ST AN3359 Application note

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.

AN3359

Application note

Low cost PCB antenna for 2.4GHz radio:

Meander design

March 2011 Doc ID 018585 Rev 1 1/28

www.st.com

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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AN3359

Table 1. Specification of the recommended substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 2. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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AN3359

Figure 1. Spherical coordinate system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 2. Layout of Meander-like PCB antennae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. Cross section of the PCB at antennae region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 4. Part of the ZigBee module's PCB with Meander-like antenna (around scale 4:1) . . . . . . . . 8

Figure 5. Bypassing impedance matching circuitry - direct RF connection . . . . . . . . . . . . . . . . . . . . . 8

Figure 6. Complex impedance of the Meander-like antenna on Smith Chart . . . . . . . . . . . . . . . . . . . 9

Figure 7. Magnitude of the S11 parameter in logarithmic scale (Cartesian plot) . . . . . . . . . . . . . . . . 10

Figure 8. Antenna's Standing Wave Ratio (SWR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 9. Three dimensional (3-D) radiation pattern overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 10. Radiation pattern on Y-Z plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 11. Radiation pattern on X-Z plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 12. Major planes used to visualize 3-D radiation pattern using 2-D plots . . . . . . . . . . . . . . . . . 15

Figure 13. Far field radiation pattern plotted on Y-Z plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 14. normalized radiation pattern on Y-Z plan (Polar plot) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 15. normalized radiation pattern on Y-Z plane (Cartesian plot)) . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 16. Far field radiation pattern plotted on X-Y plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 17. Normalized radiation pattern on X-Y plan (Polar plot) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 18. Normalized radiation pattern on X-Y plan (Cartesian plot) . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 19. Far field radiation pattern plotted on X-Z plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 20. Normalized radiation pattern on X-Z plane (Polar plot). . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 21. Normalized radiation pattern on X-Z plane (Cartesian plot) . . . . . . . . . . . . . . . . . . . . . . . . 24

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AN3359 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 Y­axis is called the "East-axis" and the Z-axis is called the "Zenith-axis".

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Layout specification AN3359

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2 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 .

Figure 3 illustrates a typical cross-section of the substrate in a PCB-antennae area.

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AN3359 Layout specification

ε

R

Figure 3. Cross section of the PCB at antennae region

A substrate with the parameters in Tabl e 1 is recommended:

Table 1. Specification of the recommended substrate

Dimension

Pos. Layer

Label Value Unit Value Unit

Constant

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

Dielectric

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Impedance matching AN3359

3 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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AN3359 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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