PEC sheet) located in free space is analyzed. The analysis reveals that the input impedance is not sensitive to the diameter of the conducting sheet, and remains constant at approximately 60π ohms. Frequencies from 1 GHz to 20 GHz are used for the analysis and it is found that the spiral radiates a bi-directional circularly polarized beam above 2.8 GHz. In addition, a method for transforming the bi-directional beam into a unidirectional beam is investigated, where the spiral is backed by a cavity whose depth is chosen to be small (0.07 wavelength at 3 GHz). The analysis reveals that wideband characteristics for the input impedance and axial ratio are obtained by attaching an absorbing strip to the wall of the cavity. The radiation patterns before and after the application of the absorbing strip are presented.