Dielectric measurements of stone materials with a wideband microwave sweep

Pekka Eskelinen Department of Electronics

Aalto University Espoo, Finland

pekka.eskelinen@aalto.fi

Terhi Pellinen Department of Civil and Environmental Engineering

Aalto University Espoo, Finland

terhi.pellinen@aalto.fi

Pablo Olmos Martinez Department of Civil and Environmental Engineering

Aalto University Espoo, Finland

pablo.olmos.martinez@aalto.fi

Abstract—Ground penetrating radars (GPR) are increasingly used for the quality assessment of asphalt work on road construction and measurements are needed to focus only on surface layers of the road. Two different aggregate slabs and reference plastic sheet were measured with the vector network analyzer to investigate the depth

resolution issues of the classic electromagnetic field reflection problem in the case of chained dielectric layers. We found that tests of stone material are inadequate at 4 GHz sweep width, which equals a GPR pulse of around 300 ps. The widest sweep from 6.5 to 18 GHz, corresponding to about 100 ps pulses in time domain radar, seems to give both repeatable and reliable information and should be used.

Keywords; asphalt; Ground peneterating radar, permittivity, dilectric value, asphalt

I. INTRODUCTION. Ground penetrating radars (GPR) are increasingly used for the evaluation of the quality of asphalt work on road construction [1]. Initially developed to give good penetration down to several meters of soil, in order to be able to study the structural supporting layers and solid rock or concrete bridges, currently commercially available systems mainly use the radio frequencies of 1-2.5 GHz. Although pulsed [1], sweeping [2] and stepped frequency [3] designs exist, they all have an equivalent pulse width of around 1 ns. For the estimation of the pulse width we assumed that the wave travels only in air (worst case scenario). In our preliminary study focusing on through-the-sample propagation of radio frequency power, we have indicated that it would be advantageous to use real microwave bands in asphalt measurements due to the highly curved permittivity characteristics of stone materials below 2 GHz [4]. Therefore, our new attempt is focused at depth resolution issues and studies the classic electromagnetic field

reflection problem in the case of chained dielectric layers. Conventional use of the radar resolution equal to the speed of the wave divided by two times the bandwidth, could not be applied. Because the speed depends on the dielectric constant of each layer and on their combination, the speed of the wave is unknown.

II. TEST SET UP AND EQUIMPENT A vector network analyzer [5] (Wiltron 360) with a custom antenna setup was utilized and results were collected in the time domain mode. The equipment was used to measure the real part of the relative dielectric constant of two stone materials, grey granite and red granite, Table 1. A plastic sheet (POM, 22 mm thick) was selected as a suitable calibration reference; in addition the calibration was made with an aluminum alloy (AlMg3) sheet. Table 1. Materials Material Dimensions (mm) POM 300x300x22 Grey granite 300x300x11 Red granite 300x300x30 The illumination was perpendicular to the sample surface at a distance of 290 mm. Three analyzer sweep widths were tested in order to get a view of the effects of pulse width in field experiments performed with the GPR. Utilizing a special jig to maintain precisely identical sample location and alignment we first measured our test samples with three frequency sweep widths, namely 6.5 to 11.5 GHz, 6.5 to 14.5 GHz and finally 6.5 to 18 GHz. The first sweep approximates a good pulsed GPR having roughly 300-500 ps pulse width and the widest sweep that of an imaginary 100 ps device.

III. RESULTS. Fig 1 shows the responses obtained from the aluminum plate together with grey granite and POM plastic sample recordings for 6.5 to 11.5 GHz.

Figure 1. Comparison of reflection amplitude from conducting plate, 11 mm grey granite sheet and 22 mm POM sheet at 6.5-11.5 GHz. Keeping everything else fixed, we move on to widen the sweep to the maximum value possible with the special antenna. Results of the same materials as in Fig 1 are shown in Fig 2. The multipath component about 40 mm behind the first reflection now becomes clearly visible.

Figure 2. Comparison of reflection amplitude from conducting plate, 11 mm grey granite sheet and 22 mm POM sheet at 6.5-18 GHz. In Fig 3, we have one of the first multiple dielectric cases with red granite in front and a POM plastic sheet behind it (as seen from the antennas). For comparison, we have plotted the experiment without the background item.

