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RE: WiFi question
From: Ake Nordin <rootmoose () telia com>
Date: Mon, 22 Nov 2004 02:26:35 +0100

(with a nod to Esmond Kane)
At 17:50 2004-11-19, Paul Schmehl thusly scribed:
--On Thursday, November 18, 2004 09:32:27 AM -0600 Paul Schmehl <pauls () utdallas edu> wrote:

--On Wednesday, November 17, 2004 12:41:44 PM -0500 "Lachniet, Mark"
<mlachniet () sequoianet com> wrote:

Could also be RF interference.  One of my coworkers tracked down a
particularly interesting problem with motion sensor lights.  

<quoted text trimmed down a bit, some lines broken in the process...>

After forwarding this to our wireless expert, he responded
with this (which he has authorized me to forward to the
I find it hard to believe that this is possible.  2.4Ghz is
the 9th harmonic.  By the time you get to the 4th harmonic
of a signal, even in very very noisy radiators, the strength
of the harmonic component of the signal is extremely minute.

Says what? Not every distortion mechanism give monotonically
falling spectral intensity. Device resonance may tilt that
spectrum substantially. If the stuff is cheap enough, it's
antenna may be a vital part of that resonator (i.e. far
better tuned at 2.4GHz than at 240MHz...)

And, given the fact that one of those sensors (which most
likely does *not* truly operate in the 240MHz portion of the
spectrum) will have a very low output (Part 15 device), the
10th harmonic of that signal will be undetectible as it will
be at or below the level of background noise.

Low output it may be, but received power is inversely
proportional to distance squared in ideal (freefield)
conditions. The AP inside the same building (room?) is
possibly quite close to the detector. Then consider the
irregularities of radio propagation inside buildings, and
the possibilities of various structures that can act as

Finally, if a device managed to get past all of the
improbabilities above, the chances of it *accidentally*
creating a signal that looked like an 802.11 beacon packet,
complete with preamble, header, etc is so off the charts as
to be laughable.

This (the preamble especially) is what _should_ eliminate
the motion sensors from the list. I'm out on this one (too
lazy to do the math), but is the 802.11b air interface that
resilient (does it really require that much redundancy)? It
should be, but that would also be some lost (usable)

One other thing...  If that device truly was operating
at 240MHz, then the first harmonic would be 480MHz.  I'm
pretty sure that frequency lies in the public service bands
(ie fire/police).  If not, its very close.  Given that and
the fact that the first harmonic would be much stronger than
the 9th harmonic, I'm pretty sure someone in those bands
would have complained loudly to the FCC as they don't take
intereference issues in those bands lightly.


1) The building will contain very much of that energy
(which never was very much on a metropolitan scale, FCC Part
15 and all that).

2) The noise characteristics as received by those services
would be intermittent, very bursty and come from many
different directions all over the city. No easy clues telling
what to complain about there.

3) I don't know about US emergency communication radios, but
typical European systems (before Terrestrial Trunked Radio)
are so bad anyway that this contributed noise hardly would
be noticed.

 /Ake Nordin       +46704-660199       rootmoose () telia com
 Duston Sickler: "There are only 10 types of people in the
 world, those who understand binary and those who don't."

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