An RFID card looks like an ordinary plastic card, but it is a small radio system with no battery inside. Hold it near a reader and a door opens, a turnstile releases, or a payment terminal responds. This guide explains what is happening: the two parts inside the card, how the card and reader exchange data without contact, and why that design replaced the magnetic stripe. If you are specifying an order rather than studying the physics, the closing section covers what these mechanics mean for a purchase.
Table of Contents
What is inside an RFID card
A passive RFID card contains only two working parts, joined into a single unit called an inlay:
- An antenna coil — a flat loop of thin copper or aluminium wire, laminated between the plastic layers. It does two jobs: it captures energy from the reader’s field to power the chip, and it carries the chip’s reply back to the reader.
- A microchip (the integrated circuit) — a silicon chip bonded to the ends of the antenna. It holds the card’s unique identifier (UID) and any application data, and it runs the small amount of logic needed to answer the reader. Its memory is non-volatile, so the data survives when the card leaves the field.
Everything else is packaging. A standard card is the ISO/IEC 7810 ID-1 format (85.6 × 54 mm, nominally 0.76 mm thick), built from PVC, PET or an eco-material with the inlay sealed between printed outer layers under heat and pressure. The artwork on the surface has no effect on how the card is read, provided the antenna is tuned for the chosen body material and thickness. That is why a wooden or metal card can behave differently from plain PVC and needs its own antenna design — the electronics are the same idea, but the tuning is not.
There is no battery, no button and no display. The card does nothing at all until a reader supplies it with power.
How the card and reader exchange data
The exchange is a short handshake that runs in a fraction of a second, faster than the gesture of tapping. It follows a fixed order:
- The reader creates a field. The reader continuously radiates an alternating electromagnetic field at the card’s operating frequency — 13.56 MHz for most smart cards under ISO/IEC 14443, or 125 kHz for older low-frequency proximity cards. The field only reaches a few centimetres, which is why you have to bring the card close.
- The card powers up. When the antenna enters that field, it captures energy by induction — the same principle as a wireless phone charger. The card rectifies that energy into a working voltage and the chip comes to life on borrowed power.
- The reader sends commands. The reader varies its field to send instructions: identify yourself, authenticate, read or write a specific memory area.
- The card replies by load modulation. This is the part that surprises people. A passive card has no transmitter and no power to run one, so it does not broadcast. Instead it changes how much energy it draws from the reader’s field, in a controlled pattern. The reader senses those tiny variations in its own field and decodes them as the card’s answer.
- The exchange ends. Once the identifier, authentication and any data operations are complete, the card leaves the field, loses power and holds its data until the next read.
When several cards sit in the field at once, the protocol runs an anticollision step so the reader can single out and address each card in turn rather than hearing a jumble. That is what lets a reader resolve one card cleanly even when a wallet holds two.
Passive by design: no battery on the card
The cards in almost every access, hotel, transit and payment system are passive: no onboard power source, energised only while inside a reader’s field. That is a deliberate choice. A passive card has nothing to wear out or run flat, costs far less to make than a powered device, and can be laminated flat enough to print and carry in a wallet.
For contrast, active tags carry a battery and their own transmitter, which buys long range and features such as sensors — at the cost of size, price and a finite battery life. Battery-assisted (semi-passive) tags sit between the two. Ordinary RFID and NFC cards are firmly in the passive camp, which is exactly why they are cheap, durable and maintenance-free.
Why RFID replaced the magnetic stripe
A magnetic stripe stores a fixed pattern that a reader plays back on contact. An RFID card holds a chip that can authenticate and, on the more capable families, run a live cryptographic exchange. Several practical differences follow from that:
- No physical contact, so no wear. There is no head to grind against and no stripe to demagnetise, so a card tolerates far more use before it fails.
- Tolerance to dirt and weather. A card reads through a wallet, a glove or a thin layer of grime, where a dirty stripe or a scratched barcode simply fails.
- Stronger security options. A stripe, once copied, is copied forever. Modern chips such as DESFire or NTAG 424 DNA answer each read with a changing cryptographic value, which a static copy cannot reproduce. Note that this depends on the chip: a basic chip that only returns a UID offers little more protection than a stripe.
- More than one application per card. A capable chip can hold separated areas for, say, door access and cashless catering on the same card.
The stripe still appears on combi and dual-interface cards where a site bridges old and new readers, but for new systems the chip is the credential.
Which chip is inside which card
“RFID card” is a family, not a single product, and the right chip depends entirely on the reader that has to accept it. A few common cases:
- 13.56 MHz smart cards — MIFARE Classic, MIFARE Plus and MIFARE DESFire for access and transit; NTAG NFC cards for phone-readable interactions. These use the ISO/IEC 14443 contactless interface; NTAG and DESFire are read by nearly every modern smartphone, but MIFARE Classic’s proprietary sector data is not accessible on iPhones and some Android phones.
- 125 kHz proximity cards — T5577 and EM4305/EM4200 chips for legacy door systems. Short range, low cost, no smartphone reading.
- Wooden, metal, paper and eco bodies — the same chip families in a different material, each needing its own antenna tuning.
The card body is a design choice; the chip is a compatibility decision. The two questions to answer before ordering are which chip your readers accept and who writes the data.
What this means when you buy
The mechanics above lead to one practical rule: the buying decision starts with the reader, not the plastic. A card only works if its chip and configuration match what the installed reader expects, and the data that makes it “open the door” is written by the system, not the supplier.
When you specify an order, confirm the exact chip your readers accept, whether cards are supplied blank or pre-encoded, and who holds the keys and performs personalisation. Then prove it before committing to a batch: request a small number of samples and test them on your own readers before approving the run. For the difference between the frequency bands, read RFID frequencies explained: LF vs HF vs UHF; for where NFC fits, see NFC vs RFID.
To move from theory to a decision, browse RFID and NFC cards, tell us the reader model and application through the inquiry form, and start with a sample pack so you can verify compatibility on your own equipment before you order in volume.
