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\begin{document}
\title{Review of Passive Wireless Sensors}
\author{Jakub M. Szypicyn}
\date{November 2017}
\maketitle
\tableofcontents
\pagebreak
\section{Introduction}

\section{Current Technologies}

\subsection{LC-Resonant Tank}

By far the most common implementation of a passive wireless sensor is that involving a resonant structure using lumped elements. The basic principle of such sensor is quite straight forward. One of the elements of the resonant structure is exposed to the measurand. As the environment around for example the capacitor varies, its capacitance varies too, to reflect the sensed changes. Depending on the measured quantity the variation may be linear, otherwise additional processing techniques are employed at the read-out stage.

Depending on the method of exposing one or more elements of the circuit to the measurand, both capacitor or inductor can play the role of a sensor. The following publications propose using capacitors as direct sensors of the environment.

Qiulin \textit{et al.} \cite{harsh} propose using a dual LC resonant circuit to simultaneously measure pressure and temperature. Both LC circuits utilise capacitors to measure the quantities. In order to sense pressure they have employed a cavity capacitor, whereas to sense temperature LTCC \cite{LTCC} was used to fill the gap between electrodes. The model is shown in Figure \ref{fig:harsh1}.

\begin{figure}[h]
\centering
\includegraphics[width=0.75\textwidth]{harsh1.JPG}
\caption{Schematic of the sensor. (a) Persective view and (b) cross-sectional view. \cite{harsh}\label{fig:harsh1}}
\end{figure}

The resonant frequencies can be accurately approximated as

\begin{equation}
f_t = \frac{1}{2\pi \sqrt{L_1C_1}}, \quad f_p = \frac{1}{2\pi \sqrt{L_2C_2}}
\end{equation}

where $f_t$ and $f_p$ are the resonant frequencies of the temperature and pressure sensing circuits respectively. In this model it is clear that by applying pressure which is normal the the largest surface in Figure \ref{fig:harsh1} (a), the air cavity height will vary, thus changing the capacitance.

Equivalently, LTCC filled capacitor is used to sense temperature, as the dielectric properties of the material will be affected by the change in temperature. LTCC material is assumed incompressible and thus it is predicted that the temperature sensor will be independent of the applied pressure. However the research group has realised that there is some dependency between pressure measurement and the temperature and therefore they propose to employ a post processing technique to compensate for the temperature in pressure measurements.

All in all, the have found that their sensor is functional in harsh environments up to 673 K. Moreover the temperature and pressure sensitivities are reported to reach 8.15 kHz/K and 1.96 MHz/Bar, respectively.

Similarly to the previous group Ren \textit{et al.} \cite{graphene} propose to use an LC tank where the capacitor is sensitive to temperature. Rather than using a parallel plate capacitor they decided to employ an interdigital capacitor (IDC) due to preferred fabrication techniques.

The space between fingers of the IDC has been filled with thin layer of graphene oxide, whose electrical properties has been found to vary with electric field, temperature or light. It is the temperature dependency that they have exploited. This is a very similar implementation of a sensor as presented by Quilin \textit{et al.} Figure \ref{fig:IDC} shows the fabrication technique used to manufacture this device.

\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{graphene1.JPG}
\caption{Fabrication process of the interdigital capacitive temperature sensor. \cite{graphene}\label{fig:IDC}}
\end{figure}

The graphene-oxide based device has been tested for temperatures ranging from 233 K to 333 K. It has been found that the sensor has two regions of linearity with different sensitivities. From 233 K to 273 K the sensitivity was measured to be 59.3 kHz/K and from 283 K to 333 K it is 46.1 kHz/K. These values are a factor of 6-7.5 higher than those reported by Quilin \textit{et al.}

The same structure as above, only without the graphene  has been employed by Jia \textit{et al.} \cite{IDC} to measure strain. Since the capacitance of the IDC depends on the spacing between its fingers, strain along one of the axis will result in change of the resonant frequency of the device. 

It is not inconceivable to imagine other similar structures that use the change of the dimensions of either capacitor or inductor to measure some physical phenomena. For instance, corrosion can be measured by exposing a capacitor to the metal surface. As corrosion occurs the capacitor will be filled with external substance - water or oxide, thus changing the dielectric property of the capacitor \cite{corrosion}.

Though this approach is fairly simple, the above research publications only interrogate the sensors inductively. This means that the maximum achieved distance is only a few centimetres for a reasonable input power. It should in theory be possible to interrogate the same devices with addition of an antenna using RF power. Lumped elements should in theory be behaving predictably up to around 20 GHz. This should therefore allow to significantly extend the operational range at the cost of increased size due to the requirement of having an antenna.

\subsection{Frequency Selective Surfaces}

Yasri \textit{et al.} \cite{corrosion2} proposed using microstrip resonant stubs to sense corrosion. The idea is somewhat basic. By exposing the resonant stub to the atmospheric conditions experienced by the object of interest, they were able to determine the shift in resonant frequency. As the microstrip stub corrodes its functional length decreases hence increasing the resonant frequency. The idea is presented in Figure \ref{fig:stub}.

\begin{figure}[h]
\centering
\includegraphics[width=0.75\textwidth]{stub.JPG}
\caption{The stub resonantor after corrosion (a) and the resonance frequency shift induced by the corrosion process (b).\cite{corrosion2}\label{fig:stub}}
\end{figure}

The challenge with using the above method is to provide realistic estimation of the corrosion taking place on the surface of the actual object rather than of the microstrip.

There has been some interest in using antenna's as direct sensors of strain and cracking. The three following publications focus on changing the geometry of the antenna and thus changing its resonant frequency.

