A radial line slot antenna (RLSA) is a high efficiency and high gain planar antenna. The efficiency of RLSA becomes lower as the aperture size reduces due to rotational asymmetry of the illumination. A concentric array RLSA (CA-RLSA) was proposed to overcome this difficulty. It adopted three new techniques; (1) not spirally but concentrically arrayed slots, (2) a rotating mode feed circuit, (3) matching slot pairs eliminating termination loss. This paper proposes the basic slot design for (1) and (2) of CA-RLSA. Excellent characteristics of very small CA-RLSAs based upon this design are confirmed by measurements.

A radial line slot antenna (RLSA) is a high gain and high efficiency planar antenna proposed for DBS subscribers. Spirally arrayed slots are excited by a cylindrical wave with the rotational symmetry. In a small sized antenna where large slot coupling is adopted, aperture efficiency reduction due to rotational asymmetry associated with a spiral arrangement of the slots becomes notable. Authors proposed a RLSA with a concentric slot arrangement excited by a rotating mode in order to enhance the rotational symmetry. This is the first report of the normal operation of a rotating mode RLSA fed by a cavity resonator. The experiments confirm the basic operation of this novel antenna; the gain of 27.8dBi and the efficiency of 68% is measured at 11.85GHz for the RLSA with 0.24mφ.