Investigation of the Influence of Panoramic Roof on ... - IEEE Xplore

The 8th European Conference on Antennas and Propagation (EuCAP 2014)

Investigation of the Influence of Panoramic Roof on Mobile Telephony Antennas Jerzy Kowalewski1 , Tobias Mahler1 , Lars Reichardt2 , Thomas Zwick1 1 Institut

für Hochfrequenztechnik und Elektronik (IHE),Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76127 Karlsruhe, Germany, (jerzy.kowalewski, tobias.mahler, thomas.zwick)@kit.edu 2 e.solutions GmbH, Lise-Meitner-Strasse 10, 89081 Ulm, Germany, [email protected]

Abstract—Due to the growing market demand, car manufacturers are steadily improving the aesthetic design of their cars and equipping them with new electronic systems. One of the aesthetic improvements is the panoramic roof. However, this changes the roof structure and thus affects car-roof antenna’s performance. Therefore the influence of different geometries and materials of panoramic roofs must be investigated to quantify its effect on communication link. On the basis of these results guidelines for antenna designer can then be made. This paper presents the results of such an investigation for the 800 MHz and 2.6 GHz LTE (Long Term Evolution) bands. Different panoramic roof variants were simulated and the results were verified with a measurement. To the authors’ knowledge this is first such investigation published for LTE bands so far.

Fig. 1.

Layout of inductively loaded monopole antenna

Index Terms—antenna, propagation, measurement, automotive.

I. I NTRODUCTION The number of antennas installed on the cars is increasing steadily as the result of the increasing number of new and future wireless services. However at the same time the possible attractive antenna placements on the car are decreasing. This is a result of aesthetic design guidelines and the introduction of other electric equipment, which might interfere with the antenna. Therefore the research on the influence of antenna placement was conducted recently in [1], [2]. Taking these results under consideration enables the design of maximal transinformation antennas, which is of importance for modern wireless services. Currently great emphasis is placed on the appearance of the vehicle. Therefore very often panoramic glass roofs are installed in the cars. Thus, the radiation pattern and other parameters of the car-roof antenna are affected by this element. Furthermore in many vehicles it is possible to open the glass roof. Therefore, a large air hole in the structure of the roof is formed. In both cases, if the antenna is placed on the roof, the shape of the ground plane is changed. Results presented in [3] show that at 6 GHz, the changes in the roof structure mentioned above can have large effects on the antenna directivity. In this work the effect on the antenna’s radiation pattern at 850 MHz and 2.6 GHz is investigated. II. A NTENNAS USED IN THE INVESTIGATION In the simulations at 850 MHz an inductively loaded monopole antenna is used (see Fig. 1). The antenna has been

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Fig. 2. Simulated and measured return loss of inductively loaded monopole antenna

designed especially for this investigation and is a modification of antenna presented by Sarabandi in [5]. The antenna presented by Sarabandi consists of a broadband conical monopole, which is capacitive. Arched lines, with the radius the same as the radius of conical monopole, connect the corners of the monopole to the ground plane and have inductive character [5]. By the use of such structure, the antenna has constant input impedance over a wide bandwidth. The radius of the monopole corresponds to 0.4 λ at the lowest frequency of operation. Such an antenna is therefore not compact at 850 MHz. However this antenna possesses a miniaturization potential. First miniaturization step is placing the structure on 1.27 mm thick Rogers 6010 substrate having high electric permittivity εr =10.2. The inductive arched lines are removed from the front side of the substrate, placed on the back side and connected

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Fig. 3.

Simulated radiation pattern of inductively loaded monopole antenna

by vias to the corners of conical monopole. The arched lines used in [5] lead to large horizontal dimensions of the antenna, therefore they are replaced by straight vertical lines. This change leads though to a limitation of the bandwidth (see Fig. 2). Nevertheless, the resulting bandwidth is still large enough for reliable measurement at 850 MHz. In the next step the angle Φ (see Fig. 1) is optimized. Due to this optimization the horizontal dimension is further reduced. In order to further reduce the operation frequency the inductance of the vertical lines should be increased. For this reason, the straight lines were replaced by meandered lines (light gray lines in Fig. 1). Hence, the antenna height is reduced to 54% of the monopoles height. The antenna is mounted perpendicular to the roof and connected to an coaxial feed, whereas the meandered lines are shorted to the ground. Furthermore this antenna has an omnidirectional radiation pattern in the azimuth plane (see Fig. 3) and is therefore suitable for the characterization of car-roof influence on radiation pattern. In the simulations at 2.6 GHz a simple monopole antenna is used. The same as the modified Sarabandi antenna, the monopole has a radiation pattern, which is omnidirectional in the azimuth plane. Both antennas used for the investigation are vertically polarized. III. S IMULATION SETUP AND RESULTS In the first step a 3D CAD model of a vehicle is imported to CST Microwave Studio [4]. Despite the progress in development of simulation tools, a simulation of the complete vehicle is still challenging. It requires a tremendous amount of computational resources and can take up to several days. For this reason, the other car body parts were removed and only the model of the car-roof (see Fig. 4) was simulated. This allows the substantial reduction of simulation time. Nevertheless a Transient Solver simulation with a coarse meshing still takes about two days. The roof railing is not considered in the model, since according to [3] it has minor effect on the antenna’s

Fig. 4.

