ib technology
Data Sheet
ANTENNA_125.PDF
4 Pages
Last Revised 21/09/09
Micro RWD 125kHz Antenna Specification
The Micro RWD has been designed to interface to a simple low Q (10-20) antenna coil
of around 700uH (micro Henry) inductance operating at a carrier frequency of 125 kHz.
For maximum range and performance the following factors should be considered:
1) Maximum range and coupling between transponder and RWD is based on the ratio
of their antenna diameters. Very approximately the RWD antenna loop diameter
should be 2-3 times the diameter of the transponder coil. The basic method of
communication is via magnetic flux linkage (like an air-cored transformer) so the
more lines of flux that intersect the transponder coil, the better the overall
performance. For ISO card transponders there is little benefit in using an RWD
antenna larger than 10cm diameter. Circular antenna coils generally give a more
uniform flux distribution.
2) The Micro RWD is designed to give up to 400ma pulse current with a peak voltage
of up to 200v. This can give a 20cm read/write range with HT2 ISO card
transponders. Lower burst current can be achieved by inserting an additional series
resistor in the antenna loop circuit (22R in series would typically reduce pulse
current/voltage by about half using example antennas).
This is recommended in
order to give the optimum signal-to-noise ratio and Q value.
3) Sample antenna supplied is for demonstration only. The characteristics of an
antenna for EMC/WT approval will vary according to shape of coil, type of wire
used, style of winding, bobbin material, compaction of windings etc.
4) If the antenna is to be positioned remote to the Micro RWD a screened twisted pair
cable can be used to connect them. The screen should be connected to the Antenna
GND pin on the Micro RWD. Cable lengths up to 1.5 metres have been tested and
perform well with no significant system degradation.
5) The capacitance and inductive effect of antenna cable and physical positioning of
the antenna should be taken into account when designing the antenna coil and if
necessary the inductance of the coil should be adjusted to compensate. The 125KHz
tuned circuit is basically an RLC network with the C element fixed on the Micro
RWD board. Tuning the antenna inductance (L) for optimum performance is quite
simple, the positioned antenna system should be connected to the Micro RWD and
power applied. An oscilloscope should be connected between the AN 2 pin and the
GND pin.
Without
a transponder in the field a pulsed 125kHz sine wave will be
seen with a peak voltage of up to +/-180v, with small switching “steps” visible near
the peaks and troughs of the sine wave.
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