(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 <[EMAIL > PROTECTED]> wrote: > > > --On Wednesday, November 17, 2004 12:41:44 PM -0500 "Lachniet, Mark" > > <[EMAIL PROTECTED]> 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 > list.) > > 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 waveguides... > 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) bandwidth. > > 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. Sorry. 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 [EMAIL PROTECTED] Duston Sickler: "There are only 10 types of people in the world, those who understand binary and those who don't." _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.netsys.com/full-disclosure-charter.html