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.
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