Appendix 1: Field Strength Adjustment - Texas Instruments 2000 Series Reference Manual

Appendix 1
Field Strength Adjustment
The magnetic field strength generated determines the charge-up distance of the
transponder. The higher the magnetic field strength, the further the transponder
charge-up distance. The charge-up distance does not, however, increase linearly
with the field strength.
The reading distance of a transponder is determined, amongst other factors, by the
charge-up distance and the local noise level. Increasing the charge-up field
strength does not necessarily increase the reading distance.
The field strength generated by the RA-RFM depends on the four factors listed
below:
1. Q factor of the antenna.
The Q factor is a measure of the efficiency of the antenna and therefore the higher
the Q factor of the antenna coil, the higher the field strength generated by the RA-
RFM, assuming that all other parameters remain unchanged. The Q factor of the
antenna itself depends on the cross-sectional area of the wire, the wire type, the
size of the antenna and the type of antenna (gate or ferrite). The larger the cross-
sectional area of the RF braided wire, the higher the Q factor of the antenna. RF
braided wire gives a higher Q factor than solid wire assuming that all other
parameters remain unchanged.
2. Size of the antenna.
The larger the antenna, the higher the field strength which is generated by the RA-
RFM, since the antenna covers a larger area and thus generates a higher flux
assuming that all other parameters remain unchanged. Large antennas have less
immunity to noise for receive functions than small antennas.
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