Pagers

[karn@qualcomm.com (Phil Karn)]

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