Textile Antenna for the Multi-sensor
Transkript
Textile Antenna for the Multi-sensor
Textile Antenna for the Multi-sensor (Impulse GPR&EMI) Subsurface Detection System A. Oral Salman Emrullah Bicak Mehmet Sezgin TUBITAK Marmara Research Center Information Tech. Inst., Sensor Tech. Group, P.O. 21, 41470, Gebze, Kocaeli, Turkey oral.salman@bte.mam.gov.tr TUBITAK Marmara Research Center Information Tech. Inst., Sensor Tech. Group, P.O. 21, 41470, Gebze, Kocaeli, Turkey emrullah.bicak@bte.mam.gov.tr TUBITAK Marmara Research Center Information Tech. Inst., Sensor Tech. Group, P.O. 21, 41470, Gebze, Kocaeli, Turkey mehmet.sezgin@bte.mam.gov.tr Abstract— The performance of a planar elliptical dipole textile antenna for the multi-sensor (impulse GPR&EMI) subsurface detection system is investigated. This is a new application of textile antenna. The textile material of antenna does not create an EMI clutter comparing solid Cu material. The antenna characteristics of the proposed antenna in the multi-sensor head are measured and compared with the simulated ones. A buried target into soil, is successfully detected by the multi-sensor system with the usage of the textile antenna in the GPR sensor. Keywords-component; Textile antennas; subsurface detection; multi-sensor systems; impulse GPR; impulse EMI. I. INTRODUCTION Textile antennas have recently received a great interest in the area of electronics, computer and biomedical engineering. There are numerous studies in literature, which have been published recently, about textile antenna designs and applications. Some of the important ones are textile antennas for UWB WBAN [1], Bluetooth [2] and ISM band [3, 4] applications, dual-band [5 - 6], dual-polarized [7] and aperture-coupled [8] textile antennas. Aerospace applications of textiles antennas and passive microwave textile components are announced in [9, 10]. Because textile antennas have a flexible structure, the effects of antenna bending [11] and crumpling [12, 13] on the antenna performance are also investigated. In on-body applications of textile antennas, the proximity effects of a human body are important and investigated in [14, 15]. Because the material of a textile antenna is fabric, the effect of moisture [16] is an important parameter. In textile antenna design, to precisely determine electromagnetic parameters of a textile material is crucial. Several techniques are presented for electrical characterization of conductive textiles [17, 18] and electromagnetic characterization of non-conductive [19] textiles. In this study, we propose a new application of the textile antennas by using of textile antenna in a multi-sensor subsurface detection system [20, 21], which is composed of a impulse GPR and a impulse EMI sensor. In such a system, dielectric contents of buried objects are detected only by GPR sensor. The metallic contents can be detected by both GPR and EMI sensors. The conductive textile material is used as the antenna patch material in this study. Textile material does not to create an EMI clutter for the EMI sensor. However, solid Cu material does. It is the advantage of textile material to the solid Cu material. A new style of antenna feeding is also proposed by this study. The design tips of creating textile antenna, the measured and simulated antenna characteristics, and the performance of the antenna in the impulse multi-sensor subsurface detection system are given in the next chapters of the paper. II. PLANAR ELLIPTICAL DIPOLE ANTENNA FOR THE IMPULSE MULTI-SENSOR SUBSURFACE DETECTION SYSTEM A. Textile GPR Antenna In the market, there are number of conductive textiles sold by several companies. We chose conductive polyester woven taffeta manufactured by Tecknit Europe Ltd. as the conductive textile material to make antenna patches and cavity walls of the multi-sensor head. In this product, the polyester fibers are plated with Ni + Cu + Ni. Because the given sheet resistance1 by the manufacturer is typically 0.05 sq and the sheet thickness is t 0.1 mm, the conductivity of the textile material is calculated as 2 10 S/m (300 times less than Cu’s conductivity). The GPR sensor requires an antenna to send a short pulse signal to target located in some environments such as soil, wall and ice etc. An elliptical planar dipole antenna is chosen as the antenna type for the GPR sensor. This antenna is a linearly polarized antenna [23, 24]. The planar structure of the antenna is important for this application because the antenna is required to have a negligible height in the multi-sensor head (Fig. 1). The GPR textile antenna consists of a receiver and a 5 1 The sheet resistance is a measure of electrical resistivity of thin materials that have a uniform thickness [22]: Rs 1 / t , where σ is conductivity (in S/m) and t is sheet thickness (in m). transmitter antenna, which are completely identical. The distance between Tx and Rx antenna is 145 mm, which is measured from the centers of two antennas. The coils of the EMI sensor are embedded into the frame of the multi-sensor head. The details of the EMI sensor are not given here because it is out of scope of this article. A detailed sketch of one of the Tx / Rx identical textile GPR antennas. All dimensions in the figure are given in mm all dimensions. The elliptical