GB214249A - Improvements in transmission systems - Google Patents

Abstract

214,249. Western Electric Co., Ltd., (Assignees of Hoyt, R. S.). April 9, 1923, [Convention date]. Artificial lines and balancing networks.-Relates to the construction of a network to simulate the impedance of a transmission line over a wide range of frequencies, for balancing purposes in telephonic repeaters or for properly terminating a telephone line. The general equation for the impedance K of a line in terms of the resistance R, inductance L, capacity C and leakance G is the leakance being comparatively unimportant except at low frequencies. Expressions for the resistance and reactance components M, N of the impedance K are obtained to various degrees of approximation. In the case of open-wire lines, R is of secondary importance, and a rough value for K is k=(L/C)<¢>, which is termed the " normal impedance." The quantities (K-k) and E/k are termed the " excess impedance " and " relative impedance." The simplest network according to the invention is a mere resistance R, equal to the nominal impedance k, but over the voice frequency range this must be supplemented by an " excess-simulator " whose impedance J is approximately equal to the excess impedance (K-k). For large gauge open-wire lines, the excesssimulator may be a capacity C, equal to 2(LC)<¢>/R, but even then is inadequate at low frequencies. More accurate values for R<1>, C<1> are obtained by multiplying by x, the real part of the relative impedance K/k, which, if leakance can be neglected, is approximately independent of the frequency and is slightly greater than unity. Figs. 11<a> and 11<b> show in series with the resistance R equivalent excess-simulators which may be proportioned to give fairly precise results over a given frequency range by tentative design, which may be considerably reduced by a semigraphical method. If J=P+iQ, this consists in plotting curves between P, (M-k) and the frequency and between Q, N and the frequency, using as parameters arbitrary functions of the elements of the excess-simulator, and selecting those curves which most nearly coincide over the required range. The values' of the elements of the excess-simulator are then determined from the specific values of the parameters obtained. Figs. 11<c> and 11<d> show networks equivalent to those of Figs. 11<a> and 11<b>. At very low frequencies, the sending end impedance of a line is affected by the distant terminating impedance. The simulating network is then modified by shunting the excess-simulator by a resistance S' approximately equal to the zero-frequency sending-end resistance diminished by R', and this entails slight alteration in the proportioning of the network. The networks of Figs. 14a and 14b are derived from those of Figs. 11<a> and 11<b>, and Figs. 14<c> and 14d show equivalent networks. For lines whose leakance is not quite negligible, this may be taken into account by a slight reproportioning of the network, with the addition of a small series inductance in those cases where the leakance increases rapidly with the frequency.