Microwave power transmission using rectenna

                                        Abstract

microwave power transmission using rectenna

A high efficient rectanna was design and tested at 5.8GHz frequency, which is use to power up applications. Patch antenna used with a low return low simulated in ADS. The micro-strip patch is a good practice for manufacturing physical antenna, as it will be easy to fabricate. The designed patch antenna have been employed in a 2×2 and 1×4 triangular antenna array so that the power and gain can be increased. The patch antenna captures the RF signal and then feed through 50 ohm feed line into the low pass filter and then into the rectifying circuit giving DC power. A low pass filter at 5.8 GHz was constructed. The diode used for the rectifying circuit is HSMS 2862 or HSMS 2080 schottky. A comparison analysis has been done between different designs of antenna. The results compared and best possible DC voltage was achieved in ADS simulation.

ADS results achieved by simulating the designs for antennas, low pass filters and rectifying circuits were efficient. The designs were manufactured. The result generated from the physical antenna and the simulation is studied.

Chapter 1

1 Introduction

As the technology is growing the world is now moving toward wireless power. We can see that now days everyone prefers to use a wireless mouse or a wireless headphone. The use of batteries can make this possible but the problem is that too many batteries are being used and there has to be a way by which these applications can run wirelessly and the best thing would be if the batteries were not used. How can this be possible? This is the problem which we will try to solve in the design. The rectanna used will convert the RF power into dc signal and instead of batteries the application will have a rectanna to produce the power. Therefore we will have a true wireless system, which has no wires and no batteries. Although we have to agree that may be so power will not be produced by these rectanna but still as the technology increase, the rectanna manufacturing will also be improved.

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The word ‘rectenna’ as we know today was first introduced by Brown [4]. The basic concept of rectanna is a “rectifying antenna”. In other word an antenna which will be used for receiving RF signal and a rectifying circuit which is used to give us DC power. Personally I view rectanna as a wireless battery, which is a very cool device.

Wireless power transmission (WPT) can be viewed as an electrical grid which generates power. WPT is the technology which is used for wireless transmission of power, this will be used in future for solar power satellites [4]. Let me explain the concept in detail. As the satellites are orbiting around the earth 24 hour and the satellites have the capability to convert the solar power into RF signals and then beam those RF signal to the earth. Array of antennas will be used to capture those RF signals and the rectifying circuits to convert those into DC power. If sufficient amount of array antennas are used, a lot of power can be produced. The electrical power station would be viewed as RECTANNA stations where RF signal would be converted into DC power.

1.1 History of Microwave Power Transmission

Tesla was the first person who introduced the idea of wireless power transmission. Tesla was not able to produce power with the RF signal because the transmitted power got diffused in all the direction with 140 KHz radio signal [4].

The problem faced by Tesla was overcome, by the fact that higher RF frequency has greater directivity and so the power can be transmitted in a particular direction. Radar technology used in world-war 2 was also very helpful in advancing the growth of wireless power. In the early 1960’s W.C. Brown used that latest technology to produce wireless power for the first time. The word “Rectanna” which we use today was first developed by W.C. Brown. He used an antenna with a rectifying circuit to produce power. The conversion was very good. Based on brown’s research work, where P.E. Glaser in 1968 introduced a solar power satellite [4] [3].

1.2 Objectives

1. The object of this thesis is to design a rectanna which will be able to receive microwave energy at 5.8 GHz and then converting that signal to DC power. This thesis will also help to provide a new ways of exploring energy resources.
2. A secondary objective is to reduce the return losses so that maximum output can be achieved.
3. A comparison analysis is done between series and parallel configuration of the 5.8ghz antenna.

1.2.1 Scope of thesis:

• Perform a good and comprehensive literature review so that all the concepts of RF electronics could be understood.
• Understand different antenna designs and test them to get the best possible result.
• Simulating and creating a functional micro-strip patch antenna design suitable for the rectenna frequency of operation of 5.8 GHz.
• Designing, simulating and creating the rectifier circuit.

1.3 Thesis Outline

The thesis was completed in two semesters. Each a certain number task must be completed.

Semester 1: Involves literature review of patch antenna, low pass filter and rectifier. The array of antenna design will also be taken under consideration and will be tested and simulated in ADS software and a prototype of 4×4 circular and triangular patch antenna will be build for testing purpose.

