Harman Becker Automotive Systems BE2728 Antenna App Note

CSR

BlueCore™

Inverted-F and Meander Line Antennas

Application Note

January 2003

Unit 400 Cambridge Science Park

Milton Road

Cambridge

CB4 0WH

United Kingdom

Registered in England 3665875

Tel: [phone]

Fax: [phone]

www.csr.com

bcant-an-001Pa © Copyright CSR 2003

This material is subject to CSR’s non-disclosure agreement.

Contents

BlueCore

Inverted-F and Meander Line Antennas

Contents

1 Introduction.......................................................................................................................................................................3

2 Inverted-F Antenna .........................................................................................................................................................4

3 Meander Line Antenna...................................................................................................................................................5

4 Real Designs.....................................................................................................................................................................6

5 Proximity to Metal Objects...........................................................................................................................................7

6 Proximity to Dielectric Materials.................................................................................................................................8

7 Network Analyser............................................................................................................................................................9

8 Final Tuning...................................................................................................................................................................10

9 Conclusion.....................................................................................................................................................................12

Acronyms and Definitions .................................................................................................................................................13

Record of Changes .............................................................................................................................................................. 14

List of Figures

Figure 2.1: Inverted-F Antenna...............................................................................................................................................4

Figure 3.1: Meander Line Antenna.........................................................................................................................................5

Figure 3.2: Input Impedance of Two Meander Line Antennas...........................................................................................5

Figure 4.1: Approximate Dimensions of Inverted-F Antenna.............................................................................................6

Figure 4.2: Approximate Dimensions of Meander Line Antenna.......................................................................................6

Figure 7.1: Preparation Before Measurement......................................................................................................................9

Figure 7.2: Assembled System Ready to Measure.............................................................................................................9

Figure 8.1: Locating Product in Far Field of Antenna.......................................................................................................10

Figure 8.2: Final Tuning Procedure....................................................................................................................................11

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This material is subject to CSR’s non-disclosure agreement.

© Copyright CSR 2003

Page 2 of 14

Introduction

BlueCore

Inverted-F and Meander Line Antennas

1 Introduction

This document outlines two types of Printed Circuit Board (PCB) antennas used by CSR.

§ Inverted-F

§ Meander Line

Also discussed in this document is the effect of placing metallic or dielectric materials near an
antenna.

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© Copyright CSR 2003

Page 3 of 14

Inverted-F Antenna

BlueCore

Inverted-F and Meander Line Antennas

2 Inverted-F Antenna

Quarterwave

Input

Output

Figure 2.1: Inverted-F Antenna

The inverted-F is a quarterwave antenna. It is bent into an L-shape. The shorter side is connected to earth. The
longer side is left open circuit at the end. The feed point is located somewhere between the earth end and the
open end. The resulting structure resembles the letter F and possesses the properties of both a loop antenna due
to the circulating current from the feed point to ground and a whip antenna due to the open circuited straight
section.

In the PCB version the antenna is printed on the top layer and a ground plane is placed near the antenna on the
top layer. There must not be a ground plane underneath the antenna.

The aim is to make the quarterwave section resonate at midband frequency (which is 2441MHz for Bluetooth™).
The feed point (which is the input/output connection) is connected to the L-Shape at the point corresponding to

50Ω. Experiment with measurement to determine correct location for the feed point and length of this antenna.

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This material is subject to CSR’s non-disclosure agreement.

© Copyright CSR 2003

Page 4 of 14

Meander Line Antenna

BlueCore

Inverted-F and Meander Line Antennas

3 Meander Line Antenna

Smith Chart

Input

Output

S

Ground

Plane

Figure 3.1: Meander Line Antenna

The length of the meander line antenna is difficult to predict. It is usually a bit longer than a quarterwave but
dependent on its exact geometry and proximity to the ground plane.

Note:

In Figure 3.1 the ground plane is shown in black. S is the distance from the ground plane. See Figure 4.2 for
approximate dimensions.

This type of antenna is always a PCB version. The antenna is printed on the top layer and a ground plane is
placed near the antenna on the top layer. There must be no ground plane underneath the radiating section of the
antenna.

A

B

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The real part of the impedance of this antenna is about 15-25 Ω, depending on geometry and proximity to the
ground plane. The impedance matching is done by adjusting the length of the antenna until the input impedance
is at the unity conductance circle (when normalised to 50 Ω), in the top half of the Smith chart (Point A). A shunt
capacitor is then connected between the antenna input and ground to match to 50Ω (Point B). Experimental
measurement is used to determine the correct design.

bcant-an-001Pa

Figure 3.2: Input Impedance of Two Meander Line Antennas

© Copyright CSR 2003

This material is subject to CSR’s non-disclosure agreement.

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