NXP Semiconductors
AN11740
PN5180 Antenna design
AN11740
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Application note
COMPANY PUBLIC
Rev. 1.1 — 19 June 2018
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3.1.2 Optimum Antenna Coil size
Fig 12 shows the coupling coefficient versus antenna radius for three different operating
distances. The scaling of the coupling coefficient does not necessarily show the correct
absolute value, since some of the fixed parameters are estimated only for this graph.
However, the relative value is important to indicate the optimum antenna size.
(1) Green: at a distance of x = 4 cm
(2) Blue: at a distance of x = 6 cm
(3) Black: at a distance of x = 8 cm
(4) The coupling coefficient scaling in this diagram is relative only. The absolute value might differ.
Fig 12. Coupling coefficient vs PCD antenna radius
The maximum coupling can be achieved, when
r
= Reader Antenna coil radius
x
= Operating distance in the center of the Reader antenna
The maximum coupling at an operating distance of 4cm can be achieved with an antenna
coil of approximately 4cm radius (i.e. 8cm diameter). Increasing the antenna radius from
4cm to 8cm decreases the coupling at 4cm distance (green curve), but increases the
coupling at 8cm distance (black curve).
However, the optimum antenna size as such does not guarantee that the coupling is
strong enough.
x
r
=
(3)
