How do antennas radiate?
Antennas radiate because a time-varying current, which is accelerating charge, produces electric and magnetic fields whose 1/r components carry power away from the source indefinitely. The radiated power appears at the antenna terminals as a radiation resistance, and structures comparable to a half wavelength radiate efficiently because their currents add in phase over a large fraction of a wavelength.[1][2]
Why does a time-varying current radiate?
Maxwell’s equations show that a static charge produces only an electric field and a steady current only a magnetic field; neither carries power to infinity. When current changes in time, the fields contain terms that decay as 1/r. Their cross product, the Poynting vector, decays as 1/r², so the power crossing any sphere around the source is constant: energy has left the antenna.[1] Balanis summarizes the requirement as: to create radiation there must be a time-varying current or an acceleration (or deceleration) of charge.
How is radiation calculated?
The standard procedure is to find the current distribution J on the antenna, compute the magnetic vector potential A = (μ/4π)∫J e−jkR/R dV′, derive H = (1/μ)∇×A and E from Maxwell’s equations, then simplify in the far field, where E and H are transverse, in phase and related by the free-space impedance η0 ≈ 376.7 Ω.[1] Radiated power is Prad = ∮ (1/2)Re(E×H*)·dS.
What is radiation resistance?
Radiation resistance Rr is the resistance that, carrying the antenna terminal current, would dissipate the radiated power: Prad = ½|I|²Rr. A short dipole has Rr = 80π²(l/λ)², about 0.79 Ω for l = λ/10, whereas a half-wave dipole has about 73 Ω.[1] Small Rr means that ohmic loss resistance competes with radiation, which is why electrically small antennas tend to be inefficient.
Why does antenna size relative to wavelength matter?
Currents on an antenna much shorter than a wavelength produce fields that nearly cancel at a distance and store much more reactive energy than they radiate, giving high Q, low radiation resistance and narrow bandwidth. Once the structure spans about half a wavelength, current elements add constructively and the antenna becomes resonant with a moderate, mostly resistive impedance.[2] See the Chu limit.
Frequently asked questions
Does DC current radiate?
No. Steady current produces only a static magnetic field; radiation requires time variation.
What is the impedance of free space?
η0 = √(μ0/ε0) ≈ 376.73 Ω, often approximated as 120π Ω.
Do fields detach from the antenna?
Yes; field-line plots of a dipole show closed loops of electric field forming and propagating outward each half cycle.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “Antenna Theory and Design, 3rd ed.”. W. L. Stutzman and G. A. Thiele, Wiley. 2012.