GB1090040A - High frequency electron discharge devices of travelling wave type and slow wave circuits - Google Patents

Abstract

1,090,040. Travelling-wave tubes. VARIAN ASSOCIATES. Dec. 6, 1965 [Dec. 11, 1964], No. 51684/65. Heading H1D. A helical or helix-derived slow-wave structure 62 is supported by longitudinal fins 63, 64 within a waveguide 65, the fins being provided with slots e.g. slots 67, 68, 69, which have the effect of perturbing certain unwanted modes of propagation. Up to eight such fins may be used (two are shown). In such structures, the slow-wave circuit comprises two (or a number of pairs of) more or less independent parts each of which propagates a wave, the two waves being coupled by the helix. The structure will, therefore, support two independent transmission modes, viz.: the " in-phase " mode in which the axial electric fields on either side of the diameter defined by the fins have the same direction at any instant, and the " out-ofphase " mode in which the fields have opposite directions. The pass bands of lowest order for the two modes are almost identical and the purpose of the slots is to shift the upper, or lower, or both cut-off frequencies of the in-phase mode, whereby stable operation in the out-ofphase fundamental mode may be obtained. The effect of the slots is to lengthen the paths of axial and radial currents associated with the in-phase mode, slots of one of the types shown being distributed along the whole delay line. The mode of operation and the effect of the slots is discussed in detail with reference to Figs. 10-17 (not shown). The slots may have oblique edges to avoid impedance discontinuities. The embodiment described is a backward-wave oscillator in which the waveguide 65 is provided with a pair of loading ridges extending from opposite points on the interior of the waveguide in a diametral plane at rightangles to the plane defined by the ridges. A modification using a single fin is described.