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Re: Net Result of Snowden

From: Beaker Meeps <beaker AT dropperbox.com>
On 2/14/2017 2:04 PM, jim bell wrote:

> *From:* Ryan Carboni <ryacko AT gmail.com>
>>I have written down so notes on the movie. Also, my cell phone works
> fine in the microwave.
> You might be very near a cell-phone tower.
> Try putting a large plastic or glass container of water in the
> microwave, with the cell phone.  (say, 1/2 gallon of water.)
> A microwave cavity, alone, is fairly well-shielded.  But it is also
> very low-loss without a "load", an object within it that will absorb the
> 2.45 GHz microwave energy.  Usually food, of course.
> One thing that would be useful is an app which showed the received
> signal strength for that cell phone, to a resolution much better than
> the usual 5-bar display. 

>Well put.   

A good radio receiver can have a dynamic range of 100 decibels.  (or, a 10**10, a factor of 10 billion) range over which a signal can be input and still give a useful, even good-quality result. http://www.radio-electronics.com/info/receivers/dynamic_range/dynamic_range.php

I could not quickly find a figure as to the typical shielding provided by a microwave oven, but let's suppose it's 60 db, or a factor of 1 million reduction.  You can see that if the signal outside the oven is, say, 80 db over the minimum detectable level, putting it into that oven would reduce it to 80-60db, or 20 db, still a very useable signal.  Thus, it would appear that the phone works fine in that oven.  Adding a large container of water into that cavity could further reduce the signal level.

Also, be aware that the effectiveness of shielding in a microwave oven may be frequency-dependent.  In some cases, I have seen a "channel" within the door-seal structure that I suspect is designed to oscillate at a microwave oven's frequency:  2.45 GHz.   Thus, it blocks that signal, but it might not do so well at blocking at a typical cell phone's frequencies,  900 Mhz, 1800 Mhz, and 1900 MHz.    https://en.wikipedia.org/wiki/Cellular_frequencies      Combining use of a microwave, with wrapping a phone in a couple layers of aluminum foil, should work okay to block it.  ×

         Jim Bell

For the curious:


tl;dr - frequency bands are different. Although some get lucky because
of the wall density.