A Complimentary Self-Structuring Antenna for use in a Vehicle - PDF document

A Complimentary Self-Structuring Antenna for use in a Vehicle Environment J.E. Ross* 1 , E.J. Rothwell 2 , and S. Preschutti 1 1 Preschutti and Associates, 204 East Calder Way, Suite 401, State College, PA 16801, e-mail: stan@preschutti.com 2 Department of Electrical and Computer Engineering, Michigan State University, E. Lansing, MI 48824, e-mail: rothwell@egr.msu.edu Introduction A self-structuring antenna (SSA) is capable of changing its structure, and thus its performance as an antenna, in response to changes in its environment [1]. A microprocessor is used to control the electrical connections between components of a skeletal antenna template using information from a sensor to optimize a desired quantity, such as received signal strength or VSWR. Because the number of possible configurations is large (2 N for N switches), efficient algorithms such as genetic algorithms or simulated annealing are used to find acceptable switch states in real time. Until recently the template consisted of wire segments interconnected by switches. This template is useful for automotive applications where the template is placed in the rear window of the automobile [2] since a vertically polarized component to the antenna pattern is present. However, this template cannot be placed in the roof of an automobile since no vertical field is produced. To allow for such an application we propose the use of complimentary antenna elements, such as slots, in the antenna template. We investigate the behavior of such a complimentary SSA (CSSA) both in free space and in an automotive environment. Modeling Tools Performance of the CSSA is determined through numerical modeling using the program GA-NEC (www.johnross.com). This is a general-purpose optimizer for the Numerical Electromagnetics Code (NEC) based on a genetic algorithm (GA). GA-NEC encodes any parameter in a NEC input file as an optimization variable. The GA fitness function is constructed using any combination of NEC output variables, such as impedance, gain, or efficiency, and may be computed based on minimization, maximization, or comparison to a desired response. GA parameters such as mutation rate and cross-over probability may be specified by the user. CSSA Template The CSSA template consists of a number of slots cut in a finite-sized ground plane of dimension 1 by 1.4 m, as shown in Figure 1. The size of the ground plane is chosen to fit within the roof rack area of a 1995 Chevrolet Blazer. The ground plane is modeled as a wire mesh, and the slots are formed by removing mesh segments. The feed point, shown as the segment at the center, is modeled using a standard NEC voltage generator. The other segments lying across the slots represent switches that are modeled as lumped resistances with 0.1 Ω for the ON state and 100k Ω for the OFF state. This model has 2260 wire elements and 41 switches, and thus 2 41 =2.2 trillion possible configurations (some redundant). No attempt was made to simulate DC power and switch control lines.

Results for Template in Free Space GA-NEC was configured to search for switch configurations that would yield minimum VSWR for a 50 Ω feed system. Using a population size of 20, the GA was allowed to run until a good VSWR was achieved. If a good VSWR was not reached, the run was terminated after 100 generations. The resulting plot of VSWR versus frequency is shown in Figure 2. It is seen that the CSSA is capable of a wide tunable bandwidth with a VSWR of less than 2:1 across the band 40 MHz to 1000 MHz. This performance is consistent with the theoretical and measured results seen previously for the SSA [3]. The VSWR below 40 MHz was poor as would be expected for an electrically small structure. Interaction with structures such as DC power lines, control lines and a vehicle body will tend to extend the lowest usable frequencies below that observed in the free-space analysis. The upper frequency limit of 1000 MHz was set by the grid size employed in the NEC model. Increasing frequency above this limit would require re-gridding the model and modification of the template to include additional detail in the region near the feed point. Results for Template above a Vehicle A 1995 Chevrolet Blazer was meshed with sufficient density to allow analysis up to 300 MHz. The CSSA template was placed ~76 mm above the top surface of the vehicle, as shown in Figure 3. There are a total of 10,000 elements in the NEC analysis, severely restricting the number of runs allowed in the GA optimization. Even using the numerical Green’s function option in NEC to compute and store the interaction matrix for the vehicle, a GA run with 20 generations at a population size of 20 took about a week of computer time per frequency point. Nevertheless, minimization of the VSWR was undertaken at several frequencies in the band 30-300 MHz. The minimum VSWR found was 2.1 at 30 MHz, 1.8 at 50 and 100 MHz, 1.2 at 200 MHz and 1.1 at 300 MHz. That these numbers are higher than those found for the CSSA in free space can be attributed to the smaller number of NEC evaluations performed for the CSSA mounted on the vehicle. Figure 3 shows the currents found on the antenna and vehicle at 100 MHz. A good deal of coupling to the roof panel and the window pillars is seen. Figure 4 shows the vertical gain pattern of the CSSA at 50 MHz. The two lobes in the horizontal plane are off the front and back of the vehicle. A greater amount of lobing is found at higher frequencies Conclusions We have demonstrated the feasibility of a complimentary SSA that uses slots as the radiating elements. Good values of VSWR were found over a 25:1 frequency band for the CSSA in free space and over a 10:1 frequency band for the CSSA mounted on a vehicle. References [1] C.M. Coleman, E.J. Rothwell, J.E. Ross, and L.L. Nagy, “Self-Structuring Antennas,” IEEE Antennas and Propagation Magazine, vol. 44, no. 3, pp.11-22, June 2002. [2] B.T. Perry, E.J. Rothwell, L.C. Kempel, J.E. Ross, and L.L. Nagy, “Simulation of a FM Band Self-Structuring Antenna in an Automobile Environment,” IEEE AP-S International Symposium and URSI Radio Science Meeting, Columbus, Ohio, June 23- 26, 2003. [3] C. Coleman, B. Perry, E. Rothwell, L. Kempel, J.E. Ross, and L. Nagy, “A Study of Simple Self-Structuring Antenna Templates,” IEEE AP-S International Symposium and URSI Radio Science Meeting, San Antonio, TX, June 16-21, 2002.

