23 mm Transponder Reference Manual
25 July 1996
Page 16 of 22
Determination of the minimum powering field strength is possible by changing the field strength
through increasing the coil current. The relation between the generated magnetic flux / field
strength and coil current can either be measured with a calibrated filed probe, or calculated as
follows:
(2)
B
N I
d
H
o
r
r
= ⋅
⋅
⋅ ⋅ = ⋅ ⋅
⋅
4
5
4
5
2
0
µ µ
µ µ
/
B: magnetic flux (Tesla=Wb/m2)
H: magnetic field strength (A/m)
N: Number of Helmholtz Coil windings
d: Coil diameter (m)
I: Coil current (A)
µo: magnetic field constant (Vs/Am) = 4×p×10
-7
Vs/Am
µr :relative magnetic field constant (in air: =1)
The Helmholtz set-up can be used for the specification of transponders in the temperature range
from -40 to +85 ºC. Tests showed, however, that deviations of the field strength caused by
temperature negligible.
The data telegram of the transponder can be captured by a pick-up coil (for example: 10 windings,
thin wire to minimize influence) which wraps the transponder. The pulse modulated signal can be
adjusted at the signal generator. The measurement of the power pulse and transponder diagram can
be done with the help of an oscilloscope triggered by the generator signal (see Figure 15). As soon
as a data telegram is completely detected the minimum necessary field strength (calculated with
equation 2) can be monitored.
Response phase
max 20msec
U
t
Power
phase
Figure 16: Received signal at the pick up coil, if power field strength is sufficient
