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UPTEC F11 066

Examensarbete 30 hp

December 2011

Design of an Antenna for a Wireless

Sensor Network for Trains

Malkolm Hinnemo

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Design of an Antenna for a Wireless Sensor Network

for Trains

Malkolm Hinnemo

An antenna for a wireless sensor network for trains is designed and built. The network will monitor temperature and vibrations of the wheel bearings on the train wagons. Doing this will allow for an earlier detection of damaged wheels, which will ease planning of maintenance and reduce wear on the rails considerably. The requirement of the system is that it is to be installed without any cables attached to the sensor nodes. This calls for wireless communication, and that for that antennas are needed. A train is a difficult environment to transmit electromagnetic (EM) waves in. It is full of metal and EM-waves cannot pass through a conducting material. Having much metal in its vicinity also affects the function of the antenna. This needs to be taken into consideration when making the design. The constructed antenna is a small dual-layer patch antenna. Dual layer means that it is constructed out of two sheets known as substrates of isolating material with different characteristics. The lower one of these substrates is made in such a way that integration with a circuit board is possible. Such integration would reduce the production cost considerably. The antenna is designed for direct placement on a conducting surface. This surface could be part of the train. It uses the surrounding metal as a ground plane in order to reduce its size. The result is a small patch antenna with good radiation qualities in metallic surroundings. The longest side is 18.35 mm, equaling 14.9 % of the wavelength that the antenna is designed for.

ISSN: 1401-5757, UPTEC F

Examinator: Tomas Nyberg

Ämnesgranskare: Anders Rydberg

Handledare: Mathias Grudén

IV

Table of Contents

Abstract ......................................................................................... Error! Bookmark not defined.

• Introduction Sammanfattning......................................................................................................................... III
  • 1.1 Project description.......................................................................................................
  • 1.2 Background..................................................................................................................
• 2. Theory
  • 2.1 Maxwell´s equations
  • 2.2 Antennas
  • 2.3 Patch Antennas
  • 2.4 Reflections and Matching.............................................................................................
  • 2.5 Polarization..................................................................................................................
  • 2.6 Friis Transmission Equation
  • 2.7 Power levels
• 3. Antenna Design Procedure
  • 3.1 Early simulations
  • 3.2 The first model
  • 3.3 Prototype Building
• 4. Measurements
  • 4.1 The return loss
  • 4.2 Total and radiated efficiency
  • 4.3 Radiation Pattern
  • 4.4 Application test 1: Open Environment
  • 4.5 Application test 2. Train
• 5. Mechanical Construction
  • 5.1 Gateways V
  • 5.2 Nodes
• 6. Measurements onboard train
• 7. Discussion and Conclusions
  • 7.1 The antenna
  • 7.2 The System
• 8. References
  • Antennas and Propagation for Wireless Communications, 200 IEEE-APS Conference on, 6- [9] S.Vajha, S.N.Prasad, “Design and modeling of proximity coupled patch antenna”,
  • nov

Introduction

1.1 Project description

The goal of this project is to design, build and test an antenna for the 2.45 GHz ISM-band. This antenna is to be a part of a temperature sensor network placed on trains. The network will monitor temperature and in a later stage vibrations of the wheel bearings on the train wagons. This is done in order to detect faulty and broken wheels. The goal is to enable better planning of wagon maintenance by detecting which wheel will break before it actually does. Today detection is only possible after the wheel gets so badly damaged that it starts to cut into the rails. When this happens the wheel will damage the rail along the whole way from the point it starts to cut until it gets repaired or replaced. Being able to replace the wheels before this happens would thus reduce the need for rail maintenance. Research on the radio environment of trains has been performed previously [1]. One sensor node is placed on each wheel bearing of each car, and a gateway is placed on the side of the car where an RFID antenna will send the information to the stationary readers that the Swedish railway authority Trafikverket has mounted along the rails of Sweden. The antennas will also be sending the information between the nodes and in this way sending the information from wagon to wagon. This is called multi hopping. The information will thus be transported forward towards the locomotive that is equipped with GSM-R, a version of the GSM network specially designed for railways.

The short term goal for the project i.e. - the goal that is to be reached at the end of thesis project - is to get a functioning prototype system with direct communication between the sensor nodes and the gateway. This is as far as antennas concern equal to a system that includes multi hopping as this is a problem concerning signal treatment, which is out of range for this thesis.

The antenna is desired to be small. It should work in metallic surroundings, a condition that puts tough requirements on the antenna design. When currents are flowing on the surface of the antenna there will be a coupling between the antenna and the metal in its surroundings. This will affect the performance of the antenna. It has to be designed in such a fashion that it will work at the

desired frequencies in a position very close to a metal wall. The design also has to make sure that the energy is actually radiated and not dissipated into the train.

The antenna has been designed by simulating its radiation characteristics and by comparing these to experimental results. It has been constructed by milling out the shapes from copper covered substrates and by then gluing these together.