CST model of the car-roof

directivity. The antennas are placed at 25 cm from the rear edge of the panoramic glass roof (see Fig. 4). This antenna position corresponds to the standard car-roof antenna placement used by car manufacturers. The same distance was also used in [3]. The number of mesh cells in the simulation at 2.6 GHz increases significantly, if compared to the simulation at 850 MHz. Thus the simulation time and effort increases. To solve this problem, a coarser mesh preference is used. The simulated structure does not have the small parts and thereby the coarser mesh preference will not affect the simulation’s accuracy. The simulation results at 850 MHz show that the radiation in the plane of the roof (xz-plane) in the driving direction for the case with glass roof is stronger than for the case if glass roof is replaced with air. However the difference is small, up to about 2 dB. This result contrasts with the simulation and measurement results from [3]. This is due to different frequency used in this work. The 25 cm distance corresponds to 5λ at 6 GHz and is greater than the far field distance. The wave impinges the glass surface with an angle 88◦ , penetrates the glass and is guided inside the glass [3]. Part of the energy is reflected and radiated at the glass-to-metal boundary. At the same time a part of the energy is absorbed and attenuated in the glass. At the 850 MHz frequency the distance between the antenna and the glass roof corresponds to 0.7λ. In this case, the glass roof is in the near field of the antenna. The relative permittivity of the glass is εr = 5 and is significantly higher than the permittivity of air. Therefore, the electric field in the direction of glass is stronger than in the opposite direction. The directivity is slightly tilted towards driving direction (see Fig. 5). The radiation in the plane of the roof in the driving direction slightly increases (see Fig. 5). It can be seen, that the radiation pattern is asymmetric. The angle between the roof and the direction of the main beam in the driving direction is about 10◦ bigger than in the opposite direction. The reason for this is the unequal length of the ground plane in the

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Fig. 7.

Car-roof model in anechoic chamber

Fig. 5. Simulated radiation pattern of the modified Sarabandi antenna mounted on car-roof at 850 MHz in elevation

from the main beam direction. Therefore it can be said that the panoramic glass roof acts as a dielectric slab antenna. As can be seen in Fig. 6, this effect does not occur for the case if glass roof is replaced with air. IV. M EASUREMENT SETUP AND RESULTS

Fig. 6. Simulated radiation pattern of the monopole antenna mounted on car-roof at 2.6 GHz in elevation

driving direction and in the opposite direction. The ground plane in the direction opposite to the driving direction is 50% longer than the ground plane in the driving direction, thus the angle between horizontal plane and the main radiation beam is smaller. The simulation for 2.6 GHz shows that the gain in the plane of the roof in the driving direction is higher for the case with glass roof than for the case without it (see Fig. 6). The local maximum of the gain occurs for the elevation angle of θ = 95◦ (see Fig. 6). The reason for this is propagation of the wave along the panoramic roof. Since, the glass has a higher relative permittivity than the air, the field concentrates in the glass sheet. A part of the field propagates undisturbed, while another part of the field is guided along and the glass and is diverted

In order to conduct the measurement and verify the simulation results a car-roof is needed. Unfortunately it is not possible to use the real car-roof for the investigation, therefore a model is built. The model is made of metallic plates mounted on a wooden frame. The frame is fabricated in the way that enables to imitate curvature and geometry of the car-roof. The model is equipped with a standard commercial sunroof (see Fig. 7). An advantage of using such sunroof instead of the normal glass is that its parameters are typical for the glass used in automotive industry. The length of the sunroof is smaller than the length of the panoramic glass roof used in [3] and in the simulation. Therefore an additional simulation using CST is conducted. The simulation shows that the length of the glass roof has only a slight influence on the results. Accordingly, the influence of the panoramic glass roof can be modeled with the sunroof. The antenna is mounted, as in the simulation, 25 cm away from the glass roof. Compared to a normal car-roof this model possesses a great advantage, all the parameters like the width of the roof, the length and the distance between the roof glass and the antenna can be chosen freely. The car-roof model including antennas is measured in the anechoic chamber of the Institute of High Frequency Technology and Electronics (IHE). For measurements of radiation pattern in the elevation the car-roof model is vertically mounted on the rotary platform and attached to the yellow tower (see Fig. 7). Due to the mounting on the rotary platform the phase center of the examined antenna, is slightly misaligned from the axis of rotation. The distance to the axis of rotation is for about 12 cm for the elevation measurement. The measurement error resulting by the misalignment is corrected by a simple

