3-148. The ratio of maximum voltage to minimum voltage on a line is called

the voltage standing-wave ratio (VSWR). Therefore--

Emax

VSWR =

Emin

3-149. The vertical lines in the formula indicate that the enclosed quantities

are absolute and that the two values are taken without regard to polarity.

Depending on the nature of the standing waves, the numerical value of

VSWR ranges from a value of 1 (ZL = Z0, no standing waves) to an infinite

value for theoretically complete reflection. Because there is always a small

loss on a line, the minimum voltage is never zero and the VSWR is always

some finite value. However, if the VSWR is to be a useful quantity, the power

losses along the line must be small in comparison to the transmitted power.

3-150. The square of the voltage standing-wave ratio is called the power

standing-wave ratio (PSWR). Therefore--

Pmax

PSWR =

Pmin

3-151. This ratio is useful because the instruments used to detect standing

waves react to the square of the voltage. Because power is proportional to the

square of the voltage, the ratio of the square of the maximum and minimum

voltages is called the PSWR. In a sense, the name is misleading because the

power along a transmission line does not vary.

3-152. The ratio of maximum to minimum current along a transmission line

is called current standing-wave ratio (ISWR). Therefore--

Imax

ISWR =

Imin

3-153. This ratio is the same as that for voltages. It can be used where

measurements are made with loops that sample the magnetic field along a

line. It gives the same results as VSWR measurements.

This chapter has presented information on the characteristics of transmission

lines. The information that follows summarizes the important points of this

chapter.

Transmission lines are devices for guiding electrical energy from one point to

another.

Input impedance is the ratio of voltage to current at the input end of a

transmission line.

Output impedance is the ratio of voltage to current at the output end of the

line.

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