Figure 3. Comparison of reflection amplitude from conducting sheet, 30 mm red granite sheet and red granite sheet with 22 mm POM sheet behind it. A further complication is shown in Fig 4 where the number of dielectric layers is increased to three. An 11 mm sheet of grey granite is placed behind the POM plate which in turn is behind the red granite slab. Interestingly, the impedance match gets worse compared with the case of POM only.

Figure 4. Comparison of reflection signatures from red granite sheet, red granite with 22 mm POM sheet behind and red granite with 22 mm POM and 11 mm grey granite sheets behind at 6.5-18 GHz. In order to get a good understanding about the importance of short enough pulses we finally show in Fig 5 a comparison of the triple dielectric setup with the widest and narrowest sweeps. The illustration should be self-evident: all unwanted multiples are completely masked by the extended “first” pulse,

easily mistaken to be a true representative of the boundary reflection only.

Figure 5. Comparison of red granite with 22 mm POM and 11 mm grey granite sheets behind at 6.5-11.5 GHz and 6.5-18 GHz. The network analyzer amplitude data indicates reflected power for the various test cases and materials. On the other hand, the dielectric constant εr is defined from the electric field components [5]. Therefore, assuming low losses, we can use (1) to convert the obtained results. 1 / 1 (1)

Here, Astone refers to measured reflection level from the test sample and Acond that obtained with the aluminum sheet at the same position. Table 2 summarizes test results. Table 2. Summary of results Material 6.5-11.5 GHz 6.5-14.5 GHz 6.5-18.0 GHz POM 3.45 2.37 2.50 POM with grey behind 3.12 2.97 2.95 POM with red behind - - 3.00 Grey granite 14.18 9.52 5.63 Grey with red behind - - 4.76 Grey, POM and red - - 4.47 Red granite - - 4.20 Red with grey behind - - 4.16

Red with POM behind - - 3.93 Red, POM and grey 11.85 - 4.04

IV. DISCUSSION AND CONCLUSSIONS As is evident from the elementary radio engineering principles, the widest sweep widths (6.5-18.0 GHz) gives the best resolution along the direction of the wave propagation, and we were thus able to separate the first reflection from the sample-air-surface from the disturbing multipath and edge effects. The test results of stone material are quite useless at 4 GHz sweep width, which equals a GPR pulse of around 300 ps. The widest sweep from 6.5 to 18 GHz, corresponding to about 100 ps pulses in time domain radar, seems to give both repeatable and reliable information when compared with our previous through-the-sample recordings at similar frequencies. As a by-product we obtained with differently sandwiched red granite samples a real-life proof of the classic transmission line theory which states that the apparent impedance (in this case permittivity) at a discontinuity depends not only on the parameters of the adjacent medium but also of those following it [6]. If only the first reflection was used, our tests would give for the same red granite sheet a permittivity from 3.93 to 4.20. In the real GPR work, it is not adequate to record the first reflection amplitude if information about the different soil layers is looked for. The only possibility is to use much shorter pulses or wider bandwidths than is available in current commercial radar devices.

REFERENCES

[1] Y.Chen, L. Lau and T.Scullion, Implementation of the Texas ground penetrating radar system, Research Report 1233-1, Texas transportation institute, The Texas A&M University System, College Station, Texas, USA, 1992.

[2] M. Zych, Measuring experiment of FMCW ground penetrating radar, Proc. International Radar Symposium IRS 2011, Leipzig, Germany, September 7-9, 2011, pp. 99-104.

[3] M. Pasternak et al, Stepped frequency continuous wave GPR unit for unexploded ordnance and improvised explosive device detection, Proc. International Radar Symposium IRS 2011, Leipzig, Germany, September 7-9, 2011, pp. 105-109.

[4] Eskelinen P. et al, Evaluation of local granite properties for radar applications, unpublished.

[5] Coombs, C. (ed), Electronic instrument handbook, 2nd ed. McGraw-Hill, New York, 1995, pp. 29.1-29.21.

[6] Ramo S. and Whinnery J, Fields and waves in modern radio, 2nd ed. John Wiley and Sons, Nwe York, USA, 1960, pp. 288-293. .