On Tue, 01 Mar 2005 15:42:32 GMT, "Eric"
<(E-Mail Removed)> wrote:
>That sounds like pure marketing. If anything, such a detector'(detecting
>raw continuous wave RF) would be less desirable than a detector that
>specifically detects 802.11 frame formating since it would be prone to false
>readings.
I have several microwave oven leakage detectors. The most sensitive
will respond to 1mw/cm^2. 5mw/cm^2 is considered a leaky microwave
oven. It will barely respond when place next to my access point
antenna. I also have some Proxim 7400 frequency hopping cards that
are setup with tweaked firmware to act like a spectrum analyzer.
They're not terribly sensitive (lack of processing gain and
synchronous detection), but do show the classic direct sequence
spectra and microwave oven leakage. I borrowed a Kensington WiFinder
and found that it would not detect 802.11g access points, was
basically comatose, and falsely responded to spread spectrum cordless
phones (Panasonic Gigarange).
>You can
>do the opposite at home by simply placing a 2.4 Ghz cordless phone handset
>inside a microwave, shutting the door, and hitting the "page button" (or
>calling from another phone) on the base unit. The handset will beep/ring
>with no problem.
Great idea. I just tried it with several cordless phones I have
around the house. All of them, including the 900Mhz flavor, responded
inside the microwave oven. I could probably calculate the attenuation
provided by the oven, but my guess(tm) based upon screen room
characteristics is that it's about 60dB. 60dB down from about 600
watts CW power is conveniently 600 milliwatts of leakage, which is not
enough to do any human damage, but is certainly more than what an
802.11 radio belches. For the cordless phone test, the average base
unit belches about +10dBm. 60dB below that is -50dBm. Path loss to
the handset drops perhaps another 40dB (at 6ft) for a -90dBm receive
signal. That should be sufficient for the cordless phone to respond.
>Or simply look at the noise floor, using Netstumbler,
>while relatively near a microwave. Microwaves play major havoc with
>802.11g/b. (One of the main reasons I'm using 802.11a for my WLAN traffic,
>as have many cordless phones and microwaves in the house.)
Actually, if you look carefully, many 5.6Ghz cordless phones are
crossband type that use 2.4Ghz in one direction and 5.6GHz in the
other. It's much easier and cheaper to build a full duplex system
(diplexer) with radically different frequencies. I don't have a list
of which ones do this. Lookup the FCCID on your cordless phone and
dig through the test results.
>Off-topic but interesting tidbit on microwaves. Common misunderstanding on
>how microwaves work is that the resonance frequency of water is at 2.4 Ghz.
>Been a while since I've taken physics, but I do recall that the resonance
>frequency of water is in the 20-something Ghz range. Water, a dipole
>molecule, can vibrate by the E/H fields caused by a wide range of
>frequencies. 2.45 Ghz was chosen for the kitchen nuker out of government
>mandate (at least in the US) since its in the "junk band". 2.45 Ghz also
>allows the cavity size of the magnetron to be managable for the
>manufacturers. Finally, if the microwave used 20-something Ghz to cook food
>it would scour the outside of that frozen burrito while leaving the inside
>completetly frozen as higher frequencies attenuate much easier.
Ugh. I had to look that one up and found this physics class notes:
|
http://faculty.trinityvalleyschool.o...icrowaves).pdf
Water molecule resonates at about 22Ghz. Apparently, the selection of
2.45Ghz was quite arbitrary.
--
Jeff Liebermann
(E-Mail Removed)
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558
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