Semester I

1. Introduction to the topic
2. Finding the research papers and resources
3. Literature review
4. Design proposal
5. Simulation in ADS
6. Operational system in ADS
7. Prototype for testing

Semester 2: The 2nd semester was utilized to make better designs and operation of antennas. Rectifying circuit will be improved as well. An application will be tested, so that the patch antenna can be used to power a small application.

Semester II

1. Tweaking of the design
2. Making system efficient
3. Measurements and results
4. Comparison analysis of design
5. Documentation and final report

Chapter 2

2 Introduction To Literature Review

This section outlines brief theory of micro-strip patch antennas .. The library resources were used extensively and the journals related to the power transmission using rectanna were studied in detail. The articles were used to get idea about the design as well as methods of adapting the microwave techniques.

2.1 Motivation

The possibility of transferring power wirelessly can open up infinite applications. The fact that wireless application will not be powered by the batteries but instead use RF signal to generate the power is so extraordinary that everyone would want to be a part of the technology.

The idea of using the solar space satellites to create power is not very new. It was first presented in 1968 by Peter E. Glaser [4] [3].

The area of wireless power is not only limited to power generation by satellites but in fact it can be used in daily electronics, such as a wireless headphone, wireless keyboard, wireless mouse and even in wireless small motors. This research will give me a glimpse of future technologies that lies ahead of us.

2.2 What I Want to Achieve

At the end of this thesis I hope to have a rectenna which will convert RF signals into DC power and that DC power will be used for any selected application.

2.3 Important Points About Antenna

An antenna is device which is made so that it can radiate and receive radiating power from the electromagnetic wave. There are some important points that we need to know about antenna before proceeding towards the main antenna design.

2.3.1 Antenna gain: The ratio of input power to the output power is called antenna gain.

2.3.2 Directivity: The property of antenna to radiate electromagnetic waves in a particular direction is called directivity. If the electromagnetic waves are concentrated in a particular direction then we can that antenna has good directivity. Directivity and gain are related to each other by the following formula. Gain = efficiency/Directivity.

2.3.3 Polarization: The phenomena of polarization can be understand as the orientation of electromagnetic waves at distance from the source. The polarization types have been show in the table below.

2.3.4 Impedance Matching: The energy transfer can only be possible if the antenna and the transmission lines are matched. Typically 50 ohm impedance is used for the radio. If the antenna is not match then the input power or the output power will be reflected back. As a result power will be lost and desired results will not be achieved.

2.3.5 VSWR and the Reflected power of antenna: The voltage standing wave ratio (VSWR) is a parameter, which tells us that how good the impedance match is done. A VSWR of 2:1 is considered good. Most of the antennas which are built have a VSWR of 1.5:1.

Chapter 3

3 Rectenna design

3.1 Introduction to rectenna

The above diagram shows a basic design of a rectanna. The antenna receives the RF signals. The signal is passed to the low pass filter by a transmission line which has an impedance of 50 ohm. The low pass filter will filter the desired frequency so that unwanted frequency does not go through the rectifying circuit. The low pass filter is also used to stop the harmonics reflected back from the diode. The rectifying circuit is used in double configuration. The double configuration is used so that maximum RF signal can be converted into DC power. Schottky diodes will be used in the rectification process as they have low voltage drop across it.

The overall efficiency of the rectanna can be determined by ?= PdcPinc, Pdc is the DC output power. Pinc is the Incident RF power

3.2 Operating frequency

The most common frequency used is 2.45GHz and 5.8GHz. The directivity of antenna is more at 5.8GHz. Over all a lot of applications are available at these frequencies.

3.3 Substrate Material

Taconic TLX-0 was used for the physical design of the antennas. It has the following properties:

• H = 0.787 mm(height)
• T = 17 µm
• er= 2.45
• TanD = 0.0019

Taconic TLX-0 laminate are low loss antennas.[8]

3.4 Design Specifications

Chapter 4

4 Array Antennas and Design:

4.1 Introduction to Array Antennas and Design

In this section is related with the antenna design in detail and the array antenna design for achieving greater power.

4.2 Micro-strip Patch Antenna

The patch antenna is triangular. It has 3 layers. The bottom layer is the ground, middle is the dielectric substrate and the upper layer which is made up of copper or gold.

