Microstrip elements and arrays with spherical dielectric overlays

A novel combination of a microstrip patch antenna and a truncated dielectric sphere having a relative permittivity 2.2 to 2.5 is analysed using a magnetic line source model for the printed patch and assuming that dielectric resonator action is a second-order effect for the cases considered. Measurements substantiate the computed gain and co-polar pattern characteristics as a function of sphere size, truncation depth and material losses. Optimal illumination of the sphere, and hence maximum gain, is approached only for spheres of radius less than 1.5 free-space wavelengths together with small truncation depths, as is also evident from simple ray tracing. The new element offers greater freedom of design as regards equality of E- and H-bandwidths, control of co- and cross-polarisation characteristics for a given bandwidth and creation of sparse element arrays with reduced feeder line radiation and losses. Detailed results are given for the application of the element as a reflector antenna feed and in arrays at microwave and millimetre wavelengths. Some ideas for future development concerning radomes and beam scanning are briefly noted, and it is concluded that the new element offers many advantages at the cost of some increase in antenna depth and constructional detail.