GB861008A - Improvements in or relating to the tuning of cavity resonators - Google Patents

Abstract

861,008. Tuning cavity resonators. STANDARD TELEPHONES & CABLES Ltd. Nov. 14, 1958, No. 36702/58. Class 40 (8). A tuner comprises a length of uniform hollow waveguide or coaxial line, a short-circuiting piston movable along the said length, and an element mounted on the said piston in such manner as to load the said waveguide or coaxial line with a shunt capacitance at a plane effectively a quarter-wavelength in front of the plane of the effective short circuit. This has the effect of providing a virtual short-circuit in front of the capacitative element, which virtual short-circuit may be located inside a physical barrier even though the piston and the capacitative element remain outside. The tuner may thus be used to tune a cavity resonator mounted inside a velocity-modulated tube in a position such that the tube envelope would restrict the movement of a conventional external tuning piston. In one form, Fig. 3, a rectangular waveguide 1 coupled to a cavity resonator 4 contains a piston 5 in the form of a box which makes contact with the broad sides of the waveguide via spring fingers 6. The front face of the box is formed by a capacitative diaphragm 7 having a seat 8 parallel to the broad sides of the waveguide, and the continuous back face 9 provides a short circuit. The axial length of the piston 5 is a quarter guide wavelength. In an alternative form, Fig. 6, a section of rectangular waveguide 1 contains a piston of circular cross-section comprising quarter-wave sections 10 and 11 joined by a quarter-wave section 12 of smaller diameter, such a piston presents a virtual short-circuit at the front face 13. A capacitative disc 14 is attached to the face 13 by a rod 15 a quarterwavelength long. A practical embodiment of this tuner is described together with its connection to a velocity-modulated tube (Figs. 8 and 9, not shown). A coaxial line type of tuner is also described connected to a velocitymodulated tube (Fig. 10, not shown). Specification 861,009 is referred to.