Subject: Poynting vector (Weller)..
Date: Mon, 14 Aug 2000 163451 -0500
From: Roy Beavers
To: guru
--------------------------------------------------
........Response from EMF-L.......
-------- Original Message --------
Subject: Poynting vector
Date: Mon, 14 Aug 2000 13:56:15 -0700
From: rweller@h-e.com (Robert D. Weller, Hammett & Edison, Inc.)
Organization: Hammett & Edison, Inc.
To: guru@emfguru.com, marjlundquist@address.com
Marjorie,
Now that you've put all of us electrical engineers on the list into a
defensive posture, I'd like to add my 2 cents.
You seem to have called into question the "plane wave equivalent" language
contained in C95 and other RF exposure standards. The "plane wave" is simply
the special (far-field) case where
S = E x H = |E|^2 = |H|^2
That is, in the far field, one assumes that the E and H vectors are at 90
degrees and of "equivalent" magnitude (there's a constant term that converts
between the units), so the magnitude of the cross product becomes simply the
square of the magnitude of either term.
There are various ways of defining the extent of the near field (that is, the
distance from the source beyond which the above and certain other far field
conditions exist). One commonly used definition for that distance, r, is
r > 2D^2 / lambda
D is the largest dimension of the radiating source. For typical cellular base
stations, D is around 1.2 meters, so the far field conditions are valid at
distances from the antenna beyond about 30 feet. There are situations where
members of the public are within this distance, but they are relatively
unusual.
You are correct that, in the near field, the E and H vectors may _not_ be at
right angles, _nor_ of the same magnitude, but this "near field effect" merely
requires that the individual (E and H) field components be measured or
calculated separately. There is a caution contained in C95 about this. That
the standards refer to a "plane wave equivalent" power density does not make
them inapplicable to non-plane wave situations, since the power density limits
are generally independent of the type of field. This doesn't hold at very low
frequencies, where the E and H field MPE limits diverge.
I don't understand your concerns about exposure from multiple sources not
being "plane-wave." In most public exposure situations involving multiple
sources each source individually usually can be treated as far-field (and
therefore plane-wave equivalent). Even if it weren't so, one could still
measure or calculate the E and H fields separately to obtain a "plane wave
equivalent" value for power density.
I guess I'll have to wait for your paper. Hopefully, Guru will post it.
Bob Weller
On Mon, Aug 14, 2000, 10:41:57 AM Roy Beavers wrote:
>.........Thanks, Marjorie, for what you semd below..... Hope you won't
>object that I have inserted (below) one comment -- for the laymen, to help
>(perhaps) see the significance of what you write......guru......
>
>
>-------- Original Message --------
>Subject: Some technical comments (e.g., for electrical engineers)
>Date: 14 Aug 00 10:12:23 PDT
>From: marjlundquist@address.com
>To: guru@emfguru.com
>
>
>I do know that, for both thermal and nonthermal hazards, the power density
>constitutes a valid exposure metric ONLY for plane electromagnetic waves.
>
>The wireless telecommunications industry presents us with two challenges that
>involve electromagnetic waves that are NOT plane: the near field of a
>transmitter, and multiple transmitters (which expose people to fields of
>comparable strength from two or more sources simultaneously).
>
>
>What should be used as an exposure metric for these fields is, at present, an
>unanswered question. The practical solution for the near field is to keep
>people out of it, and minimize the duration of their exposure, when they have
>to be in it. For multiple sources, again one ought to keep people far enough
>away that the multiple sources act like a single source and their combined
>effect is a single plane wave; and when they have to be close, minimize the
>duration of their exposure.
>
>Much work needs to be done to develop more satisfactory solutions to these
>practical problems. Such work has not even been started, because industry
and
>governments to date have been expending all their energies to try to defend
>the existing standards, which are scientifically flawed.
>
Archive provided courtesy of WaveGuide, http://www.wave-guide.org
Reprinted with permission of Roy Beavers, http://www.emfguru.com