Patent GB-830071-A (GB)
Title: Improvements relating to gas-discharge cells for controlling the phase and/or amplitude of h.f. radiation
Inventor: PRINGLE DEREK HAIR
IPC: H01J17/04
Filed: 1955-02-10
Abstract: 830,071. High-frequency discharge apparatus. FERRANTI Ltd. March 6, 1956 [Feb. 10, 1955], No. 3944/55. Class 39(1). [Also in Groups XL(b) and XL(c)] A cell for controlling the phase and/or amplitude of microwave radiation comprises a length of rectangular waveguide containing ionizable gas at low pressure in which a coldcathode or thermionic discharge is established between a filament and the walls of the guide. In one form, Fig. 1, the waveguide is closed at each end by a glass-sealed window 17, and a filament 18 of tungsten or thoriated tungsten is supported along the axis 16 by two conductors 19 and 20 each in the form of a tube of alloy glass-sheathed throughout and passing through an insulating bush 21 secured within a metal bush 22 brazed to the wall 12. A getter is provided within a cavity (not shown) communicating with the interior of the cell. The filament 18 is maintained sufficiently negative with respect to the walls of the guide by external control means (not shown) to establish a coldcathode discharge which changes the phase of radiation passing through the cell. The conductors 19 and 20 are normal to the electric field component and therefore cause minimum interaction loss. In an alternative embodiment the filament 18 is inclined to the axis 16 and the supports 19 and 20 are shorter and extend through opposite walls of the guide. Two or more such inclined filaments may be mounted parallel to each other and spaced at quarter wave intervals along the axis 16. The broader dimension of the cell may be reduced over a part at least of its axial length to enable a less intense discharge to operate the cell near cut-off conditions. A stepped transformer section is then provided at each end to match the cell to adjacent waveguides. Fig. 5 shows a directional coupler in which radiation from a main section of waveguide 35 is coupled into a side-arm guide 36 by a row of slots 37 in the common wall 38. A filament 39 is supported parallel to the row of slots 37 by conductors 40 and the guides are closed by sealed windows 42. By raising the discharge current to a sufficient level, side-arm 36 may be taken beyond cut-off and the radiation confined to the main guide 35. In any of the embodiments the filament or filaments may be heated by passing current there through. At a critical value of filament current a transition takes place from a cold-cathode to a thermionic discharge and the phase-shift and interaction loss are at a maximum. The cell may therefore be used as an attenuator if the filament current is maintained constant at the critical value, variation of the attenuation being effected by varying the negative potential of the filament. Variable attenuation may also be obtained by the arrangement shown in Fig. 6 in which radiation in an input waveguide 45 is divided into two waveguide paths 47 and 48 by a directional coupler 46. Path 47 includes a coldcathode phase-shift cell 49. A second coupler 50 delivers the radiation to an output waveguide 51. By varying the negative potential of the filament in cell 49 the phase of the radiation arriving at coupler 50 from path 47 may be varied with respect to that of radiation from path 48 and the amplitude of the output radiation is correspondingly varied. The arrangement may also be used as a modulator by applying the modulation voltage to vary the negative potential of the filament. The Provisional Specification refers to a cold-cathode embodiment in which the filament is replaced by a rod supported by a single conductor.