Standing waves:

If load is not matched to line, the current and voltage vary in the complicated way along the length of line. In some places, current is high; in other places it is low. The maxima and minima are known as loops and nodes respectively. At the current loop, the voltage is minimum, and at a current node, the voltage is maximum. Loops and nodes make it impossible to measure power by voltmeter/ammeter method, as the current and voltage are not in the constant proportion.

The loops and nodes, if graphed, form the wavelike patterns along length of the line. These patterns remain fixed over the time. They are thus known as standing waves-they just stand there.

Standing-wave loss:

At the current loops, the loss in line conductors can be exaggerated. At the voltage loops, the loss in line dielectric is increased. At the minima, the losses are decreased.  But overall, in the mismatched line, the losses are greater than they are in the perfectly matched line.

This loss takes place at the heat dissipation. It is the true power. Any true power which goes into heating up the transmission line is wasted, as it cannot be dissipated in the load. The additional loss created by standing waves, over and above the matched line loss, is called as standing-wave loss.

I The greater mismatch, the more severe the standing wave loss becomes. The more loss a line has to begin with, the more loss is caused by a given amount of mismatch. Standing-wave loss increases with the frequency. It tends to be worst in the long lengths of line at VHF, UHF, and microwaves.