GB1284455A - Impedance match for periodic microwave circuits and tubes using same - Google Patents

Abstract

1284455 Travelling-wave tubes VARIAN ASSOCIATES 6 Aug 1969 [8 Aug 1968] 39453/69 Heading H1D Unwanted band-edge oscillations in a travelling-wave tube are prevented by the provision of a matching section comprising at least two periods of the periodic structure having the same cut-off frequency which lies substantially outside the passband of the main circuit. The matching section is an odd number of quarterwavelengths long at the band-edge of the main circuit, e.g. it may consist of two elements of the periodic structure each providing a phase-shift of 45 degrees. The periodic structure in the tube shown consists of a series of annular cavities 23 coupled by apertures 26 and provided with axial re-entrant portions 24 which define a drift space and a series of reaction gaps. The tube operates in a backward-wave mode and consists of three periodic arrays, the last (downstream) of which is shown in Fig. 2. The array is terminated at the input end by a waveguide 29 coupled to a dissipative load 13 in the form of a water-cooled seal 31, and at the output end by an output waveguide 9. The last pair of cavities at either end providing matching sections #II are substantially identical and are dimensioned to have cut-off frequencies substantially above the upper band-edge of the passband of the main circuit #I. Alternatively, they could have cutoff frequencies below the lower band-edge. They provide, in effect, a quarter-wave transformer between the main circuit and the input and output. The other two arrays comprised in the slow-wave circuit of the tube are similarly provided with matching sections (Fig. 1, not shown). The beam is focused by a solenoid (not shown) provided with soft-iron polepieces 27 (one only shown).