Wave guiding properties of a periodic metal plate-loaded dielectric slab structure are investigated for obtaining surface wave antennas with broadside radiation and low voltage standing wave ratio (VSWR) characteristics. A novel technique is proposed in which dielectric phase transformers play an essential role for realizing a uniform phase distribution on the radiating aperture which is required to achieve broadside radiation. A quarter-guided wavelength spacing of the metal plates guarantees the suppression of reflections at the input port. On the basis of the theoretical results obtained, two types of antennas have been designed at 10 GHz, one having the exponential aperture distribution, the other the uniform aperture distribution. The overall antenna efficiency of the latter type has been found to be 53%, exhibiting a practical performance at microwave frequencies.