Polarization

What it is

Polarization is the direction of a wave's electric field. A vertical antenna produces a vertically polarized wave, a horizontal one produces a horizontally polarized wave. A receiving antenna picks up the most signal when it is lined up with the field it is receiving.

Why the wire's direction matters

Current moves along the wire, so the electric field it creates points along the wire too. A wave leaving a vertical whip has its field swinging up and down; a horizontal dipole swings it sideways. To catch that wave, the receiving antenna's wire must be able to follow the same swing. A receiver crossed at 90° has almost nothing to grab.

How much a mismatch costs

Tilting the receiving antenna by an angle θ from the transmitted field reduces received power by cos²θ. That is 3 dB at 45°, 6 dB at 60°, and falls away steeply near 90°. In real life the loss at 90° is limited by reflections and imperfect antennas, but it is still typically 20 dB or more: the difference between a clear contact and none.

On a clean line-of-sight path, such as a repeater, simplex or a link, matching polarization matters. By convention FM voice and repeaters use vertical polarization, because handhelds, mobile whips and base verticals are all vertical. Weak-signal SSB and CW on VHF and UHF traditionally use horizontal, so beams there are horizontal.

Circular, and when polarization stops mattering

In circular polarization the electric field rotates as the wave moves, tracing a corkscrew. It is right-hand or left-hand depending on the direction of rotation. A linear antenna hears a circular wave with a fixed loss of about 3 dB at any tilt, and two circular antennas of opposite sense are strongly mismatched. Satellites favour circular polarization because the satellite's orientation keeps changing and the wave's polarization can be rotated on its way through the ionosphere (Faraday rotation), so a fixed linear antenna would fade in and out.

Polarization matters much less in other situations:

• HF skywave. The ionosphere twists and scrambles polarization, so a signal arrives with whatever it ended up with. What matters on HF is the takeoff angle and the pattern, not the polarization.
• Reflections. A signal bouncing off buildings or terrain arrives partly changed, which is one reason a handheld works indoors even when held crooked.
• Ground wave. Low-band ground-wave signals are vertically polarized, because the ground quickly kills a horizontally polarized one; see Ground wave and surface wave.

Related

• Electromagnetic wavesElectric and magnetic fields travelling together at the speed of light.
• Ground wave and surface waveLow-frequency signals that follow the curve of the Earth.
• Radiation patternsReading azimuth and elevation plots, beamwidth and front-to-back.
• How antennas workTurning current into radio waves and back again.
• FM voice operationRepeaters, simplex and the "local" mode.
• Helical and satellite antennasCircular polarization for space work.