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I have an 134.2 khz RFID antenna for sport events which is basically a 1x1 meter square sheet of rubber with 3 loops of antenna cable embedded. The problem is that the transponder recognition range is drastically reduced when the ground below the antenna contains metal (e.g. reinforced concrete).

Would it be possible to solve this problem by attaching a layer of mu-metal or permalloy to the lower side of the antenna? I read that mu-metal draws magnetic field lines into itself and thus redirects them around what's behind (in this case the metal in the ground). Would this make sure that the magnetic field on the upper side of the antenna is the same as if there was no metal in the ground? (and that the transponder recognition range is as good as without the metal) What thickness of the material would be required, would a foil be sufficient?

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