AN3359
Application note
Low cost PCB antenna for 2.4GHz radio:
Meander design
1
Introduction
This application note is dedicated to the STM32W108 product family from
STMicroelectronics.
One of the main reasons to use a PCB antenna is the reduced overall cost of the radio
module. Well designed and implemented PCB-printed antennas have a similar performance
to the SMD ceramic equivalence. In general, the footprint for a ceramic SMD antenna is
smaller than that for a PCB-printed variant. For a PCB-printed antenna solution, the
increased size of the PCB in relation to space required for the antenna means that the radio
module is larger cost of the PCB increased. The increased cost of the PCB is smaller and
less expensive than a SMD ceramic antenna.
The STM32-RFCKIT RF control kit is based on an STM32W108xx RF microcontroller. It
implements a PCB-printed antenna to perform RF communications.
March 2011
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www.st.com
Contents
AN3359
Contents
1
1
2
3
4
5
6
7
8
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Coordinate system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Layout specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Impedance matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Radiation pattern, 3-D visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Radiation pattern, 2-D visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
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Table 1.
Table 2.
Specification of the recommended substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
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AN3359
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Spherical coordinate system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Layout of Meander-like PCB antennae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Cross section of the PCB at antennae region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Part of the ZigBee module's PCB with Meander-like antenna (around scale 4:1) . . . . . . . . 8
Bypassing impedance matching circuitry - direct RF connection . . . . . . . . . . . . . . . . . . . . . 8
Complex impedance of the Meander-like antenna on Smith Chart . . . . . . . . . . . . . . . . . . . 9
Magnitude of the S11 parameter in logarithmic scale (Cartesian plot) . . . . . . . . . . . . . . . . 10
Antenna's Standing Wave Ratio (SWR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Three dimensional (3-D) radiation pattern overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Radiation pattern on Y-Z plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Radiation pattern on X-Z plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Major planes used to visualize 3-D radiation pattern using 2-D plots . . . . . . . . . . . . . . . . . 15
Far field radiation pattern plotted on Y-Z plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
normalized radiation pattern on Y-Z plan (Polar plot) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
normalized radiation pattern on Y-Z plane (Cartesian plot)) . . . . . . . . . . . . . . . . . . . . . . . . 18
Far field radiation pattern plotted on X-Y plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Normalized radiation pattern on X-Y plan (Polar plot) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Normalized radiation pattern on X-Y plan (Cartesian plot) . . . . . . . . . . . . . . . . . . . . . . . . . 21
Far field radiation pattern plotted on X-Z plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Normalized radiation pattern on X-Z plane (Polar plot). . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Normalized radiation pattern on X-Z plane (Cartesian plot) . . . . . . . . . . . . . . . . . . . . . . . . 24
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Coordinate system
1
Coordinate system
For the purpose of this document, the spherical coordinate system illustrated in
Figure 1
is
used.
Figure 1.
Spherical coordinate system
The PCB module is orientated vertically (plane X-Z), and located in proximity to the origin of
the coordinate system. The azimuth angle radiates from the X-axis towards the Y-axis, and
the elevation angle radiates from the Z-axis towards the horizontal plane, X-Y. Sometimes,
as with geographical and navigational systems, the X-axis is called the "Nord-axis", the Y-
axis is called the "East-axis" and the Z-axis is called the "Zenith-axis".
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