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Article: The world of the near field: when Scotty is beaming up, he's working in the very far field.(EMC ANTENNA TEST)
- Article from:
- EE-Evaluation Engineering
- Article date:
- October 1, 2005
- Author:
CopyrightCOPYRIGHT 2005 Nelson Publishing. This material is published under license from the publisher through the Gale Group, Farmington Hills, Michigan. All inquiries regarding rights should be directed to the Gale Group. (Hide copyright information)
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Distinguishing between near field and far field conditions is especially important in EMC testing. Published antenna factors and related calibration data generally are intended for use in far field measurements.
The far field corresponds to an RF source-to-measurement antenna distance (r) great enough that energy radiates from the source only in a radial direction. The E and H fields are mutually perpendicular to that direction and each other, and their ratio is 377 [OMEGA], the impedance of free space.
[FIGURE 1 OMITTED]
There is no unique distance beyond which the far field exists and within which near field considerations apply. In general, ...