Firstly, Yi \textit{et al.} \cite{array} propose using a folded-patch antenna. Their system utilises a backscattering mechanism and employs a cheap RFID chip in order to allow more than one sensor to be interrogated simultaneously. The chip is powered directly by the incoming RF radiation. The sensor design is shown in Figure \ref{fig:folded}.

\begin{figure}[h]
\centering
\includegraphics[width=0.375\textwidth]{folded.JPG}
\caption{RFID tag as wireless starin sensor - design drawing.\cite{array}\label{fig:folded}}
\end{figure}

If the incident power is above certain threshold power then the RFID tag will be activated and the sensor will reflect back its ID. The strain is measured by tracking the resonant frequency of the folded-patch antenna. Its zero-strain resonance can be estimated as

\begin{equation}
f_0 = \frac{c}{4(L+L')\sqrt{\epsilon _r}}
\end{equation}

where \textit{c} is the speed of light in vacuum, \textit{L} is the physical length of the top copper cladding, \textit{L'} is the additional electrical length due to fringing effects and $\epsilon _r$ is the relative dielectric permittivity of the substrate. When the antenna experiences longitudinal strain of $\epsilon$ its resonant frequency changes in the following manner

\begin{equation}
f_{\epsilon} = \frac{f_0}{1+\epsilon}
\end{equation}

Though this is a non-linear relationship, for small strains the above equation can be approximated as

\begin{equation}
f_{\epsilon} = f_0(1-\epsilon+\epsilon ^2-\epsilon ^3 ... ) \simeq f_0(1-\epsilon)
\end{equation}

which is very much linear.

Similarly to the previous study Tata \textit{et al.} \cite{patch} exploited a patch antenna to detect strain. As opposed to Yi \textit{et al.} this research group took advantage of the 2D structure of the antenna. Namely, the realised that a rectangular patch antenna will have two dominant resonant frequencies  - one for each dimension. The key idea is to provide incident power that is polarised in a way that enables excitation of the two modes.

In the paper they did not test the sensor wirelessly, but provided the input through a microstrip line. However as a proof of concept they showed that the two propagation modes TM$_{10}$ and TM$_{01}$ are sensitive enough to detect changes in strain.

In a paper \cite{horn} published by the same research group they propose to use a horn antenna in order to provide the incident radiation with the specified polarisation. It can be seen how this can cause problems when deployed. Any relative rotation of patch antenna and horn antenna will decrease the transmitted power and hence decrease the maximum operational distance.

Figure \ref{fig:patch} shows the wireless and wired sensors designs.

\begin{figure}[h]
\centering
\includegraphics[width=0.75\textwidth]{patch.JPG}
\caption{Antenna feeding (a) 50 $\Omega$ impedance matching; (b) microstrip feed line.\cite{horn}\label{fig:patch}}
\end{figure}

\subsection{Other Resonant Structures}

There is a plethora of possible utilisations of resonant structures. In this section just one example will be presented as the general operation principle is shared by all resonant structures, i.e. either inductor or capacitor in the equivalent circuit model is used for sensing.

Zhao \textit{et al.} in \cite{evamode}  present a radio frequency evanescent-mode cavity resonator. The structure is made up of a cavity with a centre post. The resonant frequency of the structure is determined by the capacitance formed between the centre post and the top membrane of the cavity. In the paper they apply this idea to sense air flow. By attaching a deflection needle to the top of the membrane, as the air moves the needle, it inevitably increases or decreases the distance from the top membrane to the centre post. The cross-sectional view of the structure is shown in Figure \ref{fig:eva}.

\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{cavity.JPG}
\caption{Cross-sectional view of the airflow sensor unit. \cite{evamode}\label{fig:eva}}
\end{figure}

The energy is radiated inside the cavity through a coplanar waveguide (CPW). The same CPW is then used to guide the resonant signal back to the antenna. The resonant frequency of the cavity can be approximated using the standard equation

\begin{equation}
f_0 \simeq \frac{1}{2\pi \sqrt{L C_{post}}}\textrm{,}
\end{equation}

where $L$ is the equivalent inductance due to surface current and $C_{post}$ is the capacitance formed between the post and top membrane. It should be noted that this approximation holds only when $C_{post}$  is the dominant capacitance in the structure, i.e. when the gap between the post and top membrane is much smaller than the dimensions of the cavity. It therefore  has been shown that the resonant frequency depends on the deflection angle as

\begin{equation}
f_r = f_0\sqrt{\frac{C}{C'}} = \left(\frac{d}{2r\theta}\textrm{ln}\left(\frac{(d/r)+\theta}{(d/r)-\theta}\right)\right)^{-\frac{1}{2}}\textrm{,}
\end{equation}

where $C$ is the nominal, or zero angle deflection, capacitance of the structure, $C'$ is the new capacitance, $d$ is the mean gap, $r$ is the radius of the capacitive post and $\theta$ is the defelction angle.

\subsection{Intermodulation Communication Principle}

This area of passive wireless sensors has been only researched by Professor Ville Viikari of Aalto University in Helsinki. In a paper written by Viikari \textit{et al.}\cite{imc_review} three techniques of achieving intermodulation communication principle (ICP) are described.

In short ICP relies on two signals at slightly different frequencies - \textit{f}$_1$ and \textit{f}$_2$ being received by the sensor. The sensor is some passive fashion will then mix the signals together. The modulated signal which depends on some physical phenomenon or phenomena will mix back with the incident waves and re-radiate a modulated signal.

