Consideration for Optimum Antenna Placement in a Vehicle -Xx

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Page 1 of 26© Cambridge Silicon Radio Limited 2013 CS-300757-AN1Confidential Information - This material is subject to CSR’s non-disclosure agreement www.csr.com

Consideration for Optimum Antenna Placement in a Vehicle

Application Note

Issue 1

In-Car RF Lab


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Document History

Revision Date History

1 6 NOV 13 Original publication of the document

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ContentsDocument History.................................................................................................................................................. 2

Contacts ............................................................................................................................................................... 2

Trademarks, Patents and Licences ........................................................................................................................ 3

Life Support Policy and Use in Safety-critical Compliance....................................................................................... 3

Performance and Conformance ............................................................................................................................. 3

Contents ............................................................................................................................................................... 4

Tables, Figures and Equations............................................................................................................................... 4

1. Introduction .................................................................................................................................................... 6

2. Aspects of Physical Layer............................................................................................................................... 6

2.1. Antenna Characteristics........................................................................................................................... 6

2.2. Free-Space Path Loss ........................................................................................................................... 10

2.3. Reflections and Fading Aspects ............................................................................................................. 11

2.4. Absorption by Interior and Human Body ................................................................................................. 17

3. EMC Aspects ............................................................................................................................................... 20

3.1. EMC inside a Vehicle............................................................................................................................. 20

3.2. EMC to Environment of the Vehicle ........................................................................................................ 20

3.3. Vehicle to Vehicle Interference............................................................................................................... 21

4. Example of In-Vehicle RF-distribution:........................................................................................................... 22

4.1. Test Set-Up........................................................................................................................................... 224.2. Test Results .......................................................................................................................................... 23

5. Conclusions ................................................................................................................................................. 24

Document References......................................................................................................................................... 25

Terms and Definitions.......................................................................................................................................... 26

Tables, Figures and EquationsTable 2.1: Direct and Reflected Path.................................................................................................................... 12

Figure 2.1: Radiated Field of an Isotropic Antenna [1]............................................................................................. 6

Figure 2.2: Schematic: λ/2 Dipole and its Radiation Characteristic [1]...................................................................... 7

Figure 2.3: Monopole Antenna and its Radiation Characteristic [1][3] ...................................................................... 7

Figure 2.4: Inverted F Antenna and its Radiation Characteristic .............................................................................. 8

Figure 2.5: Chip Antenna [2] .................................................................................................................................. 8

Figure 2.6: Typical Chip Antenna Pattern [2]........................................................................................................... 8

Figure 2.7: Patch Antenna..................................................................................................................................... 9

Figure 2.8: Horizontal Radiation Characteristic of a Patch Antenna ......................................................................... 9

Figure 2.9: Vertical Radiation Characteristic of a Patch Antenna............................................................................. 9

Figure 2.10: Graph of Free-Space Path up to a Distance of 2 m............................................................................ 10


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Figure 2.11: Graph of Free-Space Path Loss up to a Distance of 20 m.................................................................. 10

Figure 2.12: Example of Multi-Path Horizontal Layer............................................................................................. 11

Figure 2.13: Example of Multi-Path Vertical Layer................................................................................................. 11

Figure 2.14: Cube Model for Multi-Path Investigation............................................................................................ 12

Figure 2.15: Frequency Response with Direct Connection and Multi-Path no Attenuation ...... ...... ...... ...... ...... ...... .. 13

Figure 2.16: Frequency Response with Direct Connection and Multi-Path; Direct Path 25 dB Attenuation ...... ...... .. 13

Figure 2.17: Dual Printed Antennas on a PCB...................................................................................................... 14

Figure 2.18: Reflection Impacts the Antenna Isolation........................................................................................... 14

Figure 2.19: Antenna Isolation with no Reflection Plate......................................................................................... 15

Figure 2.20: Antenna Isolation with a Reflection Plate in 1 cm Distance................................................................. 15

Figure 2.21: Antenna Isolation with a Reflection Plate in 5 cm Distance................................................................. 16

Figure 2.22: Antenna Isolation a Reflection Plate in 10 cm Distance...................................................................... 16

Figure 2.23: Human Body Absorption of Man 1..................................................................................................... 17

Figure 2.24: Human Body Absorption of Woman 1 ............................................................................................... 18



Figure 2.27: Human Body Absorption of Woman 2 ............................................................................................... 19

Figure 3.1: RF Emission of a Car using Wireless Communication with two Devices (Antennas)....... ...... ...... ...... ..... 21

Figure 3.2: RF Emissions in Crowed Traffic.......................................................................................................... 22

Figure 4.1: Test Set-Up for In-Vehicle RF-distribution Measurement ..................................................................... 23

Figure 4.2: Test Results In-Vehicle RF-Distribution............................................................................................... 23


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1. Introduction

With the introduction of wireless technology inside a vehicle aspects of RF-technology have to be considered in

order to reach acceptable performance levels.

