Re: Faraday Cage and 1/4 wavelength sized holes

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

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>


>>>>For a Faraday cage to work for some frequency, then
>>>>the holes in that faraday cage must have a diameter
>>>>less than the 1/4 wavelength of the frequency you are
>>>>trying to block.
>>         That's one rule of thumb.
>>         When i was doing EMI for computers to meet
>>         'FCC' it was 1/10 wave.  Rather than a single
>>         magic number the leakage increases as the
>>         slot/opening gets larger.

>Really, it's more of a perimeter thing.. a long skinny slot 1/4 wavelength 
>long is just as bad as a hole 1/4 wavelength in diameter.
>Doors, chassis seams, cover plates and panels are the traditional problem 
>areas (hence the popularity of conductive elastomeric gaskets, spring 
>finger stock, etc.)
         Yep.  Left that out for brevity.  8)>>


>A non-conductive slot in a conductive sheet is (EM-wise) very similar to a 
>conductor of the same dimensions in an insulated sheet: Babinet's 
>complementarity principle.  Slot antennas make use of this.

>For TC's the slots and holes are tiny.
         I would suggest:

                 This is true FOR THE FUNDAMENTAL
                 (main) freq.  For the strays, that annoy
                 the neighbors TV, the holes in eg chicken
                 wire can start getting noticeable.  For

                 Cell phone, they are wide open...


>A bigger problem will be wires that penetrate the walls (including "woven 
>wire") and carry EMI out through the shield.
         Indeed.  Hence my 'ranting' on everything must be

         filtered, and the filters must be done right.


>>>>If you build your faraday cage out of chickenwire, sure
>>>>it will keep in your arcs and lower frequency garbage, but
>>>>the higher RF frequencies (Ghz) will pass right through it.

>>>Do you or anyone else have an estimate on how many nanowatts
>>>the average coil puts out in the GHz range?

>>         An interesting question, which would be nice to
>>         know.  Wish i did, tho i suspect its LOTS of
>>         nanowatts.  (hint: in the early years, radar jammers
>>         were powerful spark transmitters.  Granted, they
>>         were optimized for it, and at modestly lower freqs.)
>>         If I Recall, cell phones (mostly?) are in the
>>         800 MHz region.

>>         (A useful approximation for noise output might be
>>         to fire up a coil, then walk away with cellphone
>>         in hand until signal is acquired.  There would be
>>         a huge variation, due to power variation, incidental
>>         nature of 800MHz output, etc.  (cut to THAT TV
>>         commercial:  Can you hear me now?  Can you hear me
>>         now?  8)>>)

>Probably not much, in a power spectral density sense.  The damped sinusoid 
>doesn't have much power up high, but the actual sparks are pretty 
>broadband (being very short duration impulses), as is radiation from the 
>spark gap and connected leads.

         Yep.


>If you figure that the spark's duration is on the order of a few tens of 
>nanoseconds, then the spectrum could easily extend up into the hundreds of 
>MHz range.
         I'd think the edges would excite random bits of wire

         further up.


>> > Anyone with a spectrum analyzer?
>>         Agreed.  Or a cell phone, or 800MHz scanner?

>If you have a computer controlled scanner like an ICOM PCR1000 with the 
>ability to read the signal strength, you could fairly easily measure this, 
>in a reasonably calibrated way.  There's software on the web for it.
         ...and each system would be different...

         Still, be interesting to know...

-- 
         best
         dwp

...the net of a million lies...
         Vernor Vinge
There are Many Web Sites which Say Many Things.
         -me