In general mixers are constructed using transistors, i.e. these are active devices. However since the power budget is limited another method of mixing is sought after. Viikari discusses three methods - MEMS, electrically non-linear elements and low-frequency resonant circuits.

Beginning with MEMS, the receiving antenna is directly coupled to a capacitive MEMS resonator. The mechanical resonator will develop voltages at both frequencies. Howeve due to its non-linearity there will also be all the harmonic frequencies of type \textit{nf}$_1$ $\pm$ \textit{mf}$_2$. The capacitance of the resonator will be modulated by the mechanical vibrations, hence the reflection coefficient will vary with time. The outgoing refelction will be at frequencies equal to \textit{f}$_1$ $\pm$ \textit{f}$_{res}$ and \textit{f}$_2$ $\pm$ \textit{f}$_{res}$. In a case when the resonant frequency is the difference of the two incoming frequencies the intermodulation signals being re-radiated are 2\textit{f}$_1$ $-$ \textit{f}$_2$ and 2\textit{f}$_2$ $-$ \textit{f}$_1$. Furthermore it has been found that the voltage at the intermodulation frequency can be estimated as follows \cite{rfidMEMS}

\begin{equation}
V_{2f_2-f_1} \simeq \frac{\omega_m^2}{\Delta \omega^2 \left(1+\left( \frac{Q_m \left(\Delta \omega^2 - \omega_m^2 \right)}{\Delta \omega \omega_m} \right)^2 \right)}
\end{equation}

where $\Delta \omega = 2\pi (f_2-f_1)$, $\omega_m$ is the angular frequency of the resonator and $Q_m$ is its quality factor. As long as the MEMS' resonance frequency or the quality factor are dependent on the measurand the the device can act as a sensor. The problem with utilising this sensor is that its cut-off frequency is around 10 MHz. It is worth noting that in the above equation the generated voltage reaches unity when the resonance frequency is equal to the difference of the interrogation frequencies. The sensor must therefore not operate at the resonance frequency of the MEMS otherwise quality factor will not be sensed. Similarly, if the sensor utilises the resonant frequency of MEMS as sensing mechanism then it is ideal that the quality factor is reasonably large, however not as large as to decrease the dynamic range of the sensor. The higher the quality factor the sharper the roll off, thus the sensing capability decreases as there isn't enough retransmitted power. The plot of the voltage as a function of quality factor and MEMS resonant frequency is shown in Figure \ref{fig:vol}.


\begin{figure}[h]
\centering
\includegraphics[width=1\textwidth]{voltage.eps}
\caption{Volatge developed on MEMS structure for varying resonant frequencies and quality factors.\label{fig:vol}}
\end{figure}

Viikari then discusses sensors based on electrically non-linear elements. This method has in fact been used by his research group \cite{icp1,icp2,icp3,icp4}. In short the same outcome as in using MEMS is achieved by employing electrically non-linear element such as a diode or micro-bolometer. More specifically he lists photo-diodes and varactor-diodes. Their non-linear behaviour and its non-idealities allow for signal mixing. It is found that the voltage at intermodulation frequency can be estimated as

\begin{equation}
V_{2f_2-f_1} \simeq Q^4(1 - |\Gamma|)^4
\end{equation}

where $\Gamma$ represents the reflection coefficient between the antenna and the varactor. The relationship between retransmitted power and either the reflection coefficient is straightforward. It is has been found by Viikari that the reflection coefficient is affected by the changes in temperature thus granting the device its sensing capabilities.

Finally, low frequency resonant circuits are proposed as means of mixing signals. It is said that utilising a single element for mixing and sensing can lead to unwanted behaviour. First of all, microwave mixing using these passive techniques is found to be inefficient. Additionally, it is difficult to realise a mixer as an efficient and accurate sensor. Lastly, the antenna impedance changes when it is placed in close proximity to a dielectric or conductive material. As such the reflection coefficient changes making the sensor virtually unusable. More sophisticated techniques are required to mix and and sense separately.

In general this sort of circuit would use an antenna coupled to a high pass filter which then feeds a mixing element. The mixing element is then connected to a low pass filter to extract the modulation components of interest. Finally, the low pass filter is loaded with a resonant circuit which employs a sensing element such as a capacitor. An equivalent circuit is shown in Figure \ref{fig:lowf}.

\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{low_f.JPG}
\caption{Simplified equivalent circuit of the intermodulation sensor based on a low-frequency resonator. \cite{rfidMEMS}\label{fig:lowf}}
\end{figure}

Due to two stage mixing process the voltage and thus the retransmitted power depends on the impedance of the resonator at the difference frequency as

\begin{equation}
V_{2f_2-f_1} \simeq A + BZ_{res}(f_{\Delta})
\end{equation}

where A and B are complex numbers independent of the difference frequency. It is therefore clear that modulation in the impedance of the resonant structure will shift the power level of the backscattered signal.

\subsection{Surface Acoustic Waves}

Surface acoustic wave (SAW) devices are known to work as simple RFID tags, but also as sensors of humidity, temperature, pressure, biologial and chemical reactions, strain or electric field variation. The surface wave also known as Rayleigh wave \cite{rayleigh} as the name suggests travels along the surface of the material and only penetrates the material roughly one wavelength deep. Hence all of the incoming RF energy can be efficiently transduced into useful mechanical energy, provided the means of transducing are efficient. 

To convert the RF signal into an elastic wave an interdigital transducer (IDT) deposited on the surface of a piezoelectric material is used. The elastic wave then propagates through the material. At this point we can distinguish two types of devices - single and double port devices.

In single port devices the same antenna and the same IDT are used to for the input and output. Analogically, double port devices use a pair of each. Most of the literature focuses on single port devices and as such all of the attention shall be given to them.

