In this paper novel miniature metamaterial-based soil moisture sensors are presented. The sensors are based on resonant-type metamaterials and employ split-ring resonators (SRR), spiral resonators and fractal SRRs to achieve small dimensions, high sensitivity, and compatibility with standard planar fabrication technologies. All these features make the proposedsensors suitable for deployment in agriculture for precise mapping of soil humidity

Goran Kitić

Vasa Radonić

Vesna Crnojević -Bengin

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Original ArticleSoil moisture sensors based on metamaterialsGoran Kiti *, Vasa Radoni , and Vesna Crnojevi -BenginFaculty of Technical Sciences,University of Novi Sad, Novi Sad, Serbia.Received  24 August 2010; Accepted  30 October 2012AbstractIn this paper novel miniature metamaterial-based soil moisture sensors are presented. The sensors are based on reso-nant-type metamaterials and employ split-ring resonators (SRR), spiral resonators and fractal SRRs to achieve small dimen-sions, high sensitivity, and compatibility with standard planar fabrication technologies. All these features make the proposedsensors suitable for deployment in agriculture for precise mapping of soil humidity.Keywords: soil moisture sensor, metamaterials, split-ring resonators, fractal curvesSongklanakarin J. Sci. Technol.34 (6), 689-693, Nov. - Dec. 20121. IntroductionOne of the factors that determine the optimal plantgrowth and therefore the agricultural production is the soilmoisture content. The yield can be increased for 40% whenpartial irrigation systems are used during the complete pro-duction  cycle  (Ludlow  et  al.,  1989).  Furthermore,  the  soilmoisture plays a very important role in determining the levelof acids and pollution in the soil.Different methods have been developed for the deter-mination  of  soil  moisture,  neutron  moderation,  electro-magnetic  methods,  and  methods  based  on  the  drying  ofthe sample, etc. (Liu et al., 2004; Muñoz-Carpena, 2009).Sampling and drying relies on the measurement of volume ormass of the sample before and after drying process. However,it requires laboratory equipment, it is very expensive and timeconsuming,  and  above  all  it  is  destructive.  The  neutronmoderation relies on exposing the soil to the emission of theneutron  beam  from  the  radioactive  source.  Although  thebeam  can  penetrate  into  the  ground  and  construct  a  3Dprofile of soil moisture, the beam radius is very small, totally30-40 cm. On the other hand, there are very strict limitationsconcerning  application  of  this  method  in  agriculturalproduction.Soil moisture can be estimated by measuring certainvalues, which depend on the soil moisture content. In a greatextend, electrical characteristics of the soil are determined byits moisture. This fact leads to the development of the elec-tromagnetic methods, which are based on the measurementof electrical properties, such as conductivity, resistivity, ordielectric  constant  of  the  soil.  Measurement  of  the  soilmoisture via the resistivity is a very simple technique basedon  application  of  two  electrodes.  Although  simple,  thismethod  has  a  big  disadvantage  of  the  resistivity  being  afunction of an ion concentration and not moisture contentexclusively. It can be concluded that the great benefits of theindirect methods are their nondestructive properties and fastresponse, but on the other hand they demand a very precisecalibration.Some  of  the  techniques  mentioned  use  relativelyexpensive equipment with high power consumption, othersare not accurate enough. These drawbacks make them poorcandidates for the mass production or application on a greatnumber of locations. Sensors for state-of-the-art applicationsin precision agriculture should have several characteristics.First, they should have small dimensions to allow measure-ments with no or minimal disturbance of the surroundingsoil. Also, since monitoring of soil moisture at certain depths(i.e.  in  the  root  zone)  is  of  great  interest  to  the  end-users,small dimensions of the sensor will facilitate its simple inser-tion into the ground. Secondly, power consumption of the*Corresponding author.Email address: gkitic@gmail.comhttp://www.sjst.psu.ac.thG. Kiti  et al. / Songklanakarin J. Sci. Technol. 34 (6), 689-693, 2012 690sensors should be low, to facilitate constant maintenance-free monitoring of soil moisture. Furthermore, the sensorsshould  be  precise,  low-cost,  reliable,  durable,  safe  to  use,easy  for  mass  production,  and  perform  nondestructivemeasurements.  The  type  of  sensor  that  meets  all  listeddemands is an electromagnetic sensor that operates on theprinciple of the permittivity measurement, due to the fact thatthe soil permittivity strongly depends on the volumetric soilmoisture content. Microstrip architecture is suitable for therealization  of  the  sensor  because  the  propagation  of  theelectromagnetic waves along a microstrip depends on themedium that surrounds it. Microstrip circuits are low-costand  easy  to  fabricate.  