Institutionen för systemteknik
Department of Electrical Engineering
Examensarbete
Underwater communications system with focus on
antenna design
Examensarbete utfört i Elektroniska kretsar och system vid Tekniska högskolan i Linköping
av
Erik Carlsson
LiTH-ISY-EX-ET–15/0444–SE
Linköping 2015
Department of Electrical Engineering Linköpings tekniska högskola Linköpings universitet Linköpings universitet SE-581 83 Linköping, Sweden 581 83 Linköping
Underwater communications system with focus on
antenna design
Examensarbete utfört i Elektroniska kretsar och system
vid Tekniska högskolan i Linköping
av
Erik Carlsson
LiTH-ISY-EX-ET–15/0444–SE
Handledare: J Jacob Wikner
isy, Linköpings universitet Per Hagström
Combitech
Examinator: J Jacob Wikner
isy, Linköpings universitet
Avdelning, Institution
Division, Department
Division of Communication Systems Department of Electrical Engineering Linköpings universitet
SE-581 83 Linköping, Sweden
Datum Date 2015-009-006 Språk Language Svenska/Swedish Engelska/English Rapporttyp Report category Licentiatavhandling Examensarbete C-uppsats D-uppsats Övrig rapport
URL för elektronisk version
http://www.commsys.isy.liu.se http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-ZZZZ ISBN — ISRN LiTH-ISY-EX-ET–15/0444–SE
Serietitel och serienummer
Title of series, numbering
ISSN
—
Titel
Title
Svensk titel
Underwater communications system with focus on antenna design
Författare
Author
Erik Carlsson
Sammanfattning
Abstract
In this thesis the possibility of building an underwater communication system using electromagnetic waves has been explored. The focus became designing and testing an antenna even if the entire system has been outlined as well. The conclusion is that using magnetically linked antennas in the near field it is a very real possibility but for long EM waves in the far field more testing needs to be done. This is because a lack of equipment and facilitates which made it hard to do the real-world testing for this implementation even if it should work in theory.
Nyckelord
Abstract
In this thesis the possibility of building an underwater communication system using electromagnetic waves has been explored. The focus became designing and testing an antenna even if the entire system has been outlined as well. The conclusion is that using magnetically linked antennas in the near field it is a very real possibility but for long EM waves in the far field more testing needs to be done. This is because a lack of equipment and facilitates which made it hard to do the real-world testing for this implementation even if it should work in theory.
Sammanfattning
I avhandlingen har möjligheten för att bygga ett undervattenskommunikationssy-stem som använder elektromagnetiska vågor undersökts. Fokus blev konstruktion och provning av antenner, även om det hela systemet också har beskrivits. Slutsat-sen är att en användning av magnetiskt kopplade antenner i närområdet är mycket möjligt, men för långa EM vågor i fjärrfältet måste fler tester göras. Detta är på grund av brist på utrustning och lokaler vilket gjorde det svårt att göra tester på den verkliga världen även om det skulle fungera i teorin
Acknowledgments
I would like to thank the following:
• Per Hagström for being my adviser at Combitech and giving me feedback on the thesis.
• J Jacob Wikner for being my examiner and setting my on the right track when I was getting sidetracked by something.
• Magnus Karlsson for helping me with the antenna measurements.
• Jon Staffeldt for aiding with ideas on what and how to measure with the antennas.
• Martin Nielsen Lönn for always being helpful when I came asking for access or help with something.
• The departments of ISY and IFM for allowing me accesses to laboratories and measuring equipment.
Contents
1 Introduction 1
1.1 Background . . . 1
1.2 Goals of the thesis . . . 1
1.3 Limitations . . . 1 1.4 General idea . . . 2 2 Theory 3 2.1 Notation . . . 3 2.2 Maxwell equation . . . 3 2.3 Propagation . . . 4 2.3.1 Radio waves . . . 5 2.3.2 Acoustic waves . . . 6 2.3.3 Attenuation . . . 6 2.3.4 Refraction loss . . . 8 2.3.5 EM-signals in seawater . . . 8 2.4 Antenna types . . . 9 2.4.1 Loop antenna . . . 9 2.4.2 Dipole antenna . . . 10 2.4.3 J-pole antenna . . . 10 2.5 Impedance matching . . . 11
2.5.1 Impedance matching a loop antenna . . . 12
2.6 Digital modulation . . . 12
2.6.1 OOK modulation . . . 13
2.6.2 BPSK modulation . . . 13
2.6.3 ASK modulation . . . 13
2.6.4 PSK modulation . . . 14
2.7 Gaussian white noise . . . 14
2.8 Commercial systems . . . 15
2.8.1 Near field communication . . . 15
2.8.2 Bluetooth . . . 16
2.8.3 Oceanreef . . . 16
3 Related Research 17
4 Method 19 4.1 The process . . . 19 4.2 The system . . . 19 4.2.1 Layout . . . 19 4.2.2 Signals . . . 20 4.2.3 Modulation . . . 20 4.3 Hardware . . . 20
4.3.1 Low pass filter . . . 21
4.3.2 Analogue digital converter . . . 21
4.3.3 Micro processing unit . . . 21
4.3.4 Signal generator . . . 21
4.3.5 RF filter . . . 22
4.3.6 RF amplifier . . . 22
4.3.7 Low noise amplifier . . . 22
4.3.8 Power detector . . . 23
4.3.9 Digital analogue converter . . . 23
4.4 Antenna/transfer system choice . . . 23
4.4.1 Discarded choices . . . 23
4.5 Loop antenna construction . . . 24
4.6 Impedance matching . . . 24
4.7 Measuring the signals . . . 25
4.8 A second system . . . 25
5 Result 27 5.1 First trial . . . 27
5.2 Second trial . . . 28
5.3 Third trial . . . 29
5.4 Conclusion of the first three trials . . . 29
5.5 Fourth trial . . . 31
5.6 Fifth trial . . . 32
5.7 Impedance measuring . . . 32
5.7.1 3-coil loop antenna . . . 34
5.7.2 2-coil loop antenna . . . 34
5.7.3 Caveats . . . 35
5.8 Longer range trials . . . 35
5.8.1 Sixth trial . . . 35
5.8.2 Seventh trial . . . 35
5.9 Trials in water . . . 37
5.9.1 Tap water ranged trial 1 . . . 38
5.9.2 Tap water ranged trial 2 . . . 38
5.9.3 Salt water ranged trial 1 . . . 38
5.9.4 Salt water ranged trial 2 . . . 39
5.9.5 Tap water short trial 1 . . . 39
5.9.6 Tap water short trial 2 . . . 40
5.9.7 Salt water short trial 1 . . . 40
Contents xi
5.9.9 Brackish water short trial . . . 42
5.9.10 Seawater short trial . . . 42
6 Discussion 45 6.1 Conclusion of the air measurements . . . 45
6.2 Conclusion of water measurements . . . 46
6.3 Conclusion of impedance measuring . . . 48
6.4 Security and integrity . . . 49
6.5 Issues with the thesis . . . 50
6.6 Conclusion . . . 50
7 Future Work 51 7.1 Building the system . . . 51
7.2 Perfecting the antenna . . . 51
1
|
Introduction
1.1
Background
The idea for this thesis was a videoclip [1] using a cellphone while submerged in water but it was still transmitting. This raised the question if you could develop a wireless communication system based on radio waves that could transmit through water and other kinds of medium that is not air. Our prior knowledge is that their is no such system on a broad and commercial level.
