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.
1
ib technology
Optimum tuning and performance is achieved when these “steps” are seen exactly at
the maximum and minimum points of the sine wave, the peak voltage will also be at
its maximum value. The antenna series resistor (R) should be set to limit the peak
voltage to 110v maximum in order to give the best Q value and optimum signal-to-
noise ratio
(For practical purposes use +/-100v peak voltage as limit).
Detuned antenna.
L too low (<700uH)
110v
peak
Micro RWD driver
switching steps
Low antenna
voltage
125kHz correctly tuned
antenna waveform with
Ra = 22 R to limit peak
voltage
AN1
Low antenna
voltage
Detuned antenna.
L too high (>700uH)
700uH
Antenna
22R
AN2
MicroRWD
antenna
connection pins
6) For optimum performance the antenna Q should not exceed 20 and to achieve
reliable tuning at 125kHz the antenna inductance should be around 700uH. Higher
Q and inductance values will still function but with a reduced range and
performance.
The formula for calculating Q = 2*π*fL / Rant = 549 / Rant
where f = Resonant frequency, 125 kHz, L = Antenna inductance, 700uH
Rant = Overall antenna resistance = Rdriver + Ra + (Rcu + Rrf)
π
= 3.14159 etc
Rdriver = 3.5 R (from IC spec) and Ra = 22 R (series resistor in antenna loop)
Rcu = Resistance of Copper (coil and cable) and
Rrf = RF resistive component (eddy current losses etc)
By measurement at 125kHz, (Rcu + Rrf) = approx 6R
Therefore Rant = 3.5 + 22 + 6 = 31.5 Ohms, Q = 549 / 31.5 = 17
Max peak antenna current (with 22R series resistor),
Iant max = 4Vdd /
π*Rant
= 20 /
π*31.5
= 200ma
Max peak antenna voltage, Uant max = Iant max . (2*π*fL) = 110v
7) The Micro RWD has been designed to work at optimum performance with
Rant = 31.5 (Ra = 22R), Uant max = 110v and Iant max = 200ma.
When designing and winding antenna coils, the wire gauge and it’s DC resistance
must be considered to keep close to the design optimum. Significant differences in
Ra will affect resilience to EMC and overall performance.
2
ib technology
8) 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
around antenna
coil
Eddy Currents
Eddy currents absorb energy
and cause detuning of antenna
Metal Plane
Magnetic field
around antenna
coil
Ferrite Plane
Optimum field distribution,
fixed antenna detuning with
minimum loss of energy
Metal Plane
5mm overlap
A useful formula is given below for calculating the approximate number of turns
required to achieve a particular inductance.
1.9
L
N = (approx)
2 . A . ln(A / D)
-9
1.9
or L = 2 . A . ln(A / D). N
L = Required Inductance (nH) i.e H x 10
A = Antenna Circumference (cm) ie.
π
x diameter (cm)
D = Wire Diameter (cm) e.g 0.0236 cm
N = Approx number of windings
3
ib technology
Example Antennas
A)
0.236mm diameter (34 swg) Enamelled Cu wire.
Antenna 14.5cm internal diameter, 45 turns.
Approx 700uH
Maximum range: approx 20cm with HT2 card transponders
B)
0.236mm diameter (34 swg) Enamelled Cu wire.
Antenna 10cm internal diameter, 55 turns.
Approx 700uH
Maximum range: approx 15cm with HT2 card transponders
C)
0.236mm diameter (34 swg) Enamelled Cu wire.
Antenna 7cm internal diameter, 67 turns.
Approx 700uH
Maximum range: approx 10-15cm with HT2 card transponders
Examples of antenna (C) and a very small antenna for applications with limited space
No responsibility is taken for the method of integration or final use of Micro RWD
More information on the Micro RWD and other products can be found at the Internet web site:
http://www.ibtechnology.co.uk
Or alternatively contact IB Technology by email at:
sales@ibtechnology.co.uk
4
stm32/stm8
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
