RFID reader behind a metal panel: spacing and window design

A solid metal panel blocks a 13.56 MHz card reader completely, and a grill of small holes changes nothing, because the eddy currents simply flow around them. The fix is mechanical. Cut the metal away over the reader coil, bridge the opening with a plastic window, and size the aperture to the coil outline plus 20 mm [3/4 in] on every side. Keep every other metal part outside that keep-out, twice as far where you can, and prove the range with a real card on a cardboard mockup before any metal is ordered. This page walks through that design for a 13.56 MHz module of the class that reads an ID card at 30-50 mm, and ends with what to put on the order.

Will an RFID reader work behind a metal panel?

No: a solid aluminium or stainless sheet detunes a 13.56 MHz coil and blocks the tag completely, and a grill of 1 mm or 2 mm holes makes no difference, because the eddy currents simply circulate around every hole you drill.

A reader at 13.56 MHz does not broadcast to the card; it couples magnetically, like a transformer whose second winding is the card antenna. Half a wavelength at that frequency is about 11 m, so the coil is electrically tiny and the card sits deep inside its near field. That is why the useful read distance is only 30-50 mm for a module of this class, and why the panel in between matters so much. The alternating field induces eddy currents in the sheet; their field cancels the coil's by Lenz's law, and the extra load pulls the antenna off resonance. The module then fails to read even with the card touching the far side.

The numbers are brutal. Skin depth in aluminium at 13.56 MHz is about 0.02 mm, so a 2 mm sheet is around a hundred skin depths thick and passes no useful flux; household foil at 0.015-0.02 mm already sits at one skin depth. Anodising is an oxide layer a few micrometres thick on a conductor, so the surface finish changes nothing. The price of the module changes nothing either: the physics treats a cheap reader and an expensive one identically.

Two tempting shortcuts fail for the same reason. A hole grid leaves the sheet a continuous conductor, and with a wavelength of about 22 m, a 2 mm hole is roughly 1/11,000 of a wave — invisible to the field. Cutting a complete ring around the coil area fails too, because the isolated island of metal is still a closed conductor acting as a shorted turn. Only metal you actually remove over the coil restores the path. Radiating links behave differently — steel still blocks them, plastic does not, and metal can even serve as a deliberate antenna ground plane — but magnetic coupling has no such trick to sell.

What do you need before you cut the panel?

Gather four things before any metal is cut: the coil outline in millimetres from the module drawing, the card or tag you will present, a 3 mm polycarbonate blank with its gasket for the window, and a test card with a cardboard mockup panel to prove the range early.

• Coil outline. Read it off the module's antenna drawing or measure the loop on the board; common 13.56 MHz card-reader modules carry coils of roughly 45 × 27 mm, and an RC522-class board sits in that band.
• Card or tag. An ID-1 card measures 85.6 × 54 × 0.76 mm (ISO/IEC 7810), and it hovers over the window rather than passing through; a key fob couples differently, so test with the stock you will actually issue.
• Window kit. A 3 mm [1/8 in] clear polycarbonate blank, cut 10 mm [0.4 in] oversize per side, with a closed-cell gasket that compresses 20-30%.
• Cable entry. A gland sized to the cable; the notes on PG7 gland alternatives and fit show how to check hole against gland before ordering.
• Mockup. A cardboard sheet with the aperture at final size, tape, and the module at its real depth behind it.

How big should the aperture over the reader coil be?

Size the opening to the coil outline plus 20 mm on every side: a typical 45 × 27 mm module coil needs an 85 × 67 mm aperture, and the margin matters more than the exact shape.

That margin keeps the nearest metal edge outside the zone where the coil's field is still strong past its outline; the edges and corners of the loop detune first, so the margin works hardest exactly there. Here is the arithmetic for a typical card-reader module with a 45 × 27 mm coil behind a 2 mm aluminium sheet:

Round the aperture up, never down: an 85 × 70 mm opening for this coil costs nothing extra and forgives placement. Corner radii of 4 mm let the gasket and window sit cleanly. An ID-1 card of 85.6 × 54 mm laid flat over the read zone covers the whole 45 × 27 mm coil with margin to spare, which is what puts the card's antenna over the reader's. If the module datasheet states a clearance for its own antenna, the datasheet wins over any rule of thumb here.

How far from metal should the coil sit?

Once the aperture is cut, keep every metal part at least 20 mm from the coil face, prefer 40 mm behind the module for standoffs, backplates and fasteners, and count paint, anodising or powder coat as nothing, because they are micrometres thick and the currents run underneath them.

The aperture edge sits at exactly that clearance by design, which is the minimum, so count everything else — DIN rail, a steel backplate, the gland body, mounting screws — and place it outside the keep-out. The field reaches backwards and sideways as well as forwards: a steel backplate 5 mm behind the module loads the antenna exactly as a front panel would. Mount the module on plastic standoffs and let metal fasteners land only where the coil outline plus the margin leaves them room.

