Compact microstrip harmonic-suppressed quadrature hybrids

In this article, compact 3-dB branch-line hybrids with harmonic suppression are presented. In comparison to the conventional hybrids, the proposed ones achieve two features: compactness and harmonic suppression. Two hybrids, one with third harmonic suppression and the other with third and fifth harmonics suppression, are designed, fabricated, and measured. T-shaped or cross-shaped equivalent lines are used to replace the quarter-wavelength microstrip lines of the conventional hybrids. The equivalent lines achieve the same fundamental characteristic and act like bandstop filters at the particular harmonics to improve suppression. Moreover, the proposed hybrid with third harmonic suppression achieves 29.5% size reduction as compared to the conventional one.