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Fig. 8. Measured radiation pattern of the modified Sarabandi antenna mounted on car-roof at 850 MHz in elevation

radiates stronger in lower right quarter sphere than in the case when glass is replaced with air. This can be explained by the fact that some energy which penetrates the glass is re-radiated in the direction of lower right quarter sphere. Finally radiation patterns of a monopole antenna in elevation are measured at 2.6 GHz. Figure 9 shows that in this case the gain in the roof level for the case with glass roof is bigger than for the case when glass roof is replaced with air. The difference of gain between the two cases, at 2.6 GHz, is significantly larger than at 850 MHz. The difference is equal to 3 dB and is though the same as in the simulation. Figure 9 shows that the main beam direction of the monopole in the driving direction in case with glass roof and with glass roof replaced with air is θ = 65◦ and θ = 55◦ respectively. The tilt of the main beam direction is usually caused by the extension of the ground plane. Assuming that the glass roof is made of a low-E glass (glass with low emission which is coated with metal oxide film), therefore it has a large reflection coefficient [7], [8]. Based on this assumption it can be said that the glass roof acts as an extension of the ground plane. Since the glass used in the simulation is not coated with metal oxide, this effect does not appear in the simulation (see Fig. 6). It is noteworthy that this effect did not occur for measurement at 850 MHz. This can be explained by phenomena discussed in [8]. It is shown that the reflection coefficient of such glass depends on the incident angle and the frequency and for certain incidence angles the wave will be totally reflected. This suggests that for 850 MHz angle conditions for the total reflection are not met and thus the electric field can penetrate the glass.

V. C ONCLUSION

Fig. 9. Measured radiation pattern of the monopole antenna mounted on car-roof at 2.6 GHz in elevation

function. The reference antenna used in the measurement is a wideband double-ridged horn antenna type 3115 from ETS Lindgren [6]. The antenna is mounted on a tripod and its height is adjusted to the vertical position of the tested antenna. The distance between the reference antenna and the tested antenna is between between 5.53 m and 5.77 m. The measurement results show that for case with the glass roof the radiation at 850 MHz in the plane of the roof is stronger by approximately 1.5 dB than for the case if the glass roof is replaced with air (see Fig. 8). It can be seen, that the gain for the elevation angles between θ = 0◦ and θ = 80◦ is lower for the case with the glass roof than for the case without it. This is not consistent with the simulation results (see Fig. 6). Figure 8 shows that for the case with the glass roof antenna

The influence of panoramic roof states on the car-roof antenna’s radiation pattern is presented in this work. The measurement results show that if the panoramic roof is made of glass coated with metal or metal oxide, there are no significant changes in the radiation pattern if compared to normal roof. The results of simulation of non-coated glass show, that due to the high electric permittivity the electric field is diverted towards panoramic roof. In this case panoramic roof can act as an dielectric slab radiator and increases the gain in driving direction. This effect might be used to optimize antenna’s radiation pattern and increase the channel’s transinformation. Furthermore, the investigation shows that in case of the open panoramic roof the gain in azimuth plane is always lower. The results of study conducted in this work, compared with results of [3], show that the influence of panoramic roof is strongly dependent on its geometry and material. Therefore such an investigation has to be done for each individual car model and should be implemented in antenna design process. Taking the results of such investigation under consideration enables the design of maximal transinformation antennas, which is of importance for modern wireless services.

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R EFERENCES [1] L. Reichardt, T. Fugen, T. Zwick. Influence of antennas placement on car to car communications channel. 3rd European Conference on Antennas and Propagation 2009. EuCAP 2009., pp.630,634, 23-27 March 2009 [2] D. Kornek, M. Schack, E. Slottke, O. Klemp, I. Rolfes, T. Kurner. Effects of Antenna Characteristics and Placements on a Vehicle-to-Vehicle Channel Scenario. 2010 IEEE International Conference on Communications Workshops (ICC), pp.1,5, 23-27 May 2010 [3] A. Kwoczek, Z. Raida, J. Lacik, M. Pokorny, J. Puskely, and P. Vagner. Influence of car panorama glass roofs on Car2Car communication. In Proceedings of 2011 IEEE Vehicular Networking Conference (VNC), November 2011 [4] CST - Computer Simulation Technology, CST Microwave Studio 2012 http://www.cst.com/. [5] N. Behdad and K. Sarabandi. A Compact Antenna for Ultrawide-Band Applications. IEEE Transactions on Antennas and Propagation, vol. 53, no. 7, pp. 2185-2192, July 2005 [6] ETS - Lindgren, Double-Ridged Horn Antenna Type 3115 Datasheet http: //www.ets-lindgren.com/pdf/3115.pdf. [7] N. Knauer, H. Doelecke, and P. OLeary. Outdoor-indoor wireless sensor communications in a modern building management system. In 4th Workshop on Wireless Sensor Networks, Nortel July 2nd 2008, July 2008. [8] M. Gustafsson, A. Karlsson, A. P. Pontes Rebelo, and B. Widenberg. Design of Frequency Selective Windows for Improved Indoor Outdoor Communication. IEEE Transactions on Antennas and Propagation, vol. 54, no. 6. pp. 18971900, June 2006.

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