RFID antennas and range: what really decides it.

What an RFID antenna does

The antenna at the reader

A reader carries either an external antenna on a coaxial cable or a built-in one, as on a handheld. It has three jobs. First: put the energy into the space where the tags are. Second: focus that energy so that it goes where the reading is meant to happen — and not into the next room. Third: pick the extremely weak return signal back up. That return signal typically sits 60 to 90 decibels below the transmitted signal, which is why the reader and the antenna have to keep the transmit and receive paths cleanly apart.

Between the reader and the antenna sits the cable, and the cable costs power. A common coaxial cable loses about 0.2 to 0.7 dB per metre at 868 MHz, depending on the type. With five metres of thin cable, half the transmit power is quickly gone before it reaches the antenna. On permanently installed systems the cable is therefore not a side issue.

The antenna in the tag

A passive tag has no battery. Its antenna is power supply and transmitter in one. It has to collect enough power to wake the chip — modern UHF chips need about −20 to −22 dBm for that, less than a hundredth of a milliwatt. After that the tag answers not with a transmitter of its own but by backscatter: it changes its antenna impedance in time with the data and reflects the reader field now more strongly, now more weakly. The reader picks that modulation out of its own echo.

The length of the tag antenna follows directly from the frequency. At 868 MHz the wavelength is around 34.5 centimetres, so a half-wave dipole comes to a good 16 centimetres. Because a label cannot be that large, the structure is folded into a meander and ends up at 70 to 100 millimetres. Every fold costs efficiency. That is why a large label almost always reads further than a small one, and why the tag antenna — not the chip — is the main reason for range differences between two labels.

Linear or circular polarisation

Why circular polarisation is usually right in retail

A UHF label is at heart a dipole. It responds to the field that oscillates parallel to its long axis. Goods on a rail, though, hang in no particular alignment: a hangtag dangles, turns, lies flat one moment and on edge the next. With a linearly polarised antenna that means part of the tags stand at the optimum and another part in a deep null — with the tag exactly crossed, the coupling is zero in theory and 20 dB or more below the maximum in practice. Those tags are then missing from the count.

Circular polarisation solves that by rotating the field a little further with every period. On average every tag then gets the same coupling, however it hangs. The price is a constant 3 dB or so, because only the field component that happens to match the tag axis does any work. 3 dB corresponds to roughly 30 per cent less range. That even loss is far easier to handle in retail than occasional total failures.

• Circular polarisation for goods in no fixed order: rails, tables, cartons, handheld stocktaking, exit gates.
• Linear polarisation where the alignment is controlled: conveyor belts, label printers, tunnel readers, applications that want maximum range with the tag always lying the same way.

Circularly polarised antennas come in left-hand and right-hand versions. Two antennas with opposite senses of rotation couple poorly to each other — which is used deliberately to keep two read zones apart.

Gain, beam width, near field and far field

Gain (dBi)

dBi is a ratio, not a power figure. 3 dB more gain doubles the radiated power density in the main direction, 6 dB quadruples it. For range, as a first approximation: four times the power gives twice the range, as long as the sensitivity of the tag sets the limit — on the way out, the power falls with the square of the distance. If instead the return signal is what runs out, matters get considerably worse, because the backscatter covers the path twice and falls off with the fourth power in total. With today's readers the way out is usually the bottleneck.

Beam width

Beam width, also called half-power beamwidth, describes the angular range in which at least half the maximum power density still arrives. Gain and beam width belong together: an antenna with 9 dBi reaches further but narrower; one with 5 dBi covers a wider area but does not get as deep into the room. For a gate on a door 2 metres wide the wide antenna is usually the better choice, for a long shelf aisle the narrow one.

Near field and far field

Close to the antenna the field behaves differently from the way it behaves further out. As a rough boundary, take one wavelength divided by 2π. At 868 MHz that is around 5.5 centimetres — everything beyond it is far field, and there UHF RFID works with radiated waves and backscatter. At 13.56 MHz, by contrast, the wavelength is about 22 metres and the boundary lies at around 3.5 metres. HF and LF systems therefore work entirely in the near field and couple inductively, that is through the magnetic field, like a loosely coupled transformer.

That explains two practical differences. First, in the inductive near field the field strength falls very steeply, roughly with the third power of the distance — which is why LF and HF end after centimetres and can at the same time be confined very sharply. Second, the magnetic field is far less sensitive to water than the UHF far field, which is why HF is used where a lot of liquid is involved.

Frequency bands and their ranges

Ranges are spans for passive tags under favourable conditions. The actual figure depends on tag size, antenna gain, transmit power, the surface behind the tag and the surroundings, and on a full shop floor it regularly sits at the lower end.

The step between HF and UHF is not a gradual one but a physical one. HF couples magnetically and ends as soon as the coil of the reader no longer carries the field. UHF radiates, and the wave carries on until it is too weak for the chip. Only that makes it possible to let several hundred tags answer one after another every second — the basis of every RFID stocktake.

