Antenna Gain
HardwareMeasure of an antenna's ability to direct RF energy in a particular direction, expressed in dBi relative to an isotropic radiator.
Antenna Gain
Antenna gain measures an antenna's ability to concentrate radiated RF energy in a particular direction compared to an ideal isotropic radiator (a hypothetical antenna that radiates equally in all directions). Expressed in dBi (decibels relative to isotropic), gain is a critical factor in the link budget that determines read range in RFID systems.
How Gain Works
An antenna does not amplify power -- it redistributes the available energy. A high-gain antenna focuses energy into a narrower beam, increasing signal strength in that direction at the expense of coverage in other directions. This is analogous to a flashlight reflector concentrating a bulb's light into a beam.
| Antenna Type | Typical Gain | Beamwidth | Use Case |
|---|---|---|---|
| Patch (single element) | 6 - 8 dBi | 60 - 90 deg | Portal doors, point-of-sale |
| Patch (array) | 9 - 12 dBi | 30 - 60 deg | Long-range dock doors |
| Yagi | 10 - 14 dBi | 20 - 40 deg | Vehicle tracking, toll lanes |
| Near-field antenna | < 3 dBi | N/A (near-field) | Encoding stations |
| Phased array | 12 - 20 dBi | Steerable | Advanced RTLS |
Gain and Polarisation
RFID antennas are available in two polarisation types:
- Linear polarization -- higher gain (typically 1-3 dBi more than equivalent circular), but sensitive to tag orientation. Best when tag alignment is controlled (e.g., conveyor systems).
- Circular polarization -- inherent 3 dB polarisation loss, but tolerant of random tag orientation. Preferred for portals and retail applications.
Regulatory Limits
Antenna gain directly affects EIRP, which is regulated by region. FCC Part 15 limits conducted power to 1 W (30 dBm), so a 6 dBi antenna produces 36 dBm EIRP (the maximum allowed). Using a higher-gain antenna requires reducing transmitter power to stay within regulatory limits.
Practical Selection
For portal reader installations, 6-9 dBi circular-polarised patch antennas are standard. For long-range applications like vehicle identification or yard management, 12+ dBi linear antennas with multiplexer switching provide extended range while staying within regulatory limits.
See also: EIRP | Link Budget | Circular Polarization
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