71083

Voltage Standing Wave Ratio

Wc now begin thc study of a parameter, namcly thc voltage standing wave ratio, of considerablc

thcoretical and practical importancc in transmission linę theory. Its theoretical importance lies in thc fact that it sharply illustrates thc complexity of thc variation and total voltagc and currcnt along thc transmission linc. Its practical importancc lies in thc fact that it is in fact a parameter which is most amcnablc to accuratc cxperimcntal mcasurcment in determining thc impedance conditions which apply on a linc.

It may be worth explaining herc just why it is that measurcments of total voltagc and current in distributed systems is not easy. This diff iculty is related to thc fact that,

as we have already leamed, interconnccting wires have both distributed inductancc and capacitance, and thc very conncctions between points on a transmission linc and a test instrument complicate thc determination of what is happening on thc linc in two ways. Firstly there is thc usual phenomenon that a test instrument places a load across thc Circuit bcing measured. and in so doing disturbs thc condition on thc Circuit so that wc arc no longcr determining what wc had wished to determine, which is thc conditions on

thc undisturbed Circuit. Sccondly. thc disturbancc to thc Circuit is not easily predictable,

because even if we know thc load presented by the measuring instrument at its input terminals, thc load becomes transformed by the interconnecting wires to a different value

at thc point whcrc they arc applied to the Circuit. Thirdly, thc voltagc at thc conncction point becomes transformed along thc interconnccting wires so thc voltagc at thc input terminals to the measuring instrument is different from that applying on the linc. Fortunatcly thc normal method of mcasurcment of voltage standing ratio is frcc of all of these dif ficulties.