ST AN3359 Application note

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	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 Y- axis is called the "East-axis" and the Z-axis is called the "Zenith-axis".

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Layout specification	AN3359

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 ε R .

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

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AN3359	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
Pos.		Layer			Dimension			Dielectric
			Label	Value	Unit	Value	Unit	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	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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