Conformity Magazine - December 2007 - (Page 50) three axes. A field probe is a broadband instrument. If more than one frequency component exist, a field probe responds to all of them. This is in contrast to antennas connected to a spectrum analyzer, where the analyzer discriminates between frequencies. It is thus critical to ensure the purity of the signal in an immunity testing setup, especially the harmonic field generated by a power amplifier. Just as antenna factors are extremely important for emissions measurement, the calibrations of probe factors are vital for immunity tests. Calibration labs typically provide a frequency correction table for each probe. This corrects the reading for the specific frequencies. Another important factor is the linearity of a probe. This is a parameter that measures how faithful a probe measures at different field levels. Modern probes have internal adjustments for linearity, making sure probe readings are correct not only at the calibration field levels (e.g., 20 V/m), but at all field strength levels. Not all probe calibrations are equal. When performing a probe calibration, a simple frequency response calibration without checking the linearity is in most cases insufficient. Conclusion A broad range of topics have been discussed in this article, including the basic parameters of antennas, the common types of antennas used in EMC, their calibrations and applications in radiated emissions and immunity tests. Intentionally left out of this article are many detail theoretical discussions, equations and formulas found in other antenna references. This is not meant to trivialize these topics. The reader can refer to the references listed below for more in-depth explanations on many important antenna topics. Zhong Chen is a senior principal design engineer at ETS-Lindgren, and can be reached at Zhong.Chen@ets-lindgren.com. Vincente Rodriguez is the Antenna Product Manager at ETS-Lindgren, and can be reached at Vince.Rodriguez@ets-lindgren.com. References 1. C. A. Balanis, “Antenna Theory Analysis and Design”, Second Edition, John Wiley & Sons, Inc., New York, 1997. Z Chen and A Cook, “Low Uncertainty Broadband EMC Measurement Using Calculable Precision Biconical Antennas,” 2000 IEEE International Symposium on Electromagnetic Compatibility, Washington, DC, 2000. Zhong Chen, “Understanding the measurement uncertainties of the bicon/log hybrid antennas”, ITEM 1999. Z Chen and M Windler, “Systematic Errors in Normalized Site Attenuation Testing,” Compliance Engineering 17, no. 1 (2000): 38–48. Z Chen and MD Foegelle, “An Improved Method for Determining Normalized Site Attenuation Using Log Periodic Dipole Arrays,” , 2000 IEEE International Symposium on Electromagnetic Compatibility, Washington, DC, 2000. J.D. Kraus, “Antennas”, Second Edition, McGraw-Hill, 1988. 2. 3. 4. 5. 6. FAST Link www.conformity.com/1270 Figure 2 50 Conformity DeCember 2007 http://www.conformity.com/1270
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