Linear polarization is smaller, cheaper and fine when both ends are fixed. Circular polarization costs 3 dB against a linear link but removes orientation sensitivity entirely — which is why GNSS satellites use RHCP and FPV video links use CP on aircraft that roll and pitch.
The E-field oscillates in one plane. A vertical whip is vertically polarized; a patch element lying flat is usually the opposite. Rules of thumb:
| Alignment | Loss |
|---|---|
| Both linear, axes parallel | 0 dB |
| 45° apart | ≈ 3 dB |
| 90° (crossed) | theoretically infinite; 20–30 dB in practice |
| Linear vs circular | 3 dB, always, regardless of rotation |
The practical 20–30 dB figure (not infinity) comes from finite axial ratio, mutual coupling and multipath. It is still enough to kill a link — which is why a meter lying on its side can lose 20 dB to a gateway it used to reach.
Circular polarization needs two orthogonal components of equal amplitude with 90° phase offset. Real antennas never hit that perfectly; the result is an ellipse, measured by axial ratio (AR):
Two buying cautions:
1. Always ask "AR at what angle?" A GNSS patch may hold 3 dB at zenith and degrade badly toward the horizon. Good designs hold ≤ 2 dB across the upper hemisphere. 2. Handedness must match. RHCP to LHCP is the same problem as crossed linear — 15–25 dB of isolation in practice.
GNSS satellites transmit RHCP by design. Each ground or building reflection flips the handedness. So an RHCP receive antenna, plus the receiver's own rejection, suppresses the LHCP multipath component. That is the primary defence against urban and foliage multipath — and the reason a "GPS antenna" is really a 1559–1610 MHz RHCP antenna, not a 1575.42 MHz part.
An aircraft rolls, pitches and yaws continuously. With linear antennas on both ends, the link fades every time the two axes cross — a 20–30 dB drop, visible as a frame freeze exactly when the pilot banks. Circular polarization removes orientation sensitivity entirely. The price is 3 dB against a linear ground station, a bulkier antenna, and correct handedness matching across the whole set (antennas on one aircraft, or one ground-station pair, must be the same hand).
| Application | Polarization | Reason |
|---|---|---|
| Sub-GHz LoRa / ISM sensor, fixed orientation | Linear (usually vertical) | Cheapest, smallest, full gain |
| NB-IoT / Cat.1 meter in a pit or cabinet | Linear, but verify orientation | Orientation may be uncontrolled — pattern matters more |
| Wi-Fi / BLE access point and client | Linear (often dual-slope or MIMO) | MIMO already handles polarization diversity |
| GNSS — GPS / BeiDou / GLONASS / Galileo | RHCP | Satellites are RHCP; CP rejects multipath |
| FPV / drone video link | CP (same hand both ends) | Aircraft roll; linear would fade |
| UWB ranging | Linear, matched | Orientation is typically controlled |
| Satellite IoT (LEO) | CP per the operator spec | Operator defines the hand |
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