Patent GB-457911-A (GB)
Title: Electrical impedance networks
IPC: H01Q5/00
Filed: 1936-03-13
Abstract: 457,911. Impedance networks. STANDARD TELEPHONES & CABLES, Ltd., 63, Aldwych, London.-(Assignees of Alford, A.) March 13, 1936, No. 7648. Convention date, March 22, 1935. [Classes 40 (iii) and 40 (v)] A transmission line conveying two or more high-frequency oscillations from a transmitter is matched to an aerial for each of the frequencies by means of shunt networks which are permanently connected to the line. A transmitter TR, Fig. 2, feeds an aerial AE through a transmission line T, and the impedance of the line has to be matched to that of the aerial for two different wave-lengths #1 and #2. It is shown that if D1H1=¢#1 and B1D1 = #1, then the impedance of the network N1 measured at A1B1 is infinite for wavelength #1, whatever the length D1F1 may be, so that for that wave-length the network N1 has no effect; but the length D1F1 is so chosen that the transmission line is matched to the aerial for the wave-length #2. Similarly the network N2 is so dimensioned that it has no effect for wave-length #2 but effects the required match for wave-length #1. In the absence of the networks N1, N2, standing waves would be formed, and the Specification shows how to calculate (1) the correct distances of A1B1 and A2B2 from corresponding current nodes and (2) the correct values of D1F1 and D2F2. These quantities are given in terms of the ratio of the current at a node to that at an antinode, and of the surge impedance of the line. The pairs of wires shown in Fig. 2 may be replaced by pairs of coaxial conductors, Fig. 3 (not shown). The end G1H1, Fig. 2, may be open-circuited, in which case the length C1G1 is #1/4, Fig. 4 (not shown). An analogous arrangement in which coaxial tubes are used is shown in Fig. 5, the end F1 being short-circuited while the end G1H1 is opencircuited. Various other combinations of open-circuited and short-circuited ends may be adopted, Figs. 6 to 9 (not shown). The arrangement shown in Fig. 2, may be simplified by omission of the section C1G1H1D1, Fig. 10 (not shown). Instead of the transmitter TR, Fig. 2, there may be a forked network 50, 51, Fig. 11, whereby the transmitters TR1, TR2 are separated. The loop V is tuned to ¢#2 and short-circuits the wave-length #2 coming from TR2, while the loop VI is so calculated as to restore the impedance-matching with respect to wave-length #1, which is disturbed by the loop V. The loops III, IV perform similar functions for #1, #2 respectively. Alternatively the networks V, VI may be replaced by a single network similar to N 1, Fig. 2, and the networks III, IV, may be similarly replaced, Fig. 11a (not shown). The system shown in Fig. 11 can be extended so as to accommodate three transmitters, Fig. 12 (not shown). Alternatively, for dealing with a set of three frequencies, three networks N1, N2, N3 may be provided, each calculated to match the line to the aerial at one of the frequencies while it has infinite impedance (and so has no effect on the system) at the other two frequencies. The method of dimensioning the networks N1, N2, N3 for this purpose is described in the Specification Fig. 13 (not shown). Alternatively one such network N3 may be used, network N1 being replaced by a single loop and network N2 by a loop of the type shown in Fig. 2 (Fig. 15, not shown). An alternative form of network which has infinite impedance for two frequencies and matches the line to the aerial for a third frequency is shown in Fig. 17. The length MN is ¢#2 so that there is effectively a short-circuit for #2 at M. The length LM is #2, so that the impedance for #2 is infinite at L. The length OP us ¢#1, so that there is an effective short-circuit for #1 at O, and the length MO is calculated as to give infinite impedance for #1, at L whatever the length of OQ. The latter is calculated so that the line when shunted by the network is matched to the aerial for #3. The principle may be extended to provide for four different wavelengths, Fig. 18 (not shown).