Physical questions

Jim Lux James.P.Lux at jpl.nasa.gov
Tue Jan 21 22:48:07 EST 2003


----- Original Message -----
From: "Robert G. Brown" <rgb at phy.duke.edu>
To: "Jack Wathey" <wathey at salk.edu>
Cc: "Ken Chase" <math at velocet.ca>; <beowulf at beowulf.org>
Sent: Tuesday, January 21, 2003 4:21 PM
Subject: Re: Physical questions


> On Tue, 21 Jan 2003, Jack Wathey wrote:
>
> > Aside from the risk of fire from a non-OTS beowulf enclosure, are there
> > any risks to life and property if the thing leaks RF as discussed above?
> > Can a shelf of radiating motherboards slowly cook (like a microwave
oven)
> > the poor nerd standing in front of it?  Can it interfere with
> > communication/navigation equipment in low-flying aircraft?  My hope and
> > expectation is that the power of such RF signals is MUCH too low to do
> > such damage, but it would be comforting to hear an authoritative answer
> > from someone who knows more physics than I.
>
> I would say no, no, no.  And no.
>

> I'd guess that total radiated power even from an unshielded unit is less
> than a watt (possibly much less) and radiates out in all sorts of
> directions and in frequencies that aren't absorbed "suddenly" in human
> tissue.  That may be plenty for a radio to pick up,

Especially since a run of the mill radio is sensitive to signals on the
order of -140 dBm (that's 1E-17 watts) without working too hard at it. The
monitor (or the cable from PC to monitor) on my desk at home radiates enough
to put -120 dBm into an antenna 5 meters away.


> SO I wouldn't worry.  I don't worry much about cell phones, either.
> Microwave ovens, I worry about.  TV sets and CRT's in general I worry
> about (they produce soft X rays and other ionizing radiation with some
> probability, especially close to the tube).
I wouldn't sweat the TV set/CRT... Color TVs used to emit Xrays back when
they used vacuum tube rectifiers and before they used lead glass in the
rectifier tube envelope (Color TVs running at anode potentials of 25 kV or
so, compared to the 10-15 kV for a B&W set..).. The front face of a CRT is
lead glass and quite thick, just for this reason, as well (since what makes
that spot is the electron beam stopping at the front...) (I seem to recall
reading that there is 2-5 kilos of lead in the average monitor dumped in the
landfills...making it a significant hazmat problem)

>
> > Along those same lines... Is there a handy-dandy (and not too expensive)
> > gadget for measuring RF emissions from an electonics enclosure in a
> > standard, quantitative way?  I'm hoping for something that can tell me
> > whether or not a prototype or DIY enclosure is sufficiently RF-tight
that
> > it would pass the official certification tests.  Are those tests simple,
> > or way to complex and expensive for an amateur to duplicate?

There is nothing cheap to purchase that can tell you with certainty that the
enclosure/setup will pass cert tests.  You can rent some gear for a few
$1000/month that would do this, or pay someone, or find someone who happens
to have the stuff (EE department?) and can make a quick and dirty
evaluation. EMI/EMC certification is truly an arcane business...  The
calibration of the measurement setup is the problem.  A qualitative
evaluation is easy, quantitative is hard, since there are lots of error
sources that can add up to 10s of dB.  You need appropriate test sites (open
field, anechoic chamber, or TEM cell), calibrated antennas, and calibrated
test gear, none of which is cheap.

For a rough and ready test, I understand that some folks have used computer
controlled broadband receivers like the ICOM PCR1000 (about $300) to make a
poorman's spectrum analyzer. With a few (tens) hours characterizing the
receiver, I think one could do a pretty good job.  One might be able to
figure out a way to do antenna calibration as well (perhaps by making an
antenna that has well known properties that can be calculated using a
numerical model).  One could make some sort of transfer standard. A suitable
open field with a wooden platform or table, while inconvenient from a test
equipment and weather standpoint, is the practical test site.

There's an ANSI/IEEE standard for all this, of course..

For what it's worth, an amateur can easily do and interpret the tests...
it's the metrology and calibration that's tricky. (There was a paper 10-15
years ago in some journal that I can't recall (IEEE Trans on Ant. and Prop?)
about backyard antenna ranges)  Relative tests are easier, too (i.e. is
alternative A or B better is a lot easier to measure than is alternative A
better than an absolute standard).  2-3 dB precision is easy to get,
although tedious with improvised gear.


For RF safety, there's a bunch of stuff on the web at the FCC site.  Search
for OET Bulletin 65, which gives Maximum Permissible Exposures, how to
calculate dose, and all that stuff.  IEEE C95.1-1998 is the latest standard
for the exposure, etc., but unlike the FCC/OET bulletin, it's not available
for free on the web.

>
> I'll let some others answer that.  A Famous List Person and I were
> kicking around this question as a possible interesting topic for EE or
> Computer engineer student types.  I don't think that they equipment is
> particularly cheap or DIY, though...

Pshaw... high school students build cyclotrons out of spare parts in the
garage....

>
>    rgb
>
> --
> Robert G. Brown                        http://www.phy.duke.edu/~rgb/
> Duke University Dept. of Physics, Box 90305
> Durham, N.C. 27708-0305
> Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu
>
>
>
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