Re: [TCML] Smoke Detector Issues / Faraday cage

[otmaskin5@xxxxxxx]

I  always thought it was the ozone setting off my detectors (ionization type, not smoke type?).  Not so?


Dennis Hopkinton MA



-----Original Message-----
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: tesla <tesla@xxxxxxxxxx>
Sent: Tue, Jan 31, 2012 7:01 am
Subject: Re: [TCML] Smoke Detector Issues / Faraday cage


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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