Typically,  they  operate  at  the  GHzfrequency range and, therefore, can be used to perform non-destructive measurements.Metamaterials present a new paradigm in microwaveengineering and have recently been used to design super-compact  high-selectivity  resonators,  filters,  and  otherpassive devices (Pendry, 1999; Baena 2004; Baena, 2005).Metamaterials are artificial structures composed of a numberof  unit  cells  with  sub-wavelength  dimensions.  They  in-herently posses small size and high-selectivity, and in thesame time are fully compatible with conventional fabricationtechnologies such as PCB (printed circuit board). This paperfocuses  on  microstrip  soil  moisture  sensors  which  usevarious  metamaterial  unit  cells,  split  ring  resonator  (SRR),multiple  SRR  with  six  concentric  rings  (M-SRR),  spiralresonator (SR), and Sierpinski split ring resonator (SS-SRR)(Crnojevic-Bengin  et  al.,  2007;  2008).  All  these  sensorsoperate at the principle of the resonant frequency shift. Theyare compared in terms of size and the influence of the soilmoisture on the resonant frequency shift of the given con-figuration.2. Sensor DesignThe  layout  of  the  proposed  metamaterial-basedmicrostirp soil moisture sensors is shown in Figure 1. Thesensor consists of a microstrip line loaded on both sides withtwo metamaterial unit cells. This structure behaves as thenotch, (Baena et al., 2005; Crnojevic Bengin et al., 2007)whose resonant frequency and stop-band characteristicsdepend on the size and type of the metamaterial unit cell.Compared to the case when only one unit cell is used, theproposed sensor topology has slightly lower resonant fre-quency  and  increased  insertion  losses  in  the  stop-band.It should be mentioned that the dimensions of all unit cellsused in this paper are smaller than g/16, which result in verycompact sensor dimensions equal to 0.07g x 0.176g (4.9 mmx 11.3 mm), where g is the guided wavelength. The sensor isplaced in an inhomogeneous medium, as shown in Figure 2,characterized by an effective dielectric constant, s. In generalthe resonant frequency of the sensor is influenced by theproperties of the used dielectric substrate, the medium thatsurrounds the sensor and sensor geometry. For a fixed di-electric thickness of the substrate, h, dielectric constant, d,and a chosen sensor geometry, the resonant frequency of thesensor solely depends on the effective dielectric constant.The effective dielectric constant is given by Equation 1, wheres, presents the unknown permittivity of the soil sample andw is the width of the microstrip line: whs d s deff12 112 2   , (1)since the soil moisture strongly influences the soil permitti-vity and therefore the effective permittivity, it can easily beobtained from the resonant frequency of the sensor.Metamaterial  unit  cells  used  for  the  sensor  designare shown in Figure 3: SRR, M-SRR with 6 rings, SR and SS-SRR. The overall dimensions of all unit cells are 4.9 mm x 4.9mm, while the ring gap, distance between the rings and thewidth  of  the  conductive  lines  are  all  equal  to  100  m.  Thecells are placed 100 m from the microstrip, to increase thecoupling. All structures are realized on 1.27 mm thick TaconicCER-10 substrate, characterized by the dielectric constantequal to 9.8 and loss tangent equal to 0.0035. The width ofthe 50  line on the used substrate is 1.4 mm.The performances of the proposed sensors are deter-mined for soil moisture varying from 2 to 20 %, as it cor-responds to the values encountered in real-life. For typicalagricultural soils, variation of soil moisture from 2 to 20%corresponds to the change of dielectric constant from 1 to20. Since the change of the soil humidity is also followed byFigure 1. Layout  of  the  metamaterial-based  soil  moisture  micro-strip sensors.Figure 2. Microstrip sensor placed in an inhomogeneous mediumwith unknown dielectric constant, s.691 G. Kiti  et al. / Songklanakarin J. Sci. Technol. 34 (6), 689-693, 2012the change of loss tangent of the medium, the latter is alsoconsidered. The exact values used in the analysis are listedin Table 1.The resonant frequency of all proposed sensors asthe function of the soil permittivity is shown in Figure 4 to 7,for various metamaterial unit cells used. All simulations wereperformed in EMSight, electromagnetic simulator in Micro-wave Office, ver 4.2. The losses in the conductor are modeledusing bulk conductivity for copper.It can be noticed that higher soil moisture causes anincrease  of  the  soil  dielectric  constant,  which  leads  to  ahigher effective permittivity. This results in the decrease ofthe resonant frequency of the sensor. However, at the sametime, the losses in the soil are also larger, which leads to adecreased  insertion  loss  and  a  reduced  selectivity  of  theFigure 3. Metamaterial unit cells that are used for the sensor reali-zation: (a) SRR, (b) SR, (c) M-SRR, and (d) SS-SRR.(a) (b)(c) (d)Table 1. Values of soil moisture and corresponding dielectricconstant (s) and loss tangents factor (tg ).Soil moisture [%] s tg 2 2.5 0.065 5 0.078 7.5 0.0911 10 0.1013 12.5 0.1115 15 0.1218 17.5 0.1420 20 0.15Figure 4. Resonant frequency of the SRR sensor as the function ofthe soil permittivity.Figure 5. Resonant frequency of the SR sensor as the function ofthe soil permittivity Figure 6. Resonant frequency of theM-SRR sensor as the function of the soil permittivityFigure 6. The resonant frequency change with the change of soildielectric constant for the case of M-SRR sensor.resonant peak, making it more difficult to detect it. In thiscase  the  use  of  detectors  with  higher  sensitivity  to  thechange of the signal strength is needed.G. Kiti  et al. / Songklanakarin J. Sci. Technol. 