For what it's worth, my pager (a Motorola Bravo (Plus?) using a frequency of 152.240 MHz) cheerfully receives pages while inside my microwave. (No, I didn't turn the microwave on. :) What that implies about the shielding capabilites of my microwave (and the safety of its continued use) is beyond my knowledge of RF and microwaves. I have some idea (please correct me if I'm wrong here) that shielding effective for energy at one frequency (like that of my pager) may be ineffective or less effective at another (like the frequency of microwaves). I'm intrigued (and alarmed) at the idea that it's possible to disable pagers via remote control. While I'll admit that there's some wee utility in being able to make sure nobody can use my pager if it's lost/stolen, it's sounding, from the posts to the list, like it'd be possible to disable many folks' pagers, were some miscreant so inclined. I know of pagers being used by police, fire department, search & rescue, private/campus security, and medical folks; if someone put some time into the "kill all the pagers" thing, it could be pretty damaging. Then again, knowing that, if I swiped somebody's pager, I'd probably just remove the battery for a few days, and hopefully miss the "stop working now" signal. I'm curious, though, how long the battery would last - my pager has a single AA battery, which lasts about a month. On average, a pager in the "find this pager" mode would have a single half-discharged AA battery to power its transmitter; and that transmitter would be sending its signal without benefit of any sort of external antenna. My very limited knowledge of RF propagation makes me think it'd be hard to get much range or duration under those circumstances. I'd be interested to see this thread move over to comp.dcom.telecom - there might be more folks who know about pagers who could comment. This is interesting, but frankly I'm still pretty skeptical. -- Greg Broiles greg@goldenbear.com Golden Bear Computer Consulting +1 503 342 7982 Box 12005 Eugene OR 97440 BBS: +1 503 687 7764
Yes, the microwave oven is designed to be especially effective at one particular frequency. Look at the gasket on the inside surface of the door. Underneath is what is called a "quarter-wave choke". It looks like a short circuit at the surface of the door, but only at 2450 Mhz, the operating frequency of a microwave oven. You probably still got a lot of shielding, it just wasn't enough. Remember that modern communication receivers (even pocket pagers) are capable of working with incredibly small amounts of signal energy by human standards. And paging systems are designed to blanket their coverage areas with a *lot* of RF from multiple synchronized transmitters, each running several hundred watts. This seeming overkill is necessary to handle the very wide dynamic range in propagation losses that terrestrial communication links can encounter due to fading, multipath, terrain blockage, changing distances, etc. Even a properly operating microwave oven that is well within all radiation safety limits is *easily* detectable with a communications receiver or spectrum analyzer (the latter is preferable because the frequency is so unstable). I think I saw about -8dbm an inch from the door seal of the oven I had back in NJ when I checked it. As I said, this is well within biological safety limits but it is, by radio communication standards, an *extremely* strong signal. Amusing anecdote: recently I took one of our CDMA cellular phones into a supposedly NSA-certified RF screen room at work (though it's not used for government work). The cell antennas are on the roof of the same building. I closed the room door and latched it, and the phone still worked! I then put the phone into a conventional metal cabinet in the room and my call finally dropped. It's conceivable that the room still met specs (something like 100 dB), but that just wasn't enough until I added a few more dB with the metal cabinet. Like I said, mobile radio systems have to deal with some *very* wide dynamic ranges. Phil
participants (2)
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greg＠ideath.goldenbear.com -
karn＠qualcomm.com