Re: More facts: Coil size to faraday cage size ratio

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Jim Lux" <jimlux@xxxxxxxxxxxxx>

At 04:20 PM 11/22/2004 -0700, you wrote:
> 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

I noticed that you used a pretty wide resolution bandwidth (3MHz).  Is this 
what's required by the spec.  Can you narrow it down?

So it looks like your problems are at 100+ MHz.  What's your cage made 
of?  How big are  the holes. Is the wire woven or welded? (300MHz=> lambda 
of 1 m => hole perimeters should be <<50 cm.)

> The frequency scale is linear, the level is a dB scale.
> The standard that we use as reference is a ISM standard, EN55011,
that would be:
EN 44011:1998 Industrial, Scientific, and Medical (ISM) radio frequency 
equipment - Radio disturbance charactersitics - Limits and methods of 
measurement, and it references CISPR 11:1998/A1:1999

(actually, there's probably a newer version of these)

I found a reference that states that since May 23,2004, FCC part15 Subpart 
B and EN55011 have the same conducted emissions standards (thats for <30MHz)
http://www.analogzone.com/pwrt0628.pdf
There's a chart that shows QuasiPeak limits of 50 dBuV/m for 30 to 300 MHz 
for EN55011 (FCC lets a bit more (55 dBuV) through from 90-300, but is 
otherwise the same.)  These are at a distance of 3m.


I assume your testing was at 3m distance (from the cage or from the coil?) 
or was scaled?


> 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.

You shouldn't need a double cage (and, if the noise is getting out via 
conduction, a second cage won't help anyway..)

What might be useful is a double wall cage, so that RF currents from 
strikes to the cage aren't carried on the outer cage.




> 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 --