Cylindrical Waveguide with Axially Rippled Wall

J. J. Barroso, J. P. Leite Neto, K. G. Kostov


Axially corrugated cylindrical waveguides with wall radius described by R0(1 + ² cos 2¼z=L), where R0 is the average radius of the periodically rippled wall with period L and amplitude ², have been largely used as slow-wave structures in high-power microwave generators operating in axisymmetric TM modes. On the basis of a wave formulation whereby the TM eigenmodes are represented by a Fourier-Bessel expansion of space harmonics, this paper investigates the electro- dynamic properties of such structures by deriving a dispersion equation through which the rela- tionship between eigenfrequencies and corrugation geometry is explored. Accordingly, it is found that for L/R0 > 1 a stopband always exists at any value of ²; the condition L/R0 = 1 gives the widest first stopband with the band narrowing as the ratio L/R0 increases. For L/R0 = 0:5 the stop- band sharply reduces and becomes vanishingly small when² < 0:10: Illustrative example of such properties is given on considering a corrugated structure with L/R0 = 1, R0=2.2 cm and ² = 0:2, which yields a stopband of 1.5 GHz width with the central frequency at 8.4 GHz; it is shown that in a ten-period corrugated guide the attenuation coefficient reaches 165dB/m, which makes such structures useful as an RF filter or a Bragg reflector. It is also discussed that by varying L/R0 and ² we can find a variety of mode patterns that arise from the combination of surface and volume modes; this fact can be used for obtaining a particular electromagnetic field configuration to favor energy extraction from a resonant cavity.

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