Lit Number: 11-06-21-068
Page (18)
4.4.2 Adapting a non 27µH Antenna
If you have to produce a loop to a particular size and the inductance is not 27 µH,
there are 3 ways to allow you to adapt that loop to a reader.
1. Changing the shape of the loop. If the loop is made squarer, the inductance
gets less. Making the loop narrower increases the inductance.
2. Use the Remote Antenna RF module. The Tuning Board for this RF Module
allows antennas from 12 to 80 µH to be connected. If the antenna is high Q
though (not a lower Q road loop), not all inductances can be accommodated as
the higher voltages may exceed the rated values of the matching capacitors.
See the Remote Antenna RFM manual – (11-06-21-047).
3. Use external capacitance to change the RF Modules matching circuit.
4.4.2.1 Using External Capacitance
In our internal matching circuit, the on-board capacitance is calculated to exactly
balance the inductance of the antenna and to create a circuit that resonates at 134.2
kHz. If the inductance value is not the expected value though, we can modify the
capacitance by adding external capacitors, either in series or parallel, to effectively
increase or decrease that capacitance and maintain balance. We sometimes have to
do this for our standard antennas when they have to be mounted close to metal (if the
inductance has decreased below 25.5 µH) or when a long tail has been added (the
inductance now exceeds 28.5 µH)
Figure 22. Out of Tune Conditions
If either the tuning core comes completely out or the top tuning LED on the S251B
reader is lit then the inductance is too high and capacitance has to be added in series
(to reduce total capacitance). If though, you cannot increase the inductance enough
by turning the core inwards, or the bottom tuning LED on the S251B is lit, the
inductance is too low and external capacitance must be added in parallel. Table 2
shows the capacitor values required for a particular inductance.
1 2 3 4 5 6
1 2 3 4 5 6
nput/ Output
Synchronisation
ON
1 2 3 4 5
ON
1 2 3 4 5
L
O.K.
L
Antenna
Tuning
RF Power Output Adj.
EMI/ Sync. Level Adj.
RS422
RS485
DAT
1
2
Antenna
Summary of Contents for 2000 LF Series
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