Subject: Re A question about Antennas (Teule)(Curry).
Date: Fri, 06 Oct 2000 112409 -0500
From: Roy Beavers
To: guru
--------------------------------------------------
........From EMF-L.......
O.K. "techies", this one is yours...... But I also recommend that we
ALL try to read through it (as I did) because I believe it will expand
your understanding of "the technical side" of what we are talking about
when we say: "It is not just that one antenna that is going up in your
neighborhood -- it is the effect of the whole Blue World.".....!!!
It is the "Blue World" that our governments are ignoring at YOUR peril
... and everybody's ... in this mad dash to smother the globe with a
biologically active "blue ether" of wireless technology...... (And --
of course -- added to the power lines and other electromagnetic
radiation already present.)
The number of locales which present the kind of situation being
discussed below are suddenly appearing in uncounted numbers ... and
in uncounted communities where people may be present in large
concentrations.....
We (EMF-L) know of a few such examples (e.g., Sutro Tower or Lookout
Mountain). But such cases are rapidly becoming a "norm" in our
hi-tech society.... No longer are they such an exception.....
I have recently posted on the website the message from Professor Michael
Kundi ("Prof. Kundi on Preece/Henshaw 'Aerosol' Effect") -- in the
"Current Messages" file..... Take a look at it.... It expands on what I
have just suggested above........guru.....
-------- Original Message --------
Subject: Re: A question about Antennas (Teule).
Date: Fri, 06 Oct 2000 10:23:00 -0500
From: "Bill P. Curry"
Reply-To: BPCurry@MCS.com
Organization: EMSciTek Consulting co.
To: roy@emfguru.com
References: <39DC5F7F.EA9685E5@emfguru.com>
Roy and Gerrit,
The question about how electromagnetic fields from many sources add is a very
comprehensive and difficult one.
In general, since electric and magnetic fields are vectors (i.e., they have
a direction associated with them), one should add all the field components
in each orthogonal direction separately (i.e., add the field components from
each source in the x-direction, then separately add all the components in the
y-direction, then add all the components in the z-direction).
If you then wish to know the power density of the resulting fields at a
location several wavelengths (based on the longest wavelength or lowest
frequency source) away from any of the sources, the power density will be proportional to
the sum of the squares of all the field components from all
the sources.
If the location where you wish to know the power density is not several
wavelengths away from the source with the longest wavelength, life gets
complicated. Then you have should add the fields from each source
vectorially, as described above and square the resulting field.
When you do so, you will find that each combination of 2 fields added
together (assuming the fields have sinusoidal time variation) will produce
fields with 2 different frequencies: one frequency will be the sum of the
frequencies of the two original fields and the other frequency will be
the difference of the frequencies of the two original fields. Thus, the
harmonic content of the original fields will be significantly enhanced in
the resultant field.
When sources are collocated on, for example, a cell phone tower, the
engineers who carry out the installation always try to assure that the
sources are far enough apart for their fields not to add vectorially.
There is a problem, in my opinion, when you have a combination of waves
from many different kinds of sources with vastly different frequencies
mixing. Since the early days of radio it has been known that radio waves
at the frequencies associated with AM broadcasts obtain part of their
coverage by traveling along the surface of the earth. These "ground waves"
die off more slowly with distance from the source than do the waves that
are radiated through the air, "sky waves."
Although the longest distance communications (thousands of miles) occur
when sky waves bounce off the ionosphere and reflect back to earth at a
shallow angle, the ground waves are primarily responsible for radio
communications over distances of tens and hundreds of miles.
The problem that I presented in Salzburg considers whether the ground wave
from a powerful radio station several miles away from a cell phone tower
with two collocated sets of antennas can actually magnify the emission
from these two sets of extremely high frequency sources on the tower.
The electric fields from the broadcast source and from the cell phone
antennas are all predominantly vertical, so it is not difficult to perform
the vectorial addition of all these fields. Since the frequency of the
field of either set of cell phone antennas is more than 1,000 times the
frequency of the broadcast station, any combination of sum and difference
frequencies of the sources would differ negligibly from the original cell
phone frequencies.
Also, since the two sets of cell phone antennas were about 44 cell phone
wavelengths apart, there was no need to consider sum and difference
frequencies associated with these two sources alone, but only with each
cell phone antenna field and the field of the broadcast source. When I
carried out the analysis and took into account the effect of the ground's
dielectric and conductive properties, I found that for typical cell phone
fields of about 3 V/m and a 50,000 AM broadcast station about 4 miles
away from the cell phone tower, the combined power density at microwave
frequencies from just one set of cell phone antennas and the ground wave
could be as much as 6 times as high as the isolated cell phone antenna
power density when the ground is dry and as much as 10 times the isolated
cell phone power density when the ground is wet.
Of course the results with both sets of cell phone antennas operating
would be approximately twice the power density from any one set of
antennas plus the low freqeuncy ground wave.
One caution needs to be noted: if we were only considering thermal
effects on biological cells, there would be no justification for
considering the power density magnification effect, because it
vanishes when you perform a time average of the resultant field.
However, my interpretation of the available laboratory evidence is that
biological cells respond to instantaneous power density variations, not
just averages.
Thus, I believe you have to evaluate the instantaneous value of the power
density stored in the combined field from the sources, including the low
fequency source.
The example I have explained above is an extreme one, but a real one, and
real people are suffering what I think can be regarded as "microwave
sickness."
Many other such circumstances may involve sources not so far apart in
frequency, and, thus, the above analysis may not be appropriate. Also, in
the real world situation that provoked my study of this problem, one set
of cell phone antennas was broadcasting ordinary cell phone transmissions.
