GB358917A - Improvements in or relating to high-frequency transmission systems - Google Patents

Abstract

358,917. Wireless and wired-wireless transmitting and receiving systems. STANDARD TELEPHONES & CABLES, Ltd., Columbia House, Aldwych, London.- (Ullrich, E. H. and Fremery, F. de ; 46, Avenue de Breteuil, Paris.) July 14, 1930, No. 21260. (Class 40 (v).] An arrangement for varying within wide limits the transmission efficiency of a high-frequency line comprises an auxiliary line associated therewith, the length of which is commensurate with the wavelength of the transmitted oscillations and the terminating impedance of which can be varied. In one embodiment, at the point B, Fig. 1, of a transmission line TL connecting devices D<1>, D" is bridged an auxiliary line TL1 terminating in a variable impedance constituted by a valve V. By proper impedance matching, standing waves are prevented on the line TL and the length of TL1 is an odd number of quarter wave-lengths. When the relay R is energized the grid bias potential of the valve V is changed and hence its impedance. The arrangement of resistances R1, R2 prevents a short-circuit of the grid bias battery if relay R makes before it breaks, and the bias is altered in two stages. The system virtually operates as a short-circuit at B which can be removed at will, without performing any direct switching in the line TL. In Fig. 2, the line TL1 is of adjustable useful length, switches K1, K2, K3 being shunted across it at different, distances from the points A, B. If the line TL is carrying a frequency F , its transmission efficiency may be varied between wide limits by switch K1 which is located an even number of quarter wavelengths from AB. For a frequency F2. switch K1 is left open and K2 operated which is located an exact number of half wavelenghts from AB, the switch K3, which is an odd number of quarter wavelengths from AB being left closed. The apparent capacity of the switch contacts and the voltage across them may be reduced by locating them the secondary circuit of a transformer T, Fig. 4, in the primary of which is tuned and bridged across the line. The switch contacts are here shown operated by relay coil R. The contacts may alternatively be located in series in the line, preferably at current nodes. An alternative method of lowering the voltage on the relay contacts comprises inserting a portion of transmission line TL<11>, Fig. 5, into the line TL of a different impedance such that standing waves are set up on it, for example by making TL<11> an integral number of half wavelengths long and terminating TL in the same way as if TL<11> did have the same characteristic impedance. The relay contacts are bridged across a potential node B of the line TL<11> indicated in the voltage curve C. This may be the arrangement at the point B in the two-way system of Fig. 3 to be described later. When the frequency handled by a line is high, the length of the latter may be reduced by placing reflectors, constituted by slidable metallic discs, at suitable points on the wires. In Fig. 3, a single transmission channel A is interconnected with a high frequency transmitter RT and receiver RR associated with a carrier suppressor CS, operators' position TOP and signalling apparatus S. The transmission line TL connects the transmitter RT with the channel A which may be an aerial, and at the point P is bridged an auxiliary line TL1, short circuited at D so that standing waves are set up in it and the bridging loss at P is low. At the point Q in line TL1 which is a potential node a further line TL<1> is bridged leading to the receiver RR. Relays R1, R2 control contacts bridged at B, D1 respectively in the lines TL, TL<1> respectively and are operated from the carrier suppressor CS, when transmission ceases, their function being to " isolate '' the transmitter RT from the aerial A, and to render the line TL1 to the receiver " conducting."