ORAL - International Conference of Agricultural Engineering

More documents

Recommendations

Info

observed from Figure 2 that the measured permittivity has a large spread. This correspondswell with silage being a heterogeneous material. Another contributing factor could be that theopen end of the probe was observed during the experiment to be small considering thecoarseness of the material. At the current probe size single stands of grass have a widthcomparable to that of the open end which gives the individual strands too large an impact onthe measurement. A larger probe would make for a better averaging of the material which inturn should lead to more consistent measurements (Filali et al. 2008). Figure 2 also showsthat the material becomes less lossy as the frequency rises but the gain from the decreasedloss should be seen in relation to the shorter wavelength. As shorter wavelengths have lesspenetration power due to increased scattering increasing the frequency would not be asbeneficial as it could initially appear.The measured permittivity is high, which corresponds well with the high water contentrecorded in Table 1.4.3 Resonance measurementsTable 2 Resonance frequency, Q and S11 measurements for both antennas in both air andsilage.Antenna f res,air(MHz)f res,silage(MHz)Δf res(MHz)Q air Q silage S11 res,air(dB)S11 res,sil(dB)Helix 424 345 79 11.5 7 -4.0 -8.1Loop 434 429 5 336.7 167.4 -16.6 -0.5Table 2 presents the measurements of the resonance frequency of the two differentantennas. From the table it is clear that moving from air to silage does change the resonancefrequency of the antennas. This is because of the large change in permittivity of thesurrounding material. It can also be seen that both antennas are affected but to a vastlydifferent extent. While the loop antenna's resonance frequency drops 5 MHz the resonanceof the helix antenna drops 79 MHz, more than a factor of 15 compared to the loop antenna.This occurs because the near field of the loop antenna is primarily a magnetic field whereasthe near field of the helix antenna is an electric field (Balanis 2005). This means that it isprimarily the permability of the surrounding material that influences the loop antenna while forthe helix antenna it is the permittivity. Assuming that the permeability of silage and air arealmost equal is not unreasonable (Ponne & Bartels 1995), whereas the relative permittivity ofair is ~1, very different from the measured permittivity, Figure 2. S11 at resonance and the Qof the antennas also change when embedded in silage, see Table 2. The Q drops for bothantennas making them less selective but while the S11 changes +16 dB for the loopantenna, S11 for the helix antenna changes by -4.5 dB. This means that almost all power isreflected back from the loop antenna leaving very little to be transmitted. Coupled with thelow radiation resistance of loop antennas in general this means almost no power is beingtransmitted. The helix antenna on the other hand actually reflects less power at the inputmaking more power go into the antenna. Whether this extra power is radiated is not knownas it cannot be seen from the measurements if the losses also increase in the antenna.Table 2 shows that adjusting the antenna to be deployed in silage is a good way to increasetransmission distance. While the range will still be shorter due to the higher loss factorcompared to vacuum, the detuning that happens if the dielectric properties of silage are nottaken into account makes the effects much worse.To aid in choosing an antenna for deployment several factors should be taken into account.Given the characteristics of the material the antenna should be as insensitive to changes inthe nearby environment as possible. As the sensor node will need to be protected from thesilage and the machines used to handle it the antenna needs to be compact to fit within thelimited space of a protective shell. The directionality of the antenna is also important as in areal situation having to arrange the sensors to face a particular way is time consuming andimpractical. Because of this an omnidirectional antenna would be preferable. From the
measurements done it can be argued that broadband antennas are preferable overnarrowband in a heterogeneous environment. While the complex permittivity measurementsdone here give a hint in which range silage lies, changes in any of several parameters(moisture, temperature, density, conductivity) could well move a narrowband antenna outsideits operating band. This could cause increased packet loss or in the worst case the WSNcommunication could be made impossible. While not all the parameters are equally likely tochange silage is often stored outside or in open barns which means that a constanttemperature over the entire storage period is unlikely. A broadband antenna could easiercope with these changes and remain within its operating band. An alternative method toovercome this would be to place sensors very close together or transmit with more power,the former being expensive in hardware and the latter putting extra strain on the alreadylimited power resources of a sensor node.Future work should involve investigating the influence of temperature on the permittivityduring the entire storage period of a silage stack. Furthermore the measurements of thecomplex permittivity should be redone with a probe capable of sampling a large volume ofsilage.5 ConclusionIn this paper the permittivity of silage was examined. The complex permittivity was measuredover a frequency span from 400 MHz to 4 GHz using an open-ended coaxial probe. Resultswere successfully obtained, but large variations suggested that a larger prober should beused for more stable results. Furthermore the effect of silage on helix and loop antennas wasinvestigated. Detuning was present for both antennas, but the helix antenna suffered a largershift in resonance frequency than the loop antenna. This effect was attributed to thedifference in nature between the two antennas, the helix being an electric antenna and theloop being a magnetic antenna. The change in Q was also measured and here the helixantenna preformed significantly better than the loop antenna.For WSN’s designed to be deployed in silage it is suggested that the antennas used arecompact, broadband and that the high permittivity of the biomass is taken into account, asthis can greatly increase the achievable communication range. Of the antennas tested thehelix antenna is recommended.Athey, T.W. & Stuchly, M.A., 1982. Measurement of radio frequency permittivity of biologicaltissues with an open-ended coaxial line: Part I. IEEE T. Microw. Theory, 82(1), p.82-86.Balanis, C., 2005. Antenna theory : analysis and design 3rd ed., John Wiley & Sons.Brady, M.M., Symons, S.A. & Stuchly, S.S., 1981. Dielectric behavior of selected animaltissues in vitro at frequencies from 2 to 4 GHz. IEEE Trans. Bio-med. Eng., 28(3),p.305-7.Filali, B., Boone, F. & Rhazi, J., 2008. Design and calibration of a large open-ended coaxialprobe for the measurement of the dielectric properties of concrete. IEEE T. Microw.Theory, 56(10), p.2322-2328.Ghannouchi, F.M. & Bosisio, R.G., 1989. Measurement of microwave permittivity using a sixportreflectometer with an open-ended coaxial line. IEEE T. Instrum. Meas., 38(2),p.505-508.Green, O. et al., 2009. Monitoring and modeling temperature variations inside silage stacksusing novel wireless sensor networks. Comput. Electron. Agr., 69(2), p.149-157.
Page 1 and 2: ORALSPC-06: APPLI-CATIONSOF SMARTSS
Page 3: Manuscript Preparation Guidelines f
Page 8 and 9: Green, O., 2010. Monitoring of stor
Page 11 and 12: FIGURE2: Example of the recipe card
Page 13 and 14: The information acquired bythe chil
Page 15 and 16: passive, 1 MB in active tags) such
Page 17 and 18: After that were transported to the
Page 19 and 20: seconds). The largest response time
Page 21 and 22: And finally, we will strengthen the
Page 23 and 24: 5. Kick-off meeting of researchersI
Page 25 and 26: FIGURE 1: Field Server and Mulch an
Page 27 and 28: Additionally, modification of the s
Page 29 and 30: utilization were examined based on
Page 31 and 32: 2. Image Monitoring2.1. CameraWith
Page 33 and 34: image data. By using these image an
Page 35: the pheromone trap to attract targe

ORAL - International Conference of Agricultural Engineering

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?