Low Frequency (LF) RFID
125–134.2 kHz
What is LF RFID?
Low Frequency (LF) RFID operates in the 125–134.2 kHz band. It is the oldest and simplest form of RFID, using inductive coupling between a reader coil and a tag coil at very short range – typically under 10 cm. LF was the first commercially viable RFID technology and remains the standard for animal identification, vehicle immobilisers, and legacy access control systems.
The key advantage of LF is its ability to work reliably near metal and through liquids. Low-frequency electromagnetic waves are less affected by these materials than higher frequencies, which is why LF is used for applications like implanted animal transponders (where the signal must travel through body tissue) and industrial settings with metal machinery.
How it works
LF RFID uses near-field inductive coupling. The reader generates an alternating magnetic field through its coil antenna. When a tag enters this field, the tag's coil antenna picks up the energy through electromagnetic induction – the same principle as a transformer. The tag uses this harvested energy to power its chip and modulate the field to send data back.
Because the coupling is magnetic (near-field), the signal drops off rapidly with distance – roughly as the cube of the distance (1/r³). This is why LF read ranges are inherently short. However, this also means the signal penetrates materials that would block higher-frequency RF signals.
LF tags typically use simple modulation schemes like ASK (amplitude-shift keying) or FSK (frequency-shift keying). Data rates are low – typically 1–10 kbps – but sufficient for transmitting a unique ID number.
Frequencies and protocols
| Frequency | Protocol | Use |
|---|---|---|
| 125 kHz | EM4100, T5577, HID Prox | Access control key fobs and cards. EM4100 is read-only; T5577 is rewritable. |
| 134.2 kHz | ISO 11784/11785 (FDX-B, HDX) | Animal identification worldwide. FDX-B (full duplex) is the dominant protocol. Used in pet microchips, livestock ear tags, and bolus tags. |
Read range
LF read range is typically 1–10 cm for passive tags, depending on antenna size (both reader and tag). Larger reader antennas with more turns can extend range slightly. Panel readers used for livestock race identification can achieve up to 30 cm with large antenna panels, but this is the practical maximum for passive LF.
The short range is a feature, not just a limitation. For access control and payment, short range prevents accidental or malicious reads from a distance. For animal identification, the reader must be brought close to the animal, ensuring the correct animal is identified.
Advantages
- Works near metal: LF signals are minimally affected by nearby metal objects, unlike UHF which is reflected and detuned by metal surfaces.
- Penetrates liquids and tissue: The magnetic near-field passes through water, blood, and body tissue with minimal attenuation. This is why injectable animal transponders use LF.
- Global frequency: 125 kHz and 134.2 kHz are available worldwide without regional frequency restrictions, unlike UHF which varies by country.
- Simple and proven: LF technology has been in use since the 1980s. The chips are cheap, reliable, and well-understood.
- No licensing required: LF RFID systems operate in unregulated spectrum in most countries.
Limitations
- Very short range: 10 cm maximum for most passive tags limits applications to close-proximity reads.
- Low data rate: 1–10 kbps means transfers are slow. Only suitable for simple ID numbers, not large data payloads.
- No anti-collision: Most LF protocols cannot read multiple tags simultaneously. Tags must be read one at a time.
- Limited security: Legacy LF protocols (EM4100, HID Prox) have no encryption and can be trivially cloned with inexpensive equipment.
- Large antennas: At 125 kHz, the wavelength is 2,400 metres. Tag antennas must use many coil turns to achieve sufficient coupling, making them larger than HF or UHF antennas.
Common applications
- Animal identification: Injectable glass capsule transponders for pets, ear tags for livestock, and bolus tags for cattle. The global standard is ISO 11784/11785 at 134.2 kHz.
- Access control: Key fobs and proximity cards for building entry. Legacy HID Prox systems at 125 kHz are still widely installed.
- Vehicle immobilisers: A transponder in the car key communicates with a reader in the ignition barrel. Without the correct LF response, the engine will not start.
- Industrial automation: Tool identification, mould tracking, and machine part recognition in environments with heavy metal and oil.