Thus, these were frequency modulated sinusoidal waves. The other set,
however, was broadcasting pulsed digital data transmissions (I think).
These transmissions are, essentially, a series of rapidly interrupted
sinusoidal waves.
In one situation, I have been asked about whether cell phone antennas
mounted on an FM broadcast station tower can cause the kind of enhanced
microwave power density as I described in the earlier example. I don't
know the answer.
The complicating factor is that the importance of the ground waves
diminishes as the frequency increases. However, the ground wave probably
isn't even necessary for this scenario, since the cell phone antennas are
mounted essentially in the aperture of the FM broadcast antenna. The
question of polarization may be important. The cell phone antennas
produce predominantly vertical electric fields, and some FM antennas are
polarized horizontally or at some other direction (e.g., some TV antennas
are circularly polarized, but I don't know whether this is the practice
for FM broadcast antennas).
In the particular question that Gerrit posed, "hot spots" may form when
there are reflecting structures near antennas. This is a problem for
those people whose occupation requires them to be in the "near field" of
the antennas. For people in the far field of the anennas, the effect of
a partially reflecting structure is usually approximated (on the basis of
average measurements made a number of years back by the U.S. EPA) by
allowing a field enhancement of 60 % when one is concerned about the
field near a structure or near the ground (but no closer than a few feet
off the ground). This means that the power density is enhanced by a
factor 2.56 (square of 1.6).
I have recently begun to question this approach, because there are precise
analytical expressions available for the field of a dipole antenna situated
a finite distance above partially conducting ground. Those interested
will find such material in C.A. Balanis's book entitled "Antenna Theory:
Analysis and Design" published by Wiley and Sons, New York (1997).
The pertinent material is found on pages 181-195. The author's primary
interest is in explaining why there is a gain in RF field strength when
a plane flies over salt water.
(Actually, the field strength varies with distance from the source so that
it is sometimes greater than would be true at the same distance from a
source with no reflecting sea surface present and sometimes less than that
value.)
In Balanis' calculations for a plane 5,000 ft. above the ocean, the "height
gain" (ratio of the field strength with and without the reflecting sea
surface) can be as much as 4 decibels. This means that the RF field is
enhanced by the sea by about a factor 2.5. For microwave antennas as close
to the earth or other reflecting structure as 20-30, I suspect that the
height gain may be even greater. Since antenna gain factors are measured
under somewhat controlled conditions in open ranges, I suspect that
antennas moounted in hilly terrain will have different effective gain
patterns from those mounted over flat surface. Probably, the same
reasoning will apply to antennas mounted on buildings amid a cluster of
other buildings.
Neil Cherry's measurements on Mt. Sutro in San Francisco showed that there
are aeas where radiation seems to "roll" down steep slopes (probably
diffraction). In the same area, I found "hot spots" around driveways.
The intertaction of RF radiation with boundary structures is always a
complex subject, and both measurements and calculations are required to
understand it.
Roy Beavers wrote:
>
> ........A very interesting question has been forwarded below from the
> Netherlands.....!!!
>
> Yes, Gerrit, the phenomenon of electromagnetic radiation "reinforcement"
> has been addressed before on EMF-L, though it cannot be said that there
> is much actual "scientific" evidence to cite. I recall some of our
> "technical" experts (Tegenfeldt, Philips, etc...) discussing it -- or
> suggesting it -- and I believe some of that discussion could be found
> in our archives. Most recently, the work that is being done by Bill Curry
> (and reported by Bill at the Salzburg Conference) is something you should
> definitely get hold of...... Bill's paper is reprinted in the Salzburg
> Conference Proceedings. Some "heavy" math there.......guru......
>
> -------- Original Message --------
> Subject: A question about Antennas
> Date: Thu, 05 Oct 2000 12:27:54 +0200
> From: teule
> Reply-To: teuleger@wxs.nl
> To: roy@emfguru.com
> CC: Wim Roskam
> References: <39DBB912.F2A91BE0@emfguru.com>
>
> Dear Roy,
>
> I got an interesting question from a reader of one of my articles. It's about
> antennas, in particular the combination of several antennas. You mentioned this
> problem several times, but this question goes a bit further on that road.
>
> It is un deniable true that we are surrounded bij several GSM antenna's and
> other sources of radiation (radar, electrical engines, speed control etc.). His
> question is this: if you throw some stones in a swimming pool at different
> positions, than the waves spread out over the whole pool. When these waves meet
> each other, than you get at certain points strong waves or even wild moving
> water. The places of these strong movements are difficult to predict. That
> depends on the shape of the pond, the places where you throw in the stones and
> of course the timing.
>
> If the same phenomenon is valid for electromagnetic waves, than we might expect
> in the real world a lot of places, where radiation really gets out of hand,
> although the distance to the GSM towers do not suggest a problem. These places
> depend on antenna-placement, the siting of buildings, reflections (glass or
> aluminium walls?), and your own position. Walking though a city can, in EM
> sense, become an interesting experience.
>
> Do we have any experience or measurements on this possibility? Did anyone ever
> do a series of measurements during a stroll though a city? The question is
> simple, but I suspect that the answer is overwhelmingly complicated. This looks
> like chaos-theory, does n't it?
>
> Gerrit Teule
> Netherlands
--
----
Bill P. Curry, Ph.D. |Physics is fun.
EMSciTek Consulting Co. |Trying to make a living!
22W101 McCarron Road, |Phone: (630) 858-9377
Glen Ellyn, IL 60137 |Fax: (630) 858-9159 with prior notice
Web page: http://www.EMSciTek.com
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Archive provided courtesy of WaveGuide, http://www.wave-guide.org
Reprinted with permission of Roy Beavers, http://www.emfguru.com