International Symposium on Antennas and Propagation
Rotating-Mode Feed Transitions from a Coaxial Line to a Three-Layered Radial Waveguide
Yuya Ida, Jiro Hirokawa, Takuichi Hirano, Makoto Ando,
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Recently, millimeter wave has received substantial attention because of its high-speed data transmission capability and generation of new frequency resource. Radial line slot antenna (RLSA)  is a candidate for high-gain and high-efficiency antenna in the millimetre wave band. In the medium gain RLSA, with a few turns of elements, concentric-array RLSA (CA-RLSA)  is preferable because the truncation effect at the end of the array is not negligible with spirally-arranged RLSA. Conical beam is produced by a cylindrical mode feeding in a CA-RLSA. A rotating mode feeding, in which the amplitude is uniform and phase is linearly tapered in the circumferential direction in the radial waveguide, is required to get a pencil-beam at the boresight in a CA-RLSA. Two types of radial waveguide structures have been proposed for the CA-RLSA in the 60GHz band. One structure uses foam material (εr=1.08) as a spacer between the bottom and top plate. 50% of efficiency and 33.0dBi of gain at 60 GHz were obtained by the measurement of the RLSA with diameter of 100mm . In this structure, fine alignment of bottom and top plate is difficult, and it causes degradation of the gain. Another structure uses a PTFE (εr=2.20) substrate, in which slots are etched on the one side of copper sheet. 55% of efficiency and 33.4dBi of gain at 60 GHz were obtained by the measurement of the RLSA with diameter of 100 mm . Though fine alignment of the bottom and top plate is accomplished in this structure, the efficiency becomes low because of the increase of dielectric loss. In addition, the bandwidth becomes narrow because of the long line effect due to high permittivity of the material. In this paper, three-layered radial waveguide , which is composed of two dielectric layers and one air layer in the middle as shown in Fig.1, is introduced to reduce the dielectric loss. The existence of the air layer can reduce the dielectric loss. The alignment between the bottom and upper plate, which are dielectric substrate with copper plate on one side, is adjusted by a penetrating conducting pin at the center of the radial waveguide in order to reduce the positioning error. Two types of rotating-mode feed transitions from the coaxial line to the three-layered radial waveguide are proposed in the paper; dielectric resonator and circular patch model. Degeneracy-separation method with perturbation element is used to obtain rotating mode by exciting two degenerate modes with 90deg phase difference. According to the principle of degeneracy-separation method, the dielectric resonator is designed so that only TM11 mode, which will radiate EM field with sinusoidal pattern in the radial waveguide, is excited. For the transition of the patch model, the perturbation element, which is notches on the patch , is designed.