125 kHz
Understanding 125 kHz RFID
When you tap your plastic key fob against the wall reader to unlock your office door, you are utilizing the 125 kHz Low-Frequency band.
Unlike Wi-Fi or cellular networks that blast propagating electromagnetic waves for miles, a 125 kHz RFID system relies purely on the Magnetic Near-Field.
The Physics of Magnetic Induction
A 125 kHz key fob contains no battery. It is completely dead plastic until it approaches the reader.
- The wall reader continuously pulses a massive, alternating magnetic field at exactly 125,000 Hertz.
- Inside your key fob is a tiny microchip attached to a coil of ultra-thin copper wire.
- When you bring the fob within 2 inches of the reader, the reader's alternating magnetic field violently sweeps across the fob's copper coil. This physical sweeping motion induces an electrical current inside the coil (Faraday's Law of Induction).
- This tiny induced current "wakes up" the microchip. The chip then uses that exact same power to alter the magnetic field, pulsing its secret ID number back to the reader.
Why 125 kHz?
| The Benefit | The Engineering Reality |
|---|---|
| Penetration of Biological Tissue | Higher frequencies (like 2.4 GHz) are violently absorbed by water and flesh. Because 125 kHz has a massive 2.4-kilometer wavelength, it passes flawlessly through water, fat, and skin. This is why 125 kHz is the exclusive frequency used for Pet Microchips injected under the skin of dogs and cats. |
| Immunity to Environmental Noise | A 125 kHz magnetic field ignores dirt, mud, paint, and concrete. You can embed a 125 kHz tag inside a solid concrete wall or a muddy wooden pallet, and it will still read perfectly. |
| Extreme Security (Short Range) | Because magnetic near-fields collapse at the cube of the distance ($1/r^3$), the fob will only power on if it is within a few inches of the reader. A hacker cannot sit in a van 100 feet away and read your 125 kHz office badge. |
Key Equations
125 kHz is a globally unlicensed Low-Frequency (LF) radio band utilized almost exclusively for near-field, passive Radio Frequency Identification (RFID) systems. Operating at a massive...
Key specifications:
125 kHz | 2.4 k
Throughput: R = Nlayers×B×ηSE×(1−OH)
Comparison
| Band | Range | Wavelength | Application | Standard |
|---|---|---|---|---|
| 125 kHz | 125 GHz region | 2.4 mm | Primary use | ITU allocation |
| Adjacent lower | 112.5 GHz | 2.7 mm | Related band | Shared spectrum |
| Adjacent upper | 137.5 GHz | 2.2 mm | Related band | Guard band |
| Harmonic 2f | 250.0 GHz | 1.2 mm | Spurious | Filter required |
| Sub-harmonic | 62.5 GHz | 4.8 mm | LO option | Mixer design |
Frequently Asked Questions
Can 125 kHz tags hold a lot of data?
No. Because the frequency is so incredibly low, the data transfer rate is abysmal (often less than 4 Kilobits per second). A 125 kHz tag typically holds nothing more than a simple 64-bit or 128-bit ID number. It cannot hold complex encrypted certificates or large text files.
Does 125 kHz penetrate metal?
No. If you place a 125 kHz key fob inside an aluminum foil wrapper or stick it to a steel plate, the massive magnetic field from the reader will induce 'Eddy Currents' in the solid metal instead of the copper coil. The metal will absorb all the power, and the tag will remain dead.
What is the difference between 125 kHz and 13.56 MHz (NFC)?
13.56 MHz (High-Frequency RFID, or NFC) is used in modern tap-to-pay credit cards and Apple Pay. Because its frequency is much higher, it can transfer data much faster, allowing for complex, heavily encrypted cryptographic handshakes. However, 13.56 MHz is much more easily blocked by human tissue and water than 125 kHz.