Also known as: polarization, polarisation
Polarization is the orientation of a radio wave’s electric field, determined by how the transmitting antenna is mounted — vertical, horizontal, or circular.1 It is a property of the wave itself, carried alongside the signal, and matching it between transmitter and receiver is one of the simplest ways to avoid throwing away signal.
How it works
An electromagnetic wave has an electric field and a magnetic field at right angles to each other and to the direction of travel. Polarization names the direction the electric field points. A vertical antenna element launches a vertically polarized wave; a horizontal element, a horizontal one. When the receiving antenna’s element lies along the arriving field, it develops maximum voltage; when it lies across the field, the induced voltage approaches zero. The loss from a partial mismatch follows the cosine-squared of the angle between them, so a 45° tilt costs about 3 dB and a full 90° cross-polarization can cost 20 dB or more in practice.2
Variants
- Linear (vertical / horizontal) — a single straight element. Most terrestrial two-way radio is vertical; over-the-air FM and TV broadcast is often horizontal or mixed.
- Circular (RHCP / LHCP) — the field rotates as the wave advances, produced by feeding crossed elements 90° out of phase or by a helix. A circularly polarized receiver loses only about 3 dB to any linear signal regardless of tilt, which makes it forgiving of tumbling or unknown orientation.2
- Slant / elliptical — intermediate cases; real signals are rarely perfectly pure.
Two related effects matter in the field. Reflections off buildings and terrain scramble polarization, so a signal that started vertical can arrive partly horizontal after multipath. And over long HF paths, Faraday rotation in the ionosphere slowly turns the plane of a linear wave, one reason satellite and long-haul links often use circular polarization instead.
In practice
Match polarization at both ends when you can. Satellites — GPS/GNSS, weather birds, many comms payloads — transmit circular, so a circular or turnstile antenna outperforms a straight whip for them. Cross-polarization is also used deliberately: it lets two signals share one frequency on orthogonal polarizations, and MIMO systems exploit polarization (and spatial) diversity to separate multiple streams. For a scanner, the practical rule is to align the antenna with the dominant traffic’s polarization and accept that reflections will blur the picture.
Relevance to SDR
A vertical antenna is the safe default for scanning land-mobile and trunked systems, matching their vertical polarization; using a horizontal antenna on the same traffic can bury a signal in the noise floor for no other reason. When a strong transmitter still won’t decode, a quiet polarization mismatch is worth ruling out before blaming the receiver. GopherTrunk itself does no polarization processing — it decodes the single stream its front end delivers — so polarization is entirely an antenna-siting decision the operator makes ahead of the ADC.
Sources
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Polarization (waves) — Wikipedia, on the orientation of a wave’s electric field and polarization types. ↩
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Circular polarization — Wikipedia, on circular/elliptical polarization and the ~3 dB and cross-pol loss figures. ↩ ↩2