Patent GB-626632-A (GB)
Title: Improvements in or relating to high-frequency attenuating devices
IPC: H01P5/02
Filed: 1943-10-15
Abstract: 626,632. Electric attenuators. SPERRY GYROSCOPE CO., Inc. Oct. 15, 1943, No. 33549/45. Convention date, July 25, 1942. Divided out of 617,236. [Class 40 (viii)] A variable attenuator for an U.H.F. transmission line comprises a wave-guide section whose cross-sectional dimensions are less than those required for free transmission at the working frequency. The section is of variable length, but is incorporated in a unit of fixed length. It may be connected between coaxial lines or between a coaxial line and a wave-guide. Thus the attenuating section 61, Fig. 1, is coupled by an input aerial 62 to a coaxial line 63, 64, and by an output aerial 68 to an adjustable coaxial section 83, 69, and so to an outgoing coaxial line 65, 63. The length of the wave-guide section .61, and in direct proportion thereto the total attenuation due to the section are varied by shifting the intermediate coaxial section 83, 69 in an axial direction by means of a nut 76 which engages pins 72 on the sleeve 69. The antenna 62 is formed of carbon or other resistive material so as to be aperiodic.. The antennµ 62, 68 may terminate in discs, Fig. 2 (not shown), and the partitions 79, 86 are then contracted to rings which surround the discs. Alternatively, the rings may be omitted and the flat discs replaced by segments of spheres, Fig. 5 (not shown). To reduce the effect of impedance mismatching at the sliding joint, the inner conductor 83 ends a quarter of a wave-length short of the outer conductor 69, and the impedance of this quarter-wave portion is the geometric mean of the impedances of the sections 65, 63 and 83, 69, on either side of it. The insulating spacers 75, 84, may have resistive coatings, Fig. 2 (not shown), to prevent reflection. Alternatively an attenuator of rectangular cross-section and capable of telescopic adjustment, Fig. 3, may be coupled to coaxial lines by means of aerials 91, 92, which are parallel to the shorter side of the cross-section. Instead of being electrically coupled by the aerials 64, 65, the wave-guide section 61 may be magnetically coupled to the coaxial lines by loops, Fig. 4 (not-shown) ; and a coaxial line 101, 102, Fig. 6, may be coupled to a waveguide 103 by means of the loop 118 which forms part of an axially slidable section, the latter comprises an inner sleeve 106 pinned to an outer sleeve 112 which is movable by rack and pinion 113, 114, to alter the length of the attenuating wave-guide section 122. The inner sleeve terminates in spring fingers 107, 108 for making electrical contact, and a terminating resistance 120 is provided. The left-hand end 123 of the inner sleeve is reduced in thickness over a length #/4 so that the characteristic impedance in this region is the geometrical mean of those on either side of it, and at the other end a similar effect is obtained by reducing the diameter of the inner sliding conductor at 124. The outgoing wave-guide 103 is terminated by a carbon-rod resistance 126 parallel to the electric vector. This resistance is adjustably positioned at a distance #/4 from the end 104 of the wave-guide.