How Antennas Radiate: Fields, Dipoles and ½ Wavelength
A radio wave is an electric field and a magnetic field that keep creating each other as they travel. Watch a dipole antenna launch one, see the wave itself, then change the dipole's length to see why half a wavelength is the classic choice.
Push an alternating current into a wire and two things happen around it. Moving charges (current) create a magnetic field, and charges piling up at the ends create an electric field. Because the current keeps changing, the fields keep changing, and a changing magnetic field creates an electric field and vice versa. Close to the wire the fields mostly slosh back and forth, but a little further out they detach and carry energy away as a radio wave, travelling at the speed of light.
1 · Watch a dipole launch a wave
A small dipole seen from the side, with the electric field lines drawn from the exact solution for an oscillating dipole. Each half cycle, a loop of electric field pinches off and travels outward. The magnetic field circles the wire, so here it points into (⊗) or out of (⊙) the screen. Tick 3D model to see the same fields all around the antenna: the loops in slices around it and the magnetic field as rings.
Speed
Electric field lines (two shades: opposite directions)⊙ ⊗Magnetic field out of / into the screen (size = strength)Magnetic field rings around the antenna (thick = strong; two shades = opposite directions)Drag the picture to turn it+ charge− chargeNear field
2 · The wave on its own
Far from the antenna, the electric and magnetic fields rise and fall together, at right angles to each other and to the direction of travel. A vertical dipole sends a wave whose electric field is vertical: vertical polarisation.
Band
3 · The dipole: why ½ wavelength?
A dipole is two straight conductors fed at the middle. Change its length (measured in wavelengths, λ) and watch the current along it, the radiation pattern and the impedance the radio sees. Thin wire, in free space.
Presets
Real size at
The antenna and its current
Radiation pattern (side view)
Impedance at the feed point
Resistance R: power that leaves as radio wavesReactance X: energy that only sloshes back and forth● resonance: X = 0Near 1 λ and 2 λ both shoot off the chart: the feed sits where the current is almost zero.
4 · Dipoles in Wi-Fi
The "omni" antennas on access points and routers are dipoles or close relatives. Standing upright, a dipole radiates in all directions around it but not along its own axis, which gives the familiar doughnut.
A ceiling AP's doughnut
Gain is reshaping, not extra power
Every pattern above sends out the same total power. A dipole's 2.15 dBi comes from sending less up and down and more sideways; a "high-gain" omni squeezes the doughnut flatter still, which helps reach far across a floor but leaves a bigger weak spot below a ceiling AP. See High Ceilings: Omni vs Directional.
The push that electric charges feel. It points from positive towards negative charge; on the antenna it stretches between the charged arms. Measured in volts per metre.
Changing electric and magnetic fields that keep creating each other and travel at the speed of light. Radio, Wi-Fi, light and X-rays are all electromagnetic waves at different frequencies.
Frequency is how many cycles pass per second (2.4 GHz = 2.4 billion). Wavelength is the distance of one cycle: the speed of light divided by the frequency, about 12.3 cm at 2.4 GHz and 5.5 cm at 5.5 GHz.
The gap in the middle of a dipole where the cable from the radio connects.
Current distribution (standing wave)
How strongly current flows at each point along the antenna. On a ½ λ dipole it is largest at the feed and falls to zero at the tips; on longer ones parts of the wire carry current in the opposite direction.
The region close to the antenna (within about λ/2π for a small one) where the fields mostly store energy and hand it back each cycle, with the electric and magnetic fields out of step.
The region further out (a few wavelengths or more) where the fields are in step, at right angles, fall off with distance and carry energy away as a wave. Path loss and link budgets are calculated here.
How much stronger an antenna's best direction is than an isotropic antenna radiating the same total power, in decibels (dBi). A ½ λ dipole has 2.15 dBi.
Directivity minus the antenna's own losses. For a well-made dipole, gain and directivity are almost the same. Gain comes from focusing power, never from adding it.
The part of the impedance that represents power leaving as radio waves (plus a little lost as heat). About 73 Ω for a ½ λ dipole, only about 2 Ω for a 0.1 λ one.
The part of the impedance that stores energy and returns it each cycle instead of radiating it. Negative (capacitive) when the antenna is too short, positive (inductive) when it is too long.
The length at which the reactance is zero, so the antenna looks like a plain resistor. For a thin dipole this is about 0.48 λ, just under half a wavelength.
Radio equipment and coaxial cable are designed for 50 Ω (TV cable is 75 Ω). When the antenna's impedance differs, some power bounces back towards the radio; a matching network can correct this.
Voltage standing wave ratio: a measure of how well the antenna matches the cable. 1.0 is perfect; 1.5 means about 4% of the power is reflected; above 3 the mismatch is significant.
"Balanced to unbalanced": a small transformer or sleeve that connects a balanced dipole to unbalanced coaxial cable, so current doesn't flow on the outside of the cable and distort the pattern.
Several dipoles stacked end to end and fed in step. Their signals add up sideways and cancel up and down, flattening the doughnut for more gain along the horizon.
↗ opens a term's full entry in the Wi-Fi Glossary, which explains over 400 Wi-Fi terms with their IEEE 802.11 references.
Notes
Near field and far field: close to an antenna (within about λ/2π for a small one) the electric and magnetic fields store energy and hand it back each cycle, and they are 90° out of step. Further out (from a few wavelengths, or 2D²/λ for a large antenna of size D) they are in step, fall off as 1/distance, and carry energy away. Free space path loss and link budgets apply to the far field.
Why the loops detach: the fields can't change everywhere at once; changes spread at the speed of light. By the time the current reverses, the field half a wavelength away still points the old way, so field lines close into loops that keep moving outward.
Impedance: calculated with the induced-EMF method for a thin, straight wire (radius 0.001 λ) in free space. A real dipole resonates a little shorter than ½ λ (about 0.47–0.48 λ) because of the wire's thickness and end effects, and nearby objects, the ground and the feed cable all change it. Cut to exactly ½ λ, the dipole measures 73 + j42 Ω: on 50 Ω cable that is VSWR 2.2, with about 14% of the power reflected. Trimmed to resonance (about 64 Ω, no reactance) it drops to VSWR 1.28 and under 2% reflected.
Directivity and gain: directivity in dBi compares the strongest direction with an ideal isotropic antenna that radiates equally everywhere. Gain is directivity minus losses; for a good dipole they are almost the same. The pattern here is the side view (elevation); seen from above, a vertical dipole is a circle.
Balun: a dipole is balanced (two identical arms) but coaxial cable is not, so real dipoles use a balun or a sleeve to stop current flowing on the outside of the cable, which would distort the pattern.
High-gain omni in section 4 is modelled as three half-wave dipoles stacked 0.75 λ apart and fed in step (a collinear array), a common way to build one.