patches of the antennas are conductive textiles whose properties were explained in the previous section. The dielectric substrate of the antenna is an FR-4 ( r 4.9 , tan 0.025 ) with 1.6 mm thickness. The dimensions of each patch are also given in the figure. The axial ratio, which is the ratio of major to minor axes of ellipse, is chosen as 1.5. This value is the optimum value, which was found for the best antenna matching and gain in [23, 24] from the measurements. The conductive textile patches are attached to the substrate with a two sided thin plastic adhesive tape. The case of the multi-sensor head is plastic. Each textile antenna is placed on a cavity. The cavity is constructed from the same conductive textile material. In application, the conductive textile material is fixed using two sided thin plastic adhesive tape to the inner wall of the multi-sensor head case. reduces the ringing effect appearing between the cavity walls and the antenna. For the feeding of textile antennas, textile material of the antenna is connected electrically using a bolt; a nut and a washer. The bolt and nut are attached to a strip line using an intermediate conductor strip. This strip line connects coaxial cable to the intermediate conductor strip. The advantage of this excitation type is the rigidity of the connection between the textile material and the coaxial cable. It is a more solid solution than soldering coaxial cable to textile material. Moreover, electrical connection is supplied from a wider surface using the washer. The bolts, nuts and washers used in antenna are made from inox, which is a specific stainless alloy, composed of carbon steel and chromium. B. The Measured and the Simulated Parameters of the Textile GPR Antenna In this section, the textile GPR antenna performance in the multi-sensor head is discussed. The measured parameters of the antenna are compared with the simulated results. The measured and simulated results of the same antenna in free space was given in [26] before. The measured and simulated 2 SWR curves of the textile antenna in the multi-sensor head (within the absorber layer and cavity) are given in Fig. 2. The lower frequency of the antenna is 645 MHz. According to the SWR curve of the antenna, the bandwidth ratio (the ratio of the upper to the lower operating ratio) is 7.55. 10 SWR 8 6 4 645 MHz 4.87 GHz 2 0 0 1 2 3 f (GHz) 4 5 6 Fig. 1. The detailed sketch of one of the Tx / Rx identical textile GPR antennas. All dimensions are given in mm. Fig. 2 The measured —– and simulated – – – SWR curves of the textile GPR antenna in the multi-sensor head (within the absorber layer and cavity). The conductive textile cavity is connected to the ground electrically by a standard electric cable. An absorber layer (Emerson&Cuming LS-24 [25]) exists between the cavity and the antenna. The cavity and the absorber in the multi-sensor head have several important duties. The first one is to decrease back lobe level of the antenna, which can cause false detections from the upper hemisphere of the multi-sensor head. They also decrease the coupling between the receiver and the transmitter antennas, which is a required case for a GPR system. Another important duty of them is to protect operator and the RF circuitry of the system from the high power of the short pulse. The absorber layer additionally For several frequencies, the E-plane (y-z plane) measured and simulated normalized Ez field patterns of the textile antenna in the multi-sensor head are shown in Fig. 3. The Hplane (x-y plane) measurements of the antenna in the multisensor head for two frequencies are shown in Fig. 4. One can see a perfect matching between the measured and the simulated patterns. Note that an elliptical planar antenna is a bi-directional antenna in free space. However, the back lobe of the antenna is decreased up to 20 dB by placing the antenna onto the 2 All simulations in the paper are performed using CST. absorber and the cavity. This result is expected and agrees with the duty of absorber and cavity, which were explained in the previous section. The measured and simulated gains of the textile GPR antenna in the multi-sensor head are given in Fig. 5. The maximum gain is measured as 4.32 dBi. The textile antenna in the head has moderate gain and broadband characteristics. In Fig. 6, for several frequencies, the simulated current distribution (magnitude and the direction) of the textile antenna in the multi-sensor head can be seen. One can realize from Fig. 6 that the current magnitude level is generally higher along the edges and feed points of the antenna. The current is homogeneously distributed and all parts of the conductive textile patches contribute the radiation for the frequencies up to 3.5 GHz. The current has also the highest level for this region. It corresponds to the high gain region (see the gain curves in Fig. 5 for a comparison). After this frequency, the current is not homogeneously distributed and low-leveled. Some parts of the patch do not contribute at the same level to the radiation comparingly to the other parts of the patch. Thus, the gain starts to decrease after this frequency. In the figure, one can see the current distribution in the parts between the patches and the rectangular border. This area is exactly the cavity walls, which is made from the same conductive textile. The absorbers are invisible in the figures. The current arrows can also be seen at the side walls of the cavity. The more frequency increases, the more current magnitude level on the cavity decreases. The reason of this situation can be explained by analyzing the operation of the absorber layer. The thickness of the absorber is not enough to absorb all electromagnetic energy at the low frequencies. Moreover, the recommended starting operation frequency of LS-24 absorber is given as 1 GHz by the manufacturer company [25]. The current magnitude level on the cavity is lower than the antenna patches due to the absorber layer between the antenna and the cavity. 