GB746798A - Improvements in or relating to wave attenuating arrangements - Google Patents

Abstract

746,798. Traveling wave tubes. TELEFUNKEN GES., [formerly TELEFUNKEN GES. FUR DRAHTLOSE TELEGRAPHIE]. Feb. 8, 1954 [Feb. 7, 1953], No. 3564/54. Class 39 (1). A travelling wave tube of the helix type is provided with loss material in the form of a strip-like layer of uniform thickness deposited on an insulating member, preferably a helix supporting rod, the layer having a linear central portion and end portions several wavelengths long extending progressively away from the helix so as to provide a progressive reduction of the attenuation, due to the rapid decrease of the high-frequency field with radial distance from the axis, and so to prevent reflection. In one form, Fig. 3, one of the three ceramic rods 11, 11a, 11b supporting the helix 3 carries a strip of loss material which contacts the helix along its central portion 12a and is bent away from the helix at its ends 12b in spiral arcs. If the rod 11 has a sufficiently large diameter, generally greater than the helix diameter, and is made of a suitable ceramic material, e.g. having a dielectric constant greater than 6, it is possible to make the line substantially dispersionless, i.e. with constant phase velocity over a wide frequency range. This is because when # increases the field spreads further radially and the retardation due to the rod material increases, which counteracts the increase of phase velocity which normally results when A is increased and similarly for a decrease in A. Thus the D.C. voltage of the helix can be kept constant. The ends of the attenuating layer are preferably substantially outside the high frequency field, so as to reduce reflection to a minimum. In a modification, Fig. 4, each of the three ceramic rods 11, 11a, 11b has an attenuating strip coating 12b. There may be a fewer or greater number of rods than three and the rods need not be of circular section, e.g. these may be three rods of rectangular section, Figs. 5, 6 (not shown). It is stated that the invention is applicable to wave retarders and other line structures in general. A prior art travelling- wave tube in which the insulating rods or tube carrying the helix have a layer of loss material the thickness of which tapers at the ends, is referred to.