1.2
Goals of the thesis
The goal of this thesis was to develop a system that will provides wireless voice communication in different kinds of water, examples are: seawater, brackish water and river water. Different systems on the market, like Bluetooth and NFC as well as own solutions will be theoretically tested, where one of them will be chosen to get a real world implementation. The voice communication should be lossless for the operator and have a high enough quality to motivate the change from a wired communication line. The specifications for the system were as follows:
• Have a range of at least 1 m.
• Be able to transfer data speech sampled at 12-bit, 11,025 KHz in a mono constellation of the sound.
• The operator will be able to interpret what is said over the system without any difficulties.
1.3
Limitations
This thesis was focused on making a working model of a system and not a fully fleshed out one, it did however take a turn towards building and testing antennas which was all that was done due to time limitations. Other limitations were:
• Both the company and the university lacks a proper testing facility, this means that theory and practice might differ a fair bit since all testing will be done in none ideal environments.
• There was no possibility of doing long range underwater testing, the longest range was in the [dm] span made in a 100 L container.
• Only one of the systems theorized was to have a real world implementation because of time limitations.
1.4
General idea
The idea of the system was that it will be consist of an analogue radio signal, which is digitally sampled and modulated by a microprocessor and then sent through an antenna (Tx). The signal will then be received in the other end by an antenna (Rx) then demodulated, reconstructed by a microprocessor and is played for the operator through a some kind of audio device, most likely a headset.
2
|
Theory
In this chapter there will be descriptions of the theory used to deliver on the subjects and goals stated in Chapter 1. It will also explain the notations and concepts used throughout the thesis.
2.1
Notation
• EM = Electro Magnetic. • = Permittivity. • r= Dielectric constant. • µ = Permeability. • σ = Conductivity [S/m]. • α = Attenuation constant [dB/m]. • VLF = Very low frequency [3 to 30 KHz]. • ELF = Extremely low frequency [3 to 30 Hz].2.2
Maxwell equation
The Maxwell equations outline the fundamentals of how electric and magnetic fields interact. If one have a polarized, linear plane and an EM wave propagating in the Z direction (random straight line), this can be described in terms of fields where the following holds:
Ex= E0· ejωt−γz, (2.1) Hx= H0· ejωt−γz, (2.2) γ = jω r µ − jσµ ω = α + jβ and (2.3) β = 2π λ (2.4) 3
where E and H represent the electric and the magnetic field, while γ is the prop-agation constant. The propagation constant is dependent on permittivity (), permeability (µ) and conductivity (σ). It can be represented as a sum of the at-tenuation constant (α) and the phase constant (β).
If an example with σ = 4 and f = 10MHz is used the sum of α + jβ will be 110 + j0.5 dB. The real part is so much larger then the imaginary that it can be ignored and then say that the propagation constant is equal to the attenuation constant.
The Maxwell equation gives a negative (kx + m) line based around the attenuation constant over distance (Figure 2.1) for seawater. What has been found by earlier research [2] is that in the near field region the real world follows the equation but when it enters the far field region, the signal decrease will level out to only a few decibels per meter, see the graph (Figure 2.1).
Figure 2.1. Signal propagation over 90 m using a RF transmitter with the power of
5 W at the Liverpool marina [2]
In this thesis there will be use of the Maxwell equations to speculate and calculate a theoretical minimum distance. For the real world application, tests must be performed to have any real idea where the maximum effective range of the system is.
2.3
Propagation
This thesis will be handling wave propagation, which is how waves travel through any medium. Electronic/light waves can also traverse a non medium which is an vacuum, no other type of wave can do this [3].
Propagation of radio signals in the High Frequency (HF) spectrum can be very limited in an underwater environment. This has to do with absorption in the
2.3 Propagation 5
medium and reflection from its surface. If the medium is a conductor, like a metal or seawater (if a poor one), the signal will generally be absorbed into the material while some of it is reflected. If one instead have a dielectric medium, for example porcelain or wood, which are none conductors, some of the signal will pass right through while some of it will be reflected back.
In seawater the attenuation [4] shows that most of the signal power will be lost over just one meter, this means that all this energy is either absorbed into the medium, reflected from the surface or distorted due to scattering, for example when it leaves the antenna and passes into the water which is a different medium.
An example of what have been mentioned above can done by looking at the pen-etration depth of the different mediums, using the skin depth. This is a mea-surement how far the signal penetrates before it has about 37% or 1e of its energy remaining [3]. When travelling through a conductor which has the permeability for vacuum (u0) and the conductivity for the medium (σ). In the example used there
is a 10 MHz signal, a rounded value for µ0 and three different kinds of medium:
δ = √ 1 πf µσ, (2.5) δair =p 1 π · 107· µ 0· 2.95 · 10−15 = 2930291 m, (2.6) δtap water = 1 p π · 107· µ 0· 0.02 = 1.12539 . . . ≈ 1.13 m and (2.7) δseawater=p 1 π · 107· µ 0· 4 = 0.07957 . . . ≈ 0.08 m. (2.8)
As seen, there is almost no loss of effect due to conductivity when in air, since it is a resistive medium. When using water on the other hand and especially seawater the signal gains a huge propagation loss just to the fact that the water is a conductor and not a resistor. Seawater is a lossy medium, meaning it isn’t a great conductor but still a decent one. If one were to take a material like copper, which is a great conductor, the penetration would be 20 µm using a 10 MHz signal like in the example above.
2.3.1
Radio waves
Radio Waves was the focus of this thesis. They are electromagnetic waves and are defined in the frequency spectrum of Hz to GHz. Radio waves do not travel very far in water and especially not in seawater due to it being a conductor or a ’lossy medium’ [5]. The higher the frequency of the signal the faster it attenuates or ’dies out’ (Section 2.3.3). This posses an interesting challenge when designing the system, the trade-off between distance and data rate.
One important part about radio waves are that they are divided into 3 different fields or ranges when they are leaving the antenna. The near field, transition zone
and the far field [6]. Most models and equations about the behaviour of radio waves are based on them reaching into the far field. This is when electric field (E) and the magnetic field (H) have (Section 2.2) become completely planar and intersect. Before this we have the near field and an undefined area, usually referred to as the transition zone. In this regions there are very few models of the signals behaviour since they are ’rounded’ close to the antenna and every case becomes unique depending on the type and structure. For example, in the very close near field a loop antenna will work as a transformer because of the strong magnetic field it generates [6].
2.3.2
Acoustic waves
Acoustic waves are a type of mechanical waves and when it comes to underwater use the frequency spectrum is in the ELF to VLF range, from Hz to KHz. This leads to very low data rate but the signals can be very far reaching in seawater. Acoustic waves have a propagation speed of 1500 m/s, this can lead to time delay in communication systems since it is rather slow compared to radio waves, which travel at 3.3 · 106 m/s in the same medium (Section 2.3.5). They also suffer from
multipath propagation in shallow waters due to reflection and refraction since they lack the ability to penetrate through objects, often creating shadow zones with no signal behind the object.