The properties of elastic waves strongly depend on the geometry of the material, shape of IDTs and the material deposited on the piezoelectric substrate \cite{SAWapps}. The main sensing paramater in SAW devices is the delay time of the wave. Firstly, an elastic wave travels three orders of magnitude slower than its electromagnetic counterpart. Secondly, the actual delay is a function of material length and wave velocity. 

\begin{equation}
T_d = \frac{L}{v_{SAW}}
\end{equation}

where $T_d$ is time delay, $L$ is path length and $v_{SAW}$ is the phase velocity. Both $L$ and $v_{SAW}$ are known to vary with the parameters listed earlier, e.g. strain. In single port devices the input signal will be converted into SAW. The elestic wave will then propagate through the surface of the material. In order to determine the time delay of the wave minimum of two reflectors are deposited in the path of the SAW. The wave will then partially be reflected by each of those structures and the signal will be backscattered to the receiver. A representation of that mechanism is shown in Figure \ref{fig:SAWbasic}. Hence by measuring the time between consecutive reflections at the read-out stage it is possible to resolve the measurand.

\begin{figure}[h]
\centering
\includegraphics[width=0.375\textwidth]{SAWdelay.JPG}
\caption{Schematic of the operating principle of a SAW-based radio-link temperature measurement system. \cite{SAW1}\label{fig:SAWbasic}}
\end{figure}

Having said that, it is also known to use wavelength as the sensing parameter \cite{SAWapps}. The SAW can only be stimulated when the incoming wavelength matches the period of the IDT finger spacing. Thus by emitting a chirp signal it is possible to determine the IDT period, which could be designed to sense strain.

Liu Bo \textit{et al.} \cite{SAWapps} discusses the significance of temperature and mass loading on the wave speed. Temperature ($T$) directly influences the phase velocity and this effect is usually referred to as Temperature Coefficient of Frequency or TCF. For a constant wavelength ($\lambda$) the wave speed ($v$) will vary as the frequency ($f_0$) varies. The frequency variation is described by the following equation

\begin{equation}
TCF = \frac{1}{f_0}\frac{\textrm{d}f_0}{\textrm{d}T} = \frac{1}{v}\frac{\textrm{d}v}{\textrm{d}T}- \frac{1}{\lambda}\frac{\textrm{d}\lambda}{\textrm{d}T}\textrm{.}
\end{equation}


Many other factors which affect the propagation velocity are based on the mass loading of the piezoelectric substrate. For example chemical sensing can be accomplished by depositing suitable layer on top the substrate that reacts with the chemical of interest, thus changing the mass loading conditions. According to Sauerbrey equation \cite{mass} mass loading has a linear effect on the frequency change

\begin{equation}
\frac{\Delta f}{f_0} \simeq -\frac{\rho _m t_m}{\rho _0 t_0} = -\frac{\Delta m}{m_0}\textrm{,}
\end{equation}

where $m_0$ is the equivalent mass of the resonant part of the device, $\rho _m$ and $t_m$ are the density and thickness of the mass on the SAW propagating path, and $\rho _0$ and $t_0$ are the density and thickness of the unloaded resonator respectively \cite{SAWapps}.

In terms of applications there are countless publications reporting the use of SAW sensors for temperature \cite{sawtemp1, sawtemp2}, pressure \cite{sawtemp2, sawpres1, sawpres2, sawpres3}, humidity \cite{sawhum}, gas \cite{sawgas}, chemical compounds \cite{sawchem1, sawchem2} and bio-agents \cite{sawbio1, sawbio2} measuring. Further details presented in the aforementioned papers won't be discussed here as the main principle of operation has been presented.


\subsection{Relaxation Oscillator}

Patterson \textit{et al.} in their paper \cite{relax} discuss the use of passive wireless sensor for what they call chem-bio agents. First of all in their research they advocate the use of inductive coupling over RF. The reason for that is more efficient power transfer, however they rightly noticed that it comes at a cost of greatly decreased range. They report peak power transfer at 1.5 cm.

The research group proposed to first convert the incident AC signal into DC using full bridge rectifier. This DC is then used to drive a tunable oscillator. The schematic of the circuit is shown in Figure \ref{fig:circuit}.

\begin{figure}[h]
\centering
\includegraphics[width=0.5\textwidth]{relax.JPG}
\caption{Schematic diagram of the sensor based on relaxation oscillator \cite{relax}\label{fig:circuit}}
\end{figure}

The circuit works by charging the capacitor connected to the anode to a threshold voltage determined by the bias resistor pair on the gate side. The input resistor limits the charge time, but it also limits valley current to cut off the the unijunction transistor during the discharge segment of operation. When the voltage across the capacitor slightly exceeds that of the bias on the gate, the unijunction transistor turns on and discharges the capacitive energy through the cathode. Since the the input charge resistor limits the amount of current that can flow once the capacitor discharges, the unijunction transistor shuts off and the cycles starts over again. The reason the transistor shuts off is that it requires a minimum current to flow through it to maintain the on state. This is referred to as valley current.

This circuit has the ability to act as a sensor only when either bias or the anode capacitance vary. In the paper they proposed to introduce a variable capacitor in parallel with a fixed value one. That way they were able to alter the frequency of the output signal with variation in capacitance. The output frequency was recorded to be around 40 kHz which makes it applicable for RF applications, however it would require a large antenna.

