Conformity Magazine - August 2008 - (Page 34)

Figure 1 illustrates the vehicle used to perform the numerical simulations. The size of the car is 4.98mx1.98mx1.48m. The hidden antenna consists on a conformal wire placed on the metallic chassis of the vehicle (underneath), close to the back tire. The purpose of the conformal shape is to both preserve the aesthetic look of the vehicle and to better hide the device. The frequency band includes the radio system communication at AM (150 KHz-1.5MHz) and FM frequencies (88-108MHz), and the range of analysis extends up to 300MHz. The alternative site of the antenna overcomes problems due to heated windscreen and metal-coated solar reflection glass (which may negatively affect the performance of the antenna). At the same time, however, the radiation may be difficult due to the proximity of the metallic chassis, which can eventually shield the antenna and sensibly reduce its performance. How does the device work? Once the antenna is installed in a vehicle, the base system uses a small hidden transceiver to send a signal to the police department, which allows them to track and recover the vehicle. Just file a stolen vehicle report, and the antenna is automatically activated. The procedure can be described by the following steps: 1) installation; 2) notification of the police department; 3) activation through a unique signal; 4) location; and 5) vehicle recovery. Due to proprietary restrictions on presenting the complete design of the antenna, a few details are not reported in the present article. Instead, a very similar antenna model is studied (see Figure 2 for the geometric details and dimensions), with the purpose of showing the general workflow process, the modeling approach, the encountered issues, and the possible solutions. The simulations reported here have been carried out using a proprietary software package [6], which uses a finite integration technique (FIT) as the basis of the solver. The software is capable of importing raw mechanical CAD data for a vehicle, that is then manipulated to create an effective electromagnetic model for the body shape. In order to reduce intensive pre-processing of the vehicle CAD data (which may contain geometry translation errors) and easily mesh the sheet metal CAD model of the vehicle, FPBA [6] is used to mesh the vehicle body. Local mesh refinement and a multi-level, subgridding scheme (MSS) is then used in the proximity of the antenna element in order to provide a better resolution of the small details. In particular, MSS reduces the number of mesh cells by a factor of 10, from about 34M to about 3.2M, while still maintaining a good resolution for the antenna. A Gaussian pulse is used as input of the time domain calculation, which is ideal for S-parameters evaluation since its transformed frequency domain signal is also a Gaussian shape. 3 Conformity AUGUSt 2008 Two main advantages can be observed: 1) a limited bandwidth: this ensures that a mesh can be created where all stimulated frequencies can be properly sampled; and 2) no zero axis crossings: this allows calculation of the Sparameters over the frequency band of the Gaussian pulse. If there were zeros in the signal spectrum, the S-parameters could not be calculated at these frequencies. S-parameters and Z-parameters are reported in Figure 3. One important point to be mentioned is that the time domain simulation needs a relatively long duration in order to avoid the truncation error, which may generate inaccuracy of the FFT in the low frequency range. Looking at the return loss, we can see the very small value over the entire simulated frequency range and a value of only -0.007283 at 170MHz. In order to investigate the radiation mechanism, surface current on the vehicle for the frequency value of interest is plotted in Figure 4. The highest currents are being induced around the top portion of the chassis of the back tire. Figure 1: Three dimensional (3D) view of the vehicle used for the numerical simulation Figure 2: Wire antenna model and geometric details

Table of Contents Feed for the Digital Edition of Conformity Magazine - August 2008

Conformity Magazine - August 2008 Contents Editor’s Note FCC Seeks Comment on Reauction of 700 MHz D Block Commission Cracks Down (Again!) on Junk Fax Marketer Canada Levies Fees for Public Safety Radio Spectrum FDA Issues Guidance on Medical Device Tracking Greenpeace Challenges Video Game Manufacturers The IEEE EMC Society: A Proud History of Accomplishments ESD Open Forum An Update on Changes to the Automotive ESD Standard, ISO 10605 Modeling of Radiated Electromagnetic Disturbances in Automotive Applications Design Issues in Automotive Radio Frequency Systems IEEE EMC Symposium IEEE 2008 Show Preview Exhibitors Buyer’s Guide Product News Most Eco-Friendly Companies Updated Standards List For the EU’s Pressure Equipment Directive Commission Issues Updated Standards List For Directive on Gas-Fired Appliances HHS Releases Fact Sheet on U.S. Imports from China Customs Agents Seize Holiday Lights IEEE PSES Symposium Set for October CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Product Reviews Advertisers

Conformity Magazine - August 2008

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