Radiation characteristics and polarisation of undulated microstrip line antennas

A numerical method is used to investigate the radiation from undulated microstrip line antennas. The undulated line is assumed to be suspended over a ground plane and its current distribution is determined using a moment method type solution. This current distribution is then used to compute the co-polar and cross-polar radiation fields. It is found that the current distribution has an oscillating behaviour along the line, with a frequency which is twice the number of undulations. The cross-polarisation is found to be high and relatively independent of the undulating shape. Its relative level, however, is reduced for large arrays, due to the array factor affecting the co-polar field. A procedure for the reduction or elimination of the cross-polarisation is then proposed, which is based on utilising two undulated lines with mutually inverted undulations. A design method for achieving low sidelobe levels is also proposed and a design example with sidelobes around the ¿¿40 dB range is presented.