2.3.3
Attenuation
Attenuation is a physical phenomena in which a flux (EM-wave for this thesis) that passes through a medium that is non-vacuum have some of its power removed along the way, when it interacts with objects in the non vacuum. This could be down to a lot of thing, like backscattering, reflection, absorption and so on. In physics there is often a pre-measured value of different materials, often denoted α and is called the attenuation factor [7]. Which is then multiplied with the distance and frequency to learn how much of the signal will be lost, for example:
Attenuation = α [db/(M Hz · cm)] · l [cm] · f [M Hz]. (2.9) Butler [7] believed that to calculate the attenuation factor in seawater with a conductivity that is denoted σ = 4 S/m and frequency which is f = 10 MHz. He applied the formula:
α = 0.0173 ·pf · σ ≈ 110 [dB/m]. (2.10) This have been expanded upon by a later scientific paper [5]. They instead list that the signal attenuation has to be split into near field and far field. In the near field the attenuation follows Maxwell’s equations (Section 2.2):
E = E0· e−σz· ej(ωt−βz) and (2.11)
α =r ωµσ
2.3 Propagation 7
This will create a huge near field attenuation loss, especially at high frequencies. This can be seen in Figure 2.1. This is where Equation 2.10 comes out to pretty much the same result as Lucas. There then follows the far field attenuation loss which is much lesser and come out as:
E E0 = e−9.29·106·f2z↔ 20 · log10 E E0 = Attenuation loss in dB. (2.13)
At this point the attenuation will less sever, even for frequencies in the MHz band. For example a signal at 25 MHz will have a far field attenuation at just 5 dB over a distance of 100 m. This is line with current theory and papers which are outlined in the related work (Chapter 3).
As seen in Equation 2.11, the attenuation for this thesis will be based around the frequency chosen, the medium the signal is sent through as well as the distance it is to propagate. Research has also show that the total path loss will be larger at deeper depths (Figure 2.3.3). This has not a clear explanation in any mathematical model but is something that has been measured [8]. This needs to be taken into consideration when designing the system.
Figure 2.2. Total path loss as a function of frequency and depth [8]
Attenuation can give us a decent estimation off how much signal power there will be after Z amount of distance. There will be more information about other factors that will affect the signal power in the receiver, Section 2.3.4 and 2.5.
2.3.4
Refraction loss
This subject will only be touched upon lightly in this thesis since the focus is mainly on pure underwater communication. Refraction loss is the signal power lost when a flux passes between two mediums with different refraction index, for example air and seawater. According to Lloyd Butler [7] this is calculated as:
Refraction loss [dB] = −20 · log10 7.4586 106 · r f σ ! . (2.14) So using the example from Section 2.3.3 there would be a refraction loss of ≈ 39 dB. This can in many applications be negated by having the signal pass between the two mediums using an antenna in one medium connected by a cable to the equip-ment in the other. To calculate general refraction there is a need to know how the signal bends when passing between mediums, the equation used is:
n = c
v (2.15)
where n is the refraction index (RI), c is the speed of light and v is the speed through the medium. For air RI ≈ 1 which means that v ≈ c.
2.3.5
EM-signals in seawater
Electromagnetic signals in seawater have a few different properties from when they pass through air. They attenuate faster, have a shorter wavelength and slower travel time [5]. This is because the speed of light changes when it passes through a medium such as water, on account of its dielectric constant (or rela-tive permittivity) (r) and relative permeability (µr). While µr= 1 when talking
about seawater, the r is a function of frequency and is calculated as a the
differ-ence between permittivity for the medium divided by permittivity of a vacuum, following:
r(ω) = (ω)
0
. (2.16)
It is known that the r of seawater varies between 72-81 depending on frequency
and for the MHz range of signals this is ≈ 81 [9]. When these values are used and put into an equation, the result is:
cwater =√ 1
r· µr
≈ 33.3 · 106m/s. (2.17)
This combined with the higher refraction index also gives the signal a much shorter wavelength. The equation for calculating wavelength in conductive mediums is the following according to peer reviewed reports [7, 2]:
λ [m] = 1000 ∗ r 10 f ∗ σ ≈ 1.56 ∗ s 4 ∗ 106 f ∗ σ (2.18)
2.4 Antenna types 9
which can create the example of using the frequency(f ) 10 MHz and the conduc-tivity (σ) being 4, giving a wavelength that will be λ = 0.5 m. This is about 60 times shorter than what was had in air. So when designing an antenna for underwater use, it will shrink to a fraction of the size that is needed for resonance in air. This could have some very interesting properties if one wants to be able to move between mediums, since the only variable that is changeable will be the frequency input to the antenna.
2.4
Antenna types
One of the first parts for the implementation of the system is to choose the hard-ware and antenna needed so it is possible to dimension the system. There are multiple antenna types that can be used in for signal propagation underwater. In this section the pros and cons of each will be discussed.
2.4.1
Loop antenna
A basic loop antenna is a metal ring where the diameter is specified to a certain frequency. Electrical loop antennas are usually classified into small and large [10]. There was a focus on the small variant since it is the mostly widely implemented and a large antenna would be extremely cumbersome to move around since its circumference is ≈ λ when a small loop would have its circumference < λ3 [11]. An example of electrically smaller loop antenna is a half-wave loop, where N is the number of turns of the loop. This is calculated with:
λ
2 = N πd ↔ d =
λ
N · 2π (2.19)
giving the loop antenna the advantage of being able to be built smaller then most of its counterparts because it only gets a fraction of the wavelength as the diame-ter. As the λ in water is already so much shorter (Section 2.3.5) than in air, this can create a very manageable diameter for the antenna.
A loop antenna typically has a very low radiation resistance and as a result of this a very low efficiency. It creates a magnetic field with a current and this is done by pushing a voltage through a resistance, what is seen is that:
Rr= puo/0 6π β 4(N A)2=pu o/0 8 3π 3 NA λ2 2 (2.20) where β = 2π λ, A = Area m
2 and N = number of turns of the loop antenna. So
to use an example that has a λ = 3.3 m, N = 2, r = 13.5 cm and is lowered in seawater there will be a radiation resistance off:
Rr= 428 3π 3 2(13.5 ∗ 10 −2)2π 3.32 2 = 0.3839 . . . ≈ 0.38 Ω. (2.21)
According to Ohm’s law for time varying currents the radiated power from a loop antenna can be calculated. Based on the radiation resistance and the current feed into the antenna:
Rr= 2Pradiated I2 0 and (2.22) Pradiated= 1 12π p uo/0∗ β4(I0A)2. (2.23)
Look at Equation 2.22 and one can clearly see that having a low radiation resis-tance will directly affect the output power, making a loop antenna very inefficient. Another drawback of the loop antenna is that it needs almost perfect impedance matching since any effect lost due to reflections will give it an even worse efficiency.
2.4.2
Dipole antenna
A dipole antenna is the oldest and most basic antenna type of them all. This section will focus on a centre-fed half-wave dipole. It simply has two parts, a positive and negative half that forms a rod capable of receiving both the positive and negative part of the electromagnetic wave or in layman terms, the "radio signal".
