Antenna polarisation is the orientation of the electric field of the radio wave an antenna sends and receives. An antenna receives best when its polarisation matches the incoming signal, so the angle an antenna is mounted at affects how well it performs.

Most antennas used in IoT have linear polarisation, meaning the electric field stays in one plane. That plane can be vertical, horizontal or slanted, usually at ±45 degrees, and an upright whip antenna is vertically polarised. Circular polarisation, where the field rotates as the wave travels, is used by navigation satellites, so GNSS antennas are usually right-hand circularly polarised to match.

A polarisation mismatch between antenna and signal costs signal. In theory, an antenna at 90 degrees to the incoming polarisation receives almost nothing. In practice, reflections from buildings and terrain mix the polarisation, so the loss is smaller, but an antenna mounted at the wrong angle still gives a weaker connection.

A cross-polarised antenna holds two elements at right angles, usually slanted at ±45 degrees, in one housing. Many mobile network masts use slant ±45 degree antennas, and a cross-polarised antenna at the router end gives two signal paths that differ enough for MIMO to carry two data streams. Most directional antennas for 4G and 5G cellular routers are cross-polarised for this reason, and are often labelled X-pol.

On a router with two separate whip antennas, angling the whips apart gives each a different polarisation and has a similar effect. A common starting point is to set them in a V shape, then check SINR and throughput on the router's status page to confirm the change helps.