SCATTERING FROM A STUB-LOADED MICROSTRIP PATCH ANTENNA

Abstract

The microstrip reflectarray antenna is a relatively new type of reflector antenna. It offers the advantages of being flat and conformal, easy to install and has a low fabrication cost. In its basic form, a microstrip reflectarray consists of a flat array of microstrip patches on a thin dielectric substrate. A feed antenna illuminates the array whose individual elements are designed to scatter the incident field with the proper phase required to form a planar phase surface in front of the aperture. One of the methods of microstrip reflectarray 'implementation is to use microstrip patches of the same size, with stubs of variable length to control the reflection phase. While implementing this type of microstrip reflectarfay antenna, precise information regarding the scattering characteristics of the individual microstrip element as well as scattering properties of a stub loaded microstrip element is required. In this dissertation, scattering properties of a microstrip antenna with and without stub have been investigated. A theoretical analysis procedure using the Method of Moments has been developed utilizing sub-domain roof top basis functions. A computer program has been written in VISUAL FORTRAN to implement the analytical procedure. First, we have determined the RCS (Radar Cross Section) for a rectangular microstrip patch antenna for both parallel as well as perpendicular polarizations and, compared the data with that available in the literature to validate our formulation. The effect of change in frequency, dielectric constant and substrate thickness on the RCS has been studied. It is found that RCS of a microstrip patch strongly depends on the frequency of operation, polarization of the incident wave and angle of incidence. Also, like any other scatterer, a patch antenna has a peak value of RCS at the resonance frequencies of the patch. Having validated our formulation, the scattering behavior of a stub loaded microstrip patch antenna for different stub lengths has been studied in terms of RCS. For the stub loaded microstrip patch antenna, it is found that use of attachment modes is necessary for the continuity of the current at the patch — stub junction in order to obtain accurate and rapidly converging results. It has been found that the resonant frequency of patch is a very strong function of the stub length.