Antenna Requirements - Texas Instruments 2000 Series Reference Manual

Antenna Requirements

4.1

Antenna Requirements

In order to achieve high voltages at the antenna resonance circuit and thus high field strength at the
antenna for the charge-up (transmit) function, the antenna coil must be high Q. The recommended Q
factor for proper operation is listed in
vary depending on the type, the construction and the size of the antenna. Furthermore, this factor
depends on the wire type and wire cross-sectional area used for winding of the antenna.
RF braided wire, consisting of a number of small single insulated wires is recommended for winding of an
antenna since it gives the highest Q factor and thus the highest charge-up field strength, for example
single wire diameter of 0.1 mm (4 mil) and 120 single insulated wires.
Note: If a high Q is not required (for example for large in-ground antennas), standard braided
wire can be used.
In order to ensure that the transmitter and receiver function correctly, the antenna must be tuned to the
resonance frequency of 134.2 kHz. For a detailed description of the antenna resonance tuning procedure,
refer to Section 4.2, Antenna Resonance Tuning.
To ensure that the antenna can be tuned to resonance with the RFM, the antenna inductance can only
vary within the limits given in
Parameter
Antenna inductance range within which the antenna can be tuned to
L_ANT
resonance
Q_ANT Recommended Q factor of antenna coil for correct operation
Note: Although a ferrite core antenna may have a high Q factor under test conditions with low
magnetic field strengths, the Q factor decreases when a high magnetic field strength is
applied to the ferrite core.
CARE MUST BE TAKEN WHEN HANDLING THE RFM. HIGH
VOLTAGE
ANTENNA RESONATOR PARTS COULD BE HARMFUL TO YOUR
HEALTH. IF THE ANTENNA INSULATION IS DAMAGED THE
ANTENNA SHOULD NOT BE CONNECTED TO THE RFM.
When low field strength for larger antennas is necessary (Vpeak <60 V), the antenna resonator can
additionally be damped by connecting an onboard damping resistor, which may be done by closing jumper
JP3 (see Figure 1-3). This jumper is open by default.
Only a certain maximum antenna resonance voltage is allowed for this option.
Please refer to
26 Associated Antenna Systems
Table 4-1, Antenna Requirements. The Q factor of the antenna may
Table 4-1.
Table 4-1. Antenna Requirements
Conditions
WARNING
ACROSS THE ANTENNA
CAUTION
Section 2.1, Recommended Operating Conditions, for details.
Min Typ
26 27
40
TERMINALS AND
SCBU022 – November 2002
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Max Unit
µH
27.9
450 -
ALL