CSR 蓝牙PIF天线和蛇形PCB天线设计指导

CSR

Unit 400 Cambridge Science Park

Milton Road

Cambridge

CB4 0WH

United Kingdom

Registered in England 3665875

Tel: +44 (0)1223 692000

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https://www.wendangku.net/doc/6412450329.html,

BlueCore ?

Inverted-F and Meander Line Antennas

Application Note

January 2003

BlueCore ? Inverted-F and Meander Line Antennas

Contents

1Introduction (3)

2Inverted-F Antenna (4)

3Meander Line Antenna (5)

4Real Designs (6)

5Proximity to Metal Objects (7)

6Proximity to Dielectric Materials (8)

7Network Analyser (9)

8Final Tuning (10)

9Conclusion (12)

Acronyms and Definitions (13)

Record of Changes..............................................................................................................................................................14List 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)

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.

BlueCore ? Inverted-F and Meander Line Antennas

2 Inverted-F Antenna

Input

Output

Quarterwave

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

BlueCore ? Inverted-F and Meander Line Antennas

3

Meander Line Antenna

Output

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.

Smith Chart

Figure 3.2: Input Impedance of Two Meander Line Antennas

The real part of the impedance of this antenna is about 15-25?, 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?), 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 50? (Point B). Experimental measurement is used to determine the correct design.

Real Designs

BlueCore ? Inverted-F and Meander Line Antennas

4 Real Designs

Ground

Plane

13.5mm

18.0mm

Not to scale Actual Size

Figure 4.1: Approximate Dimensions of Inverted-F Antenna

Feedpoint

Ground Plane

Width=0.5mm

1.5pF Capacitor

Placed immediately after Feedpoint

Not to scale

Actual Size

Figure 4.2: Approximate Dimensions of Meander Line Antenna

Proximity to Metal Objects

BlueCore ? Inverted-F and Meander Line Antennas

5 Proximity to Metal Objects

CSR recommends keeping metal objects as far away from the antenna as possible. Keeping metallic objects out of the near field is usually adequate.

Near Field = 2D 2 / λ

D is the largest dimension of the antenna. In the case of these antennas, this is approximately a quarterwave (λ/4).

Notes:

λ is the wavelength of the signal in freespace.

At Bluetooth frequencies, λ=122mm in freespace.

Substituting D=λ/4 into the Near Field equation gives Near Field = λ/8.

Near Field = 122/8 mm = 15.25mm.

Proximity to Dielectric Materials

BlueCore ? Inverted-F and Meander Line Antennas

6 Proximity to Dielectric Materials

Dielectric materials (like plastic or FR-4) detune an antenna by lowering its resonant frequency. The effect is not as serious as placing an antenna next to metal objects and can be corrected by reducing the length of the antenna. Therefore it is important for the antenna to be tuned when it is in the product. This is done during the development of the product.

Network Analyser

BlueCore ? Inverted-F and Meander Line Antennas

7 Network Analyser

A Vector Network Analyser (VNA) is used to perform the initial tuning of the antenna:

1. The PCB track (trace), just before the antenna matching network is cut to isolate the filter and previous

stages from the measurement.

2. A coaxial cable is connected between the VNA and the PCB of the product. The outer conductor of the

coaxial cable is soldered to the ground plane of the PCB as close as possible to the input of the antenna matching network. The inner conductor of the coaxial cable is left floating. The coaxial cable must have ferrite beads over the outer sleeve of the coaxial cable. The ferrite beads help to prevent RF currents

from flowing on the outside of the coaxial cable (which would disturb the measurement).

3. A One-Port calibration is performed on the VNA with Open, Short, Loads connected at the end of the

coaxial cable inside the product.

4. The inner conductor of the coaxial cable is soldered to the input of the antenna matching network.

5.

The antenna is tuned by adjusting the values of any “matching network” components, the feed point of

the antenna or the length of the antenna until the S11 trace (displayed on the VNA) is at the centre of the Smith chart at the midband frequency 2441MHz.

6. The antenna is now roughly tuned and the cut track can be repaired by putting a small amount of solder

over the cut.

Figure 7.1: Preparation Before Measurement

Figure 7.2: Assembled System Ready to Measure

BlueCore ?

Inverted-F and Meander Line Antennas 8 Final Tuning

After tuning the antenna using the VNA procedure, it is necessary to perform fine tuning. This will yield a small improvement and will be the final optimisation of the antenna. It is best to perform this procedure in an anechoic chamber, but when this is not possible an indoor or outdoor test range can be used. It is important to minimise radio signal reflections. Avoid metallic objects such as lab-benches, filing cabinets, lampposts and cars.

Figure 8.1: Locating Product in Far Field of Antenna

? Inverted-F and Meander Line Antennas

Figure 8.2: Final Tuning Procedure

BlueCore ? Inverted-F and Meander Line Antennas

9 Conclusion

Metal objects should be kept at least 15.25mm away from the Inverted-F and Meander Line types of antennas in the Bluetooth frequency band in order for the antenna to work efficiently. If that is not possible, then extra

experimentation is required to determine an acceptable trade-off between antenna performance and product size.Even if these rules are followed, antenna detuning can occur. Usually the resonant frequency of the antenna will be lowered. This can be corrected by reducing the length of the antenna.

Acronyms and Definitions

BlueCore ? Inverted-F and Meander Line Antennas

Acronyms and Definitions BlueCore ?Group term for CSR’s range of Bluetooth chips

Bluetooth ?Set of technologies providing audio and data transfer over short-range radio connections CSR Cambridge Silicon Radio

PCB Printed Circuit Board

RF Radio Frequency

VNA Vector Network Analyser

Record of Changes

BlueCore ? Inverted-F and Meander Line Antennas Record of Changes Date:Revision Reason for Change:

24 JAN 03a Original publication of this document. (CSR reference bcant-an-001Pa).

BlueCore ?

Inverted-F and Meander Line Antennas

Application Note

bcant-an-001Pa

January 2003

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CSR’s products are not authorised for use in life-support or safety-critical applications.