Experiments on circular H01 wave propagation in a curved waveguide filled with an inhomogeneous dielectric

The experiments described were designed to show that mode conversion, which normally occurs when a hollow tubular metal waveguide supporting a circular H01 mode is bent, can be reduced very considerably by the use of an inhomogeneous dielectric filling, having a distribution over the cross-section such as to keep the wavefront radial with respect to the centre of curvature of the bend and thereby to preserve the wave pattern. For this purpose a length of waveguide was filled with thin discs of polystyrene loosely packed together, giving, when straight, what was equivalent to a homogeneous dielectric filling. Subsequently, as the tube was bent the discs separated slightly on the outside of the bend and closed up still further on the inside, giving a good approximation to the inhomogeneous distribution of dielectric required in these circumstances. With a pure H01 mode at the input, the content of this mode and of the E11 mode was examined at the output for different angles subtended at the centre of curvature of the bend. The results showed that, although there was a considerable fall in the power level at the output due to losses inherent in the dielectric, the conversion to the E11 mode which would normally be expected was very substantially reduced. Thus the principle governing this technique was amply demonstrated. It remains now to find more suitable forms of dielectric filling for the purpose.