RFID

RFID – chips with no battery, read at a distance

How can a chip with no battery be read several metres away? A walk through RFID – the three parts, the frequency bands, passive and active tags, how it differs from a barcode, why metal and liquid ruin things, and why the technology did not take over everything after all.

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What this covers

The three parts of an RFID setup
How a chip with no battery manages to answer
The frequency bands, and why the choice decides everything
Passive, active and semi-passive tags
RFID against the barcode – what the difference actually is
Metal and liquid – the physics that sinks projects
Read rate, and why it never reaches a hundred per cent

What this covers (cont.)

RFID, NFC, BLE and UWB – which is which
What it is actually used for
Privacy and the criticism
Why the hype around 2005 did not come true

The three parts

An RFID setup is always three things. When a project fails, it is almost always because the third one was forgotten.

The whole chain

The tagA chip with a unique number and an antenna. Sits on the item, the pallet, the tool or the card.
The readerSends radio energy out through one or more antennas, and picks up the answer.
The systemWhat turns the number into something useful: goods receipt, stock levels, tracking.

The first two are hardware and can be bought. The third is integration, and that is where the money and the time go. A reader shouting numbers into the air with nothing receiving them has no value.

How a chip with no battery answers

This is the elegant part of the technology, and worth understanding before reading on.

The tag borrows energy from the reader, and answers by changing how it reflects the signal.

The principle is called backscatter: the reader sends out a radio wave. The antenna on the tag harvests enough energy to wake the chip. The chip does not answer by transmitting anything itself – it has no power for that – but by switching its own reflectivity on and off in a pattern. The reader sees the change in its own signal, and reads the pattern as numbers. The tag is a blinking mirror, not a transmitter.

The frequency bands

This is the most important decision in an RFID project, and it is often taken without anyone understanding the consequence.

Three bands, three uses

Band	Range	Where it belongs
LF approx. 125–134 kHz	Centimetres	Animal ear tags, car keys, access fobs. Copes with being near metal and liquid better than the others.
HF 13.56 MHz	Up to about a metre	Cards, passports, library books, payment. NFC is a close relative in this band.
UHF approx. 860–960 MHz	Several metres	Logistics: goods receipt, stock counting, pallet reading, retail. Can read many tags quickly.

UHF is the band for goods flows, and that is where the large gain lies. But note the spread: Europe uses around 865–868 MHz, the United States around 902–928 MHz. Equipment and tags bought for one market can perform noticeably worse in the other – worth knowing when the goods, the readers or the supplier cross a border.

Passive, active and semi-passive

With and without a battery of its own

Passive

No power source

Costs little – often cents per tag
Can be stuck on as a label
Lasts indefinitely in practice
Shorter range, and requires a strong reader

Active

Its own battery

Costs far more per unit
Transmits by itself, and can reach tens to hundreds of metres
Can carry sensors – temperature, shock, humidity
The battery runs out, and has to be planned for

There is a middle form too, semi-passive: the battery powers the chip and any sensors, but the answer is still sent as a reflection. The rule of thumb is simple enough – passive on goods, active on whatever is worth watching in its own right: containers, vehicles, expensive equipment.

RFID against the barcode

The most common misconception is that RFID is “a better barcode”. It is a different kind of thing, with different strengths and different costs.

Two ways to identify an item

Barcode

Cheap and certain

Needs line of sight and the right angle
One at a time
Costs almost nothing – it is printed
If you read it, the answer is right
Usually identifies the product type

RFID

Fast and unseen

Needs no line of sight – reads through packaging
Many tags in one operation
Costs per tag, every time
If you miss some, you do not know which ones
Can identify the individual item

Line of sight is the real difference. A pallet can be read without opening it, and a shop shelf can be counted with a handheld rather than one item at a time. The last bullet on the right matters too: it is the difference between knowing you have “twelve of this model” and knowing which twelve.

The physics that sinks projects

Here is the single piece of information that saves the most time, and that often arrives too late in a project.

Metal reflects radio waves. Liquid absorbs them. Both ruin things for UHF.

What it means in practice: a tag stuck straight onto a steel rack or a paint tin behaves completely differently from how it did on the test bench. There are tags made for metal, with a spacer or a ground plane of their own, but they cost more and have to be chosen deliberately. Liquid is worse: water absorbs energy in the UHF band, so a pallet of drinks shields the tags in the middle. It is also why LF is still used on animals – that band minds less about sitting on something that is mostly water.

The read rate never reaches a hundred per cent

An RFID setup is probability, not certainty. A tag can lie in a shadow, face the wrong way relative to the antenna, or be drowned out by the other tags answering at the same moment.

RFID, NFC, BLE and UWB

The four get mixed up constantly, including by people selling them.

Which is which

Technology	In short	Typical use
RFID (UHF)	Passive tags read at several metres, many at once	Goods receipt, stock counting, retail
NFC	A close relative in the HF band, with two-way communication over a couple of centimetres	Payment by phone, access cards, the tag you scan on a poster
BLE	Bluetooth with low power draw. Its own transmitter and its own battery.	Indoor equipment tracking, sensors, phone connections
UWB	Wideband that measures distance by time of flight, not signal strength	Position to the decimetre – where “nearby” is not good enough

The practical dividing line: RFID and NFC tell you that something is here. BLE and UWB tell you where it is, but need a battery in every unit. If you have a thousand items to count, RFID is the answer. If you need to find one trolley in a hall, it probably is not.

What it is actually used for

What has worked best is retail with many variants and a high need for inventory accuracy – apparel above all, where the same model comes in many sizes and colours, and where knowing what is actually in the shop is worth money.

Privacy

A tag answers whoever asks, and it does not know who is asking. That is the nature of the technology, and it is the starting point for the objections.

Why the hype did not come true

What is worth taking with you

Key points

A passive tag has no battery – it borrows energy from the reader and answers by reflecting the signal.
The choice of frequency decides the range, and it is the most important decision in an RFID project.
RFID needs no line of sight, and that is the real difference from a barcode.
UHF is the band for logistics – but the frequency is not the same in Europe and the United States.

Key points (cont.)

Metal reflects and liquid absorbs. The physics decides whether a project can succeed.
The read rate never reaches a hundred per cent. The solution has to cope with that.
NFC is a relative of RFID. Bluetooth and UWB are something else.
The hype around 2005 collapsed on tag price and read reliability, not on the idea.

Questions this answers

What is RFID, and how does it work?
How can a chip answer without having a battery?
What is the difference between LF, HF and UHF?
What is the difference between passive and active tags?
What separates RFID from a barcode?
Why does RFID work badly on metal and liquid?

Questions this answers (cont.)

Why do you never read all the tags?
What is the difference between RFID, NFC, Bluetooth and UWB?
Where is RFID used today, and where has it not caught on?
Is RFID a privacy problem?

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