Re: [TCML] Smoke Detector Issues / Faraday cage

[nickobert testein <nickobert.testein@xxxxxxxxx>]

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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