A novel miniaturized, wideband, high directivity microstrip coupler on FR4

Directional couplers realized in microstrip parallel coupled line configuration exhibits very poor directivity. Inhomogeneous nature of microstrip circuits generate even and odd modes of wave propagation having different phase velocities, which in turn leads to the degradation in directivity. This paper presents the design and development of a compact, wideband, low insertion loss 20 dB microstrip coupler on FR4 substrate having better than 25 dB directivity at L band. Idea is based on the fact that the wiggly or slotty structure raises the odd mode inductance more strongly than that of the even mode due to the different field distributions between both modes and equalizes the even and odd mode characteristic impedances. Reactive loading based slow wave structure is used to further miniaturize the circuit. Directivity of better than 28 dB, maximum insertion loss of 0.4 dB, coupling flatness of ± .25 dB and 25 % reduction in size w.r.t. conventional parallel coupled line coupler were observed in the fabricated circuit over the band of 1250±500 MHz.