The quality of wireless communication depends on several aspects. Beside the devices used for the wireless link

and their performance the environment has a huge impact, too.

A vehicle as an environment for a wireless link is a very difficult case. The cabin works partly like a Faradaý s cell

and the behaviour of reflections has to be also considered

The automotive environment means that the majority of the wireless links are short ranged, less than 5 meters.

This document provides an overview of the RF considerations needed, in particular antenna placement and design,

to achieve the best possible wireless radio performance. This document primarily considers the physical layer and

some relevant EMC aspects.

2. Aspects of Physical Layer

With respect to the OSI layer model the RF connection is within the lowest layer which establishes the physical

connection and therefore it is called the physical layer. In the following the principal aspects of the RF distribution are

introduced.

2.1. Antenna Characteristics

For wireless communication the antennas are one of the most important devices in the device chain. An antenna is a

converter. It converts the electrical power available at its input to an electric-magnetic field which is radiated in the

free space. And in other direction it picks up the electro-magnetic energy of an available RF field and converts it to

electrical power.

Based on the characteristic of that radiation there are various kinds of antennas. In the following the most important

are introduced.

2.1.1. Isotropic Radiator

The isotropic radiator is a theoretical device which radiated the total power in every direction with same density. It

has a horizontal and vertical diagram of 360 °

Figure 2.1: Radiated Field of an Isotropic Antenna [1]


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Figure 2.4: Inverted F Antenna and its Radiation Characteristic

2.1.4. Chip AntennaChip antennas are also easy to use. They could be assembled like a simple component. However, the placement

has to consider the environment of the Chip antenna because there could be some interaction with wiring and

components which are nearby to the chip antenna. A typical version with its radiation characteristic is shown below.

Figure 2.5: Chip Antenna [2]

Figure 2.6: Typical Chip Antenna Pattern [2]


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2.1.5. Antennas with Wanted Beam Patterns

In some applications it is a requirement that the RF distribution is only within certain directions. This could berealized for example by using more (active and/or passive) elements in case of Yagi antennas [3] or designing a

patch antenna

Figure 2.7: Patch Antenna

Figure 2.8: Horizontal Radiation Characteristic of a Patch Antenna

Figure 2.9: Vertical Radiation Characteristic of a Patch Antenna

Note:

This collection of antenna types is not complete but it covers the most important.


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2.2. Free-Space Path Loss

Every electromagnetic wave radiated from an antenna is attenuated with increasing distance from this antenna. Theattenuation depends on the wave length of RF signal. The formula for free-air attenuation is like following:

Path Loss [dB] = 20 log((4 π d)/ λ)

λ = Wave length [m]

d = distance [m]

Equation 2.1: Free-Space Path Loss based on Wave Length

In case of 2.4 GHz ISM band we got:

Figure 2.10: Graph of Free-Space Path up to a Distance of 2 m

Figure 2.11: Graph of Free-Space Path Loss up to a Distance of 20 m


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2.3. Reflections and Fading Aspects

Beside the increasing attenuation with distance there is another aspect which is very important for wireless systemsespecially within vehicles. Normally the body of a car is made of metal. Metal however works for RF waves like a

mirror.

In case there are at least two antennas placed in such a vehicle and both transmitting and receiving electromagnetic

waves. With the metal body around them there is a single direct connection but also quite a lot of indirect

connections. These indirect connections are caused on reflections by the surrounding metal body.

So this is a multipath situation. An example of reflections in a multipath environment is shown below:

Figure 2.12: Example of Multi-Path Horizontal Layer

Figure 2.13: Example of Multi-Path Vertical Layer


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2.3.1. Simple Model of Multipath Investigation:

As shown on the previous page the reflection behaviour inside a car is very complex. In order to get more details of

reflections and its impact to wireless communication a simple model was created:

2.3.1.1. Introduction of the Model

Within a cube two antennas are placed. As the shape of the cube is clear, reflection paths can be estimated:

With the simple assumptions that the surface of the cube reflects RF totally and the characteristic of the

antennas used is nearly isotropic, we got a certain number of paths. This number depends on the amount of

reflections within one path (grade of reflection) we want to consider:

Direct path 0 reflections

One reflection per path

Two reflections per path

Table 2.1: Direct and Reflected Path

The figure 2.14 shows all the reflection paths with one reflection and one example with two Reflections.