Length of a half-wave dipole [m] : l = 1
2· Aλ. (2.24)
The equation above is for the most widely used type of dipole, where l = length of the antenna in meters. It is rather straightforward with the exception of the
A. That is an adjustment factor to cancel the reactants in the antenna, that
appears because the signal isn’t propagating in free space but in a conductor (like a copper wire) before being transmitted. A is dependant on the diameter (d) of the conductor and the wavelength of the signal (λ), see Figure 2.3.
2.4.3
J-pole antenna
A J-pole antenna has its name from the shape of the design. It is relatively easy to build and takes a lot of its design from the dipole. It’s built around having a smaller and larger dipole configured like J-type in Figure 2.4.
The advantage of a J-type antenna is that is have a greater underwater reach then a loop antenna [13]. The drawbacks is that is has a lower SNR then the loop and as the dipole, suffers from being bulky. Reaching upwards 3 m when the signal is in the 10 MHz range even when submerged in seawater.
2.5 Impedance matching 11
Figure 2.3. Graph of A depending on the λ
d ratio [12]
Figure 2.4. J-pole Antenna and variations of same [14]
2.5
Impedance matching
Impedance matching is the technique to minimize reflection of the signal in the antenna or to put it blunt, to send the same amount energy as you input and not have anything go backwards.
γ = ZL− Z0 ZL+ Z0
(2.25)
where ZL and Z0 are the impedance values at the load and transmission line
re-spectively. γ is a measurement of how much of the signal is reflect, that goes between −1 ≤ 0 ≤ 1. If the load and transmission impedance matches up per-fectly there is no reflection according to Equation 2.25. But if they are mismatched
or one of them is zero it feeds back a portion of the out energy into the system. If the γ = −1 there is a short-circuit case and full reflection with a negative phase, if γ = 1 there is a open-circuit case and full reflection but with a positive phase this time. The implication of this is to take great care when designing the system and always keep the impedance in mind to minimize transfer loss.
Since impedance often can be imaginary, Equation 2.26 says that if the "Standing Wave Reflection" is equal to one there is zero reflections between the source and the load. For example, between a signal generator and an antenna.
|γ| =SW R − 1
SW R + 1 (2.26)
where SWR or VSWR (Voltage Standing Wave Ratio) is the value mostly com-monly referred to in data sheets and alike. This is the ratio between the highest and lowest voltage along the transmission line. This means if the V SW R = 1, there is zero reflection and the voltage is equal at all points along the transmission line.
2.5.1
Impedance matching a loop antenna
As was mentioned in the end of Section 2.4.1, there is always want for the perfect impedance matching to minimize VSWR. For an electrically small loop antenna the impedance (Z) is:
Z = Rr+ Zi+ jωLe= Rr+ Ri+ jω(Le+ Li) (2.27)
Where Rris radiation resistance, Ziis the internal impedance, Leis the reactance
of the external inductance. The terms of Ri and Li are parts of the internal
impedance, Zi = Ri+ jωLi. A equivalent circuit for the impedance is pictured
in Figure 2.5.1 with a theoretical C added. This is most often omitted since a variable capacitance is usually placed in parallel with the loop to tune out its inductance; the capacitance of the loop simply decreases the value of the parallel capacitance needed [10, p.5-3].
2.6
Digital modulation
Modulation is technique in which data is transferred from one unit to another. It’s mostly used when talking about wireless communication but most be done for any system where to units are going to be ’talking’ to each other. In this section I’ll go over the basics of the more used modulation techniques.
Modulation can in many ways be seen as the language between to systems. In simpler systems 1 and 0 might be represented as having energy present or not. With more advanced modulation techniques it will usually be a combination of energy, phase and frequency that decides which string of bits that are being sent.
2.6 Digital modulation 13
Figure 2.5. Equivalent circuit of the input impedance Z
2.6.1
OOK modulation
On-Off keying is one type of binary modulation and is the simplest form of all digital modulations. It simply works of there being signal energy present or not. If there is a signal into the receiver over t amount of time that is a 1 and no signal is a 0. This is represented as [15]:
s0(t) = 0 and s1(t) =
r 4Eavg
T · cos (2πfct) (2.28)
where the signal goes over 0 ≤ t ≤ T with a carrier frequency such as 2fcT > 0.
2.6.2
BPSK modulation
Binary Phase Shift Keying is another type of binary modulation. With this tech-nique the 1 and 0 are represented by using a shift in phase, normally π. The system will interpret the binary bits like:
s0(t) = r 2Eavg T · cos (2πfct) and s1(t) = r 2Eavg T · cos (2πfct + π) (2.29)
which gives that a system checks for a signal with a certain energy level (EAvg)
over T amount of time. When this is present, the phase of the signal is measured to detect if the system should interpret it as a 1 or 0. For this type of modulation, the receiver always has to know the exact phase of the transmitted signal else there can be a phase error and 1 turns to 0 and so on. This problem just gets worse the less phase there is between the bits, see Section 2.6.4.
2.6.3
ASK modulation
Amplitude Shift Keying is a straightforward extension of OOK with more power levels than on and off. Usually there is an even amount of signals and they are in turn evenly distributed in a signal space. An example is when there is 4 signals or
si(t) = siφ(t) for i = 1, 2, 3, 4, (2.30) φ(t) =
r 2
T cos(2πf0t) for 0 ≤ t ≤ T (2.31)
and referring to Figure 2.6.3 it can clearly be seen that a clear knowledge of which power input generates which output is needed to be able to demodulate the different strings of bits.
Figure 2.6. Signal space diagram ASK with 4 signals
2.6.4
PSK modulation
Phase shift keying is an extension to BPSK, where there is more signals in the signal constellation than 2. Since M = 2kor an even number, larger then two that
is a base of two is preferred. The example used M = 4.
si(t) = r 2Eavg T · cos(2πfct + (2i − 1) π max(i)) 0 ≤ t ≤ T (2.32)
where a constellations of four points is gained, that are all√E from origo. They
are phase shifted with π4, so to demodulate the signal the receiver needs to know the original phase from the transmitter know much it shifted.
2.7
Gaussian white noise
White Gaussian noise is special case of gaussian noise. gaussian noise is a sta-tistical noise where PDF (Probability Density Function) is equal to the normal distribution. The special case for white gaussian noise is that the values at any pair of times are identically distributed and statistically independent (and hence uncorrelated).
In telecommunication and communication channels in general, the use white gaus-sian noise is prevalent to create additive white gausgaus-sian noise (AWGN) on a chan-nel. This is used to mimic the random process and noise that can occur in nature
2.8 Commercial systems 15
while still having a uniform power over a frequency band and a normal distri-bution, making it rather easy to create a mathematical model. The stochastic variable for the Gaussian PDF, can said to be X and that is given by:
fX(x) = 1 √ 2πσe −(x−m)2 2σ2 (2.33)
where E(x) = σ is the variance of the PDF, V (x) = m is the mean. For an ideal receiver (i.e. perfect components) there will be white noise that is equal for any and every frequency. This not possible in the real world since such a signal would have infinite energy, so it starts to fall off in the [THz] region but for the purpose of this thesis it is treated as:
N0= kT0 (2.34)
where k = 1.38 · 10−23 joule/kelvin and T0 is the local temperature in kelvin. This
gives a noise level of Sn(f ) = N20 for any frequency since the noise level is split
between the positive and negative part of the frequency spectrum.
