A power combining apparatus includes a waveguide structure and a plurality of antenna elements arranged in the waveguide structure, wherein each of the antenna elements comprises a center planar antenna layer, two outer planar antenna layers arranged on opposite sides of the center planar antenna layer, a non-conductive layer between the center planar antenna layer and one of the outer planar antenna layers, and another non-conductive layer between the center planar antenna and the other one of the outer planar antenna layers. The power combining apparatus includes a waveguide structure having an input, an output, and a plurality of antenna elements arranged in the waveguide structure, wherein each antenna element is configured to transform an electric field direction of an electromagnetic field by substantially 90 degrees rotation about a longitudinal axis of the waveguide structure, wherein a bandwidth of the antenna is less than, equal to, or greater than a decade of frequency range.