ib technology
Data Sheet
ANTENNA_1356.PDF
6 Pages
Last Revised 18/02/08
Micro RWD MF (Mifare) Antenna Specification
The MicroRWD MF (Mifare) module has been designed to interface to a simple high Q
antenna coil of around 1uH inductance together with some capacitor and resistor
components. The antenna coil and passive components form the “tuned” RLC (Resistor-
Inductor-Capacitor) circuit, which is designed to resonate at the 13.56 MHz carrier
frequency and have a Q factor of 35-40. For the 125 kHz family of MicroRWD
modules, the low Q antenna (700uH) has a relatively wide tolerance of inductance so
the capacitor components are fixed and mounted on the RWD module itself – pins AN1
and AN2 connect to an external antenna coil directly. For the 13.56 MHz MicroRWD
modules, the antenna Q is higher and the inductance tolerance is narrow. For this reason
the capacitor components are external to the module to allow fine-tuning and adjustment
of the LC circuit as shown below. The external resistors are used to limit the antenna
current and correct the antenna Q to 35-40 range.
Approx 3cm max
65mm diameter coil, 2 turns 0.45mm diameter
Enamelled Cu wire (26 SWG), approx 1uH
Rext
C4 = 220pF
AN2 (pin 12)
Centre tap not required
for simple antenna
Rext
C2 = 22pF
typically < 0.5R
All capacitors ceramic 50v minimum
NPO/COG types +/- 5% or better
MicroRWD
MF
C1 = 22pF
AN1 (pin 9)
5-50pF trimming
capacitor
C3 = 220pF
GND (pins 7/13)
Directly connected antenna arrangement
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
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