Figure 2.14: Cube Model for Multi-Path Investigation

This model is useful to investigate reception performance via the location and during movement, too.

Based on the model above a spread sheet calculation was done. There were some assumptions made:

The cube has a length of 1.5 m.

The antennas were placed at the centre of two opposite surfaces.

Their distance from the outside was 0.25 m.

This example was taken to demonstrate the huge impact of reflection paths.

Antenna 2 Antenna 1


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For this calculation the direct connection and the paths with only one reflection were considered.

2.3.1.2. Results of Simulation

Figure 2.15: Frequency Response with Direct Connection and Multi-Path no Attenuation

Figure 2.16: Frequency Response with Direct Connection and Multi-Path; Direct Path 25 dB Attenuation

It can be seen that the reflections are dominant in this case.

Small changes in antenna placement could also have a significant impact on path attenuation.


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2.3.2. Impact of Reflections to Antenna Isolation

In many vehicles more than one wireless service is used, high independency of these services is required in order to

avoid any impact between them.

With respect to the physical layer it is recommended to use a separate antenna for each service. In order to get high

performance in concurrency, for example between BT and Wi-Fi, a very high isolation between both antennas is

necessary. Otherwise, in case of one service is transmitting while the other is receiving any cross-talk from the

transmitting antenna reduces the reception performance of the other and this finally results in performance reduction.

Therefore great efforts have been made to get high isolation of antennas. However the isolation is also influenced by

the environment of such an antenna configuration.

Assuming there are two antennas at a PCB (one for Bluetooth and one for Wi-Fi). Basic measurements show an

isolation of 30 dB is required. See CSR application note “M2501 Antenna Isolation” [4]

Figure 2.17: Dual Printed Antennas on a PCB

Since the radiations of the antennas are not separated from each other a piece of metal or other reflecting material

could modify the RF-fields and reduce the isolation behaviour:

2.3.2.1. Test Procedure

A metal plate was placed with a certain distance to the PCB with the antennas. The antenna isolation was tested

with no metal plate, with the metal plate in a distance of 1, 5, and 10 cm distance.

Figure 2.18: Reflection Impacts the Antenna Isolation


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2.3.2.2. Test Results

The isolation of the two antennas was measured in the frequency range from 2 to 3 GHz:

Figure 2.19: Antenna Isolation with no Reflection Plate

Figure 2.20: Antenna Isolation with a Reflection Plate in 1 cm Distance


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Figure 2.21: Antenna Isolation with a Reflection Plate in 5 cm Distance

Figure 2.22: Antenna Isolation a Reflection Plate in 10 cm Distance

With these results it is strongly recommended to keep a distance >10 cm from a dual antenna to a reflecting surface

in order to maintain suitable antenna isolation.


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Figure 2.24: Human Body Absorption of Woman 1

Figure 2.25: Human Body Absorption of Man 2


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Figure 2.26: Human Body Absorption of Man 3

Figure 2.27: Human Body Absorption of Woman 2

These results show an additional attenuation of the RF-link by 10 to 30 dB caused by human body. Also there is a

wide deviation from person to person and its position related to antennas. In order to find the causes of these

deviations additional investigations are necessary. Also the impact of the clothing has to be considered.

2.4.2. Absorption by Interior

Inside a vehicle there are a lot of materials which impact the RF-distribution as well. A quantitative estimation is very

difficult, because the parameters which are necessary for, depending on the used material, size and location.

However, as long as values of these parameters are not available, it is important to consider them in a qualitativeway.


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3. EMC Aspects

RF connections are embedded in a real environment and therefore EMC becomes important. Especially in an

environment which is more and more loaded with RF.

The basic requirements to avoid EMC issues are that a device has limited emissions on the one hand and on the

other it is immune against the remaining emissions created by its environment or other devices.

This means in case of wireless communication inside a vehicle, that these requirements have to be met within the

vehicle and its interaction to its environment.

3.1. EMC inside a Vehicle

A modern vehicle contains a lot of RF distributing devices. Either the RF are wanted signals (for example. wireless

communication) or they are unwanted emissions.Under these circumstances case has been taken to find the optimal placement for antennas.

Keep in mind antennas pick up RF-radiation in a wider frequency range than even the wanted one.