6 4 Gain (dB) 2 0 -2 -4 -6 Fig. 3. The E-plane (y-z plane) measured ● and simulated —— normalized Ez field patterns of the textile GPR antenna in the multi-sensor head for several frequencies. -8 -10 0 1 2 3 4 5 6 f (GHz) Fig. 5. The measured ─●─ and simulated ─ ─ gains of the textile GPR antenna in the multi-sensor head. III. Fig. 4. The H-plane (x-y plane) measured ● and simulated —— normalized Ez field patterns of the textile GPR antenna in the multi-sensor head for two frequencies. THE TARGET DETECTION In this section, the target detection by the impulse multi-sensor subsurface detection system with the usage of the textile antenna is discussed. In order to show the effectiveness of the proposed antenna in the impulse multi-sensor subsurface detection system, the detection results of a representative target are depicted in Fig. 7. In this figure, the raw GPR data, the processed GPR images (buried object spatial signatures)3 and the EMI detection levels (in a. u.) of the targets are given. The target is a dielectric cylinder, which contains small metallic ingredient. Its diameter and height are 7 cm and 5 cm, respectively. The target is buried in hard and dry soil (approximately r 4 5 , 10 10 S/m) and its burial depth into soil is 10 cm. In Fig. 7, the horizontal axes of the GPR images represent the movement direction index and the vertical axes represent the depth index, both are in a. u. One can see from Fig. 7 that the GPR spatial signature of the buried object was made visible by the proposed textile antenna. Therefore the viability of the textile antenna for GPR applications was also proven experimentally. 3 4 0.75 GHz 1 GHz antenna. The textile material allows us to use the proposed antenna without creating an EMI clutter. The antenna characteristics were measured and compared with the simulated ones in the multi-sensor head. In the multi-sensor head, the antenna is inserted in a cavity, which is made also from the same conductive textile material due to the same reason (not an EMI clutter). There is an absorber layer between the antenna and the cavity. The cavity and the absorber reduce the back lobe level of the antenna; protect the operator and RF circuitry from the high power of short pulse. The absorber layer is additionally responsible to reduce ringing effect. After the measurements and simulations, it is observed that the bandwidth, gain and the operation frequency band of the textile antenna in the multisensor head were found. The gain of the antenna in the head was moderate and has a broadband characteristic. The feeding style of the antenna is also new. Inox bolt, nut and washer were used for the excitation of antenna. Such kind of a feeding is mechanically stronger than the soldering cables directly to the textile material of antenna and it has also a wider excitation surface. 1.5 GHz (a) 2.5 GHz 3.5 GHz 4 GHz Fig. 6. The simulated current distribution (magnitude and direction) of the textile antenna in multi-sensor head for several frequencies. IV. CONCLUSION A planar elliptical dipole textile antenna is proposed for a multi-sensor subsurface detection system composed of impulse EMI&GPR sensors. This is a new application of a textile 3 To obtain the buried object spatial signatures, the background subtraction method [20] is used. b) (c) Fig. 7. (a) The raw GPR image, (b) the processed GPR image, and the EMI detection signal level (in a.u.) of the representative target. The simulated current distributions of the antennas in the multi-sensor head gave us the following results. The current magnitude level is higher along the edges and at the feeding points of the antenna. The current is homogeneously distributed for the frequencies up to 3.5 GHz, which corresponds to the high gain region. The current distributions on the textile cavity show that the current magnitude level decreases with the increasing frequency due to the lower performance of the absorber layer at low frequencies. The performance of the textile antenna was also tested in field area and the selected target was detected successfully. The GPR images showed that the textile antenna can be used successfully in the impulse multi-sensor subsurface detection system. ACKNOWLEDGMENT A. Oral Salman thanks to Oktay Kaya, for his help in the antenna measurements, to Nihat Kavakli, Nedret Pelitci and Mehmet Caliskan, for their helps in the creating textile antennas and to Orhan Baykan for his help in the field measurements of the multi-sensor head. REFERENCES [1] M. 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