Transmission Lines

The preceding chapter considered the transmission of electric-signal energy through networks in which the elements were lumped; that is, existed as individual units of appreciable magnitude. This chapter will consider the transmission of electric-signal energy through open-wire lines in which the resistance, inductance, capacitance, and shunt conductance are uniformly distributed, rather than lumped.

An open-wire circuit is defined¹ as "a circuit made up of conductors separately supported on insulators." The open-wire telephone line is an example. Such a line is a uniform line, defined¹ as "a line that has identical electrical properties throughout its length." For convenience these will be called transmission lines, or merely lines.

These lines have linear electrical constants, defined¹ as "the series resistance, series inductance, shunt conductance, and shunt capacitance per unit length of line." Sometimes one overhead wire and earth return are used, constituting a ground-return circuit.¹ Only metallic circuits, in which the ground or earth forms no part,¹ will be considered in this chapter.

The range of frequencies transmitted for communication purposes over such lines is great. The upper limit of the frequencies used in telegraph systems is several hundred cycles. For voice-frequency telephone purposes, the band is from about 200 to 3500 cycles. Radiobroadcast program lines for amplitude-modulated broadcasting usually transmit a band from about 100 to 5000 cycles; for frequency-modulation, a band from about 30 to 15,000 cycles is sometimes used. Multichannel carrier-telephone circuits use frequencies up to 150,000 cycles. Radio frequencies up to, perhaps, one billion cycles are sometimes used (if only experimentally) on open-wire transmission lines.

The power levels, defined¹ as "an expression of the power being transmitted past any point in a system," of the signals on communication transmission lines cover a wide range of values. For telephone transmission the level is a few milliwatts or less. On radio transmission lines, such as are used to connect a radio transmitter to its sending antenna, the power level may be many kilowatts.

In the transmission of speech and programs, the distortion caused by the lines and associated equipment must be kept low. If appreciable frequency distortion exists, then the various frequency components of a complex speech or program signal wave will not be received in the same relative magnitude that they had at the sending end. If non-linear distortion is not minimized, then frequency components that were not present in the original signal will exist at the receiving end. Delay distortion will cause a shift in the relative phase positions of the various components because all frequency components are not transmitted with the same velocity. In telephony, some distortion can be tolerated, because reliability, intelligibility, and a certain degree of naturalness, rather than high quality, are the criteria of good service.

Transmission lines used in communication are electrically long lines. For a line to be electrically long, both the physical length of the line and the frequency of the signal wave being transmitted must be sufficiently great so that a considerable fraction of a wavelength exists on the line. Then, the instantaneous current at one point in the line is not the same as the instantaneous current at another point, and the instantaneous voltage between the wires at one point is not the same as that at another point. Simple electrical theory, such as is used with many electrical circuits, is not sufficient for studying communication transmission lines. Many 60-cycle power lines are classed as electrically short lines, because, so far as the fundamental frequency is concerned, they do not have an appreciable portion of a wavelength on them.

Power efficiency is of secondary consequence in telephone transmission because only a few milliwatts of power are contained in the signal wave and because line losses can be offset by the use of amplifiers. In a radio-frequency transmission line power efficiency may be of importance, because large amounts of power may be involved.