Microwave RFID
2.45 GHz & 5.8 GHz
What is microwave RFID?
Microwave RFID operates at 2.45 GHz and, less commonly, 5.8 GHz. These frequencies sit in the ISM (Industrial, Scientific, and Medical) band – the same spectrum used by Wi-Fi and Bluetooth. Microwave RFID supports both passive backscatter and active (battery-powered) tag architectures.
Compared to UHF RFID (860–960 MHz), microwave systems offer higher data rates and smaller antenna sizes, but passive read range is shorter because free-space path loss increases with frequency. For this reason, most microwave RFID deployments use active or semi-passive (BAP) tags with internal batteries, achieving ranges of 30–100+ metres.
Microwave RFID is used in real-time location systems (RTLS), electronic toll collection, container tracking, and high-speed vehicle identification – applications where long range and fast data transfer matter more than tag cost.
How it works
At 2.45 GHz the wavelength is approximately 12.2 cm, which means even small antennas (patch, dipole, or slot) are efficient. Microwave RFID uses far-field propagation – the tag and reader communicate via radiated electromagnetic waves, not magnetic coupling.
Passive microwave tags use backscatter modulation, the same principle as UHF RFID: the tag harvests energy from the reader's signal, powers its chip, and reflects a modulated signal back. However, at 2.45 GHz the energy harvesting efficiency is lower and path loss is greater, so passive read range is typically limited to 1–3 metres.
Active microwave tags contain a battery and their own transmitter. They do not rely on the reader's signal for power, so they can initiate communication and achieve read ranges of 30–100+ metres. Active tags at 2.45 GHz are commonly used in RTLS, where the tag periodically broadcasts its ID to multiple receivers that triangulate its position.
Semi-passive (BAP) tags use a battery to power the chip but communicate via backscatter. This extends read range to 10–30 metres without the cost and complexity of a full active transmitter.
Frequencies and regulation
| Frequency | Band | Notes |
|---|---|---|
| 2.45 GHz | ISM (global) | Primary microwave RFID band. Shared with Wi-Fi (802.11b/g/n) and Bluetooth. Available worldwide without licensing. |
| 5.8 GHz | ISM (global) | Used in some toll collection and vehicle identification systems. Higher path loss but less congestion than 2.45 GHz. |
Because 2.45 GHz is an ISM band, it is available globally without regional frequency variations – unlike UHF RFID, which operates at different frequencies in different countries. However, the band is shared with Wi-Fi and Bluetooth, so microwave RFID systems must contend with interference in environments with dense wireless infrastructure.
Read range
Read range depends heavily on the tag type:
| Tag type | Typical range |
|---|---|
| Passive backscatter | 1–3 m |
| Semi-passive (BAP) | 10–30 m |
| Active | 30–100+ m |
Free-space path loss at 2.45 GHz is approximately 8 dB greater than at 915 MHz (UHF), which is why passive microwave tags have shorter range than passive UHF tags despite using the same backscatter principle.
Advantages
- Small antennas: The 12.2 cm wavelength allows compact patch and slot antennas, enabling smaller tag form factors.
- High data rates: More bandwidth is available at 2.45 GHz, supporting faster data transfer than LF or HF systems.
- Global frequency: 2.45 GHz ISM band is available worldwide without regional restrictions, unlike UHF which varies by country.
- Active tag range: Battery-powered tags can communicate at 100+ metres, enabling RTLS and wide-area tracking.
- Directional antennas: At shorter wavelengths, it is easier to build highly directional antennas for focused read zones.
Limitations
- Shorter passive range: Higher path loss compared to UHF means passive tags read at 1–3 m versus 10–12 m for UHF.
- ISM band congestion: Sharing spectrum with Wi-Fi and Bluetooth can cause interference in dense wireless environments.
- Higher tag cost: Active and BAP tags are significantly more expensive ($5–$50+) than passive UHF inlays ($0.05–$0.15).
- Battery life: Active tags require battery replacement every 2–7 years depending on beacon rate.
- Water and body absorption: 2.45 GHz is strongly absorbed by water (the same principle that makes microwave ovens work), making these tags poorly suited for wet environments or on-body applications.
Common applications
- Electronic toll collection: Active 2.45 GHz and 5.8 GHz tags (e.g. DSRC-based transponders) are used in highway tolling systems in Europe, Asia, and South America.
- Real-time location systems (RTLS): Active 2.45 GHz tags broadcast periodically to a network of receivers that triangulate position. Used in hospitals (patient/staff tracking), warehouses (forklift tracking), and manufacturing (WIP tracking).
- Container and vehicle identification: ISO 18000-4 defines the air interface for microwave RFID used in intermodal container tracking and vehicle access control.
- Asset tracking: Semi-passive tags on high-value assets (IT equipment, tools, medical devices) that need longer range than UHF passive but lower cost than full active systems.
How microwave compares
| HF | UHF | Microwave | |
|---|---|---|---|
| Frequency | 13.56 MHz | 860–960 MHz | 2.45 / 5.8 GHz |
| Passive range | Up to 1 m | Up to 12 m | Up to 3 m |
| Active range | N/A | Rare | 30–100+ m |
| Data rate | Up to 424 kbps | 40–640 kbps | 1+ Mbps |
| Antenna size | Coil (large) | Dipole (medium) | Patch (small) |
| Water tolerance | Good | Poor | Very poor |
| Global frequency | Yes | No (varies) | Yes (ISM) |
| Passive tag cost | $0.10–$1 | $0.05–$0.15 | $1–$5 |