Figure 1. Mesh model of CSSA template. VSWR vs Frequency for CSSA in Free Space Environment 10 9 8 7 6 5 VSWR (50 Ohms) 4 3 2 1 0 20 40 50 100 150 200 300 400 500 600 700 800 1000 Figure 2. VSWR versus frequency for a CSSA template in free space.

Figure 3. Vehicle mesh with CSSA mounted on roof. Current shown at 100 MHz. Figure 4. Vertical gain pattern at 50 MHz of CSSA mounted on roof of vehicle.

A Complementary Self-Structuring Antenna for Use in a Vehicle Environment E.J. Rothwell, Michigan State University J.E. Ross, John Ross & Associates, LLC S. Preschutti, Preschutti & Associates, Inc. APS Session 85 Evolutionary Optimization Techniques in Applied Electromagnetics Wednesday June 23, 10:40 am June 23, 2004 2004 IEEE AP-S/URSI Symposium 1

Overview � SBIR Program � Overview of Self-Structuring Antenna (SSA) � Modeling Software � Complementary Self-Structuring Antenna (CSSA) � CSSA Model in Free Space � CSSA Model on Vehicle � Conclusions June 23, 2004 2 A Complementary Self-Structuring Antenna for Use in a Vehicle Environment

Navy SBIR NO3-097 “ Ultra-Wideband High-Efficiency Low-Profile Antennas” Strategic Objective “Apply the SSA concept to provide a low-profile wideband mobile antenna suitable for a wide range of military, civilian and foreign communications equipment.” � Design Goals Reduce visual � Frequency range 20 MHz to 4 GHz signature! � Power levels up to 200 Watts � Emphasize replacement for vertical antennas on vehicles � Emphasize omni-directional patterns in horizontal plane � Status � Phase I – Funded July – Dec 2003 � Phase I Option / Phase II – Funding pending June 23, 2004 3 A Complementary Self-Structuring Antenna for Use in a Vehicle Environment

SSA Introduction The Self-Structuring Antenna (SSA) is a new class of adaptive antenna that changes its electrical shape in response to the environment by controlling electrical connections between the components of a skeletal “template.” June 23, 2004 4 A Complementary Self-Structuring Antenna for Use in a Vehicle Environment

SSA Block Diagram SSA automatically adjustments electrical shape to maintain optimum signal quality Array of wires or . patches interconnected SELF- with electronically m control lines . STRUCTURING . controlled switches . ANTENNA TEMPLATE antenna feedback feed control line SENSOR MICROPROCESSOR Receiver/Transmitter with feedback signal for VSWR, S-Meter, BER, etc Uses smart (evolutionary) algorithms to select switch positions which optimize feedback signal June 23, 2004 5 A Complementary Self-Structuring Antenna for Use in a Vehicle Environment