In 1973, the U.S. Department of Defense conceived the Global Positioning System (GPS). Thirty years later, GPS receivers are everywhere—from the military’s use of the technology in Operation Iraqi Freedom, to its application in automobiles, aircraft, cellular phones and personal handheld units.
Positioned almost 11,000 miles above the earth’s surface, a GPS satellite transmits a signal in many directions. This primary signal usually bounces (reflects) off of various natural and manmade obstacles, such as buildings, trees and bodies of water, before reaching the receiving antenna. Any given radio may receive both the primary signal and one or more reflections of the same signal. This phenomenon is called multipath interference, which often has adverse effects on the primary signal, such as inducing an error in the calculated position.
Because multipath effects depend on the surrounding environment, they are difficult to quantify, and available signal-processing techniques don’t help in solving the problem completely under all conditions. One way to overcome multipath is to simply relocate a wireless device and its antenna to avoid obvious obstructions. But if relocation doesn’t work, using an innovative antenna with superior multipath-rejection capability is a possible solution.
Two professors and a student at the University of Calabria’s Dipartimento di Elettronica, Informatica e Sistemistica, in Rende, Italy, used HFSS™, Ansoft Corporation’s 3D electromagnetic finite-element simulation tool, in the design of a low-profile, lightweight, GPS patch antenna that is capable of reducing multipath error.
To simplify the design process, they used a simple analytical model as a starting point to roughly estimate the antenna resonant frequency and feed location. Then they optimized the design through extensive finite-element-model (FEM)-based simulations with HFSS, obtaining accurate simulations by manually refining the mesh for each geometrical element of the antenna. According to the January 2003 Microwaves & RF article* featuring the University of Calabria paper, “Due to the precision of the simulator and the accuracy of the fabrication process, it was possible to achieve a fairly precise design that provided predictably high performance.”
The final antenna design, based on a shorted annular patch (SAP) antenna geometry, turned out to be inexpensive and lightweight while offering an extended radiation pattern flexibility that can be used to optimize the multipath rejection performance. Performing both numerical and experimental tests, the University has verified the characteristics of the antenna; the simulated results were in excellent agreement with the experiments.
To read the Microwaves & RF article about the University of Calabria’s GPS antenna design, download a PDF version here, or visit MRF’s site at www.microwavesrf.com.
To learn more about HFSS, go to www.hfss.com; for information about Ansoft Corporation, please visit www.ansoft.com.
*L. Boccia, G. Amendola, G. Di Massa, “Design a High-Precision Antenna for GPS,” Microwaves & RF, January 2003, pp. 91-93.
Author: Tom Interval
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