size of (a) is smaller than (b), the half-value angle is smaller. But side lobes of (a) are larger than those of (b). When the incident wave is in higher mode than TE01, the beam is sharper. TE$n-TEn10 MODE CONVERTER Figure 11 shows the configuration of a circular-TE()n to rectangular-TE1(l mode converter. The linearly polarized output wave beam (h0 x 4/) is deflected by a plane conductor and enter a rectangular waveguide («i x b). When the height of the waveguide («,) is appropriate, most of the power of the beam is converted to TEl0 mode. Figure 12 shows numerical results for circular TE(I1 incidence, x = is normalized frequency (x = Fp' On/lTr). The efficiencies have peaks near ax/h0 = 1.3-1.4. ELLIPTIC CYLINDER REFLECTOR ANTENNA ‘ An elliptic cylinder can be used to focus wave beams in the transverse direction. To focus in the longitudinal direction, another parabolic cylinder reflector is used. Figure 13 shows the configuration of the reflector antenna. The beam can be concentrated at an arbitrary point by proper settings of the two reflectors. CONCLUSIONS Two types of parabolic reflector antennas have been compared, (a) the stair-cut type and (b) the obliquely-cut type. The latter antenna (b) can be made compact when h/f is equal to 2 or 3. If the sizes of reflectors are the same, the beam of (a) is sharper than that of (b). The side lobe level of (b) is lower than that of (a). The obliquely-cut type antennas (b) have been used in Gamma-10 (Tsukuba Univ.) and other fusion plasma devices, and satisfactory results have been obtained (3). This research was supported by the Grant-in-Aid for Fusion Research, Ministry of Education, Japan. REFERENCES 1. S.N. Vlasov and I.M. Orlova, Radiophysics and Quantum Electron. 17(1), 1974 (Eng. Trans.). 2. M. Nakajima and O. Wada, Report of the Special Research Project on Nuclear Fusion, 1983 (in Japanese). 3. T. Saito et al., Conf. Digest 9th Int. Conf, on Infrared and Millimeter Waves, Osaka, 1984.
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