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8) The strength of the magnetic (inductive) field of the antenna coil is proportional to
the number of turns of wire used. Therefore, for maximum performance and range,
the number of turns should be as high as possible and the gauge (thickness) of wire
chosen to ensure the total Rant value (DC resistance) is kept around 31.5 ohms.
Therefore, for antenna coils with DC resistance of 10 ohms, the series resistor
should be 22 ohms.
For antenna coils with DC resistance of 20 ohms (better performance and range), the
series resistor should be around 10 ohms.
9) 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.
Metal Plane
Magnetic field
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
3
fixed antenna detuning with
minimum loss of energy
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