Microwave Engineering Europe - June 2008 - (Page 16)

16 ANTENNA DESIGN Designing and simulating a wireless LAN antenna Electromagnetic and EM-circuit co-simulation of a 2.4-GHz polarization diversity printed antenna with switching network By How-Siang Yap and Bart Van-Hecke, Agilent EEsof EDA W LAN antennas can be fabricated on low-cost PCB substrates with good receiver performance by employing polarization diversity. This article describes how a pair of printed dipoles operating at 2.4 GHz with orthogonal polarization can be designed and simulated with the latest 3D electromagnetic (EM) simulation tools to predict surface currents and associated farﬁeld radiation patterns. The article also addresses the use of EM-circuit co-simulation to incorporate the effects of baseband circuit elements responsible for switching the antenna polarization. This enables antenna excitation to be directly fed from linear or non-linear circuit simulation without the need for manual data transfer. signal, we can make use of this polarization diversity to reduce the effects of multi-path reﬂections and interference on WLAN operation. For a detailed design and analysis of this antenna see Reference 1. In this article we will show how the use of electromagnetic (EM) simulation can provide quick insight into antenna behavior; and how EM-circuit co-simulation can be used to analyze the effect of the switching circuitry on antenna performance. EM Simulation for gaining fast insight into antenna design The geometry and dimensions of the dipole is shown in Figure 2. Figure 3: Effect of FR4 PCB dielectric constant variation on antenna resonant frequency. Introduction Consumer wireless applications require antennas to be incorporated unobtrusively into the wireless product and yet have to deliver good performance at low cost. Figure 1 shows an example of how this can be implemented for a 2.4 GHz WLAN application by printing 2 orthogonal dipole antennas on a FR4 PCB board. When operated with the PCB board standing upright, the vertical or horizontal dipoles will preferentially transmit and receive vertically or horizontally polarized signals respectively. By building circuitry to detect and switch in the antenna with the stronger Figure 2: 3D geometry and dimensions of printed 2.4-GHz dipole. Figure 1: Polarization diversity with two printed dipoles and switching network on a PCB. The antenna structure was simulated using the Agilent Momentum planar 3-D EM simulator. The results achieved matched accurately the published data in Reference 1 and were obtained within one minute using a typical laptop PC comprising an HP xw4400 Intel Dual Core 6600 2.4 GHz Win XP 64bit with 2 GB of RAM. Because of this fast simulation, we can obtain rapid insights into the antenna behavior with respect to variations in geometrical or material parameters. Figure 3 shows the impact of FR4 dielectric constant variation between 4.2 and 5.0 on the dipole resonance frequency. Here we note that a higher dielectric constant results in a lowering of the resonance frequency, which is to be expected because the dipole is electrically larger relative to the reduced wavelength in the higher dielectric constant substrate. Such considerations are especially important when designing for low-cost manufacturing where such variations often exist. Additional insights can be obtained by viewing the impact of geometry changes on antenna surface current ﬂow as shown in Figure 4. The surface current plot is useful when troubleshooting the cause of mismatches or undesired couplings because the current density is mapped to visible colors and can be animated by sweeping its phase over 360 degrees. We can now see and correct where currents are being induced in neighboring structures or trapped in unintended resonant geometries. This is surgically more precise and efﬁcient Figure 4: Surface current animation on the printed dipole is useful for identifying and correcting the locations where undesired coupling to neighbouring structures, reﬂections or resonance occur. Microwave Engineering Europe ● June 2008 ● www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - June 2008 Contents Comment News Cover Feature Designing and Simulating a Wireless LAN Antenna 60GHz: Achieving the Ultimate Wireless Dream New Radar Developments Include HFETs to Challenge DMOS/LDMOS and a 77-GHz CMOS PA for Automotive Applications Testing Raises Concerns Over 802.11-Based High-Speed Bluetooth IP2 & IP3 Design Considerations with Direct Conversion I/Q Demodulator Receiver Products Calendar

Microwave Engineering Europe - June 2008

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