When a dense panel forces metal close behind the reader, fit a ferrite-backed coil or a ferrite sheet between coil and metal — the standard fix for mounting NFC antennas on metal, and the one the chip makers' antenna guides recommend. Ferrite keeps the field out of the metal instead of letting the eddy currents cancel it, so metal a few millimetres behind the module becomes workable. The card side gets no such help: metal in front of the coil blocks the read regardless of what sits behind it.

Alloy changes the numbers, never the answer. Stainless carries eddy currents less readily than aluminium, but any metal inside the strong field still loads the coil, so design by distance and let the card test close the argument.

How do you cut and fit a read window?

Cut the aperture to the coil outline plus the 20 mm margin a side, bridge it with the 3 mm polycarbonate window sealed on all four edges, bring the reader cable in through a rated gland, and keep every fastener clear of the keep-out — five steps, in order.

1. Mark the aperture from the coil outline plus the margin per side, with 4 mm corner radii, and put the window edge and gland position on the same drawing.
2. Cut the opening. On a panel we build, the aperture comes from your cutout drawing and the finished box is delivered in 10 business days from drawing approval; on a panel you cut yourself, deburr both edges so the gasket seals on clean metal.
3. Fit the window: 3 mm clear polycarbonate over a closed-cell gasket compressed 20-30% around the full perimeter, with fasteners every 75 mm or so.
4. Bring the cable in through a gland rated for the environment, at least 50 mm [2 in] from the coil outline; a metal gland body inside the keep-out loads the coil like any other metal.
5. Close the rating: the window gasket, the door gasket and the gland now define the assembly rating, because the weakest part sets it — the notes on what rating a panel keeps after you cut it and mount devices and gasket design for IP64 and IP65 panels carry the detail.

The window itself is radio-invisible. Polycarbonate carries no eddy currents, so 3 mm of it costs essentially no range, and even radiating links lose little through a few millimetres of plastic, as the 433 MHz plastic-enclosure comparison shows. The cutouts, glands and windows guide covers the general art of cutting a rated box; the pass test for this one comes next.

How do you prove the read range before you commit?

Mock the finished panel up in cardboard with the aperture cut to size, tape the module behind it at its real depth, and accept the design when a test card reads at the centre and all four corners from a flat touch out to a 10 mm hover.

Run 20 consecutive reads per position and let the worst corner govern. A corner that drops out points at metal inside the keep-out — usually a screw, a label or a steel rule left on the bench. Record the maximum hover distance per corner, then repeat the whole test on the real panel with the window, gasket and gland fitted, because the gland brings metal into the box that the cardboard never had.

A short range is a symptom, not a nuisance. When the module reads at 5 mm but fails at 20 mm, something conductive sits closer to the coil than the design allows. When it fails everywhere at any distance, look at the module side first — power, wiring and the SPI link — and the write-up on RC522 readers that read no tags at all works through those checks.

Which mistakes cost range or the panel rating?

Range dies on forgotten metal — grill holes, foil-backed labels, stainless screws and metal gland bodies inside the keep-out — while the rating dies on sealing, because an open aperture or an unrated gland drags a Type 4X assembly down to splash-resistant.

• A hole grid instead of an aperture: currents close around every hole, and 2 mm is 1/11,000 of the 22 m wavelength.
• A ring cut that leaves an isolated island: it still behaves as a shorted single turn.
• Foil-backed or metallised labels over the read zone: one foil layer already sits at the 0.02 mm aluminium skin depth.
• Metal standoffs, screws, hinges or the gland body within the 20 mm clearance of the coil outline.
• The module clamped hard against a steel backplate with no ferrite between.
• An aperture left without a sealed window, or an open breather added to a washdown panel: the assembly takes the lowest rating among its parts.

Every one of these shows up on the cardboard mockup for free, which is why the mockup comes before the metal.

What do you put on the order?

Order the enclosure with the aperture cut to your drawing, list the 3 mm polycarbonate window with its gasket and the cable gland, and note that our stainless bodies are rated NEMA 4X / IP66 by the manufacturer; not UL listed, delivered in 10 business days from drawing approval.

Three cases cover every reader behind metal. If you are adapting an existing panel, cut the aperture, fit the sealed window, and pass the cardboard test before the real cut. If the contact panel is new and lives in washdown, salt air or dust, order a stainless body with the factory aperture and window: custom enclosures take the material, size and every cutout from your drawing, and the stainless cutout page shows what to send. If no aperture is possible on an existing sealed face, move the read point to the tag side: a small plastic pod stood off the panel so the coil keeps its clearance, a ferrite sheet behind the coil if the pod must sit close, and one sealed gland through the panel for the cable.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions