What makes an on metal tag different?
A layer of ferrite material between the antenna and the surface. It redirects the field so the metal underneath stops cancelling it. This is a construction rather than a coating, which is why an on metal tag cannot be improvised from an ordinary one.
Why an ordinary tag fails
A conductive surface in an alternating field develops currents that generate an opposing field. Put a tag flat on steel and the field it needs is cancelled exactly where the antenna sits.
The failure is total rather than partial. The tag does not read at a shorter distance, it does not read at all, and moving the phone closer makes no difference.
It also detunes what little remains, because the metal changes the electrical environment the antenna was tuned in. Two mechanisms, both working against you.
What the ferrite does
Ferrite is a magnetically permeable material that offers the field an easier path than the metal below. Placed between the antenna and the surface, it channels the field so it loops through the coil rather than being cancelled.
The metal is still there and still conductive. The ferrite does not neutralise it, it gives the field somewhere better to go.
This is why thickness matters. A thin ferrite layer redirects less, so on metal tags are thicker than ordinary ones and the good ones are noticeably so.
What it costs you
Thickness, first. An on metal tag stands proud of the surface in a way a label does not, which matters on a smooth product and not at all on a machine housing.
Price, second. The ferrite layer is a real material cost, so these tags are several times the price of a plain label.
Range, third and mildly. An on metal tag reads a little less far than the same antenna in free air. Compared with an ordinary tag on the same surface, which reads nothing, this is not a meaningful complaint.
They work off metal too, so a mixed deployment can standardise on them rather than stocking two products. That is often the cheaper answer once the labour of sorting tags is counted.
Buying one
Ask whether the tag is on metal by construction. Some products are described as suitable for metal when they are ordinary tags with a foam spacer, which is a different and weaker thing.
Test on the actual metal. Steel, aluminium and coated surfaces behave differently, and a tag validated on one is not validated on the others.
Test at the real thickness of whatever it is stuck to, since a thin sheet and a solid block are not equivalent.
And test with the phone, not with a dedicated reader, because a reader with a strong field will succeed where a phone will not.
Frequently asked questions
Can I make my own on metal tag?
Not reliably. A foam spacer helps a little and a proper ferrite layer is a manufactured component, so improvising gives inconsistent results across a deployment.
Do on metal tags work on other surfaces?
Yes, generally. They are overkill on wood or plastic and they still read, which is why standardising on them can be simpler than stocking two types.
Does aluminium behave like steel?
Both cancel the field and they differ enough that a tag validated on one deserves a check on the other. Test the actual material.
Is a thicker on metal tag always better?
More ferrite generally redirects more field, and thickness has practical limits on the product. Choose the thinnest that reads reliably on your surface.