Broadband, high power, dual polarized phased arrays constructed from waveguide type elements generally require a 90 degree rotationally symmetric waveguide cross sectional geometry that can support single-mode propagation over the desired bandwidth. This dissertation presents a novel method of obtaining bandwidth enhancement of a square waveguide using both dielectric loading and mode filters that retains the 90 degree rotational symmetry. By placing two orthogonal dielectric slabs transverse to the guide, one obtains what is referred to as the crossed-septum waveguide (CSW). The bandwidth enhancement of the square waveguide is achieved by exploiting the modal symmetry properties of the electric field about a given axis within the guide. The objective is to increase the modal separation between the TE₁₀ (or TE₀₁) mode having even/odd (or odd/even) symmetry and TE,, mode having odd/odd symmetry in the guide. The TE₁₁ and TM₁₁ modes having even/even symmetry are forced to attenuate rapidly by the use of mode filters. An analysis of the CSW is performed using the mode matching technique to determine the electromagnetic fields, phase constants and modal cutoff frequencies. A numerical study of the modal cutoff frequencies as a function of septum thickness and dielectric constant is performed. The TE₂₀, mode is found to split into two distinctly different modes upon the introduction of the dielectric septum and are referred to as the 'U'-upper and 'L'-lower modes. The analysis shows that one can obtain increased cutoff frequency separation between the TE₁₀ and TE_20L modes, however the TM₁₂ mode may limit bandwidth performance. A process for designing and experimentally verifying a mode filter for supporting the dual polarization requirement is described. The mode filter consisting of a thin conductive film is demonstrated using the resonant cavity technique, and is experimentally shown to suppress unwanted TM₁₁ and TE₁₁ higher-order mode resonances. A general case multi-mode radiation analysis is used to identify dominant mode and higher-order mode far-field radiation characteristics. Co- and cross-polarization measurements are performed and show that energy can be largely confined to dominant mode. Under these conditions, the patterns are reasonably well behaved and degrade gracefully at the high frequencies. The concept is demonstrated over a 2.2:1 bandwidth. A potential application of the CSW is a phased array antenna radiating element that supports dual polarization. / Ph. D.