UWB stripline coupler with low loss and ripple

An asymmetric multi-section ultra-wide band
stripline coupler with two coupled parallel stripline is proposed.
The primary features of the coupler include operating frequency from 2 GHz to 18 GHz to cover S, C, X, Ka and Ku bands; coupling coefficient below -18 dB; return loss of -20 dB for all the ports; minimum directivity of 25 dB; small size and low losses, which make it appropriate for applications, where the available space is limited. The coupler parameter values are obtained after a two
step optimization process using i) a genetic algorithm with Matlab; and, ii) a particle swarm algorithm with HFSS software. The whole structure is simulated, fabricated and measured. There is an excellent agreement between measured and simulated results.