Basic Radio is a free introductory textbook on electronics based on tubes. See the editorial for more information....

Author: J.B. Hoag

When an alternating current starts to flow through a conductor, its electromagnetic field builds up, with the lines of force in a certain direction, until the current has reached its maximum. Then, as the current decreases, the field continues to collapse back into the wire until it is zero at the end of the first half-cycle. Then the field builds up again, but with the lines of force pointed in the opposite direction,

However, not all of the energy returns to the conductor each time the field collapses. Just why this is so is not known. The percentage of energy which dissociates itself from the original field is very small when the frequency of the alternating current is low. It increases more and more as the alternations of current take place in shorter and shorter intervals of time. One of the important functions of a radio transmitter is to generate currents of sufficiently high frequency so that significant amounts of energy are dissociated from the original fields. The portion of the field which returns to the conductor is called the induction field and that portion which is freed is called the radiation field.

 Fig. 8 A. The electric and magnetic fields of a radio wave are at right angles to each other and also at right angles to the direction in which energy is propagated

The radiation field travels away from the conductor with the speed of light. This is symbolized by c and is equal to 3 · 1010 cms. per second, = 3 · 108 meters per second, = 3 · 105 = 300,000 kilometers per second, or is equal to 186,000 miles per second. Fig. 8 A shows the relationships between the electric and magnetic fields and the direction in which energy is propagated.

Last Update: 2009-11-01