Therefore the following aspects of suitable placement should be considered.

Keep distance to a potential interferer

Avoid that a device which is working at the wanted frequency or could generate harmonics in this

range, is located near to the antenna

Take care of antenna beaming and avoid unwanted emissions

3.2. EMC to Environment of the VehicleIf wireless communication is used inside a vehicle, a certain part of the RF power is transmitted to the outside, too. In

contrast to the metal body the windows do not (or much less) reflect the RF. And so it could pass to the environment.

In case there is another location (house, shop, people using mobile phones, etc.) at which wireless communication is

used, too, a potential interference scenario is created.

For a better illustration we assume a car in which wireless communication is used to connect two devices with one

antenna for each device.

With the further assumption that the RF passes the car through the widows we got the following pattern. Where the

yellow areas caused by antenna 1 and the blue ones are caused by the antenna 2.


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Figure 3.1: RF Emission of a Car using Wireless Communication with two Devices (Antennas)

It is also very important to consider that the communication inside the car is sensitive to potential interferer located in

the displayed beam areas.

3.3. Vehicle to Vehicle Interference

Further on assuming that the there is heavy traffic and each of the cars use wireless communication in the same way

as described above, there is quite a lot of interference potential.

The following figure illustrates a scenario with 6 cars on a multi lane street. Four cars drive from South to North andtwo in the opposite direction.


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Figure 3.2: RF Emissions in Crowed Traffic

There is a high probability that with increasing number of cars using wireless communication near to each other

interference effects happen. These effects could cause disturbances and even break downs of the wanted

communication inside of one or more of these cars.

4. Example of In-Vehicle RF-distribution:

4.1. Test Set-Up

In a car, Chrysler Grand Voyager (left hand drive), one transmitting (IFA) and six receiving antennas were placed

according the schematic shown below. RF in the range from 2.4 to 2.5 GHz was transmitted and via a multi-switch

one of the six receiving antenna was connected to a receiver. Then the switch made one step ahead (the next

antenna is connected and the measurement was repeated. This was done for all six receiving antennas.

The testing was done for an empty cabin, a driver, driver and passenger, additionally a person behind the

driver and finally additionally a person behind the passenger.


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Figure 4.1: Test Set-Up for In-Vehicle RF-distribution Measurement

4.2. Test Results

These figures are the frequency response cures of the different receiving antenna locations. They show the

attenuations with different numbers of passengers inside the vehicle.

Figure 4.2: Test Results In-Vehicle RF-Distribution


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5. Conclusions

A general conclusion on antenna placement in a vehicle cannot be reached, however antenna

placement can clearly be optimised per vehicle based on the required behaviour and the vehicle

characteristics.

Because of the complexity of a vehicles shape its RF reflections behaviour and the various impacts of

absorptions by human bodies and the interior a prediction of the wireless link quality is not possible.

EMC aspects will become more important with the number of vehicles using wireless communication.

Consider that the areas with high RF emissions are bi-directional. This means that devices inside the

vehicle could be interfered from external devices located in these areas and vice versa.

But based on the considerations above the following recommendations can be made:

Place the antenna on a location which minimize the amount of reflection paths:

Reduce the presence of human bodies within the RF-link (a human body absorption of 10 to30 dB should be assumed).

Antenna locations inside the cabin towards the roof, next to B- or C-pillar may advantageous.

Use antenna with beam pattern. Orient the beam to the inside of the vehicle (for example

downwards if it is located next to the roof).

Do not place dual antenna, which have to have high isolation just in-front of reflecting

materials.

Lower the TX power as much as possible to avoid interference of the environment of the

vehicle. The distances inside of a vehicle cabin are short and do not need much power to

pass them (see chapter 2.2). The main part of link quality reduction is done with absorption

and reflection.


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Document References

Document Reference

Designing an Inverted-F Antenna CS-217944-AN

[1] Wikipedia

[2] Application Note AN048 TI

[3] Antennen.pdf, Fachhochschule Nordwestschweiz

[4] M2501 Antenna Isolation CS-235304-AN

[5] Designing an Inverted-F Antenna CS-217944-AN


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Terms and Definitions

CSR Cambridge Silicon Radio

dB decibel

EMC Electromagnetic Compatibility

IFA Inverted F Antenna

PCB Printed Circuit Board

RF Radio Frequency

RX Receive or Receiver

TX Transmit or Transmitter

Documents

Consideration for Optimum Antenna Placement in a Vehicle -Xx