\subsection{Metamaterial Structures}

Metamaterials are artificial materials engineered to exhibit properties which are not readily found in nature. Generally isotropic dielectrics at a given frequency have positive permittivity and permeability, thus resulting in a real and positive refractive index. However, metamaterials exhibit negative permittivity and permeability and thus have a real but negative refractive index. For completion, we can also distinguish two other sets of material, i.e. plasma or metals at optical frequencies and ferrites. The former has negative permittivity and positive permeability, whereas in the latter the signs are reversed. Either way, the refractive index ends up being negative and imaginary. In those cases there is no propagation and we only observe an evanescent wave.

There are no known naturally existing materials that exhibit meta properties. However, structures that behave as such have been successfully engineered. One such example is a split ring resonator or SRR shown in Figure \ref{fig:srr}. Note that both of the squares have gaps - larger one at the top and the smaller one at the bottom.

\begin{figure}[h]
\centering
\begin{subfigure}{.5\textwidth}
  \centering
\includegraphics[width=0.75\textwidth]{srr.JPG}
\caption{Traditional split ring resonator structure.\label{fig:srr}}
\end{subfigure}%
\begin{subfigure}{.5\textwidth}
  \centering
\includegraphics[width=0.75\textwidth]{nested.JPG}
\caption{Nested split ring resonator structure.\label{fig:nested}}
\end{subfigure}
\caption{Plan view pictures of two SRR structures. \cite{srr}}
\end{figure}

These types of structures are usually very narrowband. Most of the time they behave as right-handed materials.

Melik \textit{et al.} \cite{srr} proposed using metamaterial structure for strain sensing in medical application space. The advocate using it over traditional materials as metamaterials tend to have lower resonant frequency, which was of advantage to them since higher frequencies would be absorbed by soft tissue. Moreover, these artificial materials exhibit much higher Q-factors compared to standard spiral structures. This enables tracking of the resonant frequency more comfortably. Thirdly, linearity is much easier achieved when compared to traditional spiral structures, thus allowing to infer the measured quantity directly form the slope. Final point wraps back to the first one in that metamaterials can support much larger wavelengths for the same size, hence their size can be greatly reduced.

The research group then used the splits in the rings as the means of measuring strain. As the gap size varies the resonant frequency of the structure varies with it, and hence it is possible to determine the magnitude of stress or strain.

In the same paper, they propose to go a step forward and introduce a nested metamaterial structure, which is shown in Figure \ref{fig:nested}. This structure is reported to have further increased the Q-factor, and sensitivity. At the same time it allowed them to further decrease the operating frequency by some 23 MHz from 529.8 MHz to 506.2 MHz.

\section{Potential Technologies}

\subsection{Sensing Based On Impedance Mismatch}

By performing the scattering or S-parameter analysis of any device we can determine key values such as $S_{11}$ or $S_{21}$, which represent the return and insertion losses respectively. For a lossless device the reflected power and transmitted power should add up to the original signal power.

For a perfectly matched, i.e. impedance complex conjugate matched, systems the values of the S-parameters are $S_{11} = -\infty$ dB and $S_{21} = 0$ dB. However, it is virtually impossible to be able to always perfectly match devices such as a 50 $\Omega$ antenna with the rest of the circuit. As such there will always be some portion of the input signal that is going to be reflected. The magnitude of the backscattered signal will depend on how poorly the systems are matched. Thus, by exploiting this idea it is possible to make a circuit whose impedance will vary relative to the antenna, or vice-versa.

\subsection{Sensitive Slow Wave Structures}

Similar to surface acoustic waves, it would be possible to slow down electromagnetic waves and employ similar techniques to those of SAW. The key differences between this solution and SAW are as follow; firstly, surface acoustic wave is a mechanical, elastic wave whereas slow wave structures are still based on electromagnetic propagation. Secondly, the reported slow down factor in SAW was in the excess of 1000 over the free space counterpart. In slow wave structures we should not expect as high slowing factors. It is reasonable to assume a 100 fold decrease in group velocity.

We can distinguish high-$\kappa$ and low-$\kappa$ dielectrics. These are essentially materials with high and low relative permittivity. An example of a low-$\kappa$ dielectric is silicon dioxide SiO$_2$, which has a dielectric constant of just 3.9. This means that an electromagnetic wave will only slow down by a factor of $\sqrt{3.9}$. A high-$\kappa$ materials include compounds such as strontium titanate which is reported to have relative permittivity of 186 \cite{strtit}.

\subsection{Semi-Passive Sensors}

\section{Conclusion}

\bibliography{mybib1}{}
\bibliographystyle{unsrt}

\end{document}


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   publisher = {IEEE},
   pages = {819-20},
   DOI = {10.1109/CRMICO.2014.6959647},
   url = {http://dx.doi.org/10.1109/CRMICO.2014.6959647},
   type = {Conference Proceedings}
}

@inproceedings{srr,
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   title = {Metamaterial-based wireless RF-MEMS strain sensors},
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   pages = {2173-2176},
   ISBN = {19300395},
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   url = {http://dx.doi.org/10.1109/ICSENS.2010.5690582},
   type = {Conference Proceedings}
}


@inproceedings{relax,
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   title = {A passive wireless sensor platform for chemical and biological agents},
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   publisher = {IEEE},
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   url = {http://dx.doi.org/10.1109/NAECON.2012.6531038},
   type = {Conference Proceedings}
}