My part of the project also includes assembling the enclosure for the gateway and assisting in the assembling of the nodes. This includes deciding what box is to be used and securing them so that they will be able to withstand vibrations.

1.2 Background

There are today a very large variety of antenna models. There has been an explosion of new models due to the introduction of simulation software.

Many antennas made today are designed to work in empty space. This is the simplest environment, where only the antenna needs to be taken into consideration. Those antennas will not work for this application, since the antenna is mounted on a large piece of metal and has an electric circuit close to it. Both of these factors will affect the performance of the antenna. There are antennas designed for hostile environments. Some are designed to have a large bandwidth in order to make them work even after detuning [2]. Detuning is the unwanted change in operating frequency that takes place due to the surroundings of the antenna. Others are made in such a way that they will be very robust to changes in their surroundings and can work in difficult environments. The goal of those designs is to keep the fields within the structure of the antenna and in that way minimize the sensitivity of the antenna to its surroundings [3]. There is however no general solution to this problem and there are no antennas available to buy that will work well in any hostile environment. This means that for each application one needs to find the solution that works the best in that particular environment.

conductor ends but the waves will continue [5].

Fig 1: Illustration of waves leaving antenna

The reason that not all conductors radiate perfectly is that there are losses, for example resistivity, and for conducting devices that are not meant to radiate these losses are usually much greater than the radiation (which is itself considered a loss in those cases).

2.3 Patch Antennas

Patch or microstrip antennas are one kind of antennas. A patch antenna design is presented in this report which means that a more detailed explanation of their function is in order. The classic patch antenna consists of a ground plane and a thin layer of conducting material that are separated from each other by a dielectric sheet called substrate. Current is led to the thin layer or patch and will induce an electric field between the patch and ground. This field will directly beneath the patch be straightly directed from the patch to ground or the other way around depending on the phase of the

current. But at the edges it will be more spread out since there is more room. This effect is called fringing and it is this that makes patch antennas work.

Fig 2: Patch antenna with fields

To describe this a basic half wavelength rectangular patch antenna, which is shown in Fig 2, will be used as an example. Half wavelength means that at resonance frequency the wavelength will be about double the length of the antenna and the current will thus form half a wave. In this kind of antenna the electric potential will reach its maximum values at the edges but with a sign change. This is the situation shown in Fig 2. What happens is that near the patch the electric field will be directed towards positive y on both sides of the patch. That means that they will have the same phase and for the radiation normal to the patch surface the fields will add up [6]. This is the direction of the strongest radiation. For waves traveling in the y direction the interference will be negative since there is one half wavelengths distance between the two edges where most of the radiation originates meaning that a wave that has traveled from one side to the other will experience a sign change and the interference will therefore be negative. Theoretically no radiation should be possible to detect in the positive or negative y directions [5].

At the edge of a conductor the current needs to be zero. This is because current is transportation of charge and if the conductor ends the charges need to stop. For an antenna with open ends such as the patch antenna described above this means that the longest standing wave will be twice the size of the antenna. If there is a way for the current to continue to flow, it can instead reach its maximum value at the end. This can be accomplished by

one of the load part of the wave it will be reflected so that the requirement of impedance of the load will be met [8]. This reflected part will, together with the incident current, form a standing wave in the transmission line and is lost to the antenna.

To avoid this loss one of the main goals of antenna design is to match the impedance of the antenna to that of the transmission line. In an antenna this is mostly done by changing the geometry. A long thin line will give a higher inductance, while a slot tends to build up electric charge on each side and thus increases capacitance. The standard impedance for transmission lines is 50 Ω real. In this project matching means matching to this impedance.

2.5 Polarization

The electrical field has at any given point in time and space a direction. How these directions vary is called polarization [4]. When the wave reaches a conducting material it will induce a current on it in the direction of its polarization. If the conducting material is part of an antenna this current will be recorded, but only to the extent that the current can propagate in that particular direction. This is the receiver´s polarization and if there is a polarization mismatch between the incoming wave and the receiver less or no energy will be received. For this reason polarization is very important to bear in mind when considering radiation characteristics. The electromagnetic waves that propagate large distances in space are all transverse electromagnetic waves. There are a few main types of polarization:

• Linear
• Circular
• Elliptic
• Unpolarized

Linear polarization is the kind that will be of interest in this article. It means that the direction of the electric field will be along one axis at all times. This axis does not have to be one of the principal axes in the coordinate system, it just means that the field will point in direction or in the direct opposite direction. This definition is valid for one wave, another wave close to it might be linearly polarized in another direction.

Circular polarization is a special case of elliptic polarization. In this kind of polarization the direction of the electric field rotates around the axis of propagation. This is desirable in applications when it is difficult to know the polarization of the transmitter/receiver that one wants to communicate with.

Unpolarized radiation is when the polarization of different photons is uncorrelated. To an observer this will seem like the polarization of each photon is random. An example of unpolarized radiation is sunlight.

