Standing Waves

Standing or stationary waves are usually waves that interfere with each other but occur at a frequency that creates an alternating waveform of constructive and destructive interference. Furthermore, it can actually be demonstrated by a rope attached to the wall. If a person is to send a pulse to the wall and the pulse is large enough, it will be seen that a reflected pulse can be observed to occur along the rope. As the incident pulse is developed along the line at a higher frequency, a unique waveform will be observed that is, the wave will appear to be not moving that is the crests and troughs of the waveform will not flow as in a regular sinusoidal waveform. These are stationary waves where the highest parts are called antinodes and the lowest points called nodes.

Standing waves are also present in transmission lines where characteristic impedance of a transmission line is not matched with the load impedance. When mismatch occurs, some of the power is reflected back to the load and the voltage also travels back along the line. Since transmission lines are very long cables, these incident and reflected voltages interfere with each other and standing waves occur inside the transmission line. The degree of the standing waves that occur in a transmission line is characterized by coefficient called standing wave ratio or SWR. The ideal SWR is equal to one. A higher SWR is not good as this equates to power losses along the transmission line. The equation for SWR is equal to the maximum voltage of the standing wave developed divided by its minimum voltage.

Posted 2011-01-29 and updated on Jan 29, 2011 8:39pm by crisd

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