-6 Sn(f ) N0 2 f
Figure 2.7. White Gaussian Noise
2.8
Commercial systems
2.8.1
Near field communication
Near field communication is a version of Radio Frequency Identification (RFID). Developed and classified in 2002 by Sony and Philips (today NXP Semiconduc-tors). NFC is used to communicate between small electronic devices and just as RFID, has the ability for the transmitter to power up the receiver through wireless power transfer. The radius for the system is in the maximum range of 10 cm in air, works with the frequency of 13.56 MHz and with a maximum bandwidth of 424 Kbit/s [16].
The possibility of using both RFID and NFC for underwater applications was studied in 2013 by Benelli and Pozzebon [17]. While they confirmed that low frequency RFID has a space to fill for identification of fish and rocks under water, high frequency RFID as well as NFC had an effective range of 3 cm. For such a low power interface with the near field attenuation present in seawater, they
would in my opinion require to be in physical contact to guarantee a stable data connection. This is the reason that NFC was ruled out for this thesis.
2.8.2
Bluetooth
Bluetooth is a wireless communications interface in the 2.45 GHz with a trans-fer rate of up to 54 Mb/s [18, p.168] , using a frequency hopping technology to minimize interference in the communication. It has a maximum specified power of 100 dBm that gives it a range of about 100 m in air [18, p.346]. It works by pairing two devices with each other and having them follow the same jumping pattern, this way they are essentially only talking with one another but can work in the same frequency range as quite a few other devices without any interference between them.
Advantages of Bluetooth is that it is easy to use, have a built in encryption in form of frequency hopping and still has a relative low power consumption, using only a few mA. The disadvantages is that in the 2.45 GHz range it is rapidly approaching penetration depths (Equation 2.5) of zero when submerged in seawater. There have been some trials with systems in this frequency range [19], though none of them have been with Bluetooth. What was found was that they could not penetrate beyond 17 cm without suffering large packet loss and even when increasing the power this range would not move. This makes Bluetooth unsuitable for the system since the goals stated (Section 1.2) was to have an effective distance of at least 1 m.
2.8.3
Oceanreef
Oceanreef is a commercial product used for underwater voice communication using wireless ultrasonic technology [20, p.27]. This system works around the frequency of 37.768 kHz and is specified to have a range of up to 200 m. This system fills up most of the requirements that are specified in Section 1.2 but it is an acoustic wave (Section 2.3.2) system and therefore not in the goals of this thesis. Since this thesis focuses on the use of RF waves (Section 2.3.1). Because of the advantages detailed in the mentioned sections.
3
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Related Research
In this chapter, related research will be covered and their differentiated conclusions are listed. Since part of the work builds on this information it is relevant to have a basic grasp of what they include for full understanding of the thesis.
A. Shaw and A.I. Al-Shamma’a
These Liverpool based researchers have been writing reports from 2004 on this subject [4] and still are to this day. They have done a lot of research on real life behaviour of electromagnetic signals under water. They have mostly been using loop antennas to explore on how an EM signal behaves in a test environment or in shallow seawater like a harbour.
Much of their work has laid the foundation for this thesis since they have proven [2] that a tone can be sent over a distance of at least 90 m when using a 5 MHz carrier in DSB-LC modulation. This make it seem plausible that you should be able to transfer full audio using 10 MHz carrier with a yet to be decided modula-tion.
In their paper from 2009 [9] they also showed that 6.0 and 10.7 MHz could be ideal frequencies for transmitting data under water, see Figure 3.1. Using an easy to build system with loop antennas and OOK modulation (Section 2.6.1) they were able to have a signal level of about -60 dB into the receiving antenna. A caveat was that this experiment was done in a lab environment and not outside, in an actual harbour or equivalent.
J Lucas and Ck Yip
These (also) Liverpool based researchers have written a very interesting paper on signal attenuation in seawater [5]. Their focus is to find which frequency is suitable for electromagnetic transmissions when the distance is in the +100 meters range. They also want to find a solution to the disparity in signal behaviour between the near and far field range.
The report differentiates it self from a few others by having different r values
and going into more detail in behaviour between sea- and fresh water. Though it
Figure 3.1. Attenuation graph from the concluding part of the paper [9]
comes the same conclusion that frequencies up to 10 MHz are suitable to propagate signals up to 100 m in seawater.
Carlos Uribe and Walter Grote
The authors of this report [21] is out to try and define a propagation model for EM signals in seawater. Their equations make a decent bit of sense when reading the whole thing and what they achieve is to sum up much of the research up until 2009 and forward, as can be seen in Figure 3.2.
4
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Method
4.1
The process
When this thesis started the original goal was to create a fully functional system for underwater use. Due to complications in the process and time limitations only a few antennas was created and tested, under non ideal environments. The process went from starting to outline the system, then looking into antenna choices and coming up short. This lead into design of own antennas for use which took more time and effort then was anticipated at the start of it all. This lead into the realization that there was a lack of proper testing equipment for antenna usage and especially for underwater usage. Which meant that the testing was made which what equipment that was available, in suboptimal environment.
4.2
The system
In this section there will be a general description of the system and layout. How the information will be transferred inside the system and how the signal should be modulated to have greatest efficiency in water and for the systems design.
4.2.1
Layout
As seen in Figure 4.1, the layout of the system would have been rather straight forward with a ADC sampling the signal, transferring it to the MPU which will break it down into individual bits, pass these through to a multiplier that adds the carrier frequency from the signal generator (Figure 4.3.4), it then passes through RF-filer and RF-amplifier before going up into the antenna.
Figure 4.1. Sketch of the system with all signals represented
4.2.2
Signals
As seen in Figure 4.1 there will 3 stages of the signal. A sample with 12-bit resolution from the analogue signal that is transferred as square waves to the receiver. What needs to be said about the square wave in Figure 4.1 is that it isn’t a true square wave that goes between a Vhigh and Vlow, it instead would have
onge between Vhigh and 0 . Since the system is using OOK keying it represents if
have an energy over the channel or not.
4.2.3
Modulation
For the hardware implementation of the system it supposed to go with OOK modulation (Section 2.6.1). Because of its ease to implement and since it has no phase or anything of the sort, the receiver only has to measure Pin over a
period of T time to see if there is a 1 or a 0 being sent. Another large advantage of using OOK modulation is the ease to change the frequency of the signal in both the transmitter and receiver. Since the antenna properties will change in the different mediums there is an advantage to be able to have different frequencies, the potential is that the system can use the same antenna for all mediums, it’s just the frequency that changes. A problem with OOK modulation is that it requires a larger bandwidth then most modulations. Since the thesis never reached a point where implementation become a reality there is no way of knowing how well OOK modulation had worked out in comparison to other modulations.
