Antenna definition and single wire radiation conditions

An antenna is a metallic structure (wire, rod, horn, dish) that converts guided electromagnetic energy from a transmission line into free-space waves, and vice versa. IEEE defines it as "a means for radiating or receiving radio waves".

Radiation mechanism in a single wire

Radiation is caused by time-varying current, i.e. by accelerated or decelerated charges. For a wire of length l carrying charge density q (C/m) moving with velocity v:

I = q · v
dI/dt = q · dv/dt
l · dI/dt = l · q · dv/dt      (basic radiation equation)

So a time-changing current (left side) is the same as accelerated charge (right side). Both mean radiation.

Conditions for radiation from a single wire

1. No charge motion: no current, no radiation.
2. Charge moving with uniform velocity:
   • on a straight, infinitely long, smooth wire – no radiation;
   • on a wire that is curved, bent, discontinuous, terminated or truncated – radiation occurs, because the charge changes direction or speed there.
3. Charge oscillating in time (time-harmonic current), even on a straight wire – radiation occurs.

uniform v, straight  ------->   no radiation
bent wire            ---->\     radiation at bend
                           \
end of wire          ------>|   radiation at end
oscillating current  <-> <->    radiation all along

So a practical antenna is fed with an alternating current; the charges accelerate continuously, and the wire radiates EM waves. Radiation is strongest for wire lengths comparable to the wavelength (e.g. λ/2 dipole).