Single wire antenna and field line detachment

A single wire acts as an antenna when an alternating current flows on it: the charges are continuously accelerated and decelerated, and accelerated charges radiate (l·dI/dt = l·q·dv/dt). A straight wire with steady DC or uniform charge motion does not radiate; a time-varying current, a bend or an open end causes radiation.

Detachment of electric lines of force from a dipole

Consider a small centre-fed dipole driven by a sinusoidal source of period T.

First quarter cycle (0 to T/4):

• Charges flow outward; positive charge builds up on one arm and negative on the other.
• Electric field lines form between them and spread outward, reaching a distance of about λ/4 by t = T/4.

Second quarter cycle (T/4 to T/2):

• The current reverses; the charges move back and neutralize.
• The old field lines, which have already travelled out, lose their ends on the conductor.
• Since there are no charges left to end on, the lines join and close on themselves, forming closed loops.
• At the same time, new lines of opposite direction start forming near the dipole.

Next half cycle: the same process repeats with reversed polarity. The new lines push the closed loops away.

 t = T/4           t = T/2           later
   +  ___             ___             _   _
   | /   \          /     \          ( ) ( )
   ||     |   -->  |  ( )  |  -->     loops travel
   | \___/          \_____/           outward at c
   -              (lines close)

The detached closed loops of E field, with an associated H field, travel outward at the speed of light as free-space waves.

The transmission line guides energy as a TEM wave; at the open end (the dipole arms) the field lines spread out, detach and propagate. The detached fields are the radiation field; fields that stay near the antenna and return energy each cycle are the reactive (induction) field.