Patent GB-640089-A (GB)
Title: Improvements in ultra high frequency wavemeters
IPC: G01R23/04
Filed: 1946-04-15
Abstract: 640,089. Wave meters. BRITISH THOMSON-HOUSTON CO., Ltd. April 15, 1946, No. 11527. Convention date, April 16, 1945. [Class 37] [Also in Group XL (b)] In a high frequency wavemeter having a cylindrical cavity resonator including means for selectively suppressing undesired modes of the electromagnetic waves such means comprises a recess extending in radial direction between an end wall and a side or longitudinal wall of the resonator which are in electrically conductive relationship beyond the recess. A wave-absorbing material, e.g. a plastic material loaded with carbon, may be arranged in the recess. In the wavemeter shown in Fig. 1, high-frequency electromagnetic waves from a source 1 are applied through a wave-guide 2, which may be of rectangular conductive material, to an off-centre aperture 3 in the fixed end wall 4 of a cylindrical cavity resonator 5 having its other end wall formed by a movable piston 6 which may either make conductive contact with the side walls of the cylinder 5 or may be separated therefrom by an air-gap and thus be capacitatively coupled thereto. The plunger 6 is adjustable by a rod 7. The waves in the guide 2 are supplied through an exciting aperture 8 to a wave-guide 9 across which is connected a rectifier 10. The output of this is supplied to a meter 12, decrease in the reading of which indicates resonance in the resonator 5. A scale 13 and pointer 14 indicate the dimensions of the cavity resonator 5 when resonance is established, and thus the wave length. In order to displace the piston positions for resonance of undesired lower order waves, a radial recess or choke 16 is introduced between the end wall 4 and the cylinder 5. This choke has a depth approximately equal to a radial quarter wave length in the frequency length through which the meter is to be employed. The quarter wave choke inserts an appreciable gap between the end wall 4 and the end of the cylinder 5 which interferes with current flow between the end wall 4 and the side walls of the resonator and thereby displaces the wave pattern for the unwanted waves within the cavity so that the wanted wave may be more easily distinguished therefrom. The undesired waves may be dissipated by partially filling the recess 16 with high loss material 17 such as plastic material loaded with carbon. In a modification, Fig. 2, the rectangular input wave-guide 2 communicates with an aperture 18 in the side wall of the cylinder 5 to establish waves therein. The end wall 4, which projects into the cylinder 5 to establish a quarter wave choke 20 with the inner wall, is provided with a cap 19 which abuts against the cylinder 5 and forms the closed end of the choke 20. The waves of the resonator 5 are supplied through an aperture 21, diametrically opposite the aperture 18, to a wave-guide 22 across which is connected the rectifier 10 used for detecting the waves in the guide 22 to supply current to the meter 12. The condition of resonance within the resonator 5 is indicated by an increase in the reading of the meter 12. Preferably the apertures 18, 21 are located a quarter wave length from the end wall 4. The choke 20 may include power absorbing material 17. The movable end wall is formed by a piston 23 having its inner end 24 spaced from the side walls of the cylinder 5.