STMicroelectronics AN3359 This application note is dedicated Datasheet

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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Contents
AN3359
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1
Coordinate system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2
Layout specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3
Impedance matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4
Radiation pattern, 3-D visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5
Radiation pattern, 2-D visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6
Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8
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.
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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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1
Coordinate system
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 Yaxis is called the "East-axis" and the Z-axis is called the "Zenith-axis".
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Layout specification
2
AN3359
Layout specification
PCB antennas, including the electrical parameters of PCB materials used, are layout
sensitive. STMicroelectronics recommends using a layout as close as possible to that
shown in Figure 2.
Figure 2.
Layout of Meander-like PCB antennae
The electrical parameters and performance of the PCB antenna are also determined by the
substrate used, in particular the thickness of the core and dielectric constants ε R .
Figure 3 illustrates a typical cross-section of the substrate in a PCB-antennae area.
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Layout specification
Figure 3.
Cross section of the PCB at antennae region
A substrate with the parameters in Table 1 is recommended:
Table 1.
Specification of the recommended substrate
Dimension
Pos.
Layer
Label
Value
Unit
Value
Unit
Dielectric
Constant ε R
1
Solder Mask, Top
S1
0.7
mil
17.78
µm
4.4
2
Copper Trace
T
1.6
mil
40.64
µm
---
3
Core
C
28
mil
711.2
µm
4.4
4
Solder Mask, Bottom
S2
0.7
mil
17.78
µm
4.4
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Impedance matching
3
AN3359
Impedance matching
Meander-like PCB antenna can be tuned to the required 50 Ohm impedance by matching
the impedance circuitry with the π topology. In Figure 2 the impedance matching area is
marked with a dashed line. Under nominal conditions, this antenna should exhibit
impedance very close to the required nominal impedance (50 Ohm).
To check the performance of this design, a sample antenna was manufactured (according to
the specifications covered by this document). Figure 4 shows this antenna.
Figure 4.
Part of the ZigBee module's PCB with Meander-like antenna (around
scale 4:1)
Assuming that the manufactured sample exhibits the expected performance (no impedance
matching necessary), the impedance matching circuitry was bypassed by two 100 pF
capacitors connected in series, as shown in Figure 5:
Figure 5.
Bypassing impedance matching circuitry - direct RF connection
All electrical parameters of the meander-like antenna have been measured at connection to
the Band Pass Filter with the frequency span covering frequencies from 2.4 GHz to 2.5
GHz.
Complex impedance of the antenna is shown in the Smith diagram in Figure 6:
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Impedance matching
Figure 6.
Complex impedance of the Meander-like antenna on Smith Chart
Figure 7 shows the magnitude of the S11 parameter (in log scale).
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Impedance matching
Figure 7.
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Magnitude of the S11 parameter in logarithmic scale (Cartesian plot)
Figure 8 shows the Standing Wave Ratio (SWR).
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Impedance matching
Figure 8.
Antenna's Standing Wave Ratio (SWR)
The following changes will affect the radiation impedance of the PCB antenna:
●
slight board size variation
●
metal shielding
●
use of plastic cover
●
presence of other components in proximity of the antenna
The best performance impedance matching circuitry will compensate these effects so that
for operating frequencies, the optimum 50 Ohm impedance is achieved.
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Radiation pattern, 3-D visualization
4
AN3359
Radiation pattern, 3-D visualization
A three-dimensional (3-D) visualization of the radiation pattern (magnitude of the electrical
far field |E|) is done for the center ISM band frequency 2.44175 GHz.
Figure 9.
Three dimensional (3-D) radiation pattern overview
Figure 10. Radiation pattern on Y-Z plane
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Radiation pattern, 3-D visualization
Figure 11. Radiation pattern on X-Z plane
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Radiation pattern, 2-D visualization
5
AN3359
Radiation pattern, 2-D visualization
In this chapter all radiation patterns are related to the magnitude of electrical far field E,
which is normalized and shown in the logarithmic scale (in dB). This means that the
maximum global radiation pattern (maximum magnitude of the electrical far-field E) is
represented by 0 dB level. To show antenna radiation patterns in detail, three two
dimensional (2-D) major cuts are presented. Consider the orientation of the module in the
spherical coordinate system as shown in Figure 1.
A three dimensional (3-D) far field radiation pattern is visualized as three two dimensional
(2-D) cuts through a 3-D pattern. Three major planes are used for these cuts (Figure 12):
●
One horizontal X-Y plane
●
Two vertical planes: X-Z plane and Y-Z plane.
