More facts: Coil size to faraday cage size ratio

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Finn Hammer" <f-h@xxxx>

I have some more facts of the measurements made on the coil.

The purpose of the measurements was to get a rough idea of what kind of 
shielding would be needed to comply with rules.
The measurements were made with a bi-conical antenna, looking much like 2 
egg-beaters combined at the shafts. The frequency responce of the antenna 
is 30-300MHz, and the signal is fed into a HP spectrum analyzer type 
8591EM. This instrument is corrected for the responce of the antenna, and 
this is the reason for the apparent dip in radiated level on these curves, 
where the first curve is the signal from the coil, the second is the 
background noise.
http://home5.inet.tele.dk/f-hammer/emc.pdf
The frequency scale is linear, the level is a dB scale.
The standard that we use as reference is a ISM standard, EN55011,
Industrial,scientific, medical,  line, radiated, equipment (I didn`t get 
the rest)
The guys that I work with say, that pulsed equipment is given an advantage, 
but that it is perhaps about 10dB, and not something to be concerned about 
at this moment.

We could pass DC into the cages, and feed it into a swichmode converter, 
and we are investigating the feasibility of this solution.

We realize that a dual cage is probably needed, and are going o make a 
prototype cage this way.

Steve, the shielded pulse transformer you mentioned is definately going to 
be needeed, so I`l appreciate your making one.

I will keep you all informed in the future of the development of this cage, 
as well as all measurements when they are available.

Cheers, Finn Hammer

Tesla list wrote:

> Original poster: "Bert Hickman" <bert.hickman@xxxxxxxxxx>
> 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 --