AN3359

AN3359 STMICROELECTRONICS | Alldatasheet

Document overview

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• PDF pages: 28

Technical content

Datasheet sections

• 1 Introduction
• 1 Coordinate system
• 2 Layout specification
• 3 Impedance matching
• 4 Radiation pattern, 3-D vi sualization
• 5 Radiation pattern, 2-D vi sualization
• 6 Performance
• 7 Summary
• 8 Revision history

March 2011 Doc ID 018585 Rev 1 1/28 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.

1 Coordinate system

Figure 1. Spherical coordinate system axis is called the "East-axis" and the Z-axis is called the "Zenith-axis".

2 Layout specification

Figure 2. Layout of Meander-like PCB antennae 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.

Figure 3. Cross section of the PCB at antennae region Table 1. Specification of the recommended substrate

3 Impedance matching

impedance very close to the required nominal impedance (50 Ohm). 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 Figure 5. Bypassing impedance matchi ng circuitry - direct RF connection

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

• 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.

4 Radiation pattern, 3-D visualization

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

Figure 11. Radiation pattern on X-Z plane

Radiation pattern, 2-D visualization AN3359 14/28 Doc ID 018585 Rev 1

5 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:
• 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 Z- axis 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.

Figure 12. Major planes used to visual ize 3-D radiation pattern using 2-D plots This chapter uses short dipole for comparison and clarification purposes only. this plot is shown in Figure 13.

Figure 13. Far field radiation pattern plotted on Y-Z plane plane. For a vertically orientated dipole, this pattern is equivalent to the horizontal radiation.

Figure 15. normalized radiation pattern on Y-Z plane (Cartesian plot)) the X-Y plane and this plot is shown in Figure 16.

Figure 16. Far field radiation pattern plotted on X-Y plane the "dips" (between -10 and -14 dB) are much less critical than for the dipole.

Figure 18. Normalized radiation patt ern on X-Y plan (Cartesian plot) versus the X-Z plane and this plot is shown in Figure 20.

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

Figure 21. Normalized radiation patt ern on X-Z plane (Cartesian plot)

6 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

7 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.

8 Revision history

Table 2. Document revision history 17-Mar-2011 1 Initial release.