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A TMo mode surface wave excited Rotman lens is explored for micro/millimeter-wave applications. The TMo mode lens can be designed to have a significant portion of the electric field distributed in the lossless air-cladding region to avoid both conductor loss and dielectric loss. A prototype lens has been implemented with seven beam ports and seven array ports. Measurements show that the efficiency of the lens is about 15%-30% in the range from 18 to 23 GHz. The synthesized beam patterns show that the lens can generate clear beam patterns over a range of approximately plusmn20deg but its scanning capability can be limited by relatively large longitudinal electric field. Several types of dielectric Rotman lenses are compared and discussed in terms of their excitation methods, losses, and beam patterns.