As you can see in the figure, the 3 layer are shown. The patch antenna radiates because of the fringing fields between the ground and the patch. For good performance the thick dielectric should be used with a low dielectric constant [13]. As the design does not allow us to use a thick dielectric, otherwise the size of the antenna would be very big, so in our design a thin dielectric with high dielectric constant would be used.

A micro strip antenna has some advantages and some drawback. Some of the advantages and drawbacks are given in the table.

ADVANTAGES OF MICROSTRIP ANTENNA

• Light weight and have low volume with low profile.
• Fabrication cost is low, easily manufactured in big quantities.
• Circular and Linear polarizations can be made in them.
• Dual frequency and dual-polarization is also possible with this.
• Microwave integrated circuits can be integrated with them as well.
• The antenna structure can be fabricated with Feed lines and matching networks within.

Figure 7 Advantages of microstrip

DISADVANTAGES OF MICROSTRIP ANTENNA

• Bandwidth Narrow
• Lower gain
• Large losses with the feed structure.
• Cannot be high power handling capacity

Figure 8 Disadvantages of microstrip

4.3 Feed Technique

There are two types of feeding techniques

• Contacting: In this type feed network the RF signal is feed directly into the patch antenna.
• Non contacting: In this configuration electromagnetic field coupling occurs due to transfer of energy between the line of mirco-strip and the patch.

4.3.1 Micro-strip Line Feed

As you can see in the figure the simplest way of having a feed line is to attach a transmission line feed with the edge of the antenna. This feed line technique is very useful as the feed line and the antenna are both on the same plane. The structure is on the same surface and area is also reduced. It has a better utility as now the antenna (with the feed line) can be place in numerous applications.

The thickness of the feed line determines the impedance of the line.

4.4 ANALYSIS

4.4.1 Triangular patch antenna :

As we know that lots of work have been on rectangular patch antennas and the circular patch but for my design I will be using a triangular patch so that the radiation pattern and the return losses can be studied. The size of the triangular patch is smaller than the rectangular patch, so a much more efficient design. The design formulas are, in the later sections of the report.

4.4.2 Design specifications

4.4.2.1 TRANMISSION LINE MODEL :

In my design I have used the transmission line model to develop the concepts and theory behind the triangular patch antennas the equilateral triangular patch was also design using the transmission model.

Resonant frequency

The resonant frequency is given by [14]

fr=ckmn2pEr =2c3aEr(m2 +mn+n2)1/2 ————–1

In the above equation c is the velocity of the light in the free space. Er is the dielectric constant and K(mn) are the different modes of harmonic order. The lowest order resonant frequency given by [14]

fr=2c 3aEr——————————2

The effect of fringing field was not consideration in the above equation. The fringing field occurs at the antenna patch edge. In our case the fringing field will occur at the edge sides of the equilateral triangle. The fringing fields are depended on the physical dimension of the triangular patch and the height of the substrate material.

The figure below shows the field lines of a micro-strip antenna. The maximum amount of field lines are going into the substrate and some of them are in the air. The side of a patch is increased due to the fringing field effect. The side length of a triangular patch antenna will no longer will be the same as we have to take account of the effective dielectric constant Ee. The modified equations are given below.

The equation above does not take given very accurate results, as it does not consider the fringing fields produced by the antennas. The Er in equation (1) and (2) can be replaced by effective dielectric constant. The dielectric constant given by [14]

Ee=Er+12+Er-14(1+12h/a)-12 —————3

Similarly the a can be replaced by aeff (effective). The value of the aeff (effective) given by [14]

aeff=a+hEr ————————————-4

Where h is the height of the material

Thus equation 2 can be replaced with the effective value of aeff and Ee giving us the final equation given by [14]

fr=2c 3aeffEe——————————5

4.4.3 Excitation technique:

4.4.3.1 Micro-strip feed:

The location of feed point is determined according to dimension of the antenna. Once we have calculated the accurate dimension of the antenna then we have to connect it with a 50ohm transmission line. In my design I will be using the center fed for the patch antenna. There are two ways to do this. The first is using a quarter wave transformer and the 2nd is to use the inset model.

4.4.3.2 INSET MODEL:

A triangular patch antenna was designed using the inset model. The advantage of using the inset model is that the size of the antenna reduces as compared to the quarter wave transformer design [14].