@article{evamode,
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}


@inproceedings{sawbio2,
   author = {Zakaria, M. R. and Omar, M. F. and Azman, A. H. and Hashim, U. and Md Arshad, M. K.},
   title = {Fabrication and characterization of SAW IDT biosensor for biomolecule detection},
   booktitle = {2015 IEEE Regional Symposium on Micro- and Nanoelectronics (RSM), 19-21 Aug. 2015},
   series = {2015 IEEE Regional Symposium on Micro- and Nanoelectronics (RSM). Proceedings},
   publisher = {IEEE},
   pages = {4 pp.},
   DOI = {10.1109/RSM.2015.7354960},
   url = {http://dx.doi.org/10.1109/RSM.2015.7354960},
   type = {Conference Proceedings}
}


@inproceedings{sawbio1,
   author = {Goto, M. and Yatsuda, H. and Kondoh, J.},
   title = {SH-SAW biosensor on quartz},
   booktitle = {2016 IEEE International Frequency Control Symposium (IFCS), 9-12 May 2016},
   series = {2016 IEEE International Frequency Control Symposium (IFCS)},
   publisher = {IEEE},
   pages = {6 pp.},
   DOI = {10.1109/FCS.2016.7546725},
   url = {http://dx.doi.org/10.1109/FCS.2016.7546725},
   type = {Conference Proceedings}
}


@article{sawchem2,
   author = {Pan, Y. and Yang, L. and Mu, N. and Shao, S. and Wang, W. and Xie, X. and He, S.},
   title = {A SAW-based chemical sensor for detecting sulfur-containing organophosphorus compounds using a two-step self-assembly and molecular imprinting technology},
   journal = {Sensors (Basel)},
   volume = {14},
   number = {5},
   pages = {8810-20},
   ISSN = {1424-8220 (Electronic)
1424-8220 (Linking)},
   DOI = {10.3390/s140508810},
   url = {https://www.ncbi.nlm.nih.gov/pubmed/24854058},
   year = {2014},
   type = {Journal Article}
}


@article{sawchem1,
   author = {Xu, F. Q. and Wang, W. and Xue, X. F. and Hu, H. L. and Liu, X. L. and Pan, Y.},
   title = {Development of a Wireless and Passive SAW-Based Chemical Sensor for Organophosphorous Compound Detection},
   journal = {Sensors (Basel)},
   volume = {15},
   number = {12},
   pages = {30187-98},
   ISSN = {1424-8220 (Electronic)
1424-8220 (Linking)},
   DOI = {10.3390/s151229793},
   url = {https://www.ncbi.nlm.nih.gov/pubmed/26633419},
   year = {2015},
   type = {Journal Article}
}


@article{sawgas,
   author = {Wang, W. and Hu, H. and Liu, X. and He, S. and Pan, Y. and Zhang, C. and Dong, C.},
   title = {Development of a Room Temperature SAW Methane Gas Sensor Incorporating a Supramolecular Cryptophane A Coating},
   journal = {Sensors (Basel)},
   volume = {16},
   number = {1},
   ISSN = {1424-8220 (Electronic)
1424-8220 (Linking)},
   DOI = {10.3390/s16010073},
   url = {https://www.ncbi.nlm.nih.gov/pubmed/26751450},
   year = {2016},
   type = {Journal Article}
}


@article{sawhum,
   author = {Sayar Irani, F. and Tunaboylu, B.},
   title = {SAW Humidity Sensor Sensitivity Enhancement via Electrospraying of Silver Nanowires},
   journal = {Sensors (Basel)},
   volume = {16},
   number = {12},
   ISSN = {1424-8220 (Electronic)
1424-8220 (Linking)},
   DOI = {10.3390/s16122024},
   url = {https://www.ncbi.nlm.nih.gov/pubmed/27916870},
   year = {2016},
   type = {Journal Article}
}


@inproceedings{sawpres3,
   author = {Grousset, S. and Augendre, E. and Benaissa, L. and Signamarcheix, T. and Baron, T. and Courjon, E. and Ballandras, S.},
   title = {SAW pressure sensor based on single-crystal quartz layer transferred on Silicon},
   booktitle = {2013 Joint European Frequency and Time Forum and International Frequency Control Symposium, EFTF/IFC 2013, July 21, 2013 - July 25, 2013},
   series = {2013 Joint European Frequency and Time Forum and International Frequency Control Symposium, EFTF/IFC 2013},
   publisher = {IEEE Computer Society},
   pages = {980-983},
   DOI = {10.1109/EFTF-IFC.2013.6702082},
   url = {http://dx.doi.org/10.1109/EFTF-IFC.2013.6702082},
   type = {Conference Proceedings}
}


@article{sawpres2,
   author = {Hashimoto, S. and Kuypers, J. H. and Tanaka, S. and Esashi, M.},
   title = {Design and fabrication of passive wireless SAW sensor for pressure measurement},
   journal = {Transactions of the Institute of Electrical Engineers of Japan, Part E},
   volume = {128},
   number = {5},
   pages = {230-4},
   ISSN = {1341-8939},
   DOI = {10.1541/ieejsmas.128.230},
   url = {http://dx.doi.org/10.1541/ieejsmas.128.230},
   year = {2008},
   type = {Journal Article}
}


