Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 108

the permeability of the ferrite material, and it may therefore be necessary to readjust the
resonant frequency or even redimension the matching network (in readers) altogether
(see Section 11.4).
4.1.12.4 Fitting transponders inmetal
Under certain circumstances it is possible to fit transponders directly into a metallic
environment (Figure 4.53). Glass transponders are used for this because they contain
a coil on a highly permeable ferrite rod . If such a transponder is inserted horizontally
into a long groove on the metal surface somewhat larger than the transponder itself,
then the transponder can be read without any problems. When the transponder is fitted
horizontally the field lines through the transponder's ferrite rod run in parallel to the
metal surface and therefore the eddy current losses remain low. The insertion of the
transponder into a vertical bore would be unsuccessful in this situation, since the field
lines through the transponder's ferrite rod in this arrangement would end at the top of
the bore at right angles to the metal surface. The eddy current losses that occur in this
case hinder the interrogation of a transponder.
It is even possible to cover such an arrangement with a metal lid . However, a
narrow gap of dielectric material (e.g. paint, plastic, air) is required between the two
metal surfaces in order to interrogate the transponder. The field lines running parallel
Figure 4.53 Right, fitting a glass transponder into a metal surface; left, the use of a thin
dielectric gap allows the transponders to be read even through a metal casing (Photo: HANEX
HXID system with Sokymat glass transponder in metal, reproduced by permission of HANEX
Co. Ltd, Japan)