In this paper, a split-ring-resonator-loaded sub-wavelength waveguide with H-plane-bend (HPB) radiator structure is proposed. Improved matching is applied to minimize the reflected power at the input. This proposed waveguide is used as part of a high-power narrow-wall slotted waveguide antenna. The waveguide is loaded with an optimized split-ring resonator (SRR) array applying H-plane discontinuity. The S parameters of the SRR-loaded waveguide with HPB-radiator indicates resonance below the cutoff frequency. The transverse dimension of this waveguide is reduced significantly. An improved waveguide to waveguide transition design is proposed to be added at its input to reduce the insertion loss. The HPB-radiator [N.R. Devarapalli, Electromagnetic Compatibility, vol.53, no.2, pp.380–389, May 2011] is added to the output of the waveguide to minimize the reflected power at the input.