What is RFID and how does a tag talk to a reader?
An RFID system has two halves. The reader sends out a radio signal through an antenna, and the tag, a small chip joined to a coil or antenna, responds with data. In most access cards that data is little more than a serial number. The reader passes the number to a controller or server, which compares it with a list of permitted credentials and then decides whether to release a lock, lift a barrier or simply log the event.
The important point is that the tag is only a label. It does not know who is holding it or whether the door should open. All of the decision making lives in the back end. That is why two cards that look identical can have very different security: one may merely announce a number to anyone who asks, while another may prove its identity with a cryptographic exchange before it reveals anything.
Passive tags versus active tags: where does the power come from?
A passive tag has no battery. When it enters the reader's field, the energy it collects is just enough to wake the chip. In the lower frequency families the reader and tag behave like two coils of a transformer: the tag changes how much load it places on the reader's field, and the reader detects that change as data. In ultra high frequency systems the tag instead reflects a controlled portion of the incoming wave back, a method called backscatter.
An active tag carries its own battery and often its own transmitter. It can announce itself from much further away and can add sensors or buttons, but it costs more, needs battery replacement and is bulkier. Semi-passive designs sit in between, using a battery to run the chip while still reflecting the reader's signal. For doors, gates and badges, passive tags dominate because they are cheap, small and last for years.
Low frequency, high frequency and UHF: how do the RFID families differ?
Three families cover most security work. Low frequency, around 125 kilohertz, is the older proximity card world: simple, tolerant of nearby materials, but short in reach and usually sending a fixed number. High frequency, at 13.56 megahertz, is the family behind smart cards and near field communication, and it supports data storage, encryption and mutual authentication. Ultra high frequency, roughly in the 860 to 960 megahertz region depending on country, reads tags across a much larger zone and suits vehicles and inventory.
Lower frequencies couple through magnetic fields and stay close to the reader. Ultra high frequency uses a travelling radio wave, so reach grows but the signal becomes more sensitive to reflections, bodies and orientation. No family is better in every way. Choosing one means weighing how far you need to read, how much protection the credential needs and what the tag will be attached to. In practice, ask what range, data capacity and security level each credential really offers before choosing a family.
Can RFID cards be cloned? Cloning, replay and eavesdropping
Weak credentials are the main exposure. A basic low frequency card transmits the same number every time and proves nothing, so a cheap device placed near it can record the number and write it to a blank tag. This is cloning. Replay is related: an attacker captures a valid exchange and repeats it later. Eavesdropping simply listens to the exchange from a distance, which is easier than people expect because a reader's own signal is strong.
Better credentials answer these with challenge and response. The reader sends a random value, the card signs it with a secret key, and a recorded answer is useless the next time. Some older high frequency cards used proprietary ciphers that have since been broken, so the cryptography itself matters, not just the frequency. Mitigations include modern cards with strong mutual authentication, an encrypted link between reader and controller, a second factor such as a code or a camera check, prompt revocation of lost credentials and a log reviewed for odd patterns.
Why do metal, water and interference upset RFID?
Radio fields behave badly near conductive materials. Metal can detune a tag's antenna, reflect the signal, or absorb the magnetic field and shield the chip altogether. A card taped to a steel panel or held against a phone case with a metal plate may fail even though it works in the air. Specialist tags use a spacer or a shielding layer so they can be mounted on metal, though at a cost in size.
Water and the human body soak up radio energy, which matters most at ultra high frequency where a tag in a wallet against the chest can read poorly. Nearby electronics, motors and other readers can add noise or cause readers to talk over each other. Regulation also varies: permitted power and channels differ between countries and regions, so check local rules before installing anything. Test with real tags in the real mounting position, not on a bench.
How does RFID fit a gate, construction site or other temporary location?
Temporary and remote sites suit RFID because credentials are cheap to issue and revoke and readers have no moving parts. A construction gate can use cards for workers and windshield tags for regular vehicles, while a ballot drop box or equipment cage can log who opened it. The reader needs reliable power and a communications path to the controller, so a solar and battery site must budget for that load and for weather exposure.
Reading a credential proves very little about who is actually standing there. Pairing the read with video closes that gap: an access event can be matched with a camera view so a person can check that the badge holder and the visitor are the same. Plan for lost cards, tailgating behind a vehicle and cloned tags, and treat RFID as one layer rather than the whole control. Keep a simple procedure for revoking a card the moment it goes missing, since the reader itself cannot tell a lost card from a valid one.
Key terms
- Passive tag
- A tag with no battery that uses energy from the reader's field to wake its chip and send back its stored data.
- Active tag
- A tag with its own battery and transmitter, able to announce itself over a longer distance but costing more and needing maintenance.
- Backscatter
- A method where a passive tag modulates and reflects part of the reader's own radio wave back as its reply.
- Inductive coupling
- Magnetic linking between reader and tag coils, like a loose transformer, used in low and high frequency RFID at short range.
- Cloning
- Copying a credential's data onto another tag so that it is accepted as the original by the reader and controller.
- Challenge and response
- An authentication exchange where the card proves it holds a secret key by answering a fresh random value from the reader.
- Detuning
- A shift in a tag antenna's resonance caused by nearby metal, liquids or bodies, which weakens or blocks reading.
A site that uses RFID for gates or cages still needs eyes on it, because a badge read says nothing about who is present. A trailer like this can watch the entry point with cameras, and if a site needs it, gateway events can reach the video platform so an access event sits beside the footage. Operators at the 24/7 operations center verify alerts, and IP speakers or strobes can respond live.
Common questions
Is RFID the same thing as NFC?
No. NFC is one specific kind of high frequency RFID, working at 13.56 megahertz over very short distances with defined modes for phones and cards. RFID is the wider family, including low frequency cards and ultra high frequency tags that can be read much further away.
Are RFID access cards secure?
It depends on the card. Simple fixed number cards are easy to clone. Modern cards using mutual authentication and strong encryption are far harder to copy. Security also depends on the reader link, key management, and whether a second factor or video check backs up the read.
Do RFID tags work on metal surfaces?
Ordinary tags often fail on metal because it detunes the antenna and shields the field. Tags built for metal include a spacer or special antenna so they can be mounted directly on steel. Always test the exact tag in its final position.