Hi Mark:
Don gave you a good list of sources, but let me add a few subjective
tidbits!
--- On Mon, 23 Jun 1997 23:48:31 -0700 Mark Montrose <[email protected]>
wrote:
>Greetings.
>
>I am interested in locating a web page, resource, or on-line information to
>provide answers to the following..
>
>1. Typical self-resonant frequency range of the human body? How to
>calculate it.
I think you'll find a number of resonant frequencies. The lowest relates to the
vertical height of the whole body; adult humans are about 60-65 MHz, children
are higher, dogs even higher, rats higher still.
>
>2. What kind of "high frequency" RF fields, either magnetic or electric
>cause biological harm?
All. But first, I need your definition of harm. Is it leukemia in 25 years,
melanoma in 3 months, distraction, annoyance, discomfort, fear, heating,
cooking, blistering, charring or vaporization?
>
>3. At what frequency ranges (microwave, radar, etc.) is biological harm
>noted?
Again, harm can be documented at most all frequencies. What varies is the
absorption rate. IMHO, the most dangerous range is about 100 MHz through 18
GHz. This covers a lot of spectrum, but I'm thinking about the availability and
distribution of emitters as well as the body's absorption. Let's take a hint
from the microwave oven guys; 950 MHz gives good penetration into a thick block
of meat, while still yielding good absorption (quick cooking) into the meat.
Your residential microwave oven, using 2450 MHz, gives a bit poorer
penetration, but still heats pretty well.
>
>4. What power levels are required in the GHz range, or even in the 30-300
>MHz range to cause harm, (25V/m, 60V/m, 100V/m, etc.)?
>
>5. How long does one need to be exposed to high intensity radiated fields
>before effects are noted?
Two stories:
First:
I was generating a 100 V/M CW field at a distance of one meter from an
X band (8 to 12 GHz) standard gain pyramidal horn antenna. I was feeding the
antenna with a 200 Watt TWT amplifier through flex waveguide. I walked into the
shielded enclosure to rotate the polarity of the antenna. As I grasped the
antenna, placing my left hand across the entire aperture, I felt warm air
blowing out of the antenna! I thought that this was strange, because I knew
that the horn antenna had a plastic barrier across the throat. So, why the hot
air? Then I realized that I had left the signal turned on! Thus, I can
personally attest that, with a hundred or so Watts, in near field, at 8 GHz,
you can note the presence of RF almost instantly. It is actually a pleasant,
warm, evenly distributed glow (no pun).
Second:
A series of letters followed a story on RF exposure in one of the
microwave trade magazines (sorry, can't remember which one) about 10 years ago.
It seems that one writer was dismissing the long-term biological hazards of RF
exposure. He related that he was part of a several person project researching
arcing within waveguides at high altitudes. He explained that their
experimental setup consisted of a 250 KW X band transmitter pulse modulated to
a duty cycle of about 0.1%. (So that was the power equivalent of a continuous
250 Watt emitter.) He said that the transmitter was waveguide coupled to a horn
antenna located in a large bell jar. The jar was vacuum pumped to the desired
atmospheric pressure, and the RF turned on. In order to study the waveguide
throat, a researcher had to stand in front of the horn antenna, peering into
the antenna throat with his face pressed up against the bell jar glass. The
writer said that the researchers would trade places looking into the ante!
nna when their faces got unbearably hot!
The writer was commenting that he had kept track of the group, and that
so far (40+ years) none had developed any unique maladies that he could
attribute to the RF exposure.
>
>
>7. What are typical symptoms if one has been exposed to dangerous field
>levels,
>whatever they may be?
The Eastern Bloc countries, during the Cold War, kept leaking studies which
indicated that RF exposure subjects experienced memory loss, co-ordination
problems, concentration difficulties, etc, to levels far below the tissue
heating levels thought dangerous by Western research. These studies were
intriguing, because maybe they were on to something we just didn't see, but
there never seemed to be any repeatibility to their results. Maybe it was just
disinformation. Maybe...
So we keep coming back to tissue heating based criteria. As I said, initial RF
exposure is perceptible as a general, warm, almost fluid feeling. Altogether
pleasant, like a couple of quick stiff shots (alcohol dosage units). But once
you understand what that glow means, there's no surer inducement to motion (ie,
run like hell in any direction) that I can recall.
And yes, I've heard the anecdotal military myths about BMEWS personnel with
melted candy bars in their pocket. Might have some basis in truth.
Personally, I believe that applying an electric potential or a magnetic
gradient across a cell structure will cause chemical changes. And them changes
are most likely to be rather unhealthy from the perspective of the cell. Yet,
there must be a large safety margin, or threshold, or else us RF engineers (and
hams) would be dropping like malignant fruit flies.
>
-----------------End of Original Message-----------------
--------------------------
Ed Price
[email protected]
Electromagnetic Compatibility Lab
Cubic Defense Systems
San Diego, CA. USA
List-Post: [email protected]
Date: 6/25/97
Time: 9:43:56 AM
--------------------------
