Patent GB-906172-A (GB)
Title: Improvements in or relating to radio frequency dielectric heating apparatus and medical diathermy apparatus
Inventor: TIBBS CHRISTOPHER EVAN MUNDELL
IPC: H05B6/48
Filed: 1957-12-05
Abstract: 906,172. Cavity resonators. RADYNE Ltd. Dec. 5, 1958 [Dec. 5, 1957], Nos. 37901/57 and 40082/57. Class 40 (8). [Also in Groups XXXVI and XL (c)] An oscillation generator for radio-frequency heating comprises a cavity resonator 40 having disposed therein a tubular axial conductor 44 and an intermediate-shell conductor 42 which co-operates with the end wall of the cavity to provide a variable capacitance, the opposite end wall of the cavity upon which the conductors 42 and 44 are mounted being slidable within the side walls of the cavity to vary the capacitance between the conductor 42 and the first-mentioned end wall of the cavity and also serving as an end wall of a screened compartment 50, comprising side wall 46 and opposite perforated end wall 66, within which the oscillator valve is housed. The diameter of the cavity is made at least 0.8 of its axial length; it being found that harmonic production is reduced thereby. The open end of the inner conductor 44 extends through into the compartment 50. The anode of the valve 16 is connected through a capacitor 20 to a point on the inner surface of the conductor 42. Energy is fed back to the grid of the valve from a loop 14' one lead of which is connected to the grid of the valve while the other is connected through an R.F. choke, a series resistor 28 and a lead through capacitor 31 to a point external to the cavity and compartment. The series resistor 28, Fig. 5, which is not visible in Fig. 1, comprises four lamps in series, Fig. 2 (not shown), and serves to limit grid current flow under condition of no load. The anode is also connected to the positive terminal of the H.T. supply through a choke 22 and lead-through capacitor 24. A choke 32 and capacitor 33 are similarly arranged in the filament supply circuit for the valve. Output is taken from a loop 12 through a variable capacitor 34 to the heating electrodes 36, 38. As shown in Fig. 1, the capacitor 34 comprises a movable electrode 56 hinged about its lowest point and moved with respect to a fixed electrode by means of a cable 57. The cable 57 is attached to a lever 74, Fig. 4, which passes through slots in a sleeve 72 and a threaded rod 70, being pivoted to the latter by a pin 76 and held at its lowest point by a pin 78. On rotation of knob 68, the rod 70 moves, thus causing the lever 74 to move and so acting upon the electrode 56 through the cable 57. Cooling air is blown through the compartment 50 by means of fans 64. Temperature compensation may be provided by making different parts of the cavity tank circuit of different metals, e.g. copper and brass, so as to take advantage of their different coefficients of expansion. Modified forms of construction for the intermediate conductor 42 and inner conductor 44 are described, in particular Fig. 13 (not shown), illustrates an arrangement in which the oscillator valve is housed entirely within the inner conductor 44. Fig. 11 shows an alternative form of output coil for use when heavy currents are to be carried and consisting of a wide metal strip 12 which is made in three pieces and bolted or otherwise fixed together after the central loop has been inserted in place into the cavity. The use of a plurality of short slots 94 avoids the severe distortion of the field within the cavity which would result if a pair of long slots were used. In a further embodiment, Fig. 12 (not shown), a plurality of loops coupled into the cavity supply energy directly to separate points on the heating electrode.