The radiation zone fields of the stepped dielectric rod antenna excited in the HE₁₁ mode

Abstract

In this work analytical expressions are determined for the radiation zone fields of the stepped dielectric rod antenna with HE₁₁ mode excitation. This type of antenna is similar to both the cylindrical dielectric rod antenna and the tapered dielectric rod antenna. This particular type of dielectric rod antenna consists of several cylindrical sections of dielectric material with different radii. The transition from one section to an adjacent section is achieved by means of an abrupt step in rod diameter. The analytical expressions for the fields of the antenna are obtained by two methods. In the first method, a continuous array of point sources is used as a model for the antenna. This approach allows the field pattern of the antenna to be determined by using an empirical expression for an unknown function of the angular coordinates. This function has little effect on the pattern in the vicinity of the principle lobe. In the second method, the exact form of this function and resulting fields of the antenna are determined from an application of an equivalence principle. In the equivalence principle approach, the fields on the antenna surface are assumed to be those of a forward travelling HE₁₁ waveguide mode. It is found that only the equivalent currents on the lateral surfaces of the antenna need to be considered except in the case of very short antennas. When the contributions of the equivalent currents on the section ends are required, the integrals for these contributions must be evaluated numerically. The field patterns calculated from the analytical expressions are compared with experimentally determined field patterns. The limitations on the analytical expressions for the fields with respect to the section lengths, overall antenna length, and the magnitudes of the steps in rod diameter are investigated. The measured field patterns are found to compare favorably with the computed field patterns provided the limitations placed on the analytical expressions are observed.