Analysis into Proximity-Coupled Microstrip Antenna on Dielectric Lens

Abstract

Analysis into proximity-coupled microstrip antenna on dielectric lens for characteristics with high gain and pencil beam is presented. Earlier to this work, the University of Michigan developed double-slot antennas on a silicon material lens (Filipovic et al., 1993) and the Swiss Federal Institute of Technology developed aperture-coupled patch antenna on a substrate lens (Eleftheriades et al., 1997). In order to achieve the stringent requirements for efficiency and low cost necessary for a practical radio communication system, we propose our lens antenna for which lens is made of polyethylene plastic for commercial applications. Since the objective is to result in an efficient dielectric lens antenna system, substrates for the proximity-coupled microstrip antenna and the material for the dielectric lens are chosen that they have the same dielectric constant so as to eliminate surface waves and thereof no surface-wave power losses attributed to these selections. We have shown theoretical investigation for how the lens’ geometry comes up with dielectric ellipsoid lens and subsequently modified to extended hemispherical dielectric lens as synthesized ellipsoid. Determination of antenna return loss is carried out using method of moments (MoM) which involves spectral domain Green’s functions for a proximity-coupled microstrip antenna residing in homogeneous dielectric half space. The Sommerfeld type double integrals are numerically solved and obtained MoM impedance and excitation voltage matrix elements. After solving MoM matrix equation, current column matrix is resulted in and one of which elements is the equivalent current reflection coefficient at the open-end of the microstrip feedline. The return loss is calculated from this factor over a frequency band of 38 GHz and compared with measurement and found very good agreement. Radiation pattern theory is derived in such a way that the proximitycoupled microstrip antenna mounted on the back of the lens radiates into it and illuminates the interior side of the hemispherical surface. We ensure that the fields arrive at this surface are far fields so that we can apply ray tracing method in order to calculate the fields. We then calculate the fields on the exterior side of the hemisphere using appropriate transmission coefficients (parallel and perpendicular polarizations) for the interface between lens’ dielectric and free space media. Equivalent surface electric and magnetic current densities on the hemispherical surface are obtained from these fields. These two current densities are incorporated into radiation integral equations and far field patterns in free space are determined. This method for determination of far field patterns is called Schelkunoff Principle or Huygens’ Principle or Field Equivalence Principle. Once again we achieved good agreement between theoretical and measured radiation patterns.