The length of the triangular patch is d. the length of transmission patch is l and the width of the transmission patch is w. the width and length of transmission patch if design do that a 50ohm impedance can be achieved.

4.4.3.3 Quarter Wavelength Transformer

The quarter wave transformer is a system which is used to match the impedance. As we know that the resistance Rin of the antenna will not matched with the feed line which has a impedance of 50 ohm. A formula is used to calculate the impedance of the quarter wave transformer.

4.5 Antenna design

The calculation for different antennas was designed in ADS. And the result will be shown in much detail the later sections of the report. The operating frequency is 5.8GHz. The result show in the table below is for triangular patch.

4.6 Introduction to Array theory

The range of the system can be increased by array of antennas which are working together and the focus of the reception or transmission of energy also increase in a particular direction[10].

The antenna in an array can be manufactured on a micro-strip with a feed network and a power divider. There are two kinds of feed network in array antennas

• Single line(series feed network)
• Multiple feed network

The series-feed network in an array of antennas is limited with a fixed beam[13]. We can see that series feed is easy to manufacture as it has the same configuration which repeats. There is a major disadvantage, any change to a single element can affect the remaining elements.

The antenna elements are fed by a 1 to N (in corporate network).The power divider network has a identical path lengths to all the elements, from the feed point [13]. The phase of the element can be controlled in the corporate network with the help of phase shifters. Amplifier can be used to fix the amplitude [13].

4.6.1 The Array Factor

The array factor depends on

• Number of elements
• Element spacing’s
• Amplitude
• Excitation phase which is applied to each elements within the array

4.6.2 Array design

As we see in the previous section that that array design is deeply affected by the element of spacing, so for 5.8 GHZ antenna the wavelength is 51.72mm, based on these factors the element spacing for 5.8GHZ antenna the element spacing should be 38.79mm.

We can see that the element spacing is 0.75 % of the given wavelength.

As we can see in the below figure that the quarter transformer is matched with antenna which has a impedance of 100ohm. The 100ohm line are combined together and gives a resulting impedance of 50ohm at the meeting point of the line. In the last the two 100ohm lines are combined to give a 50ohm impedance which is same for a feed line.

4.6.3 Metering of Corners

In our design we are using a microstrip. So with the mircostrip antennas, the 90% angle at any corner can cause large reflection from the 90% degree angle. So a smooth edge has to be made, so that there are no reflection losses. To reduce the reflection factor the edges are metered at the corner so that there is a smooth flow of current. A equation can be used to metered the corners which is given by the equation below.

4.7 Simulation results in ADS

4.7.1 Single triangular patch

4.7.1.1 Single triangular patch :

4.7.2 Single patch with inset model:

4.7.2.1 Single patch with inset model:

4.7.3 2X2 triangular patch with inset model:

4.7.4 1×4 linear triangular patch:

4.7.5 Single patch with quarter wave transformer

4.7.6 2×2 triangular patch with quarter wave transformer

CHAPTER- 5

LOW PASS FILTER

5 Low Pass Filter

5.1 Introduction to low pass filter

In this chapter we will discuss the low pass filter and why is it important for the rectanna design. The techniques which will be used and calculation used to make a low pass filter. The results which are calculated by the ADS calculation will also be show in this section.

5.2 Theory behind LPF

The antennas which were designed in the previous section was for 5.8Ghz but still some extra frequencies have to be filtered. The rectifying circuit also cause reflection from the first harmonics. To stop those reflection from the diode, LPF is used. LPF is extensional for the design as it can increase the power of the antenna. The LPF block the signal coming back from the diode and so the phenomena of re-radiation does not occur.

The LPF is usually designed for the lower frequencies. We can use lamped component for the design but there is a problem with that. The lamped component can also cause reflection, so SMT components should be used to avoid the reflections from the inductor or the resistors.

There is another good way, instead of using lumped component, the LPF can also be designed on the same transmission line by using Richard transmission. I think it would be the best thing for our design because then we would be able to make the design on the same surface. The whole design would have the same physical platform and planer surface can be achieved.

There are two kinds of low pass filter which we can use in our design.

1. Equal ripple
2. Maximum flat

5.3 LPF Design

The filter can be designed by using “Richardson method from chapter of Micro-wave engineering by david pozar[9].

