A 5-slot Yagi-Uda surface-wave launcher operating at 20 GHz is proposed in this paper. The launcher, which is composed of a driven slot, a reflector slot and three director slots, fed by a spoon shaped microstrip line, and manufactured on a 0.6 mm thick substrate with ε_r=2.65, is used to excite surface wave in a grounded teflon slab, whose thickness is optimized as 5 mm for maximum surface-wave power excitation. The Numerical results show that the relative bandwidth in terms of-10dB return loss is increased from <11.4% to 13.85%, and the maximum forward/backward directivity ratio is increased from 16.7 dB to 24.7 dB, when compared to those of a three-slot launcher with only one director.
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