For a typical microstrip antenna placing the radiating elements and feed lines on one side of the substrate realises the advantages of planar fabrication. It can be shown that the currents in many parts of the feeder circuit have magnitudes comparable with those in the radiators and make a significant contribution to the total radiation. In this thesis the radiation from both the radiators and the feed lines will be examined. The radiation from microstrip was calculated from the current distribution. This method allows the calculation of the radiation from the nominal radiators and the feed circuits. Theoretical and experimental results verifying this approach will be given for examples of relatively low Q resonators on thick, low permittivity substrates required for efficient antennas. Unwanted radiation from the feed circuit could be reduced by the use of coaxial lines below ground or a multiple-layer strip line assembly, but such complications are unattractive. Reduction in unwanted feed line radiation has been investigated by the use of a balanced line feeder, combined with a suitable balun.