Need info on Ferrite shielding in metallic environment

[kern_paillard]

Dear all,

This is my first post in the forum.

I am working on a RFID technology used in door opening systems. The wall readers, we use are typically placed behind the door, as is the case with the metal being used on the front side of the door and in the key hole casings there is the obvious problem of intereference by the metal in communication between the RFID tag & reader.

So we thought of experimenting, by bringing the antenna of wall reader a little forward i.e. to be placed inside of the KEY HOLE CASING (made of metal) on front of the door while keeping the main electronics of the wall reader behind in hopes that bringing the antenna forward would help in the communication between tag and reader, BUT it doesn't work as the reader antenna coil is placed directly on the KEY HOLE CASING courtesy Eddy currents et al.

Acc to the RFID H-book, by introducing a highly permeable ferrite btw the antenna coil and metal surface it´s posssible to prevent the occurence of eddy current (Sec 4.1.12.3)

I would like to know about the highly permeable ferrite.
exactly what kind of ferrite should I use [dimension (MAX. PERMITTED THICKNESS: 3-4mm, 52mm x 35mm), material etc]?
If anyone has worked on this problem, can you share some details?

Retuning the system, isn't allowed.

Please suggest about the ferrite material, ways to enhance the T<--> R communication and any other ideas you have.

I hope, I was able to communicate my problem clearly.

Thanks in advance,

Kern P.

 

[berkeman]

Welcome to the PF, Kern. I hadn't heard of the technique you mentioned, about using the ferrite to try to prevent eddy currents in the metal from attenuating the RF signal. I don't have the book you refer to -- can you locate a web page that talks about the technique?

Doesn't the RFID system that you purchased (and cannot return) come with mounting instructions? It should use a plastic enclosure that is mounted on the outside of the door in order to avoid the attenuation problems that you are describing.

 

[mdelisio]

Using ferrites as asborbers in microwave and RF systems is often done, but a bit of a black art. You have to experiment with the material, location, and size a bit. I would suggest using a sheet of ferrite-loaded rubber. These sheets come loaded with various ferrites and they come in various thicknesses. They have an adhesive on the back so you can stick it down.

Cuming Microwave has a "Problem Solver Kit" that has a varitey of materials and thicknesses:

http://www.cumingcorp.com/pdf/310-Rubber%20Sheet%20Absorbers/TB301_Problem%20Solver%20Kit.pdf

Emerson and Cuming Company also offers a free kit:

http://www.emersoncumingmp.com/sales/sample_tube.htm

As does ARC Technologies:

http://www.arc-tech.com/

You may want to get some of these and experiment to see what works best for your application.

Hope this helps.