2.6 Friis Transmission Equation

The Friis Transmission Equation is one of the most commonly used equations when testing antennas. It gives the power received as a function of the power transmitted, the geometry, and various loss factors.

The subscripts t and r refer to transmitter and receiver respectively. The e- terms are radiation efficiency, the gamma terms are the reflection losses, D is directivity, and the term to the utmost right is polarization losses. (λ/4πR)^2 is called the free space loss factor, which shows the losses that appear due to the waves spreading in a spherical way. This is, as the name hints, only valid for free space. Radiation efficiency takes into account the combined effect of conduction and dielectric losses. These are lumped together since it is most often very hard to separate them. Radiation efficiency is defined as the ratio of the power radiated to the power that reaches the antenna. Directivity is a measure of how much power is transmitted in a particular direction. The free space loss factor assumes isotropic radiation, which is never the case in reality. The directivity adjusts for this. It is defined as 4piU/Prad. Where U is the radiation intensity in the direction of interest and Prad is the total radiated power. What this does is that it gives a coefficient of how much the intensity in the given direction varies compared to the average value. Sometimes the parameter gain is mentioned. Gain is the radiation efficiency multiplied by the directivity. Total gain is the total efficiency multiplied by the directivity.

3. Antenna Design Procedure

3.1 Early simulations

The first step in designing the antenna was to find a basic idea of how the antenna should work. There are many different ways to make a radiating structure and the first step in the design process is to determine which structure would suit this application best. This was done by reading articles about other antennas and when a promising design was found, simulating that design and looking at what kind of behavior it showed. Simulations were done in CST Microwave studio. For the initial tests all boundaries were modeled as open boundaries, which mean that the behavior in free space was be simulated. The simulations were done in the time domain. In these early simulations the goal is to get a quick look at how the design works, and so the accuracy is not very important. The mesh settings are therefore kept at the default settings which are 10 lines per wavelength in the regions of interest and a lower limit of 5 lines per wavelength. For the same reason the ports used are discrete ports. The frequency spectrum is large, since many of the designs presented in the articles do not work as expected when simulated. By simulating over a very large range of frequencies it is possible to analyze how the antenna might work but not in the area specified by the article in which it was mentioned. The aspects that were given most importance in this early stage were bandwidth, size, directions in which the radiation was the strongest and, the most important aspect for this application, how it reacted when large metal objects were nearby.

3.2 The first model

The model that was found to have the best characteristics was a kind of dual layer patch antenna [9]. This antenna had a large enough bandwidth even though patch antennas usually are narrow banded. The largest radiation was in the normal direction of the patch which in this test also meant that it was in the normal direction of the train. This is the only direction where there is no receiver so that energy is wasted. There was however sufficient radiation along the sides of the train for the design to be interesting. What really made this design stand out was the fact that it was not very sensitive to metal being placed behind its ground plane. This is because the near fields do not reach outside of the region defined by the ground plane. For this reason it

This would however not have been practical to make, so short circuiting with a patch was tried instead and results were similar.

Fig 6: Antenna with shortened wall

With one end short circuited the behavior of the antenna changed drastically. First of all the current changed in the way that it was intended to do, and changed from having an half wavelength resonance to having a quarter of a wavelength resonance. Secondly and much more unexpected the direction of radiation changed. It became more isotropic but with two directions of stronger radiation, one normal to the shortening patch and one away from the feeding point. This radiation pattern is perfect for the application on the train where there will be antennas in many directions. Fig 7 shows the system setup on one wagon. In the final system the intention is that each node is to be connected to both gateways on the wagon where it is placed and to the gateways on the neighboring wagons. This requires that each node antenna radiates to the left, right, and inwards where the directions are the ones of the picture.

Fig 7: System setup on one wagon. Sensor nodes(blue) gather data and sends it to the gateways(red). The processed data is sent to the supervisor through RFID

The materials that was used for the substrate modeling was taken directly from the article on which the first model was based. The material choice for the upper substrate turned out to be a very costly one as no substrates with that electric permittivity are made as thick as 3 mm. In order to avoid having to order a custom made substrate, a search of the office was made and a new material was found. This was a Rogers 3003 substrate with thickness 1.524 mm. This material is a low loss dielectric with electric permittivity 3. This was higher than the 2.2 that was used in the simulations but it was the lowest that could be found. New simulations were conducted to see how this material would work. It was clear that radiation efficiency and bandwidth both suffered from this change. The radiation efficiency sank by about 1 dB or 21%. The bandwidth decreased dramatically but it was still large enough to meet what was thought to be the requirements and so this was considered acceptable.

The goal this far had been to make an antenna that would be as efficient as possible. This forced a turning point in the design process. Either these results were considered to be too bad and a new design would be drafted, or new goals for the project had to be made. The latter option was decided upon. The new goal of the design was to make the antenna small that is to