In this article, an overview is given for the development of dielectric-resonator antennas. A detailed analysis and study of the hemispherical structure, excited by a coaxial probe or a slot aperture, is then given, using the dyadic Green's functions pertaining to an electric-current source or a magnetic-current source, located in a dielectric sphere. The integral equation for a hemispherical dielectric-resonator antenna (DRA), excited by either a coaxial probe or a slot aperture, is obtained. The integral equation is solved using the method of moments (MOM). The antenna characteristics, such as input impedance, radiation patterns, directivity, and efficiency, are computed numerically, around the resonant frequency of the TEI 1 1 mode (the HEM11 mode for cylindrical coordinates). The computed input impedance is compared with numerical and experimental data available in the literature.
Introduction2.1 Review of the development of the dielectric-resonator antenna he early experimental study of the dielectric-resonator antenna T (DRA) by Long and McAllister [l, 2, 31, followed by a numerical computation of the radiation characteristics of dielectric resonators with new applications [4-51, have shown that the dielectric-resonator antenna (DRA) is an efficient radiator at microwavefrequencies. Furthermore, attempts have been made to increase the bandwidth of the cylindrical DRA, excited by a coaxial probe, from 10% for a single dielectric resonator excited in the HEMI 1 mode, to more than 25%, by stacking two resonators on top of each other [6]. Other types of excitation have been investigated, such as a microstrip-transmission line [7], an aperture-coupled microstriptransmission line [8-91, and a coplanar waveguide [lo]. Also, geometries other than the cylindrical DRA have been investigated, such as the rectangular parallelepiped [2], hemispherical [3], and half-split cylinder [11][12]. To improve the radiation characteristics of the dielectric-resonator antenna, the dielectric was loaded with a conducting ring [ 131. This structure increased the antenna directivity for the HEMl I mode, which gives a broadside radiation pattern.The early studies have proven that the DRA can be excited efficiently with different excitation mechanisms. The resonant frequencies of the structures were computed approximately, in the earliest studies, using the magnetic-wall model [ 141. This approximate model may result in large errors in predicting the resonant frequency for the desired mode [14, 151. A more-rigorous numerical method has been used to compute the resonant frequencies in the complex plane, for symmetric-dielectric resonators, such as discs [15,16]. This method was found to be reliable and accurate for computing the resonant frequencies, quality factors, and radiation patterns of different modes [15-171. A comparison between the dielectric-resonator antenna and the microstrip antenna was presented in [ 181. This comparison shows that the dielectric-resonator antenna can be a more-efficient radiator than the microst...