@article{sawpres1,
   author = {Schimetta, Gernot and Dollinger, Franz and Weigel, Robert},
   title = {A Wireless Pressure-Measurement System Using a SAW Hybrid Sensor},
   journal = {IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES},
   volume = {48},
   number = {12},
   pages = {2730-2735},
   year = {2000},
   type = {Journal Article}
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@article{sawtemp2,
   author = {Borrero, G. A. and Bravo, J. P. and Mora, S. F. and Velasquez, S. and Segura-Quijano, F. E.},
   title = {Design and fabrication of SAW pressure, temperature and impedance sensors using novel multiphysics simulation models},
   journal = {Sensors and Actuators A (Physical)},
   volume = {203},
   pages = {204-14},
   ISSN = {0924-4247},
   DOI = {10.1016/j.sna.2013.08.021},
   url = {http://dx.doi.org/10.1016/j.sna.2013.08.021},
   year = {2013},
   type = {Journal Article}
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@article{sawtemp1,
   author = {Zhgoon, S. and Shvetsov, A. and Ancev, I. and Bogoslovsky, S. and Sapozhnikov, G. and Trokhimets, K. and Derkach, M.},
   title = {SAW Temperature Sensor on Quartz},
   journal = {IEEE Trans Ultrason Ferroelectr Freq Control},
   volume = {62},
   number = {6},
   pages = {1066-75},
   ISSN = {1525-8955 (Electronic)
0885-3010 (Linking)},
   DOI = {10.1109/TUFFC.2014.006840},
   url = {https://www.ncbi.nlm.nih.gov/pubmed/26067041},
   year = {2015},
   type = {Journal Article}
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@article{mass,
   author = {Sauerbrey, G.},
   title = {Use of crystal oscillators for weighing thin films and for microweighing
Verwendung von Schwingquarzen zur Waegung duenner Schichten und zur Mikrowaegung},
   journal = {Zeitschrift fuer Physik},
   volume = {155},
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   pages = {206-222},
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   type = {Journal Article}
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@inproceedings{SAW1,
   author = {Reindl, L. and Shrena, I. and Kenshil, S. and Peter, R.},
   title = {Wireless Measurement of Temperature Using Surface Acoustic Waves Sensors},
   booktitle = {Proceedings of the 2003 IEEE International Frequency Control Symposium and PDA Exhibition Jointly with the 17th European Frequency and Time Forum, May 4, 2003 - May 8, 2003},
   series = {Proceedings of the Annual IEEE International Frequency Control Symposium},
   publisher = {Institute of Electrical and Electronics Engineers Inc.},
   pages = {935-941},
   ISBN = {01616404},
   DOI = {10.1109/FREQ.2003.1275216},
   url = {http://dx.doi.org/10.1109/FREQ.2003.1275216},
   type = {Conference Proceedings}
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@article{SAWapps,
   author = {Liu, Bo and Chen, Xiao and Cai, Hualin and Mohammad Ali, Mohammad and Tian, Xiangguang and Tao, Luqi and Yang, Yi and Ren, Tianling},
   title = {Surface acoustic wave devices for sensor applications},
   journal = {Journal of Semiconductors},
   volume = {37},
   number = {2},
   ISSN = {1674-4926},
   DOI = {10.1088/1674-4926/37/2/021001},
   year = {2016},
   type = {Journal Article}
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@article{rayleigh,
   author = {Lord Rayleigh, D.C.L., F.R.S.},
   title = {On waves propagated along the plane surface of an elastic solid},
   journal = {Proceedings of the London Mathematical Society},
   volume = {17},
   pages = {4-11},
   year = {1885},
   type = {Journal Article}
}


