Patent GB-649761-A (GB)
Title: Improvements in electron discharge devices of the electron velocity modulation type
Inventor: FREMLIN JOHN HEAVER
IPC: H01J25/08
Filed: 1946-02-08
Abstract: 649,761. Velocity modulated tubes. STANDARD TELEPHONES & CABLES, Ltd. Feb. 8, 1946, No. 3987. [Class 39 (i)] A velocity modulated tube comprises inner and outer conductors defining between them a hollow resonator in which oscillations may be set up by the passage of an electron beam across two gaps between the inner and outer conductors, the inner conductor system being formed in part from at least one pair of opposed substantially rectangular metal plates between which the electron beam is adapted to be projected, the said inner conductor system comprising a bimetallic system adapted when heated to distort it in the neighbourhood of said gaps so as to alter the capacity thereacross. The bimetallic system may be arranged to eliminate changes of frequency with changes of temperature or may be arranged to adjust the frequency of the device. In the hollow resonator shown in Fig. 3 the outer conductor has fins 5 in line with two rectangular metal plates 7 forming the inner conductor which is spaced from the fins 5 by gaps 9. Each of the plates 7 has a central longitudinal slot 10 and two strips 11 of material, for example tungsten, of lower coefficient of thermal expansion than the rest. In a modification the strips are arranged diagonally. Fig. 6 shows a V.M. tube in which the resonator system of Fig. 3 is incorporated in order to maintain constant frequency and which comprises an annular metallic disc 14 bonded to the glass envelope 15 and to a glass cap 16 and fixed to the resonator 4 of Fig. 3. The disc 14 provides means for mounting the tube on the wall of a wave-guide so that the antenna 8 of the resonator 4 projects inside the cavity. The electrode system comprises an indirectly heated cathode 19 behind which is mounted a screen 20, a control grid 21 and a screen grid 22 which produce a ribbon-shaped beam of electrons which is projected through the resonator system to an anode 23. The primary focussing means is a magnetic field provided by field coils external to the tube. In a modification changes of frequency can be effected by providing the tube with a second electron beam arranged to heat the resonator system directly or indirectly by electron bombardment. The cathode could be enlarged and independent focussing means and control electrodes added to form the second beam. The temperature of the resonator system and hence the frequency is adjusted by variation of the potentials applied to the control electrodes of the second beam. Figs. 8 and 9 show a modified construction for changing the frequency in which each of the plates 7 is replaced by two parts 24 which overlap to form small gaps 26 and which are supported by members 25 formed of two lengths of bimetallic strips end-to-end and reversed in their direction of bend. The two parts 24 move in opposite directions when the bimetallic strips are heated by a second electron beam. In one arrangement the bimetallic member 25 nearer the cathode is bombarded by a portion of the second beam and has holes therein arranged to allow a further portion of this beam to impinge on the other bimetallic member.