Texas Instruments 2000 LF Series, Примечание к применению

Страница: 33 / 38

Lit Number: 11-06-21-068
Page (27)
Appendix A – MicroReader Antenna Designs
Because of the Micro-reader’s modest power output and the requirements for
antennas to perform next to metal, certain constraints have been imposed by design
on antenna construction.
•
The antenna Q factor must be less than 20
•
The inductance must be between 46 ~ 48 µH
•
The maximum recommended size is 200 mm x 200 mm
The Q factor of an antenna is a measure of its effectiveness, unfortunately, the higher
the Q the more easily the antenna is de-tuned by proximity to metal. The micro-reader
is designed for antennas with a Q factor less than 20. If the Q factor exceeds 20:-
•
The output capacitors may receive over-voltage and long term damage could
result.
•
The antenna may still be resonating when the response from the tag is
received. Without built-in damping, the data may not be received correctly.
•
The antenna may be de-tuned by metal when in situ.
Increasing the resistance has the effect of reducing the Q factor and is why in the
following designs high resistance wire is used, or extra resistance added.
The table below lists the parameters of four antennae that meet the design rules and
whose constructions are described in detail in later sections.
Antenna Size
(mm)
Turns Q L
(µH)
Range
(mm)
32 mm Tag
Notes
1 10Ø n/a 17 47 40 Off-the-shelf Inductor
2 40Ø 28 14 47 110 Automotive lock barrel size
3 75Ø 15 18 47 160 General purpose antenna
4 200 x 200 8 20 47 270 Largest size recommended.
Table 3. MicroReader Antenna Designs