___________________________________________________ Principles of Transmission Lines
Voltage Standing-Wave Ratio
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.
Power Standing-Wave Ratio
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.
Current Standing-Wave Ratio
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.
SUMMARY
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.
3-47
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