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BT121 RANGE TESTS

APPLICATION NOTE

Tuesday, 10 March 2015

Version 1.0

Bluegiga Technologies Oy

Copyright © 2000-2015 Bluegiga Technologies

All rights reserved.

Bluegiga Technologies assumes no responsibility for any errors which may appear in this manual. Furthermore, Bluegiga Technologies reserves the right to alter the hardware, software, and/or specifications detailed here at any time without notice and does not make any commitment to update the information contained here. Bluegiga’s products are not authorized for use as critical components in life support devices or systems.

The WRAP is a registered trademark of Bluegiga Technologies

The Bluetooth trademark is owned by the Bluetooth SIG Inc., USA and is licensed to Bluegiga Technologies. All other trademarks listed herein are owned by their respective owners.


VERSION HISTORY

Version Comment

1.0 Release


TABLE OF CONTENTS

1 Introduction ....................................................................................................................................................5

2 Test Setup .....................................................................................................................................................6

3 Results and Analysis .....................................................................................................................................7

3.1 Test Results ..........................................................................................................................................7

3.2 Analysis .................................................................................................................................................9

3.3 Conclusions ....................................................................................................................................... 12

4 Contact Information .................................................................................................................................... 13


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1 Introduction

This document provides practical open field measurements of the performance of BT121 at various ranges, as well as a rough means of estimating achievable ranges in theory. Also included are the radiation patterns for the BT121 carrier board. The results in this document do not guarantee a reliable practical range for a real application. The result should be considered as an example of an achievable range in near optimal conditions. In most practical applications the range can be much shorter because the orientation and height of the antenna are not optimal and typically there are obstacles within the RF path which will attenuate the signal significantly. The tests were performed with classic Bluetooth only, for Bluetooth Low Energy the range will be approximately 30% lower due to the slightly lower output power.

In a practical application the range is impacted by:

Any obstacles within the RF path

PCB layout around the antenna

Objects and materials close to the antenna, causing reduced radiation efficiency

The shape of the PCB

The mechanical design of the end product

The access point used

Proximity of walls, building, other big objects, even when not directly on the radio path. This is due to multipath propagation, reflections from other objects interfering with the wanted signal causing local signal fading.

On long ranges, the height from ground, due to ground reflection causing local signal fading. Typically, the higher from the ground the devices are, the better the achievable range.

Because the range is impacted by many factors which are difficult to control, the practical range must be tested with the end product and the application should not be design based on the maximum theoretical range because the practical range will always be shorter.


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2 Test Setup

To determine the range for BT121, it was tested in an airfield using a data connection between two BT121 modules on DKBT evaluation boards. The modules were running a custom BGScript application showing the link data throughput in one direction and the status of the connection.

The range was determined by measuring the data rate at various distances. When the throughput starts to drop, it drops quite steeply with further range until the connection fails.

Figure 1: Range test setup for BT121


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3 Results and Analysis

3.1 Test Results

The results are given as throughput in kilobits per second on the vertical axis and range in meters on the horizontal. The ranges were measured with four module orientations. The absolute value of the data throughput shown by these measurements should not be taken as an example of achievable data throughput due to the special application and development phase firmware used.

Figure 2: BT121+DKBT directional combinations


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Figure 3: The longer range with BT121 antenna ends of the DKBT board pointing towards each other (case 1 in above drawing), the shorter with the antennas on opposite directions (case 2)

Figure 4: BT121+DKBT board broadside ranges (cases 3 and 4)


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3.2 Analysis

The horizontal radiation pattern of BT121 soldered on a DKBT carrier board (though without the DKBT motherboard itself) is shown in the Figure 6. The chip antenna used has an average mid-band efficiency of -5 dBi and a peak gain of -3 dBi, with narrow notches reaching a worst case gain of about -25 dBi in one direction. Figure 7 and Figure 8 show a fairly omnidirectional pattern in other directions.

The TX power at the antenna input of BT121 is about +12 dBm.

The link budget for two BT121 modules with average antenna efficiency is:

+12 dBm (TX power) - 5 dBi (TX antenna) - 5 dBi (RX antenna) – (-95 dBm) (sensitivity) = 97 dB.

Based on the signal attenuation curve in Figure 5, this would imply a maximum range of almost 400 m. Using the peak gain instead of average would give a link budget of 101 dB and a maximum range of 450 m.

The absolute worst case is when the narrow directional minimums in both modules would point towards each other, which has a very low probability. A case with the other side pointing the minimum gain towards the other and the other an average gain would give the following link budget:

+12 dBm (TX power) – 5 dBi (one side) – 25 dBi (second side) – (-95 dBm) (sensitivity) = 77 dB

Looking at Figure 5, we get a range estimate of about 120 m.

However, a cell phone or other Bluetooth device the BT121 connects to may have a lower transmit power and a lower sensitivity than BT121 and thus a lower total link budget, reducing the range to some extent. Using BT121 against a device with +8 dBm transmit power and a sensitivity of -89 dBm with an antenna of similar performance would give the following link budgets in each direction:

+12 dBm – 5 dBi – 5 dBi – (-89 dBm) = 91 dB (BT121 to other device)

+8 dBm – 5 dBi – 5 dBi – (-95 dBm) = 93 dB (other device to BT121)

The lower of these determines the maximum range, which according to Figure 5 would be around 200m.

The calculated theoretical maximum range signifies the end of any communication and should be viewed as a maximum possible range rather than a reliable prediction of what will be achieved in practice. Though the connection might not be lost until the maximum range, proper data transfer may be practical only up to a somewhat shorter range than this, and the connection will be unreliable and very sensitive to device orientation at the range extremes.

The measured practical ranges are about a quarter lower than the calculated ranges, but approximate the radiation pattern of Figure 6. Based on the results, the achievable reliable range between two BT121 devices with most module orientations is more than 200 meters.


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Figure 5: RF path loss in an open field with antennas 1.4 meters above ground, demonstrating the effect of ground proximity compared to free space

Figure 6: Radiating pattern projection viewed from carrier board top


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Figure 7: Radiating pattern projection viewed from carrier board front

Figure 8: Radiating pattern projection viewed from carrier board side


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Figure 9: 3D radiating pattern

3.3 Conclusions

The range of two BT121 connected to each other was tested in an open field with antennas located 1.4 meters above ground. The results were compared to theoretical range based on the measured TX power, RX sensitivity and the antenna radiation pattern and were found to be quite well in line with the calculated estimates.


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4 Contact Information

Inquiries / Support: www.bluegiga.com/

Orders: www.bluegiga.com/

Head Office / Finland:

Phone: +358-9-4355 060

Fax: +358-9-4355 0660

Sinikalliontie 5A

02630 ESPOO

FINLAND

Postal address / Finland:

P.O. BOX 120

02631 ESPOO

FINLAND

Sales Office / USA:

Phone: +1 770 291 2181

Fax: +1 770 291 2183

Bluegiga Technologies, Inc.

3235 Satellite Boulevard, Building 400, Suite 300

Duluth, GA, 30096, USA

Sales Office / Hong-Kong:

Phone: +852 3972 2186

Bluegiga Technologies Ltd.

Unit 10-18

32/F, Tower 1, Millennium City 1

388 Kwun Tong Road

Kwun Tong, Kowloon

Hong Kong

http://www.bluegiga.com/support

http://www.bluegiga.com/support

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