Two-port higher mode circular microstrip antennas

For circular microstrip antennas, single-mode design techniques work well for the dominant mode (n=1, where n is the azimuthal dependence of the fields in the antenna), but this is not the case for zero-order (n=0) and higher order (n>1) modes, where a modal expansion is necessary. The design of higher order dominant mode circular microstrip antennas is addressed, with an example and measurements for the n=3 case. Measurements for the n =3 modes show reasonable agreement with theory in the forms of the impedance loci, but the frequency dependence is in error by the order of the bandwidth of the antenna. A design example is given for a roof-mounted circular patch for vehicular communications. The example indicates that an n=3 dominant mode patch can be used as a two-port antenna for diversity applications or with a 90° phase shifter and adder as a single-port antenna for omnidirectional coverage with (essentially) circular polarization