High-Gain Characteristics of a Dielectric Tube Antenna

An endfire antenna is known as a simple high gain antenna in the microwave and millimeter-wave regions [1]-[4]. The attainable gain is often estimated by the Hansen-Woodyard condition [1]. Although the estimation by the Hansen-Woodyard condition is relatively good in the conventional endfire antennas, this often discourages us to realize a higher gain antenna with an acceptable axial length. Note that the radiation mechanism of the endfire antenna is explained by the so-called discontinuity radiation concept, in which the antenna is regarded as an array of two aperture sources situated at the feed and open ends [3]. We should also recall that the HansenWoodyard condition corresponds to an ideal situation that the two aperture sources are identical (an excitation efficiency of 50%). This means that the ideal situation leads to a gain increase by approximately 3dB from the original gain evaluated from a single aperture source. In other words, the total gain should be limited by the original gain from each aperture source. Therefore, a high gain may be obtained, provided that the aperture field with a planar phase distribution is sufficiently extended in space and that the two aperture fields are generated almost evenly. In this article, we investigate high gain properties of a dielectric tube antenna fed by a metallic waveguide [5]. Since the effective index of the dielectric tube is close to unity, it serves to extend the surface wave in space. To efficiently excite the dielectric tube, we introduce a small dielectric sphere [6] between the tube and the metallic waveguide. We numerically demonstrate that the proposed antenna achieves a gain higher than that estimated by the Hansen-Woodyard condition.