@article{harsh,
   author = {Qiulin, Tan and Tao, Luo and Tanyong, Wei and Jun, Liu and Liwei, Lin and Jijun, Xiong},
   title = {A Wireless Passive Pressure and Temperature Sensor via a Dual LC Resonant Circuit in Harsh Environments},
   journal = {Journal of Microelectromechanical Systems},
   volume = {26},
   number = {2},
   pages = {351-6},
   ISSN = {1057-7157},
   DOI = {10.1109/JMEMS.2016.2642580},
   url = {http://dx.doi.org/10.1109/JMEMS.2016.2642580},
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@inproceedings{LTCC,
   author = {Pietrikova, A.},
   title = {Potentiality of LTCC for sensor applications},
   booktitle = {24th International Spring Seminar on Electronics Technology. Concurrent Engineering in Electronic Packaging. ISSE 2001. Conference Proceedings, 5-9 May 2001},
   series = {24th International Spring Seminar on Electronics Technology. Concurrent Engineering in Electronic Packaging. ISSE 2001. Conference Proceedings (Cat. No.01EX492)},
   publisher = {IEEE},
   pages = {112-16},
   DOI = {10.1109/ISSE.2001.931025},
   url = {http://dx.doi.org/10.1109/ISSE.2001.931025},
   type = {Conference Proceedings}
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@inproceedings{graphene,
   author = {Qing-Ying, Ren and Jian-Qiu, Huang and Li-Feng, Wang and Shu, Wan and Li-Tao, Sun and Qing-An, Huang},
   title = {Temperature sensing properties of the passive wireless sensor based on graphene oxide films},
   booktitle = {2014 IEEE Sensors, 2-5 Nov. 2014},
   series = {IEEE Sensors 2014. Proceedings},
   publisher = {IEEE},
   pages = {432-5},
   DOI = {10.1109/ICSENS.2014.6985027},
   url = {http://dx.doi.org/10.1109/ICSENS.2014.6985027},
   type = {Conference Proceedings}
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@article{IDC,
   author = {Yi, Jia and Ke, Sun and Agosto, F. J. and Quinones, M. T.},
   title = {Design and characterization of a passive wireless strain sensor},
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   volume = {17},
   number = {11},
   pages = {2869-76},
   ISSN = {0957-0233},
   DOI = {10.1088/0957-0233/17/11/002},
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   year = {2006},
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@article{corrosion,
   author = {Zarifi, M. H. and Deif, S. and Daneshmand, M.},
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   ISSN = {0924-4247},
   DOI = {10.1016/j.sna.2017.04.006},
   url = {http://dx.doi.org/10.1016/j.sna.2017.04.006},
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@inproceedings{corrosion2,
   author = {Yasri, M. and Gallee, F. and Lescop, B. and Diler, E. and Thierry, D. and Rioual, S.},
   title = {Passive wireless sensor for atmospheric corrosion monitoring},
   booktitle = {2014 8th European Conference on Antennas and Propagation (EuCAP), 6-11 April 2014},
   series = {2014 8th European Conference on Antennas and Propagation (EuCAP)},
   publisher = {IEEE},
   pages = {2945-9},
   DOI = {10.1109/EuCAP.2014.6902445},
   url = {http://dx.doi.org/10.1109/EuCAP.2014.6902445},
   type = {Conference Proceedings}
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@inproceedings{array,
   author = {Xiaohua, Yi and Chia-Hung, Fang and Cooper, J. and Chunhee, Cho and Vyas, R. and Yang, Wang and Leon, R. T. and Tentzeris, M. M.},
   title = {Strain sensing through a passive wireless sensor array},
   booktitle = {20th Analysis and Computation Specialty Conference, 29-31 March 2012},
   series = {20th Analysis and Computation Specialty Conference. Proceedings},
   publisher = {American Society of Civil Engineers},
   pages = {117-26},
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@article{patch,
   author = {Tata, U. and Huang, H. and Carter, R. L. and Chiao, J. C.},
   title = {Exploiting a patch antenna for strain measurements},
   journal = {Measurement Science and Technology},
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   ISSN = {09570233},
   DOI = {10.1088/0957-0233/20/1/015201},
   url = {http://dx.doi.org/10.1088/0957-0233/20/1/015201},
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@article{horn,
   author = {Tata, U. and Deshmukh, S. and Chiao, J. C. and Carter, R. and Huang, H.},
   title = {Bio-inspired sensor skins for structural health monitoring},
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   volume = {18},
   number = {10},
   pages = {104026 (8 pp.)},
   ISSN = {0964-1726},
   DOI = {10.1088/0964-1726/18/10/104026},
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   year = {2009},
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@inproceedings{imc_review,
   author = {Viikari, Ville and Song, Jinsong and Pesonen, Nadine and Pursula, Pekka and Seppa, Heikki},
   title = {Review of passive wireless sensors utilizing the intermodulation communication},
   booktitle = {2014 IEEE RFID Technology and Applications Conference, RFID-TA 2014, September 8, 2014 - September 9, 2014},
   series = {2014 IEEE RFID Technology and Applications Conference, RFID-TA 2014},
   publisher = {Institute of Electrical and Electronics Engineers Inc.},
   pages = {56-61},
   DOI = {10.1109/RFID-TA.2014.6934200},
   url = {http://dx.doi.org/10.1109/RFID-TA.2014.6934200},
   type = {Conference Proceedings}
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@article{rfidMEMS,
   author = {Viikari, V. and Seppa, H.},
   title = {RFID MEMS sensor concept based on intermodulation distortion},
   journal = {IEEE Sensors Journal},
   volume = {9},
   number = {12},
   pages = {1918-23},
   ISSN = {1530-437X},
   DOI = {10.1109/JSEN.2009.2031809},
   url = {http://dx.doi.org/10.1109/JSEN.2009.2031809},
   year = {2009},
   type = {Journal Article}
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@article{icp1,
   author = {Rasilainen, K. and Ilvonen, J. and Hannula, J. M. and Viikari, V.},
   title = {Designing harmonic transponders using lumped-component matching circuits},
   journal = {IEEE Antennas and Wireless Propagation Letters},
   volume = {16},
   pages = {246-9},
   ISSN = {1536-1225},
   DOI = {10.1109/LAWP.2016.2570938},
   url = {http://dx.doi.org/10.1109/LAWP.2016.2570938},
   year = {2017},
   type = {Journal Article}
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@article{icp2,
   author = {Viikari, V. and Seppa, H. and Dong-Wook, Kim},
   title = {Intermodulation Read-Out Principle for Passive Wireless Sensors},
   journal = {IEEE Transactions on Microwave Theory and Techniques},
   volume = {59},
   number = {4},
   pages = {1025-31},
   ISSN = {0018-9480},
   DOI = {10.1109/TMTT.2011.2108309},
   url = {http://dx.doi.org/10.1109/TMTT.2011.2108309},
   year = {2011},
   type = {Journal Article}
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@article{icp3,
   author = {Viikari, V. and Seppa, H. and Mattila, T. and Alastalo, A.},
   title = {Wireless Ferroelectric Resonating Sensor},
   journal = {IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control},
   volume = {57},
   number = {4},
   pages = {785-91},
   ISSN = {0885-3010},
   DOI = {10.1109/TUFFC.2010.1482},
   url = {http://dx.doi.org/10.1109/TUFFC.2010.1482},
   year = {2010},
   type = {Journal Article}
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@inproceedings{icp4,
   author = {Viikari, Ville and Song, Jinsong and Pesonen, Nadine and Pursula, Pekka and Seppa, Heikki},
   title = {Review of passive wireless sensors utilizing the intermodulation communication},
   booktitle = {2014 IEEE RFID Technology and Applications Conference, RFID-TA 2014, September 8, 2014 - September 9, 2014},
   series = {2014 IEEE RFID Technology and Applications Conference, RFID-TA 2014},
   publisher = {Institute of Electrical and Electronics Engineers Inc.},
   pages = {56-61},
   DOI = {10.1109/RFID-TA.2014.6934200},
   url = {http://dx.doi.org/10.1109/RFID-TA.2014.6934200},
   type = {Conference Proceedings}
}