How Antennas Radiate: Fields, Dipoles and ½ Wavelength
See how a radio wave is created from changing electric and magnetic fields, what a dipole antenna is, and why a half-wavelength dipole is resonant, with an animated field simulation.
How to use this tool controls and what to look for
Watch a dipole launch a radio wave, then change its length and see why about half a wavelength works best.
Controls to change
- Electric field: show or hide the electric and the magnetic field.
- 3D model: switch to 3D. Use Turn and Tilt to rotate it, and Reset view to go back.
- Speed: choose Slow, Normal or Fast, or Pause and Step through it.
- Presets: compare a short dipole (0.1 λ), the resonant ½ λ, and longer ones up to 2 λ.
- Real size at: see the real length at 2.4, 5 or 6 GHz.
What to look for
- Close to the antenna the fields are out of step (near field). Further out they fall into step and travel as a wave (far field).
- Near ½ λ the feed resistance is about 73 Ω, and at about 0.48 λ the reactance reaches zero: the dipole is resonant.
- Longer dipoles split the pattern into several lobes, and nothing radiates off the ends of the wire.
- A higher-gain omni isn't stronger, it's flatter: the same power is squeezed into a thinner doughnut.
About this tool
A radio wave is an electric field and a magnetic field that keep creating each other as they travel. This tool animates a dipole antenna launching a wave, drawn from the exact solution for an oscillating dipole, then shows the travelling wave on its own. Change the dipole's length to see the current along it, its radiation pattern, its feed impedance and match to 50 Ω cable, and why about half a wavelength is the classic choice.
What you'll learn
- How a changing current creates changing magnetic and electric fields that detach and travel as a wave
- What the near field and far field are, and why the fields fall into step far from the antenna
- Why a dipole of about ½ wavelength is resonant, with about 73 Ω feed resistance and 2.15 dBi gain
- Why Wi-Fi omni antennas have a doughnut pattern, and why higher gain means a flatter doughnut
Common questions
Why is a half-wave dipole the standard antenna?
At about half a wavelength the dipole is resonant: its feed impedance is almost purely resistive (about 65–73 Ω), close to 50 or 75 Ω cable, so nearly all the power is radiated. It has one clean lobe and is a convenient size.
How long is a dipole for Wi-Fi?
About 6 cm in total at 2.4 GHz, 2.7 cm at 5 GHz and 2.3 cm at 6 GHz. Real antennas are trimmed slightly shorter than exactly half a wavelength, to about 0.47–0.48 λ.
What does 2.15 dBi mean?
A half-wave dipole's strongest direction is 2.15 dB stronger than an ideal isotropic antenna that radiates equally in every direction. The extra comes from sending less up and down, not from more power.
Free for private and educational use. This interactive model is a simplified teaching aid and may contain mistakes. It is provided as is, without warranty. Use it to understand the concepts, not as a substitute for a site survey, vendor documentation or professional design advice.