The legal framework in Europe and the United States

Europe: ERC/REC 70-03 and ETSI EN 302 208

The harmonised standard EN 302 208 sets out the conditions for RFID equipment in the 865 to 868 MHz band, which CEPT recommendation ERC/REC 70-03 makes available across the EU and the EEA. The key points are the ceiling of 2 watts ERP, a channel raster of 200 kHz and procedures that keep several devices in the same room from blocking one another. What matters in practice: the 2 watts apply to the radiated power, that is to transmit power plus antenna gain minus cable loss. Connect a 9 dBi antenna and you have to set the reader lower than you would with a 6 dBi antenna. Because the band is harmonised, the same device is legal in every EU and EEA country; what differs is the national regulator that administers the general authorisation — the Bundesnetzagentur in Germany, ARCEP in France, UKE in Poland, RDI in the Netherlands, and so on. Normal operation needs no individual licence and no registration, and no spectrum fee is payable.

Newer versions of the standard also provide for an upper band at 915 to 921 MHz, which several European countries have released and which permits higher power. Whether you may use it depends on the national frequency allocation and on the approval of the device. For an ordinary fashion shop it makes no difference — practically all hardware there works in the lower band.

The United States and the rest of the world

North America uses 902 to 928 MHz, a considerably wider band in which the devices change frequency constantly. Up to 4 watts EIRP is permitted there. That sounds like twice the European limit, but it is not: the European 2 watts ERP correspond to roughly 3.3 watts EIRP, so the US limit sits only about 20 per cent above it. Range figures from American datasheets therefore cannot be carried over one to one to a European shop. Other regions again have allocations of their own, Japan and China for instance in the range above 900 MHz.

For tags this is mostly uncritical: ordinary UHF labels are built as worldwide-band versions for 860 to 960 MHz and work in both regions, if not equally far in both. The reading side is where it matters. A reader bought in the United States may not be operated in Europe, and a handheld for Europe does not reach the ranges that are possible in North America. Devices sold for other regions are hardware variants, so order the EU variant explicitly. The United Kingdom sits outside this framework as well: since leaving the EU it applies its own rules, with Ofcom interface requirement IR 2030 and UKCA marking in place of CE.

This page is no substitute for advice on radio law. What governs is the version of the standard in force and the national frequency allocation, administered by the regulator of the country in question.

What limits range in practice

• Metal. A conductive surface directly behind the tag all but cancels the field. A standard label lying flat on sheet metal is often not read at all. Metal shelving, clothes rails, mirrors and air-conditioning ducts also produce reflections, and with them zones in which waves cancel one another out.
• Liquids. Water absorbs strongly in the UHF range. Drinks, cosmetics and damp goods attenuate the signal noticeably. Wet clothing reads worse than dry clothing.
• The body. People consist largely of water and therefore act as an absorber. Carry the handheld in front of your stomach and you give away range to the rear; a tag between the goods and your body often goes uncounted.
• Tag size. The larger the antenna area, the more energy the tag collects. A small label for jewellery reads 1 to 2 metres, a large hangtag 4 to 10 metres.
• Alignment. Even with a circularly polarised antenna a residual factor remains: if the dipole axis of the tag points straight at the antenna, it lies in the null of its own radiation pattern. A tag “on edge” reads worse than one that shows the antenna its face.
• Shadowing in the pile. Densely packed goods shield themselves. The bottom piece in a stack is often only picked up on the second pass.
• Reflections. In rooms with many hard surfaces, standing waves form with maxima and minima a few centimetres apart. That is why it helps to move the handheld a little while counting rather than holding it still.

In practice, read rates per pass on tagged clothing lie between 97 and over 99 per cent, depending on the set-up and on how you work. The remaining pieces are found with a second pass, or by walking the critical zones — metal shelving, mannequins, corners of the stockroom — individually and on purpose. Anyone promising 100 per cent does not have physics on their side.

Antenna placement in the shop

The gate at the exit

Two to four circularly polarised antennas to the left and right of the door, usually at two heights. The gate detects which articles leave the shop — as article surveillance and at the same time as an automatic outbound booking. The challenge is the boundary: the field must not reach into the shop floor, or the system reports goods as having left when they are merely hanging near the door. That is solved with reduced transmit power, antennas with a narrow beam width and by evaluating the order in which the antennas see a tag.

Ceiling antennas

Antennas in or under the ceiling cover zones from above. They are suited to tracking roughly which area an article is in — shop floor, fitting room, stockroom. The accuracy is at zone level, not at shelf level, and the effort grows with every additional zone.

Smart shelf

Here the antenna sits in the shelf itself, often as a near-field structure with a very short range, so that only the articles in its own compartment are picked up. That delivers stock data in real time and without anyone doing anything, but it is the dearest variant: every compartment needs an antenna, cabling and a channel at the reader. It makes sense with a small number of high-value articles, watches or jewellery for instance.

Why the handheld is usually enough

A handheld carries its antenna with it. Range then stops being a problem you have to solve by building something — you simply walk closer. For a shop with a few thousand pieces that means a complete count takes one to two hours, without a single antenna being mounted. That is exactly why HANGCOUNT works with a handheld rather than a fixed installation; gates and smart shelves only pay off once stock is to be monitored permanently rather than periodically.

NFC compared with UHF RFID

The confusion is understandable, because both methods use the same basic principle: a reader powers a battery-free data carrier and reads its number. The difference lies in the physics behind it. NFC is deliberately built for a short range, because proximity is a security feature here: whoever pays should hold exactly one card to exactly one terminal. UHF is designed for the opposite.

In practice that means a smartphone can read an NFC tag in the label and show the customer a product page. What it cannot do is count a clothes rail. Anyone who wants both works with two tags or with dual-frequency labels carrying an HF and a UHF chip — that costs more and is unnecessary for stock control alone.

Common questions about RFID antennas

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