Our plates have anti-metal shielding: what is it for?

Why a metal surface stops an NFC chip from answering

An NFC chip has no battery. All of its energy comes from the phone, as an alternating magnetic field at 13.56 MHz. That field passes through the chip's antenna — a copper spiral etched around it — and induces the current that wakes it up. Without the field, the chip is an inert piece of plastic.

The trouble is that an alternating magnetic field does not stop at the chip. It carries on and reaches whatever sits behind. If that is metal, two things happen at once, and both work against you.

Eddy currents absorb the energy

The field entering the metal makes loops of current circulate inside it, known as eddy currents. Those loops produce their own magnetic field, pointing the opposite way to the phone's. The two fields subtract. Part of the energy meant for the chip turns into heat in the metal — far too little for you ever to feel your counter warm up — and what remains is no longer enough to power the chip.

It is the same principle as an induction hob, except the power involved is millions of times smaller. An induction hob is trying to heat the metal; your phone is simply trying to talk to a chip, and the metal is stealing its voice.

The antenna is detuned

The second effect is subtler, and it is often the one that finishes off the read. An NFC antenna is a tuned circuit: its geometry is calculated to resonate very precisely at 13.56 MHz, the NFC frequency. Bringing a mass of metal close changes the inductance of that spiral, exactly as resting your hand on a guitar string changes its note.

The antenna still resonates, but no longer at the right frequency. The phone transmits at 13.56 MHz and the chip is now listening beside it. So even if enough energy were left, the two no longer understand each other. This is why a plate placed on metal does not give a "weaker" or "slower" read — most of the time it gives no read at all.

That detuning also explains a puzzling behaviour many shop owners report: the plate works perfectly while held in the hand, then stops dead once stuck to the counter. Nothing is broken. The surface changed everything.

What other suppliers do

Most often, nothing

Most NFC plates on the market have no anti-metal protection at all. That is not always bad faith: on a painted wall, a glass panel or a wooden counter, the missing shielding never shows. The product works, the customer is happy, and the question simply never comes up.

The gap appears the day the ideal spot happens to be metal — and that is common, because the most visible places in a shop often are. The supplier then suggests moving the plate "a little further along", onto a neutral surface. You end up choosing the location around a technical limitation of the product instead of around your customers.

A reliable clue before buying: if a product page does not mention anti-metal, there almost certainly is none. And if the seller explicitly advises against metal surfaces, the question is settled.

A ferrite patch to stick on yourself

The other approach is to sell you — or let you buy separately — a ferrite patch: a flexible square a few hundredths of a millimetre thick, to slide between the chip and the surface. The physics is sound; it is exactly what built-in shielding does. The execution raises three problems.

The difference is less about physics than about reliability. A well-fitted ferrite patch works. The problem is that it has to be well fitted, and stay that way for years on a surface wiped several times a day.

What we do: anti-metal built in from manufacture

On our plates the shielding layer is not an accessory: it is placed between the chip and the adhesive during manufacture, then set within the thickness of the plate. You never see it, you have nothing to fit, and there is nothing that can come loose independently of the rest.

In practice this changes three things.

You pick the spot for your customers. The right place for a plate is the one the customer looks at while waiting: next to the card terminal, on the counter, at eye level. Whether that spot is stainless steel, zinc or laminate no longer enters into it.

There is no test to run before sticking it down. A plate's 3M adhesive goes on once. Without shielding you have to check first, which means holding the plate in place without bonding it — and that does not faithfully reproduce the final contact. With built-in shielding, you stick it straight on.

Behaviour does not drift. A plate that works on day one works on day one thousand. There is no patch shifting, no corner lifting, no read turning temperamental a year later for no apparent reason.

Where it makes all the difference

These are not textbook cases: they are the four setups we see most often among our customers, and in each of them the most effective spot is metal.

• A bakery's stainless steel counter. The customer pays, waits for change and looks straight ahead. That is exactly the moment they can leave a review — and the surface in front of them is stainless steel. See our page on Google review plates for bakeries.
• A bar top. A bar counter is a continuous metal surface, wiped all day long. It is the most visible spot in the room, and the most hostile to an unshielded chip. See Google review plates for bars and cafés.
• A pharmacy till. Counter space is tight and often taken up by metal fixtures. The plate has to fit the few centimetres available, with no choice of material. See Google review plates for pharmacies.
• A garage bonnet or workbench. Here everything is metal. An unshielded plate is simply unusable, wherever you put it. See Google review plates for car garages.

We document twelve trades this way, each with the placements that work best: the full list is on Google review plates by trade.

The thirty-second test

If you already own an NFC plate and wonder whether it is shielded, you need no tools at all.

1. Hold the plate in the air, away from any surface, and bring your phone close. Note that it responds.
2. Now lay it flat on a metal surface — a stainless steel counter, the top of a till, a car body panel. Do not stick it down, just rest it there.
3. Bring the phone close again, holding it there for two or three seconds.

If the read disappears or becomes erratic, the plate has no anti-metal protection. If it responds as quickly as it did in mid-air, it has. The test is reliable because both effects described above are immediate: there is no gradual degradation to watch for.

You can run the same check before buying, with a borrowed plate or in a shop. Thirty seconds settle a question that product pages often leave vague.

Buyer's tip: before buying an NFC plate elsewhere, check whether it mentions anti-metal protection. If it is not stated, there almost certainly is none — and you will be limited in where you can put it. Better still: if a seller advises you to avoid metal surfaces, that is proof their plate is not anti-metal.

The other mark of quality on our plates: the epoxy resin finish. And to understand how it all works, read our complete guide to the Google review plate.