STANDING WAVES

When a transmission line is not correctly terminated ,the travelling wave at the receiving end is reflected completely or partially at the termination. The combination of incident and reflected waves give rise to standing waves of current and voltage along the line, with definite maxima and minima of current and voltage along transmission line.

At some points , the incident and reflected signals are in phase and both the components add together and at these points voltage and current will be maximum . on the other hand,at some other points ,the two components may oppose each other and at these points voltage and current will be minimum.

The resultant graphical profile of both these incidents and reflected waves is called standing waves.

The points where the resultant signal (voltage or current) is maximum are known as voltage or current maxima.On the other hand, the points where the resultant signal (voltage or current) is minimum are called voltage or current minima. Thus

|V_max|=|V_i|+|V_r|

|V_min|=|V_i|-|V_r|

|I_max|=|I_i|-|I_r|

The magnitude of standing waves provides an idea of the amount of reflection.

Standing Wave Ratio

The ratio of the maximum and minimum magnitudes of current or voltage on a line having standing waves is called the standing wave ratio.

SWR=|V_max|/|V_min|

SWR=|I_max|/|I_min|

Voltage standing wave ratio:VSWR

The ratio of maximum and minimum magnitude of voltage on a line having on a line having standing waves is called voltage standing wave ratio.

Therefore                         VSWR=|V_max|/|V_min|

Generally , VSWR is always greater than 1 and when it is equal to 1,the line is correctly terminated and there is no reflection.

Current Standing Wave Ratio: CSWR

The ratio of maximum and minimum magnitude of current on a line having standing waves is called current standing wave ratio.

Therefore                                         CSWR=|I_max|/|I_min|

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