Analysis of the dielectric-loaded helical antenna and the dielectric-loaded slot antenna

Student thesis: Doctoral Thesis

View graph of relations

Author(s)

  • Hon Tat HUI

Related Research Unit(s)

Detail(s)

Awarding Institution
Supervisors/Advisors
  • Kai Ning Edward YUNG (Supervisor)
Award date8 Jun 1998

Abstract

In this thesis, two types of dielectric-loaded antennas are rigorously analysed. One is a representative from wire antennas: the dielectric-loaded helical antenna and the second is a microstrip driven dielectric-loaded slot antenna. The dielectric-loaded helical antenna is a conventional helical antenna loaded by a cylindrical dielectric core. The dielectric-loaded slot antenna is a slot cut on a ground plane and covered by a multilayered dielectric hemisphere. The slot is excited by a finite-width substrate microstrip from below the ground plane. Two basically similar analysis methods are applied to these two antennas. For the dielectric loaded helical antenna, the dielectric loading is analysed by using the equivalent source method. The effect of the dielectric loading material is simulated by an electric polarization current density. This results in an equation for the electric field inside the dielectric load which couples with the equation for the current distribution along the helical wire. These two equations are then solved by the method of moments. Special treatment is used to handle the singularity of the electric dyadic Green's function. Antenna input impedance and far-field characteristics are obtained and compared with experimental measurements. For the dielectric-loaded slot antenna, the feeding microstrip is analysed in the same way as the dielectric-loaded helical antenna by using the equivalent source method. The microstrip is modeled by a finite-length section excited by an ideal delta-gap source and the finite-width substrate is treated as a dielectric load. The current distribution and the voltage along the microstrip, which is proportional to the electric field inside the substrate, are then solved. These current and voltage are used to calculate microstrip characteristics such as the open-end impedance and reflection coefficient. More importantly, the knowledge of these current and voltage enables us to evaluate, by a transmission line method, the impedance of the dielectric-loaded slot antenna. The multilayered hemispherical dielectric load is analysed by using the Green's function approach. This provides a systematic method for studying a dielectric load with any number of dielectric layers. On the slot, an equivalent magnetic current density is sought. Through this equivalent magnetic current, the electromagnetic fields on either side of the ground plane are coupled together. This leads to the formulation of three coupled integral equations for three unknowns: the microstrip current, the microstrip substrate electric field, and the equivalent magnetic current over the slot. On solving these equations by the moment method, we are then able to obtain the antenna impedance and radiation pattern. Numerical results for one-, two-, three-, and four-layer dielectric loads have been generated. Comparisons of theoretical calculations with available published data are made. To summarize the results of this study, it is found that for a dielectric-loaded helical antenna, directivity can be increased, while other antenna characteristics being less affected, by a suitable positioning of the dielectric core. Or by another method, the directivity can be increased by using a higher permittivity dielectric core. For the dielectric-loaded slot antenna, a high permittivity hemispherical dielectric load can generally result in a smaller slot size while an increasing number of dielectric layers can enhance the bandwidth. The price to pay for a multilayered dielectric load is a lowered radiation efficiency. It is hoped that through the methods developed in this study and the results so obtained, some knowledge may be provided for engineering designs of dielectric-loaded antennas.

    Research areas

  • Antennas (Electronics), Dielectrics