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Large area, diffusion-cooled, RF excited, waveguide CO2 lasers with output powers in the multi-kilowatt regime were demonstrated several years ago [1] and are low cost devices which are relatively simple to construct. The annular format for diffusion-cooled CO2 lasers forms an alternative to planar slab geometries for producing ultra-compact lasers in the high power regime [2]. Some of the advantages of annular lasers over slab geometries include a reduction in the overall volume for the same electrode area, and the availability of low cost high tolerance metal tubing which can be used for the coaxial electrodes. However due to the high frequency of RF excitation required to produce high output powers from CO2 waveguide lasers, usually around 100 MHz, transmission line effects become important when scaling these lasers to large areas. In the case of planar slab CO2 lasers, this problem can be avoided by the technique of distributed parallel resonance to eliminate voltage variations along the length of the discharge. In annular lasers it is not usually possible to use this technique without blocking some of the annular aperture; hence length scaling of these lasers has previously been somewhat limited.