A radio wave's electric field points in a direction, and an antenna only picks up the part of the field that lines up with it. See the different kinds of polarization, turn two antennas against each other and watch the signal drop, then see why indoors it matters much less than on an outdoor link.
The polarization of a wave is the direction its electric field points. A vertical dipole sends a vertically polarized wave, and a receiving antenna collects the part of that field that lies along it. Line the two antennas up and you get the full signal. Turn one by 45° and you lose half the power (3 dB). Turn it by 90° and, in open space, almost nothing arrives.
Lined up (0°)0 dBall the signal
45° apart−3 dBhalf the power
60° apart−6 dBa quarter of the power
90° apart−20 dB or worseoutdoors, in line of sight
1 · What polarization looks like
A wave travelling to the right, with the electric field drawn as arrows along it. The small circle shows the same wave head-on, as the receiver sees it: the tip of the field traces a line for linear polarization and a circle for circular. Drag the picture to turn it.
Polarization
Electric field (its direction is the polarization)Transmitting antennaDrag the picture to turn it
2 · Turn the antennas: polarization mismatch
Both antennas are seen head-on, from behind the receiver looking back at the transmitter. The receiver collects only the part of the incoming field that lines up with it. This is a clean line-of-sight link with no reflections, where polarization matters most.
Transmitter
Receiver
Angle between them
Antenna quality (cross-polar discrimination)
The two antennas, head-on
Loss against the angle between them
3 · Indoors, reflections blur it
Every time the signal bounces off a wall, floor or piece of furniture its polarization is twisted a little. Indoors the receiver gets many bounced copies at many angles, so turning the antenna costs much less. Here the access point's antenna is vertical and you tilt the client's antenna.
Surroundings
Direct and bounced signals
Loss against the client's tilt
4 · Polarization in Wi-Fi
Where polarization helps, where it hurts, and what to do about it.
Polarization diversity in MIMO access points
Many APs mix vertical and horizontal (or +45° and −45°) elements. The two polarizations fade independently, so the radio gets less-correlated copies of the signal: better for spatial streams and for combining at the receiver. A client at an odd angle also lines up better with at least one element.
Point-to-point links: line up both ends
Outdoor bridges have almost no reflections, so polarization is kept all the way. A dual-polarized dish carries one stream on vertical and one on horizontal. Rotate one end by 10° and each stream loses only 0.1 dB, but 15 dB of the other stream leaks in, which limits the modulation. Check the port labels and mount both ends the same way.
External antennas: follow the vendor's diagram
On a ceiling-mounted AP, dipoles usually point straight down: vertical polarization and a flat doughnut across the room. Bent flat against the ceiling, they turn horizontal and point their doughnut at the floor and ceiling. Use the orientation the datasheet shows for how the AP is mounted.
Clients sit at every angle
A phone is held upright, tilted or flat on a desk, and a laptop's antennas run along the top of the lid. Indoors, reflections soften the effect, so a turned phone typically loses a few dB, not 20. That's also why Wi-Fi doesn't use circular polarization: against a linear client it always costs 3 dB.
The direction the electric field of a radio wave points, as seen looking along the direction of travel. It is set by the transmitting antenna's orientation.
The part of the wave that pushes on charges, and so drives current in a receiving antenna. The magnetic field is always at right angles to it; polarization is defined by the electric field.
Linear polarization tilted 45° to the left or right. Common on dual-polarized AP and cellular antennas, because +45° and −45° are at right angles to each other.
The electric field keeps its strength but rotates once per cycle, so its tip traces a circle. Made, for example, by two crossed dipoles fed a quarter of a cycle apart, or by a helix.
Right-hand and left-hand circular polarization. Point your right thumb in the direction of travel: if the field turns the way your fingers curl, it is right-handed (the IEEE convention). Each reflection swaps the hand.
The general case between linear and circular: the tip traces an ellipse. The axial ratio is the ellipse's long axis divided by its short axis: 1 (0 dB) is perfectly circular; real "circular" antennas are slightly elliptical.
Signal lost because the receiving antenna isn't lined up with the incoming field. For two linear antennas at angle ψ it is cos²ψ: 3 dB at 45°, 6 dB at 60°. Also called the polarization loss factor (PLF).
Co-polar is the polarization an antenna is designed for; cross-polar is the one at right angles to it (horizontal for a vertical antenna, or the opposite hand for circular).
How much better an antenna responds to its own polarization than to the cross-polar one, in dB. It sets how deep the 90° null really is: with 20 dB XPD, crossed antennas still get 1% of the power.
The twisting of a wave's polarization by reflection, scattering and diffraction. Strong indoors, weak on an open outdoor link. It is also described as the channel's XPD.
Using antennas with different polarizations (vertical and horizontal, or ±45°) so that their signals fade independently. It improves MIMO and receiver combining.
One antenna with two ports, one for each of two perpendicular polarizations. Point-to-point dishes use it to carry two streams at once on the same channel.
↗ 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
The loss formula: the fraction of power a receiving antenna collects is |êt·êr*|², the squared overlap of the two polarizations. For two linear antennas that is cos²ψ; linear to circular is always ½ (3 dB); circular to circular is 1 for the same hand and 0 for opposite hands.
Real antennas never reach zero: section 2 adds the antenna's cross-polar discrimination as a floor, so crossed antennas lose about the XPD value (20 dB by default) instead of everything. "Ideal" removes the floor.
Section 3 is a simplified model: the surroundings are described by a channel XPD, the fraction of power the bounces move into the other polarization (typical values used: 20 dB outdoors in line of sight, 10 dB open indoor, 6 dB office, 3 dB cluttered with no line of sight). Real values vary widely with the building and the path, and the loss changes as you move.
Why Wi-Fi hardly ever uses circular polarization: (1) clients are linear (phones, laptops and IoT devices all have linear antennas), and a circular wave received by a linear antenna always loses about 3 dB, so a circular AP would give away 3 dB to every client; (2) each reflection reverses the hand (right becomes left), so indoors, where most of the signal arrives by bouncing, a circular antenna rejects many of the reflections Wi-Fi relies on; (3) MIMO gets more from two linear polarizations (vertical and horizontal, or ±45°), which give independent paths for spatial streams. Circular polarization pays off only when both ends are circular, with the same hand, on a mostly line-of-sight path: drones and moving vehicles on 2.4 and 5.8 GHz (their orientation keeps changing), some long-range outdoor links with helical or circular patch antennas (a single bounce off the ground arrives with the opposite hand and is largely rejected), and, outside Wi-Fi, satellites and GPS.
Range: the "outdoor range" figure assumes free space, where range scales with 10^(loss/20): 6 dB of loss halves the range. Indoors path loss rises faster with distance, so the same loss costs less range.