4.3
Hardware
For this system to work it needs to have an ADC with a sample depth of 12 bits that can at least handle more then 11025 samples, see Section 1.2. This equation then becomes that:
T ransf er Rate = 12 · 11025 = 132300 Hz = 132.3 Kb/s (4.1) and for sending out the information it uses a RF-filter and a RF-amplifier to remove any distortions that could be applied to the signal and then amplified it with enough transmission strength to penetrate even through a lossy medium, such as seawater. A note here is that the decision of the amplifier will be dependant on how sensitive the power detector for the system is. An advantage is that there will be very little noise since it is working under water, so even if the system have a large attenuation it will just need a sensitive enough power detector and possible an LNA (Low Noise Amplifier) to get rid of any harmonics in the signal. The bits transferred will then input and saved in the MPU, making it possible to rebuild the signal by sending the 12-bits through a DAC, generating a sound to an output device, like a headset.
4.3 Hardware 21
4.3.1
Low pass filter
The system would have used a simple low pass filter to remove any input frequen-cies outside of the human speech range before it enters the system. The human range of speech is between 50-3400 Hz, even though we can hear up to 20 kHz. The low pass filter could therefore have a cut off frequency at 3.4 kHz, this would lower the bandwidth in use by a great deal. If a simple RC filter is used, where there is a 50 Ω resistor present, it would have the function off:
fc= 1
2πRC ↔ C =
1
3.4 · 103· 2π50 ≈ 936 nF. (4.2)
4.3.2
Analogue digital converter
This piece of electrical equipment samples an analogue signal and converts each of them into a string of bits, where the resolution is decided on how many bits are used. 12-bits means that the value of the signal can be represented with a depth between 0-2048, since one bit is used to represent if the signal sample is negative or positive. The systems sampling rate should be at least 11025 Hz (Section 1.2) which at least a bit depth of 12 for each sample, which can easily be achieved with off the shelf hardware, for example [22].
4.3.3
Micro processing unit
Their will be two micro processing unit (MPU) in the circuit. The first ones main task will be to take the bits incoming from the ADC, split them up and output them to the RF-amplifier and the transmitting antenna (Tx), at a pace where the
power detector will follow the timing of the OOK modulation. Where the second recreate the string of bits dependant on what the power detector detects as the incoming bits. The focus will be on keeping them small, power efficient and with good timings so that the OOK modulation doesn’t drift off and get bit errors.
4.3.4
Signal generator
The signal generator can either be a stand alone apparatus or a smaller integrated part, like an oscillator. In this system it would have been used to generate the carrier frequency sine wave that is multiplied with the square waveforms coming out of the MPU. This will create a waveform that looks alike the line off Fig-ure 4.3.4. The sinus wave inside of each 1 for the power detector, no signal is equal to no power. This is an idealized description of the generator, the multipli-cations won’t be perfect and there will be noise in the spectrum, that might have to be compensated for.
Figure 4.2. OOK Modulation [9]. Energy over T time indicates a ’1’.
4.3.5
RF filter
The RF-filter used before the amplifier will usually be a bandpass filter. This should be frequency matched or be even more narrow to avoid that any amplifying of resonance or noise occurs.
4.3.6
RF amplifier
The RF amplifier should would have been tuned to the frequency used in the antenna, preferentially with a narrow amplification band that has a steep drop off. This means that in a perfect world it would act as in Figure 4.3. In a none perfect world the RF-filter will have a larger BW then wished for and that is why a RF-filter (4.3.5) is used beforehand. This removes as much of the added noise as possible, preventing it from being lumped together and amplified in the transmitted signal. -6 ' $ High dB Zero dB fc
Figure 4.3. Theory of a RF-amplifier
4.3.7
Low noise amplifier
A low noise amplifier (LNA) is used to amplify very weak signals, for example, those captured by an antenna. The main advantages of the LNA is that it has a very low noise figure but a large gain. Then look at the Frii’s formula for noise and see that the total noise factor F will be small since the gain suppresses it in every stage.
4.4 Antenna/transfer system choice 23 Ftot= F1+ F2− 1 G1 +F3− 1 G1G2 + . . . + Fn− 1 G1G2. . . Gn . (4.3) What is seen in Equation 4.3 is that each cascading stage adds less noise power then the one before it. Since nearly all the noise power is created in the first step, it can summed into one term Frestand then it becomes that the noise power for
the LNA as:
Freciver= FLN A+
Frest− 1
GLN A , (4.4)
so when choosing the LNA one will have to take into account the gain and noise figure in the first hand. As well as bandwidth, stability and input, output VSWR (Section 2.5).
4.3.8
Power detector
A power detector circuit is most often used to sense what signal input is present and then convert it into a voltage on a linear scale specified by the manufacturer [23]. It is needed to do specific readings with the antenna, amplifier and LNA mounted to know what the general output (dBm) is at. This is then used to decide where to draw the line between the 1 and 0 bit. This is of course done in the MPU (Section 4.3.3), where it will have a voltage level generated from the detector and decide what it count that bit as.
4.3.9
Digital analogue converter
A digital analogue converter (DAC) that takes digital data and converts into an analogue signal on the other side, it’s the reversal of an ADC (Section 4.3.2) . The form and accuracy of the output signal is dependent on the sample rate of the DAC as well as the bit depth received.
4.4
Antenna/transfer system choice
Based on the pros and cons of each antenna/system type (Section 2.4.1, 2.4.2, 2.4.3 & 2.8) as well as what researchers have used beforehand (Chapter 3), it was chosen to go with the loop antenna. The first advantage is the size which shrinks to almost a fraction of either the dipole or the J-pole which will be a huge advantage when it comes to implementation of the system. Even if the other antennas will give us a slightly better received voltage [13]. There is also the fact that nearly all research up to this point have been using loop antennas [4, 5, 9, 21, 24] which lead to the belief that this had to be the more advantageous choice.
4.4.1
Discarded choices
As mentioned in theory chapter (Section 2) there were multiple choices including both antenna constellations as well as complete commercial systems. The other
choices of antenna was mostly deemed to bulky to be really feasible for use by a person moving underwater. The commercial system were all interesting but failed in some aspect. The first one to fall was Blutooth due to it being in a to high a frequency spectrum. There have been tests with 2.4 GHz signals in seawater and the maximum distance achieved was 18 cm [19]. Oceanreef is a fully functional underwater communication system used by diver but it uses ultrasonic waves which are acoustic waves and not in the scope for the goals of this thesis. Lastly is the NFC communication system which was the hardest to rule out among all of them. Tests have been done with RFID which is the predecessor to NFC [17], they confirmed that the power output and combined with the attenuation at the frequency is to much to transmit any distances. Combine this with general information found among enthusiasts that say NFC never reaches its peak potential for data transfer raised the question what would be the advantage over just a cable with a magnetic clamp. This last part was taken from forums and no tests have been found that would verify or dismiss the information.
4.5
Loop antenna construction
The loop antenna is built out of copper pipes with a diameter of 15 mm and the copper itself is 1 mm wide. It was built with a diameter of 27 cm with 2 coils which should have been the resonance for a half wave antenna working at about 90 MHz (Equation 2.19).
