Texas Instruments 2000 LF Series, Примечание к применению

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Lit Number: 11-06-21-068
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Example 1. RI-ANT-G01E antenna,
Where:
ƒ = 134200 Hz (134.2 kHz)
L = 0.000027 H (27 µH)
R = 0.2 Ohms
Q = (2 * 3.142 * 134200 * 0.000027)/ 0.2 = 114
Example 2. MicroReader antenna,
Where:
ƒ = 134200 Hz (134.2 kHz)
L = 0.000047 H (47 µH)
R = 2.4 Ohms
Q = (2 * 3.142 * 134200 * 0.000027)/ 2.4 = 16.5
4.3 Controlling the Antenna’s Q
We have seen from Equation [3] that the resistance (R) controls the Q. When R is
low, an antenna has a high Q and when R is high, the Q is low.
By selecting the correct wire type we can vary the Q.
4.3.1 Wire Selection.
At RF frequencies, the behavior of an AC current through a wire is different from the
flow through a DC circuit. What might be considered a low resistance wire in a DC
circuit can become high impedance when in an AC circuit because of the ‘Skin Effect’.
4.3.1.1 Skin Effect
At RF frequencies e.g. 134 kHz, when a signal passes through a wire, eddy currents at
the centre of the wire inhibit flow and the current tends to flow close to the
circumference (skin) of the wire. This is the ‘Skin Effect’ and the higher the frequency,
the thinner the depth of the skin through which the current flows.
ƒ
2
1000
[4]
Skin depth(m m) =
So, at 134.2 kHz, we get a skin depth of:
Depth = 2 / (sq root (134200/1000)) = 0.173 mm (0.007”)