A novel structure of high directivity broadband microstrip coupler

This work describes two methods for improving the directivity of an edge coupled microstrip based coupler with weak coupling. The proposed couplers have a coupling factor of about -30 dB. The coupler with interdigital capacitors achieved for excess loss of 0.5 dB a fractional bandwidth of 44 % and directivity of 25 dB. The second structure uses floating stubs between the coupled lines, which increases the fractional bandwidth up to 67% with the same directivity. The insertion loss of the proposed structures is lower than 0.3 dB.