λ = 2 · 2π0.27 = 3.4m → f = 88.4 MHz and (4.5)
λ = 2 · 3π0.27 = 5.1m → f = 58.9 MHz. (4.6) The antennas had an air core, the reason being the skin effect (Section 2.3), which means that 63% of the current will travel in a few µm of the cladding at the frequencies that was to be used in the system, in MHz that is. A thing to note about the tests below is that they are done in a very noise environment that also includes a lot of close by metal objects. This can very easily change the radiation pattern since the signals can be absorbed or reflected by nearby materials. The tests were therefore only an indicator of how well the antennas would be working in a real world environment but the resonance frequencies should match up well enough towards the real world values.
4.6
Impedance matching
As show in Section 2.5.1, the impedance match for the antenna one need to gain a zero sum reflection which means that γ = 0. Referring back to Equation 2.25 it is stipulated that ZL = ZS for this to happen, this means there has to be
measurements over the antenna after it is built to know how much is needed to compensate with. This was supposed to be done using a impedance analyser where the antenna at first will be measured in air, then put into tap-, sea- and brackish
4.7 Measuring the signals 25
water, to make an estimation of how large capacitance is needed to match the impedance in each medium. Because of the lack of access to a network analyser in the region the experiments had to be cut short and could only be achieved for the results for air during this thesis. This means that calculating the impedance match for water became speculation.
4.7
Measuring the signals
Measuring was started with sending just a pure sinus to test the connection, the first measurement test was with an oscilloscope that was connected between the Tx and Rx of the system. This allowed for understanding how much of the power was being transferred and allowed one to see how the transmission medium dis-torts the signal.
There also needed to be measurement for any kind of EM propagation through the air down into the water [5]. This could either be done by having a separate spectrum analyser or having an antenna port on the oscilloscope used, the later one would be the most effective since it is easier can compare all 3 signals at the same time, in the same graph.
The goal is to have a signal level stronger then the noise level into the receiving antenna (Rx). This only required that the power detector (Figure 4.1) is sensitive enough to pick up the very attenuated signal after filtering, since it will have a noise level in the range of -100 dBm or less.
4.8
A second system
During the thinking process about this system it came up another layout for the system. This layout is less cluttered and could be an interesting for further use. Ultimately it was chosen to not go with the second system since it’s (in my opinion) harder to find fitting hardware for it. It will also be more difficult to change the carrier frequency for the system without a switch in hardware.
Figure 4.4. Layout of the second system.
The difference main of the system described in Figure 4.8 to the system in Fig-ure 4.1 is that signal is transmitted somewhat directly from the MPU to the antenna. This is a form of OOK modulation but now it’s sending pure square
wave over the antenna, which changes the detection to a signal level and not just detection of a signal being present.
5
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Result
5.1
First trial
The first pair of loop antennas are a 3 turn loop as the transmitter and a 5 turn loop as the receiver. They are made of coiled copper pipe with the two ends having the plastic removed to make room for the connector. The connector was made out of a peeled coax cable with a BNC connector in one end and in the other, the ground and conductor has been split up, then each of them has been soldered to one end of the antenna. This makes the voltage travel through the entire length of the coil and creates the signal which is passed over to the receiving antenna. The transmitter is connected to a high bandwidth signal generator, generating a signal with an amplitude of -10 dBm and the receiver is connected to a spectrum analyser with a DC-stop in place (Figure 5.1).
Figure 5.1. Layout of the first trial. Two antennas suspended by wires.
For the first trial it had the best SNR values at the signal level of 30 MHz. The signal was completely buried in the noise at 90 MHz (Figure 5.2). This was rather expected since the two loops had resonances frequencies at 60 MHz respectively 35 MHz. What should also be noted is that the amplitude of 30 MHz didn’t change when even when bringing the antennas further together or having them almost 2 meters apart. It was a constant -30 dBm.
Figure 5.2. Signal levels in Rx for Trial 1
5.2
Second trial
The second trial has a change in antenna placement since it was determined that the wires suspending them might be affecting the higher frequencies (Figure 5.7). The signal configuration is the same as Sections 5.1 but with the antennas changed for a 2 turn loop as the Txand a 3 turn loop antenna as the Rx. The signal power
from the transmitter is still -10 dBM.
Figure 5.3. Layout of the second trial
What was found here is that it had a signal level at -30 dBm for the frequency of 38 MHz but now it also had a better SNR for the frequency of 91.2 MHz (Figure 5.4). This is in close to the calculated results in (Section 2.4.1) for the Tx.
λ = 2 · 2π(0.27) ≈ 3.4 m ↔ f = C
5.3 Third trial 29
Figure 5.4. Signal levels in Rxfor Trial 2
5.3
Third trial
This experiment was very close to Trial 2 in its set up (Section 5.2). This time only the receiving antenna (Rx) was changed to a 2 loop antenna, which was to
be closer to an ideal.
Figure 5.5. Layout of the third trial
5.4
Conclusion of the first three trials
What was found was that all the antennas used have resonant frequencies around the 30 MHz mark. It was found that one constellation (Section 5.2) has an even stronger peak at 90 MHz which is around the theoretical resonant frequency of
Figure 5.6. Signal levels in Rx for Trial 3
that system (Equation 5.1). It can also been seen, that at peak performance it had a drop of 20 dBm from the original signal, this can be noted as the near field losses for the antenna for air. This is important information going into the underwater measurements, to see if the equations in the theory chapter holds up in the real world (Section 2.3.5 and 2.3).
The conclusion of this trial was full of caveats but still gives a general idea on what antenna constellation to use, as well as which frequencies that are ideal. Something that had to be noted is that these trials was done in a none ideal environment, with both large metal objects and electronic equipment in close range. It is also acknowledged that the wires suspending the antennas in the first trials might have interfered with the signal. This was only discovered after 5 turn loop had been destroyed the to create the smaller loops used in trial 2 and 3.
5.5 Fourth trial 31
Figure 5.7. Measuring results for Trail 1,2 & 3
5.5
Fourth trial
Trial 4 and 5 were done after the impedance testing (Section 5.7). The reason for this is that it was discovered just how much the cable placement as well as positioning of the antenna affected the VSWR and signal levels of the system. The setup was close to that off the first 3 with the same distance of 1 meter between the antennas and a signal level at -10 dBm. In trial 4, 2-coil loop antennas was used as both the Tx and Rx. What was seen in Figure 5.9, is that there are now higher
signal levels across the board then previous trials. The test was also measuring up to 200 MHz which made the falloff beyond 100 MHz much more visible then in the previous trials. The reason for the change in frequency range was to see if there would be higher range resonance frequencies to go with the lower levels of VSWR in that area (Section 5.7).
5.6
Fifth trial
The fifth trial was done with a 2-coil loop as the Tx and a 3-coil loop as the Rx.
As in the fourth trial, the distance between the two antennas was 1 meter, the signal level into the Tx was -10 dBm and the frequency ranged swept was 10-200
MHz. This setup gave a slightly lower levels of signal in the region of 50-100 MHz. It has been acknowledged it could be an error that depends on the environment.
