Re: Coil size to faraday cage size ratio

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-net> 

Hi Finn,

Was the measurement made using a shielded loop or a dipole antenna, and at 
what frequency range(s), and at what distance(s)? In any event, the EMI 
measurement was made within the "near field" (a distance less than 
Lambda/2*Pi from the source). And, although it's likely that your current 
Faraday cage may work fairly well for damping the E-field around your coil, 
unless your shield uses high Mu material the near field magnetic induction 
field is probably passing right through your Faraday shield.

The low frequency near-field magnetic component can be a very significant 
problem, especially when dealing with equipment that uses low frequency 
(<200 kHz) and high current loops (i.e., induction heating, air core RF 
transformers, and SSTC's). The problem becomes worse as you lower the 
operating frequency. Fortunately, the near H-field declines as the cube of 
the distance, so doing the EMI measurement at a further distance may help 
bring you closer to spec... but probably not by a factor of 100. If the EMC 
test that failed used a loop antenna, you were picking up the H induction 
field. You'll probably need to use magnetic shielding material in your 
Faraday cage in order to effectively shield it.

Best regards,

-- Bert --
-- 


Tesla list wrote:

>Original poster: "Finn Hammer" <f-h-at-c.dk>
>All,
>I`ve had my OLTC coil tested for EMI, and since it exceeds the legal limit 
>a 100 times, inside a faradays cage with 50db damping. This corresponds to 
>a 4Km (3miles)radius inside which the legal limit is violated.
>I am going to need a better cage.
>Question: For the same coil, will a big faraday cage offer better damping 
>than a small one.
>My reasoning says yes, since the longer distance to the cage, in the case 
>of a big cage, will result in a weaker field than as in a small cage.
>If the coil produces 600kV and the distance to the cage is 1meter in the 
>small cage and 2 meters in the big cage, then the small cage has to damp 
>from 600kV/meter, whereas the big cage only sees 300kV/meter.
>Is this correct?, and will it matter? If yes, by which factor 1/2? :-|
>1/4? :-\    1/16? ;-)
>Cheers, Finn Hammer
>
>
>.