Well, This has been a good learning experience for me. I didn't know that there was more than the E/H ratio to be concerned with. Now I know.
Dave Cuthbert From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Friday, April 30, 2004 8:03 AM To: [email protected] Subject: Re: Near/far field definition I like this definition which includes both induction effects and the fully formed pattern effects because it is practical. The concern about "near field" for EMC measurements almost always has to do with the question of field strength vs. distance, with the distance being smaller than the standard. The concept of the pattern not being fully formed intuitively conveys one of the problems at hand. Don Borowski Schweitzer Engineering Labs Pullman, WA drcuthbert@micron .com Sent by: To owner-emc-pstc@ma <[email protected]> jordomo.ieee.org cc Subject 04/29/04 04:34 PM Near/far field definition Please respond to drcuthbert@micron .com It seems there are two definitions of the near/far field transition. The definition "an area where the ultimate pattern is not fully formed" appears to apply to my simulation out to a few meters, which is over 100 wavelengths. But the region where the induction fields exist (the definition I usually use) extends less than one wavelength from the antenna. Beyond this the E/H ratio is 377 although the pattern is not "fully formed". From less than one wavelength to over 100 wavelengths we are in the Fresnel zone, with the antenna in question. Cut and pasted from the website of Tom Rauch, W8JI: Nearfield The nearfield area is an area where the ultimate pattern is not fully formed, and where induction fields (from charge distribution and charge movement) have a noticeable effect. It is possible, with large arrays of small elements, to be out of the induction field region but still find the area called the "nearfield" area or zone. Let's consider individual groups of elements as "cells", and the array a combination of small directional cells occupying a very large physical area. Each cell has formed a radiation field. Depending on the size and type of radiator in each cell, the induction fields that charge distribution plays a role in may be attenuated so much as to be negligible, yet the radiation pattern of the entire array may not be totally formed. This is the case with my phased Beverages and phased verticals, where the individual antennas making up the array are so distant that the effects of charge distribution (electric induction field, sometimes called the electrostatic field) or steady movement (considered at one infinitely brief instant of time, or magnetic induction field) have no effect. For example, at about 1 wavelength distance the electric and magnetic induction fields are negligible from either my circle of eight verticals or 780-foot Beverages, yet the pattern of the overall array established by the phasing of multiple cells is not fully formed. The pattern would only be fully formed several wavelengths from each array, where the distance between cells or elements is a small fraction of the viewing distance. The total pattern of two 780-foot long Beverages spaced 350 feet apart is not fully formed even at distances of several thousand feet, yet nearfield induction effects are totally gone at much shorter distances. The field impedance is established, yet the antenna pattern is not. The nearfield generally refers to or includes the area where "static" or induction fields still have a noticeable influence. Fresnel Zone The Fresnel (fre-nel, no "S" sound) zone is the area where pattern is still being formed. It may or may not include induction field areas. Physically large arrays almost always have a physically large Fresnel zone. Even simple omni-verticals have a Fresnel zone extending out a few wavelengths. The field impedance may or may not have already been established in the Fresnel zone. You may have heard about Fresnel zones during discussions of vertical antenna loss at low wave angles, or Fresnel lenses for lighthouses or other beacon lights. Farfield The farfield is the area where any changes in distance result in no noticeable change in pattern or field impedance. Losses are lower in the farfield area because field density is lower, not because we call it farfield. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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