Metallic waveguides have major advantages, such as low propagation loss and high power transmission in the microwave frequency range. However, one disadvantage is that the usable frequency range is restricted to fc < f < 2fc, because the TE20 mode is possible in a frequency region higher than 2fc for rectangular metallic waveguides. A ridge waveguide (Cohn, 1947) (or double-ridge waveguide) has an advantage in that it can spread the propagating frequency range as a result of reduction in the cutoff frequency for the TE10 mode. However, one disadvantage is that the attenuation constant becomes large. Power sources, such as watt class IMPATT diodes or Gunn diodes, are readily available, and for high frequency use, power sources are sometimes combined, due to their low power rating. However, power combiners consisting of cavity resonators usually have narrow bandwidths (For example, Matsumura et al., 1987). Power dividers and power combiners may be easily setup using mode converters. For example, a TE10–TE30 mode converter easily offers a three-port power divider, and a three-way power combiner can be composed by reversal. A power combiner is useful for application to Gunn diodes in a waveguide array (Bae et al., 2000), because it converts the TE30 mode to the TE10 mode.