Electromagnetic compatibility (EMC) and electromagnetic interference (EMI) analysis is crucial in the development of electronic and telecommunication devices for both domestic and international markets. The successful development and marketing of such devices is predicated on meeting both domestic and international regulations and requirements on both emission and immunity. In response to this national need, the Institute is involved in several research efforts in the areas of electromagnetic compatibility (EMC) and electromagnetic interference (EMI). In the past year our EMC/EMI efforts encompassed electromagnetic characterization of composite materials, absorbing materials, anechoic chambers, radiated emissions from printed circuit boards, rf interface for wireless communication systems, and standards activities.
Dedicated short-range communication (DSRC) systems have been proposed for automatic motor vehicle toll collection operations at locations across the United States in the 5850- to 5925-MHz band. Various high-power search and weather radars operate at or near this frequency band and are a source of potential interference. The Institute has performed a series of interference measurements to determine the electromagnetic compatibility of DSRC systems and high-power 5-GHz radars. From these measurements, the time (wait time) required for a successful transaction between a motor vehicle and a DSRC toll collection system in the presence of radar signals with various pulse characteristics and power levels was calculated. The table shows radar power levels that were found to yield unacceptable wait times for a typical radar signal.
Anechoic and semi-anechoic chambers provide an accurate and convenient environment for EMC/EMI testing and are important cost-effective tools for achieving EMC/EMI compliance. The Institute has developed finite-difference time-domain (FDTD) models for predicting the performance of these test facilities. The FDTD technique can be used to predict the performance of absorber-lined chambers for various testing scenarios. This allows for the validation of the performance of chambers with particular types of absorber prior to construction which provides a great cost effective tool for chamber manufacturers. Figure 1 shows typical results obtained from the FDTD model. Depicted here is the performance of a 3-meter semi-anechoic chamber with various absorber types.
| Peak Pulsed Interference Power Levels Resulting in Excessive Wait Times |
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| Vehicle Speed km/hr (mph) |
Max Allowable Wait Time s |
Interference Power Level Resulting in Excessive Wait Times pulse width = 1 ms duty cycle = 0.1% |
| 64.4 (40) |
0.89 | -68 dBm (0.5%) |
| 48.3 (30) |
1.19 | -58 dBm (3.3%) |
| 32.2 (20) |
1.79 | -58 dBm (0.4%) |
(Percentage of trials exceeding the maximum is shown in parentheses.)
In the installation of ferrite tile absorbers on anechoic chamber walls, gaps between individual tiles are inevitable. These gaps have a detrimental effect on the overall performance of the ferrite tile absorber, which in turn can have a substantial effect on the overall chamber performance. In fact, gaps as small as 0.3mm can cause 1.5dB changes in the chamber performance. Since only ±1dB is usually budgeted for chamber imperfections these variations can have a dramatic effect on the ability of the chamber to meet its design specifications. We have recently developed a model that accurately predicts the performance of ferrite tiles with gaps. For example, Figure 2 shows the reflection coefficient of a ferrite tile absorber with various gap sizes. Notice that as the gap size increases, the performance of the ferrite tile absorber decreases (i.e., the reflection coefficient increases).
Other EMC/EMI efforts at ITS this year have been related to the development of an effective material property model for advanced fiber composites and for various other electromagnetic absorbing materials. For example, an effective material property model was developed for hollow pyramids this year. The variations of the material properties obtained from this model as the wave propagates into the hollow absorber are depicted in Figure 3.
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| Figure 1: Predicted chamber performance using the FDTD technique. |
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| Figure 2: The effects of gaps in ferrite tiles on absorber performance. |
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| Figure 3. Effective material property as a function of x, where x is the depth into the absorber, for different hollow absorber parameters. |
R.A. Dalke, F.H. Sanders, and B.L. Bedford, "EMC measurement and analysis of C-band radars and dedicated short range communications systems," in Proc. IEEE International Symposium on EMC, Seattle, WA, Aug. 1999, pp. 974-979.
C.L. Holloway, J.R. Baker-Jarvis, R.T. Johnk, and R.G. Geyer, "Electromagnetic ferrite tile absorber," in The Wiley Encyclopedia of Electrical and Electronics Engineering, J.G. Webster, Ed., New York: John Wiley & Sons, Inc., 1999, vol. 6, pp. 429-440.
C.L. Holloway, M. Johansson, E.F. Kuester, R.T. Johnk, and D.R. Novotny, "A model for predicting the reflection coefficient for hollow pyramidal absorbers," in Proc. IEEE International Symposium on EMC," Seattle, WA, Aug. 1999, pp. 861-866.
C.L. Holloway, P. McKenna, and R.T. Johnk, "The effects of gaps in ferrite tiles on both absorber and chamber performance," in Proc. IEEE International Symposium on EMC, Seattle, WA, Aug. 1999, pp. 239-244.
For more information, contact: Dr. Christopher L. Holloway