LF — 125 kHz
The old, slow, everywhere band — badges, pet chips and car keys, read from a few centimetres by a coil with hundreds of turns.
Low-frequency RFID lives at 125 kHz, plus 134.2 kHz for animal tags. A wavelength here is about 2.4 km, so a tag's antenna can't be anything like a "fraction of a wavelength". Instead, the tag works entirely in the near field: its coil and the reader's coil share a magnetic field like the two halves of a transformer.
#Why so many turns
A coil picks up more voltage the more turns it has and the faster the field changes. At 125 kHz the field changes slowly, so LF tags make up for it with hundreds of turns of hair-thin copper, or with a ferrite core that concentrates the field (that's the pet chip trick). It's why LF tags tend to be chunkier than HF ones.
#Good at
- Cheap and tough. Simple chips that have been made for decades.
- Doesn't mind water. Body tissue and liquids barely affect a magnetic field at this frequency, so LF works for animals, and in wet or dirty places.
- Short range by design. A few centimetres, maybe 10 cm with a big reader. That's usually a feature.
#Bad at
- Speed. A few kilobits per second, so there's no room for serious cryptography in the time a tag is in the field.
- Brains. Many LF tags are read-only and simply repeat a fixed ID the moment they're powered, with no handshake and no secret. That's fine for a cow's ear tag and a real problem for a door (see Why old cards are a bad idea).
#Who lives here
| Family | Where you meet it |
|---|---|
| EM4100-style ID tags | gym and apartment key fobs, cheap badges |
| HID Prox, Indala, AWID | office badges, mostly in North America |
| FDX-B (ISO 11784/11785) | pet microchips, livestock |
| Immobiliser transponders | inside car keys, so the engine knows the key is real |
Each family encodes its bits a bit differently (switching the load fully on and off, shifting frequency, or shifting phase), which is why an LF reader has to try several decoders before it knows what it's holding.
With Proxmark3
lf search powers the LF antenna and tries every demodulator it knows. It
tells you which family the tag is and prints its ID.
With Flipper Zero
125 kHz RFID → Read, then hold the card against the back of the Flipper. It names the protocol (EM4100, HID Prox, FDX-B…) and shows the data.
With ChameleonUltra
The Ultra's LF side reads EM4100-family tags, and the companion app shows the ID.