RFID blocking/sheliding

[Buzz84]

Hello all,

Wondering if some of the clever people on here might be able to assist. I know what I need to do but the technical/Math knowledge is out of my realm to work out the specifics.

Where I work we have installed and are commissioning a new system to log movements of pallets from the factory, into stores and then through to despatch. It uses RFID tags in the pallet labels to achieve this.

The antennas for this system are a directional type, so they "send" most of their "detection field" in the direction that they are aimed. But in reality there is some "leakage" to the rear and side so they are able to read tags when passing close to the rear and side.

This could causes a potential issues as due to the design of the factory, pallets transit close by one despatch bay when on the way to the other, also there is a workstation between the bays where tags may end up and we are concerned that there is potential for miss reads (we have seen it happen during the current commissioning phase).

While the easy method would be to say no tags to be stored at this location and tell the forklift drivers to take a wide birth, its always better to engineer out the issue completely.

We have some metal protection around them to prevent knocks and have made allowances to be able to attach some shielding material to them when they were made, We could easily just fit a solid plate to block the signal, but I feel that perforated plate would be a lot more aesthetically pleasing!

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This is where the assistance is needed, I have looked into it and searching RFID blocking material generally just lists fine copper mesh, but we would need a more sturdy plate.

there is very little information that I have found relating to this and one site seem to suggest calculations of

physicsforum said:

At 870 MHz, λ is 345 mm; The rule of thumb for hole dimension is λ / 20 = 17 mm.

Reference: https://www.physicsforums.com/threads/mesh-hole-si...

(our antennas spec is 865-868 Mhz)

If this is correct, then presumably 17mm is the biggest size that would block the signal and anything smaller would too? does distance from the antenna play any part? Would the material make a difference?

Very grateful for any help or advice anyone can give.

[FMOB]

A simple antenna will radiate efficiently when the length is a half or quarter wavelength, what you are trying to do with the hole size is make it very inefficient at allowing energy through.

Using a 1/20 of a wavelength for the max hole size will do the job as will smaller hole diameters. Think of a solid sheet as containing an infinite amount of infinitely small holes.

Some others suggestions, can you turn the RFID reader transmitter power down or attenuate it to reduce the range so tags in the wanted direction are read but not in the unwanted directions, changing the mounting angle of the tag relative to the read antenna may have a similar effect.

Because of the frequency you could use copper or aluminium as the shield material, if you are trying things then get some aluminium tape from screwfix for £7 that you can stick on some cardboard and mount in different positions, etc.

[Buzz84]

FMOB said:

A simple antenna will radiate efficiently when the length is a half or quarter wavelength, what you are trying to do with the hole size is make it very inefficient at allowing energy through.

Using a 1/20 of a wavelength for the max hole size will do the job as will smaller hole diameters. Think of a solid sheet as containing an infinite amount of infinitely small holes.

Some others suggestions, can you turn the RFID reader transmitter power down or attenuate it to reduce the range so tags in the wanted direction are read but not in the unwanted directions, changing the mounting angle of the tag relative to the read antenna may have a similar effect.

Because of the frequency you could use copper or aluminium as the shield material, if you are trying things then get some aluminium tape from screwfix for £7 that you can stick on some cardboard and mount in different positions, etc.

Thank you,

So my googling along with what you have said with the 1/20, seem to agree that 1.65 to 1.7cm holes would be enough to block it. So some 10mm perforated plate should do the job. (As I have to buy the plate either way the perforated would simply look better!)

I do have access to the settings and do plan to adjust the transmitter power to prevent over reading. I cant reduce it down too much as because the loading bay is just an open shutter and not a dock, the pallets will not always pass the readers at the same distance - The position of the lorries can vary and the pallets get loaded in two rows so they can be are with on the left or right of the trailer. meaning that there is probably a 3m window in the middle in which they could pass.

But as of yet its not possible to adjust the settings as the system has yet to be used with pallets containing stock, the commissioning is still all currently focused on the software system behind it all and the RFID equipment has only recently gone in, So that's all to come later.