Standing Waves

Author: J.B. Hoag

An electric light bulb is designed to radiate as much light as possible, with a minimum loss of energy in other forms.

A flatiron is built to give out heat and not light. An antenna is used to radiate as much energy as possible in the form of electromagnetic waves of radio frequency. If the fields from one part of a resonant circuit overlap those from another part they may well cancel each other. It is necessary to spread the circuit out in space if the radiation is to be appreciable. Such an " open circuit " is called an antenna.

The radiation of energy is greatest from those parts of an a.c. circuit where the current is greatest. The current can be increased at certain parts of a circuit by making its dimensions comparable to the operating wave-length. When this is done, standing waves of current and of voltage are established in the circuit. This is analogous to the case of sound waves moving back and forth in a pipe where, by suitable choice of the length of the pipe and the length of the sound waves, it is possible to build up large volumes of sound from a feeble source. We shall now see how standing waves can be produced electrically.