We have to get the values from the table then Richard transformation is used to get the series inductors and the stub used and the shunt capacitors to shunt stubs. For Richard ?/8 at ?= ?C. Then the 2nd step would be to use the kuroda identity to series stub and shunt stubs. For the frequency of ?/8 at 2.45 GHz) and (?/8 at 5.8 GHz) we need to apply the impedance and the frequency scaling to get the accurate value.

5.3.1 Equal ripple low pass filter

5.3.2 Maximum low pass filter

5.4 Simulation results from ADS

5.5 LPF AT 5.8GHZ

5.5.1 LFP EQUAL RIPPLE (5.8GHZ)

5.5.2 LPF MAX FLAT(5.8GHZ)

CHAPTER 6

6 RECTIFYING CIRCUIT FOR RECTENNA

6.1 INTRODUCTION TO RECTIFYING CIRCUIT FOR RECTENNA

The final stage of the rectenna design is the rectanna which actually converts the RF signal into the DC voltage. As the diode has its own impedance so we have to match the impedance of the rectifying circuit as well. The method by which the impedance was match and the results obtained from the ADS simulation will be looked.

6.2 Single Rectifier design

The single diode configuration is very easy to understand. In the positive wave of the AC signal the diode d5 is forward biased and the capacitor is being charged. When the negative wave come, the diode is reverse biased and no current crosses the diode and at that time the capacitor is providing the voltage. As the capacitor is providing the voltage we have small ripples in the simulation and in practical applications.

6.3 Single voltage quadrupler:

The quadrapler provides 4 times the voltage as campared to the single rectifying circuit. The configuration is easy to fabricate and easy to understand. In the positive cycle the 2nd diode and the 4th diode is forward biased allowing the capacitor c4 and c1 to be charged. In the negative cycle the 1st and the 3rd diode are reversed biased and the capacitor c3 and c2 are being charged. The four capacitor voltages adds up before the resistor, giving us four times the voltage at the resistor.

6.4 Four 5.8 ghz with quadraupler in parallel configuration:

In this design we have again used a quatrupler configuration bt this time we are using 4 sources in parallel with each other. The four parallel source were used to replicate the design, when 4 parallel antennas are used and then feeding into a single quadrupler rectifier configuration. The results are show below.

6.5 Four 5.8 ghz with four individual quadraupler rectifier in parallel configuration:

The difference between the above design and this design is that, this time individual rectifier is being used with a single 5.8ghz source. The configuration is used in parallel combination.

6.6 Four 5.8 ghz with quadraupler in series configuration :

In the design below a series combination was used. Four 5.8ghz source which is acting like a 5.8ghz patch antenna are configured in series conbination. The output from the source is then feed into the voltage quadrupler. The results are shown below.

6.7 HSMS 2862k Diode parameters

Chapter 7

7 Conversion efficiency

7.1 Introduction to Conversion efficiency

The design with different configuration was tested in ADS simulation. Now we have to move towards the physical antennas and we need to determine how we can mearsure the efficiency of a rectanna. When the antenna designs are manufactured we will tested the parrallel combination with the series combination.

We will be comparing two designs.

• 1×4 array of antenna
• 2×2 array of antenna

Chapter 8

8 Complete Rectenna design

In this section we will see the complete rectanna design on a single surface. Which means that the low pass filter and the rectifying circuit will be on the same surface as that of the array antennas

The dimension and the simulated results of all the component were shown in the above sections of the report.

Chapter 9

9 CONCLUSION

As we have seen that all the design of the antennas and the other component were tested at maximum in ADS and the results obtained from the ADS simulation shows that we are on the right track and we will be cable of manufacture a good efficient antenna is the next semester. The antenna will be able to convert the RF signal in DC power.

The low pass filters were designed and we saw that the equal ripple filter show a much better results and so we will be using that with our design. The rectifying circuit were build and tested and a practical results show that we need a minimum of 3dbm power, so that the SMT led can be turned ON.

In the future a 5.8 ghz antenna with array of 2×2 and 1×4 will be made. The simulation of 2×2 and 1×4 antenna array is already done in ADS. The designs are ready for manufacturing.

9.1 Prototype and gerber Files for Manufacturing

10 Appendix

References

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