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Ferrite Shielding
When the RWD antenna coil is positioned close to metal objects such as the reader
housing or even the PCB ground plane then the RF field induces eddy currents in the
metal. This absorbs the RF field energy and has the effect of detuning the antenna
(reducing the inductance). Both these factors can very significantly affect the
performance of the RWD system. Therefore for operation in metallic environments it
may be necessary to shield the antenna with ferrite. This shielding has the effect of
concentrating the magnetic field lines close to the ferrite material, which introduces a
fixed field component that detunes the antenna system (which can be accounted for in
antenna design). Practically, there is a compromise between the shielding effect
(reduction of eddy currents) and the concentration of the magnetic field (reduced range),
so the ferrite plane should only slightly overlap the antenna coil. The optimum size of
the ferrite plane, the distance from the coil and the degree of overlap are very hard to
calculate and must be determined practically. Tests have shown that best performance is
achieved when the antenna coil and ferrite plane overlap by around 5mm.
Magnetic field
Metal Plane
around antenna
coil
Eddy Currents
Magnetic field
Eddy currents absorb energy
and cause detuning of antenna
Optimum field distribution,
Ferrite Plane
Metal Plane
around antenna
coil
5mm overlap
fixed antenna detuning with
minimum loss of energy
Antenna Design
A useful formula is given below for calculating the approximate number of turns
required to achieve a particular inductance.
1.9
N = (approx)
L
1.9
or L = 2 . A . ln(A / D). N
2 . A . ln(A / D)
-9
L = Required Inductance (nH) i.e H x 10
A = Antenna Circumference (cm) ie. Pi x diameter (cm)
D = Wire Diameter (cm) e.g 0.0450 cm
N = Approx number of windings
4
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