NXP Semiconductors
AN145715
PN544 Antenna Design Guide
145715
© NXP B.V. 2006. All rights reserved.
Application Note
Rev. 1.5 — 28th August 2009
9 of 43
4.1
12B12B
Equivalent circuit
The following subchapters describe the matching procedure. It starts with the
determination of the antenna parameters and ends with a fine tuning of the antenna
circuitry.
4.1.1
26B26B
Determination of series equivalent circuit
The antenna loop has to be connected to an impedance or network analyzer to measure
the series equivalent components.
The equivalent circuit (see
60H60H60H
Fig 7) must be determined under final environmental
conditions especially if the antenna will be operated in metal environment or a
ferrite will be used for shielding.
Antenna
R
a
L
a
C
a
Fig. 4 Series equivalent circuit
Typical values:
L
a
= 0.3...3µH
C
a
= 3...30pF
R
a
= 0.3...8
Ω
f
ra
= self-resonance frequency of the antenna
The antenna capacitance C
a
can be calculated with:
(
)
a
ra
a
L
f
C
2
2
1
⋅
⋅
=
π
The antenna parasitic capacitance Ca should be kept low to achieve a self-resonance
frequency > 35 MHz.
4.1.2
27B27B
Calculation of damping resistor RQ
The quality factor of the antenna is calculated with
a
a
a
R
L
Q
⋅
=
ω
(1)