Figure 5.8. Layout of the fourth and fifth trial
Figure 5.9. Signal levels in Rx for Trial 4 and 5
5.7
Impedance measuring
The impedance measuring was done at campus Norrköping, LIU using a Vector Network Analyzer of the brand Rhode and Schwarz. Since the antenna lab was
5.7 Impedance measuring 33
temporary de-constructed it was needed to do the measurements inside an office, this creates the scenario that was had in Section 4.5, with a lot of metal close by and any tug on the cable would change the VSWR. The measurements were done over a range from 10-200 MHz with a 1601 points accuracy and the antenna placed on a cardboard box (Figure 5.7). More detailed explanation is be provided below but one can say it is possible to push the two measured antennas down to a VSWR of 2 even with all the metal and human bodies close by. Another thing that was learned from the tests is that the coaxial cables used isn’t as well isolated as one would think, since they be could wrap around the antenna and that would create another coil, moving the frequency where the VSWR was at its smallest towards a lower point in frequency. This has afterwards been replicated when doing the signal transfers as in Section 4.5, making the measurements rather moot since all that was needed is a small flip of the cable to change it.
Figure 5.10. VSWR results scaled for visibility. Lower is better.
The impedance matching was not in line with the current theory, showing the antennas having their lowest VSWR at completely different frequencies then pre-dicted. This will be expanded upon in the discussion (Section 6). The reason that only two antennas were impedance tested is that one of the 2-coil antenna’s connector was damaged during transport and was not available for measurement.
Figure 5.11. The setup used for impedance measuring
5.7.1
3-coil loop antenna
The 3-coil antenna was tested with a piece of the cable striped to the side of it. This was done through to trial and error, giving the ideal VSWR that can be seen in Figure 5.7. The lowest values of VSWR was found at the frequency of 146 MHz with a value of 2.5. Then take Equation 2.26 and make it a function of γ.
V SW R = 1 + |γ| 1 − |γ|↔ 2.5 = 1 + |γ| 1 − |γ| , (5.2) |γ| = 2.5 − 1 2.5 + 1 = 0.4285 . . . ≈ 0.43 (5.3) and good area for the VSWR was at the point off 34M Hz (Figure 5.7) where the VSWR was about 3. So that gave:
V SW R = 1 + |γ| 1 − |γ|↔ 3 = 1 + |γ| 1 − |γ| and (5.4) |γ| = 3 − 1 3 + 1 = 0.5. (5.5)
5.7.2
2-coil loop antenna
The 2-coil loop antenna was tested with a piece of the cable laying under it. With this structure it gained the lowest VSWR at the frequency of 180 MHz with a value of about 2, as can bee seen in Figure 5.7. Using yet again Equation 2.26, it gives: V SW R = 1 + |γ| 1 − |γ|↔ 2 = 1 + |γ| 1 − |γ| and (5.6) |γ| = 2 − 1 2 + 1 = 0.33333 . . . ≈ 0.33. (5.7)
5.8 Longer range trials 35
This antenna also showed a dip in the VSWR around 36 MHz (Figure 5.7). The values here was as for the 3-coil antenna around 3.
V SW R = 1 + |γ| 1 − |γ| ↔ 3 = 1 + |γ| 1 − |γ| and (5.8) |γ| =3 − 1 3 + 1 = 0.5. (5.9)
5.7.3
Caveats
As mentioned in Section 5.7 there are quite a few things that affects these results from being conclusive. The antennas placement was imitated as best possible but with the signal leakage through the cable as well as being two human beings combined with a lot of metal close by, it is rather impossible to say what the signal was interacting with. This is something that needs to be accepted for this thesis. The thesis can and will use the results as a baseline for how the antennas are working. New signal measurements was also done in a broader frequency range to see if a better result can be gained then the ones found in Section 5.4.
5.8
Longer range trials
After consulting with one of Jon Staffeld from Combitech there was a realization that the first trials were stuck in the near field (Section 2.3.5), this should have been realized because the receiving antenna could be moved around the transmitter without any noticeable difference in signal. As in it could be held above it, moved between 1-4 m in distance and polarised in the other direction. This lead to new trials being done with a longer distance and confirmed change in signal when moving the antennas. Both tests are done with a distance of 10 m between the transmitter and receiver, a -10 dBm signal with a 10 MHz bandwidth and the noise floor was -60 dBm.
5.8.1
Sixth trial
What can be seen is that the signal level has dropped and now it’s clear where the antenna is resonant, it is in the frequency range of 40-44 MHz. There is also a slight spike in the region of 150-154 MHz. In the first frequency there top value is -39 dBm which gives a SNR of 60 − 39 = 21 dBm. This means a drop from the transmitter to the receiver by 39 − 10 = 29 dBm. This is a rather large difference from the third trial (Section 5.3) which can be attributed to leaving the near field and having an increased distance between the antennas.
5.8.2
Seventh trial
In this seventh trial there is a signal level drop equal to that of the sixth (Sec-tion 5.8.1) but in the 37-44 MHz region of frequencies. So the first peak for this
trial has a slightly larger bandwidth in the lower region, while not reaching the full peak in the higher one. Admitted this could be down to a measuring error since the testing was done by hand with a cursor being moved over the spectrogram to gather the data.
Figure 5.12. Layout of the sixth and seventh trial
5.9 Trials in water 37
5.9
Trials in water
Because of the limited space of the containers used, the long trials were done by submerging each antenna in a container filled with water, then separating the containers with 10 meters. These trials can somewhat be compared to the ones done in Section 5.8 since they are done in the same range and close to the same environment. Differences are that the signal strength had been increased by 10 dBm making it 0 dBm, the antenna polarization was also changed to vertical, to make the fit inside the containers. A reference test was done in air, which can be seen in the figure.
The short ranged trials were done in a plastic container filled with 100 L of water. The signal level used was once again -10 dBm because of the short distance of 2 dm between the antennas. The measurements were done in both tap and different kinds of salt water. One thing to note about these trials is that they go from 1-200 MHz. The spectrum analyzer is supposed to go down to 10 KHz but there might be some measuring errors closing into the lower end of the spectrum.
Figure 5.14. Layout of the long range water testing
5.9.1
Tap water ranged trial 1
This trial was done with 2-loop antennas being used as the Txand Rx, the signal
output from the generator was 00 dBm and noise floor was around -58 dBm. What can be seen in the graph is that the energy of the spectrum have moved down in frequency, which is in line with the theory from Section 2.3.5. The peaks are focused around 20-25 MHz as well as a few lesser ones in the 100 MHz and 150 MHz range. The least signal lost is -37 dBm which still is a loss down to a thousand part of the signal energy. Something else that needs to be considered is the refraction loss (Section 2.3.4) from changing mediums two times, water to air to water.
5.9.2
Tap water ranged trial 2
This trial was done with a 2-loop antenna as the Tx and a 3-loop antenna as
the Rx. The signal output from the generator to the Tx was 00 dBm and range
between the two containers with the antennas was around 10 m. Comparing this to the first trial (Section 5.9.1), the signal energy has a slightly higher peak at 24 MHz but also a sharper drop off after that. Using this the least signal lost is -34 dBM which in is in the same range as the previous trial.
Figure 5.16. Long range tap water trials with reference
5.9.3
Salt water ranged trial 1
In this trial we used 2-loop antennas as both the Tx and Rx. The tap water was
