Contactless Payment
How the 13.56 MHz Near-Field Link Works
A contactless payment session is fundamentally an inductively coupled RF transaction rather than a radiated one. The reader, often called a proximity coupling device, drives a tuned loop antenna with an unmodulated 13.56 MHz carrier. The card or phone contains its own resonant loop, tuned with a parallel capacitor to sit near 13.56 MHz, and the reader's alternating magnetic field induces a voltage across that loop by Faraday's law. The card rectifies this induced AC to power its secure element, so no battery is required. Because the wavelength at 13.56 MHz is roughly 22 m and the antennas are only 40 to 80 mm across, the entire interaction occurs in the reactive near field where the two coils behave like a loosely coupled, air-cored transformer.
Data flows in both directions on that single carrier. From reader to card, ISO/IEC 14443 Type A uses 100 percent amplitude-shift keying with a modified Miller code at 106 kbit/s, while Type B uses 10 percent ASK with NRZ coding; higher data rates of 212, 424, and 848 kbit/s are negotiated for faster EMV transactions. From card to reader, the card cannot generate its own carrier, so it replies by load modulation: an internal transistor periodically switches a resistive or capacitive load across the card antenna, which the reader detects as small sidebands at 13.56 MHz plus or minus 847.5 kHz. EMV contactless then layers the payment application, dynamic cryptograms, and tokenization on top of this physical and protocol stack.
Reliable operation depends on hitting an operating field-strength window. ISO 14443 specifies that a compliant card must function with magnetic field strength H between 1.5 and 7.5 A/m RMS, and the reader must not exceed the upper bound to protect the card front end. Antenna designers therefore size the reader coil and its matching network for a deliberately low loaded quality factor, typically Q of about 10 to 20 after damping, so the field is strong enough at a few centimeters yet the bandwidth stays wide enough to pass the 847.5 kHz load-modulation sidebands without excessive attenuation. An undamped 13.56 MHz coil can self-resonate at a much higher Q, so a damping resistor is added precisely to trade some field strength for the bandwidth those sidebands require.
Near-Field Coupling and Power Equations
λ = c / f = 3×108 / 13.56×106 ≈ 22.1 m
Induced EMF (Faraday's law):
Vind = − N × dΦ/dt ≈ − N × A × μ0 × (dH/dt)
Load-modulation subcarrier:
fsub = fc / 16 = 13.56 MHz / 16 = 847.5 kHz
Antenna −3 dB bandwidth from quality factor:
BW = fc / Q → for Q = 15, BW ≈ 904 kHz (about ±452 kHz)
Where N = antenna turns, A = loop area, μ0 = 4π×10−7 H/m, H = field strength (A/m), fc = 13.56 MHz carrier, Q = loaded quality factor. Field strength must stay within H = 1.5 to 7.5 A/m at the card per ISO 14443.
Contactless Payment Standards Compared
| Standard / Mode | Carrier | Reader-to-Card Data Rate | Modulation (uplink) | Typical Range | Where Used |
|---|---|---|---|---|---|
| ISO 14443 Type A | 13.56 MHz | 106 to 848 kbit/s | 100% ASK, Miller | 0 to 4 cm | Most EMV bank cards, MIFARE |
| ISO 14443 Type B | 13.56 MHz | 106 to 848 kbit/s | 10% ASK, NRZ | 0 to 4 cm | ID cards, some bank cards |
| NFC (ISO 18092) | 13.56 MHz | 106 to 424 kbit/s | ASK, peer or card emulation | 0 to 4 cm | Phones, host card emulation |
| FeliCa | 13.56 MHz | 212 to 424 kbit/s | Manchester, 10% ASK | 0 to 4 cm | Transit and retail in Japan |
| ISO 15693 (vicinity) | 13.56 MHz | 1.65 to 26 kbit/s | ASK, pulse position | up to ~1 m | Item tagging, not payment |
Frequently Asked Questions
What carrier frequency and standards do contactless payment cards use?
The carrier is 13.56 MHz, defined by ISO/IEC 14443 for proximity cards and ISO/IEC 18092 for NFC. The reader sends data with amplitude-shift keying at 106 to 848 kbit/s, and the card replies by load modulation, producing sidebands at 13.56 MHz ± 847.5 kHz. EMV contactless adds the payment application and cryptography, so one reader serves cards, phones using host card emulation, and wearables.
Why does contactless payment only work within a few centimeters?
At 13.56 MHz the wavelength is about 22 m, so a 40 to 80 mm reader coil operates in the reactive near field. Coupling is inductive, and on-axis field strength falls roughly as 1/r3 beyond the coil radius. ISO 14443 needs the card to see H between 1.5 and 7.5 A/m, a window only met within about 0 to 4 cm. Short range also blocks remote, unintended reads.
How does a passive payment card get power without a battery?
The card is a passive transponder: a multi-turn loop tuned near 13.56 MHz with a parallel capacitor. The reader's alternating field induces a voltage by Faraday's law, which the card rectifies into a DC supply of a few milliwatts up to roughly 15 mW to run its secure element. Reader antennas are damped to a low loaded Q of about 10 to 20 so the bandwidth still passes the 847.5 kHz load-modulation sidebands rather than maximizing field at the expense of bandwidth.