Re: [TCML] Smoke Detector Issues / Faraday cage

[Henry Hallam <henry@xxxxxxxxxxxxxxxx>]

A microwave oven doesn't try to be a Faraday cage.  The first
generation of ovens did so, but the difficulty of maintaining a good
connection all the way around the door seal over many years of use,
grease buildup etc meant that they frequently leaked a lot more than
desired.  Subsequent designs have used a 1/4 wave choke with a 3cm gap
at the door which results in an effective short at 2.4GHz and is quite
efficient at preventing much power from being leaked.  It doesn't do
much at other frequencies.

Ref: http://www.freepatentsonline.com/3956608.pdf

H

On Tue, Jan 31, 2012 at 10:34 AM, nickobert testein
<nickobert.testein@xxxxxxxxx> wrote:
> yes, thank you for correction, i meant slot. also sorry if you felt it
> off-topic.
>
> On Tue, Jan 31, 2012 at 8:55 AM, Jim Lux <jimlux@xxxxxxxxxxxxx> wrote:
>> On 1/31/12 8:17 AM, nickobert testein wrote:
>>>
>>> take a sheet of aluminum foil and wrap phone. call. dead. now take a
>>> knif and cut a small hole, call again. dead, now increase cut size and
>>> call again. repeat until the phone rings. open phone- voila, the
>>> antenna will be smaller than the cut in the foil.
>>
>>
>> I'm not sure what this exercise is designed to show.
>> The bigger issue with shielding cages and boxes is things like slots, not
>> holes, and the thickness of the wall.  But shielding design isn't really
>> on-topic here.
>>
>> For TC frequencies (wavelengths of hundreds of meters), your cage is going
>> to be in the near field of the coil, and more importantly,a good fraction
>> the magnetic field will go right through it.
>>
>> Cages are terrible shields, but good at containing sparks (that is, the
>> spark will generally NOT penetrate the cage wall) and directing spark return
>> currents in a way that reduces interference.  They also reduce electrostatic
>> coupling pretty well  (interestingly, a sheet of paper or cardboard works
>> almost as well for that), but don't do much for propagating waves.
>>
>>
>>
>>
>>
>>>
>>> the enterior of a microwave oven in the off-mode is a wave guide, not
>>> a faraday cage.
>>>
>>
>> Huh? A microwave oven is a shielded box, designed to attenuate the RF at 2.4
>> GHz enough to meet safety standards (roughly 1 mW/sqcm at the surface of the
>> oven).  As it happens, it doesn't take great shielding to do that, when
>> considered in the context of a radio receiver (cell phone) that has a
>> dynamic range of something like 60-100dB.
>>
>>
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