Methods of Measuring the Insulation of Telephone Lines at High Frequencies

Abstract

Synopsis-This paper outlines the problem of measuring the The paper includes a description of a special line which has been insulation of open-wire telephone circuits in the frequency range constructed for the testing of insulators, an explanation of the essenfrom 3000 to 50,000 cycles, and discusses a method which has been tials of the measuring technique, and a brief summary of the results used in the experimental study of insulator losses at these frequencies. which have been obtained. M/[EASUREMENTS involving the transmission of their magnitude. These methods and apparatus, and high-frequency currents over open-wire teletheir application in1practise, form the subject of this phone lines began in the Bell System about 10 paper. years ago, as a preliminary to the application of the THEORY OF LEAKAGE MEASUREMENT first carrier telephone and telegraph systems.2 Since that time, more or less continuous study has been given Transmission over wires at high frequencies is .. . . . . . . . . . . . . accomplished in precisely the same ma-nner as transto the different problems involved in the transmission m pise in precise thesame anner as trn over line circuits of carrier frequencies ranging from m a about 3000 to 50,000 cycles. From the beginning it was guiding medium for the energy in both cases, andithe abot 30 to csame fundamental equations may be applied to both. apparent that the attenuation of open-wire line circuits at these higher frequencies iS very much greater than inTertmfatnainfrauiomln ici tthesehiger frequencies,iavr uch egrea t in which is terminated so as to avoid reflection effects may, thvoice range of frequencies, and undergoes wide therefore, be determined by means of familiar transvariations due to changing weather conditions. Inas-m f T . .~~~mission formulas. The attenuation constant ae determuch as the attenuation is one of the most important ... . . . mines the decrease in magnitude of the voltage and factors in high-frequency transmission; its investigation y~~~~u rrent transmitted over the circuit, according to the has been very actively prosecuted along both theoretical cenatran and experimental lines. equations The fundamental problem which originally presented E2 I2 la itself was that of segregating the different losses which E = I I ( are experienced by the high-frequency energy trans-mitted over an open-wire circuit. It was obvious that a where 1 and Io are the voltage and current at the substantial part of the increased attenuation at high sending end of a section of the circuit and F2 and 12 are frequencies resulted from the increase in wire resistance the voltage and current at a point distant 1 units from due to skin effect, but it was equally obvious that other the sending end. sources of loss were also contributing in large measureThe value of the attenuation constant at any freto the attenuation. It was known that radiation was a quency may be derived from the so called "primary negligible factor in the line losses. It was known also constants" of the circuit, which are as follow...