34 (6), 689-693, 2012 692The largest resonant frequency shift equal to 910 MHzoccurs in the case of SRR-based sensor, while the smallestshift equal to 600 MHz is exhibited by the SR-based sensor.Also,  among  all  proposed  geometries,  SRR  shows  thehighest insertion losses for high soil moisture content.All proposed sensors are compared in Figure 8, wherethe relative frequency shift in respect to the initial resonantfrequency obtained for the case when no soil is placed abovethe unit cells is shown as the function of the soil moisture.It can be noticed that the relative frequency shift is a linearfunction of the soil moisture, in the case the sensors basedon SRR, M-SRR and SS-SRR. However, the SR exhibitsnonlinearity for low soil moisture contents, which makes itunsuitable for the sensing applications.The SRR-based sensor exhibits the largest change ofthe resonant frequency in the case of varying soil moisture,i.e.  it  can  be  used  to  determine  the  soil  moisture  with  thehighest resolution of all proposed geometries. This sensoralso exhibits the highest insertion loss, which facilitates thedetection of the resonant peaks. Although SR shows thesmallest range of frequency shift and the lowest resonant fre-quency, it is also characterized with relatively small attenua-tion for high soil moisture values and nonlinear frequencyshift. SS-SRR has a linear change of the resonant frequency,but like SR, it also shows relatively small attenuation for highsoil moisture content, which further complicates the detec-tion process. M-SRR represents the solution, which showslinear  change  of  the  resonant  frequency  with  small  rangeof change and insertion losses that are similar to the SRRresonator.3. ConclusionIn this work metamaterial-based soil moisture sensors,which  operate  on  the  principle  of  the  resonant  frequencyshift, were presented. The influence of the soil moisture onthe performances of the proposed sensors was considered.The proposed sensors are characterized by extremely smallsize, linear change of the frequency with the change of thesoil moisture, and compatibility with standard low-cost planarfabrication  technologies.  The  drawback  of  some  of  theproposed  sensors  is  relatively  small  insertion  loss  at  theresonant frequency, especially in case of high soil moisturecontents. However, this can be overcome by a proper detectordesign.AcknowledgmentsThis  work  was  partially  supported  by  EUREKA!MetaSense project - Miniature Metamaterial-Based SensingDevices  for  Agricultural,  Environmental,  and  GeologicalApplications.ReferencesBaena, J., Bonache, J., Martin, F., Marqués, R., Falcone, F.,Loptigei, T., Laso, M., Garcia, J., Gil, I., Portillo, M. andSorrola, M. 2005. Equivalent-circuit models for split-ring resonators and complementary split-ring resona-tors coupled to planar transmission lines. Institute ofElectrical and Electronics Engineers (IEEE) Transac-tion on Microwave Theory and Techniques. 53, 1451-1461.Baena, J., Marqués, R., Medina, F. and Martel, J. 2004. Arti-ficial magnetic metamaterial design by using spiralresonators. Physical Review B: Condensed Matter andMaterials Physics. 69, 14402–14402.Crnojevic-Bengin,  V.,  Radonic,  V.  and  Jokanovic,  B.  2007.Left-handed microstrip lines with multiple complemen-tary split-ring and spiral resonators. Microwave andOptical Technology Letters. 49. 1391-1395.Crnojevic-Bengin, V., Radonic, V. and Jokanovic, B. 2008.Fractal Geometries of Split-Ring Resonators, Instituteof Electrical and Electronics Engineers (IEEE) Tran-sactions of Microwave Theory and Techniques. 56,2312-2321Figure 7. Resonant frequency of the SS-SRR sensor as the functionof the soil permittivity.Figure 8. Comparison of the proposed sensors in terms of relativeresonant frequency variation with soil moisture content.693 G. Kiti  et al. / Songklanakarin J. Sci. Technol. 34 (6), 689-693, 2012Liu, R., Chen, X., Li, J., Xing, H., Chen, J., Ekbote, A. andWen, B. 2004. Development of Soil Moisture Sensorfor Measuring Moisture. Research Report 0-4415-2.Department of Electrical and Computer Engineering.Houston, TX, U.S.A. August 2004.Ludlow, M.M., Sommer, K.J., Flower, D.J., Ferraris, R. and So,H.B. 1989. Influence of root signals resulting from soildehydration and high soil strength on the growth ofcrop plants. Current Topics in Plant Biochemistry andPhysiology. 8, 81-99.Muñoz-Carpena, R. 2009. Field Devices for Monitoring SoilWater Content. University of Florida – IFAS extension#BUL343, 1-17.Pendry, J.B., Holden, A.J., Robbins, D.J. and Stewart, W.J.1999. Magnetism from conductors and enhanced non-linear phenomena. Institute of Electrical and Electro-nics  Engineers  (IEEE)  Transaction  on  MicrowaveTheory and Techniques. 47, 2075-2084.

Soil moisture sensors based on metamaterials

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