Linear Polarization
HardwareAntenna polarization where the electric field oscillates in a single plane, providing maximum range when aligned with tag antenna.
Linear Polarization
Linear polarization (LP) is an antenna polarisation mode where the radiated electric field oscillates in a single fixed plane as the wave propagates. In RFID, linearly polarised reader antennas deliver maximum energy transfer when the tag antenna is aligned in the same plane, offering higher antenna gain and longer read range than circular polarization at the cost of orientation sensitivity.
Polarisation Alignment
The performance of a linearly polarised system depends entirely on the angular relationship between the reader antenna's polarisation plane and the tag antenna's orientation:
| Alignment Angle | Power Transfer | Effect on Range |
|---|---|---|
| 0 deg (co-polarised) | 100% (0 dB loss) | Maximum range |
| 45 deg | 50% (-3 dB loss) | ~70% of max range |
| 90 deg (cross-polarised) | ~1% (-20 dB loss) | Tag unreadable |
This steep penalty at 90-degree misalignment means LP antennas are only appropriate when tag orientation can be controlled -- for example, when all tags are applied in the same direction on cartons moving through a conveyor tunnel.
Advantages Over Circular
- 3 dB higher gain for the same antenna aperture, directly extending read range.
- Simpler antenna construction -- single feed point, no 90-degree hybrid or sequential rotation needed.
- Lower cost -- typically 20-30% less expensive than equivalent CP antennas.
- Narrower beamwidth available, useful for isolating specific read zones.
Common Deployments
LP antennas excel in controlled-orientation environments:
- Conveyor tunnels -- tags are consistently applied to the same face of each carton. LP antennas mounted perpendicular to the conveyor belt maximise read rate.
- Smart shelves -- tags on products face a consistent direction. LP shelf antennas provide precise zone discrimination.
- Vehicle identification -- windshield tags have a known orientation relative to overhead reader antennas.
- Printer-encoders -- the tag passes the near-field antenna in a fixed orientation during label printing.
Dual-Linear and Switched Polarisation
Some advanced readers offer switchable or dual-linear polarisation: the reader alternates between horizontal and vertical LP to approximate the orientation tolerance of CP while retaining higher gain. This approach adds hardware complexity but can be effective in semi-controlled environments.
See also: Circular Polarization | Antenna Gain | Orientation Sensitivity
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