Harmonic suppression of a dual-polarized microstrip array by using microstrip stubs

A compact dual-polarized two-element microstrip antenna array has been designed at 3.5 GHz frequency band and the higher order modes up to second harmonics have been successfully suppressed for two orthogonal modes. Two pairs of microstrip stubs of electrical lengths lambda(g1)/4 (lambda(g1) = wavelength of 1st harmonic signal) and lambda(g2)/8 (lambda(g2) = wavelength of 2nd harmonic signal) respectively have been integrated with the microstrip feed line and two harmonics and a sub-harmonic of the fundamental mode have been reduced. The design offers a port isolation of more than 23 dB over the operating band and the measured bandwidths of the two polarizations of 250 and 275 MHz, respectively, are reported. Simulated and measured results show that the harmonic components exist at 7.05 GHz (1st harmonic, TM20), 10.45 GHz (2nd harmonic, TM30) and also a sub-harmonics at 5.5 GHz (TM12 of the fundamental frequency have been successfully suppressed. The harmonic properties of microstrip antenna with and without stubs have been studied and the result indicates that the harmonic components of the array for two orthogonal polarizations have been well eliminated because of the effect of the tuning stubs. To validate the concept the prototypes were fabricated, tested and compared with the simulated results.