A reader came across the online version of the article “The World of the Near Field,” by former senior technical editor Tom Lecklider. Unfortunately, the images don’t appear, or the equations don’t render properly, so we are posting a scan of the original print version here.
“Distinguishing between near-field and far-field conditions is especially important in EMC testing,” Lecklider writes. “Published antenna factors and related calibration data generally are intended for use in far-field measurements.” He adds, facetiously, “When Scotty is beaming up, he’s working in the very far field.”
Lecklider explains that there is no unique distance beyond which the far field exists and within which near-field considerations apply.
However, he continues, “In the reactive near field, energy is stored in the electric and magnetic fields very close to the source but not radiated from them. Instead, energy is exchanged between the signal source and the fields. Should a device capable of coupling energy from the fields be nearby, a received signal will be developed by that device. This is the mechanism behind near-field radio frequency identification (RFID) tag coupling, for example. A resonant circuit in the tag is tuned to the frequency being transmitted by a nearby antenna and absorbs power from it.”
You can expect to see more on near- and far-field measurements in a cover story in our upcoming August print issue, which will be distributed at the EMC+SIPI Symposium July 30 through August 3 in Long Beach. Meanwhile, read the scanned version of the October 2005 article here:The World of the Near Field.