For the colors of the plots in Figure 12:
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●
The "Blue" plot is drawn on the horizontal X-Y plane, where azimuth φ radiates from 0°
on the X-axis towards the Y-axis until it reaches 360° on the X-axis.
●
The "Red" plot is drawn on the X-Z plane, where elevation θ radiates from 0° on the Zaxis towards the positive part of the X-axis until it reaches180° on the negative part of
the Z-axis. In this plot (cut by X-Z plane), elevation θ is negative for X < 0.
●
The "Green" plot is drawn on the Y-Z plane, where elevation θ radiates from 0° on the
Z-axis towards the positive part of the Y-axis until it reaches 180° on the negative part
of the Z-axis. For this plot (cut by Y-Z plane), elevation θ is negative for Y < 0.
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Radiation pattern, 2-D visualization
Figure 12. Major planes used to visualize 3-D radiation pattern using 2-D plots
This chapter uses short dipole for comparison and clarification purposes only.
The first radiation patterns in Figure 14 and Figure 15 show a normal electrical field
radiation pattern |E| (far field) on the Y-Z plane. The module orientation versus Y-Z plane and
this plot is shown in Figure 13.
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Radiation pattern, 2-D visualization
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Figure 13. Far field radiation pattern plotted on Y-Z plane
Notice the nearly constant level of the radiation—nearly omni-directional radiation on this
plane. For a vertically orientated dipole, this pattern is equivalent to the horizontal radiation.
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Radiation pattern, 2-D visualization
Figure 14. normalized radiation pattern on Y-Z plan (Polar plot)
Figure 15 shows the same radiation pattern as in Figure 14, presented as a Cartesian plot.
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Radiation pattern, 2-D visualization
AN3359
Figure 15. normalized radiation pattern on Y-Z plane (Cartesian plot))
The second far-field radiation pattern (Figure 17 and Figure 18) represents a normalized
magnitude of the electrical field |E| plotted on the X-Y plane. The module orientation versus
the X-Y plane and this plot is shown in Figure 16.
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Radiation pattern, 2-D visualization
Figure 16. Far field radiation pattern plotted on X-Y plane
For a vertically orientated dipole, this pattern is equivalent to the vertical radiation. Note that
the "dips" (between -10 and -14 dB) are much less critical than for the dipole.
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Radiation pattern, 2-D visualization
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Figure 17. Normalized radiation pattern on X-Y plan (Polar plot)
Figure 18 show the same far |E|-field radiation pattern on the X-Y plane as in Figure 17,
presented as a Cartesian plot.
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Radiation pattern, 2-D visualization
Figure 18. Normalized radiation pattern on X-Y plan (Cartesian plot)
The third and last radiation pattern (Figure 20 and Figure 21) represents a normalized
electrical field radiation pattern |E| (far field) on the X-Z plane. The module orientation
versus the X-Z plane and this plot is shown in Figure 20.
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Radiation pattern, 2-D visualization
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Figure 19. Far field radiation pattern plotted on X-Z plane
For a horizontally orientated dipole, this pattern is equivalent to the vertical radiation. Note
that the "dip" (about -18 dB in worse case) is not as deep, in comparison to the dipole
radiation pattern.
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Radiation pattern, 2-D visualization
Figure 20. Normalized radiation pattern on X-Z plane (Polar plot)
Figure 21 shows the same far electrical field radiation pattern on the X-Z plane (Figure 20),
presented as a Cartesian plot.
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Radiation pattern, 2-D visualization
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Figure 21. Normalized radiation pattern on X-Z plane (Cartesian plot)
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6
Performance
Performance
At center ISM Band frequency 2.44175 GHz, antennae show the following key performance
parameters:
– Directivity
2.21 dB
– Gain
1.95 dBi
– Maximum intensity
0.125 W/Steradian
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Summary
7
AN3359
Summary
The designed antenna occupies a small part of the module's PCB. It is inexpensive and
simple to produce and shows very good performances, confirmed by measurements of the
manufactured samples. Keeping the manufacturing process as close as possible to the
specification detailed in this document produces an antenna that does not need any of the
additional components usually required for impedance matching circuitry (cost reduction,
increased reliability). In addition, a no tuning procedure or similar is required. The antenna
impedance is close to the nominal 50 Ohm value, with excellent SWR < 1.35 together and
wideband capabilities, where log (|S11|) < -10 dB is satisfied for more than 150 MHz.
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Revision history
Revision history
Table 2.
Document revision history
Date
Revision
17-Mar-2011
1
Changes
Initial release.
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AN3359
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