GB591765A - Modulator system - Google Patents

Abstract

591,765. Electric transmission lines ; modulating. STANDARD TELEPHONES & CABLES, Ltd. Dec. 3, 1943, No. 20394. Convention date, Aug. 6, 1942. [Classes 40 (iii) and 40 (v)] Relates to modulating devices of the kind described in Specification 540,871 for radio beacons, in which a rotating slotted condenser plate 40 varies the resonance frequency of a quarter-wave-length section 20, 21 which is electrostatically coupled to a transmission line 10, 11 so as .to block the transmission of current of the resonance frequency in the manner described in Specification 517,935, and comprises means for broadening the responses so that the blocking effect may persist in spite of frequency-drift or of modulation of the kind described in Specification 563,076. The production of standing waves between the source and the coupled section is eliminated when the coupled section is detuned. To broaden the response, the coupling between the quarterwave-length section 20, 21 and the line 10, 11 is increased, without a dangerous decrease in the spacing between them, by enlarging the conductors 20, 21 and so shaping them, for instance as hemi-cylinders, Figs. 1, 2, or as quarter-cylinders or flat strips (Figs. 3, 4, not shown) that they cut an extended area of the electrostatic field and so increase the capacity between the coupled section and the line. In order that the coupled section may not increase the sending-end impedance by setting up standing waves when the coupled section is detuned, the coupled section is half a wave-length long, but one quarter-wavelength is open-circuited at one end 30, 31 and short-circuited at the other by a bar 32, while the other quarterwave length is short-circuited throughout at intervals as by bars 34, 36. Wave-reflections in the line 10, 11, due to change of impedance adjacent to the ends of the coupled section cancel one another when the section is detuned so that standing waves are avoided, while if the section is resonant to the transmitted current the two quarter-wave-lengths have dissimilar effects and transmission is blocked. The section 20, 21 may be mounted so that it can be adjusted in relation to the line 10, 11 by means of screws 28, 29, Fig. 2. The inner surfaces of the section may be provided with a material having a large dielectric constant.