On Wed, 17 Aug 2011 12:19:11 +0800, TonyS wrote:

Microwave: no reception

Try a 2.4GHz cordless phone in the microwave oven. Punch the "find
handset" button on the base unit. The isolation of the oven is
usually insufficient and the phone will ring.

Let's run the numbers. A microwave oven belches about 1000 watts of
RF. The leakage spec is very roughly about 4 mw.
10 log (1000 / 4*10-3) = 54 dB isolation

In a strong signal area, the local cell site will probably produce
about -80dBm. (I cheated and just used the test mode on my cell phone
to get the number). The minimum sensitivity for the cell phone is
about -102dBm. In order to stop the phone from ringing, you need only
about 22dB of isolation. Since the microwave oven shielding is 32dB
better, which is 1400 times better, the microwave oven does quite well
at blocking cell phone signals.

However, the cordless phone is another story. The base transmitter
puts out about 10 mw into a really crappy 1/4 wave antenna, which will
product about -20dBm at the handset (with lots of assumptions that I
don't want to get into). The handset has a sensitivity of about
-104dBm, requiring an isolation of 84 dB to completely block the
signal. That's 30dB or 1000 times more than the 54 dB that the
microwave oven shielding can provide, so the cordless phone rings.

Note: All the numbers I supplied are bad guesses and are probably
wrong. However, since the isolation difference is 1000 times, it's
highly improbably that my numbers are in error by a factor of 1000. If
you have better numbers, that's fine, but the results will be the
same. The cordless phone will ring, and the cell phone will not.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558