Patent GB-682652-A (GB)
Title: Improvements in or relating to modulators employing electromagnetic wave guides
Inventor: STARR ARTHUR TISSO
IPC: H03D9/06
Filed: 1951-02-01
Abstract: 682,652. Modulation systems; generating high-frequency oscillations. STANDARD TELEPHONES & CABLES, Ltd. Feb. 1, 1951, No. 2542/51. Classes 40 (v) and 40 (vi). [Also in Group XL (b)] Relates to micro-wave frequency-changers or modulators of the kind shown in Fig. 1 in which a magic-T wave-guide junction is employed, the microwaves being fed to an input section 6, a signal or second source 20 being connected to vary in opposite senses the admittance of two variable admittances 13, 15 placed near the closed ends of two lateral sections 4, 5 and the resultant waves being taken from the section 7. The invention consists primarily in providing reactance devices in each lateral section between the junction and the variable admittance devices for modifying the effective admittance as seen from the junction. The reactance devices may comprise adjustable screws 23 ... 26 extending through the guide walls on which the electric vectors terminate, and placed centrally of those walls, each pair being spaced a quarter-wave apart from each other and from the variable admittance, which is itself a quarter-wave distant from the closed end of the lateral guide. The reactance devices 23 ... 26 are designed or adjusted in such manner that the reflection coefficients of each lateral section presented to the junction, and corresponding to the two conditions of high and low admittance of the device 13 or 15, are substantially of equal magnitude and phase angles 180 degrees apart. Moreover, the susceptance of each device 13, 15 is so adjusted that the tangents of the phase angles of the reflection coefficients corresponding to the admittance of the device in its two respecconditons are substantially equal; and the transforming network between the device and the T-junction is arranged to multiply the admittance of the device by a real quantity. These conditions are discussed mathematically in the Specification, and are shown to be satisfied by the adjustable screws 23 ... 26. The variable admittance devices 13, 15 may be germanium crystal rectifiers, which may be similarly placed and connected in push-pull to the source 20, or one rectifier may be inverted relatively to the other, with parallel connections to the source. In either of these cases, each rectifier in turn has high admittance whilst the other has low admittance. In a further arrangement, the two lateral guide arms differ in length by an odd number of quarter wavelengths, and both rectifiers are simultaneously in the same condition. If the guide arms differ in length by an odd number of eighths of a wavelength, the source 20 is connected to supply the respective rectifiers in quadrature, a phase-shifter 27 being provided in one of the leads for this purpose. The variable admittance rectifiers may be replaced by triode valves 28, Fig. 4, with their grids coupled to the source 20, their cathodes connected to the guide wall, and their anodes biased positively, the grids and anodes being capacity-coupled to the guide wall by flanges and interposed dielectric washers. A tetrode valve may be similarly arranged, Fig. 5 (not shown). A resonator may be provided for the valve 29 by two equi-distant posts 37, 38 placed centrally on either side of the valve ; or by two pairs of rectangular plates extending towards each other from the shorter sides of the guide, Fig. 6 (not shown). Reflex klystrons may be substituted for the triode or tetrode valves, the source 20 being connected in push-pull to their reflecting electrodes, and the wave-guide forming the resonator; the adjustment being such that the klystrons do not oscillate. Specification 260,067, [Class 40 (v)], is referred to.