GB455492A - Improvements in or relating to electric signal transmission lines - Google Patents

Abstract

455,492. Loading. BLUMLEIN, A. D., 32, Audley Road, Ealing, London, and HARDWICK, J., 21, Drayton Park Avenue, West Drayton, Middlesex. March 7, 1935, Nos. 7206 and 32791. [Class 40 (iv)] A transmission line is corrected for variations of wave velocity and attenuation at the low and high frequencies of a wide band by the insertion of spaced loading networks which modify the constants of the line for those frequencies. In Fig. 2, the constants of the line are represented by the elements in the dotted rectangle, and it is shown that the correction for low frequencies may be obtained by the addition of series capacity C<1> equal to 4L/R<2>, these quantities being per unit length, or, if the leakance G is not negligible, by series capacity C<1>= C / (R/2 #C/L - G/2 #L/C)<2> shunted by resistance G<1>=GC<1>/C. At high frequencies, due to the skin effect, the line is represented by the elements in the dotted rectangle in Fig. 3, and the correction for these frequencies is obtained by the addition of series resistancecondenser networks R1, C1, &c., where C1L1 = R12 &c. The loadings should be so spaced that the added impedance is small compared with the characteristic impedance at a frequency corresponding to a wave length of four times the loading distance. Corrections may be made for capacities of the networks to earth and temperature effects. Dielectric loss at very high frequencies may be allowed for by considering it as an increase of effective series resistance. Instead of the series resistancecondenser networks of Figs. 2, 3, equivalent shunt resistance-inductance networks as shown in Figs. 4, 5 may be employed, or the correction for low or high frequencies may be made partly by one form of network and partly by the other form. The loading distance for resistanceinductance loading should be small compared with the wavelength corresponding to a frequency A /B, A being a loading resistance and B the associated inductance. If the high frequency correction is such that GL=RC, no correction for low frequencies is necessary. As shown in Fig. 1, the corrected line is fed through a coupling condenser 6 connected to one wire, the other wire being earthed. The termination comprises condensers 11, 12 having a capacity equal to the equivalent capacity of the line and a series resistance 13 equal to the high frequency surge impedance. In the modified termination shown in Fig. 6, an autotransformer 17 is employed and the resistance 13 is shunted by a condenser 18, so that a voltage step-up for the higher frequencies is obtained. A similar auto-transformer may be inserted at the sending end to give a voltage step-down for the higher frequencies. In a cable of the concentric type, the loading is preferably applied only to the central conductor. In a balanced line, the series loads are inserted in each wire. This applies also to a line comprising three wires, two of which are used as a side circuit and as one leg of a phantom circuit completed by the third wire.