Proper faraday cage mesh size
Finn Hammer, Tue Mar 12 2019, 12:23PMAll,
I have just had a discussion with a scholarly engineering type kind of guy (as opposed to myself who is more of a "poke it and see" kind of a guy.
Anyway, we talked about faraday cages, and I alledged that the mesh size should be 50mm by 50mm (two by two inch).
This sent him through the roof, claiming that the mesh size was effective if it was 5% of the wavelength.
Coil Fres os 50kHz, so wavelength is 6000meters, and so the cage mesh size would not have to be smaller than 300 by 300 meters.
I don't know, but something doesn't seem to add up, here. I do realize that this only deals with the fundamental resonant frequency, and there may be noise higher in the spectrum, but how high and how much?
Can you help me with a solid argument for the proper size of mesh in a faraday cage for DRSSTC's in the 50kHz range?
Cheers, Finn Hammer
Re: Proper faraday cage mesh size
Sulaiman, Tue Mar 12 2019, 12:53PM
If you want to screen far-field electromagnetic waves then what your friend argued is close to correct.
But we operate TCs in near-field (scalar) mode
(e.g. at 50 kHz if you are 6m away that's only 1/1000th wavelength)
and
it is not an electromagnetic wave, it is a current carrying thread of plasma at varying potentials relative to 'earth'.
I GUESS that the maximum spacing is about the same as the distance of your body parts from the mesh
- but I would not bet my life on it.
===========================================
T Cs generally perform very porly as radiowave transmitting antennas due to their small dimensions relative to operating wavelengths,
so there is surprisingly little transmitted r.f. power. (I've checked)
(but more than enough r.f.i. to cause some problems)
(ask an amateur radio enthusiast how easy it is to launch 1W at 137kHz even.)
And operating wavelength is not (normally) determined by wire length or antenna height.
So a TC is mostly just an L.C resonator with essentially capacitive coupling of the topload and inductive coupling of the primary to nearby objects.
... no magic ... other than the awesome arcs ''n' sparks (and their physics) :)
P.S. ... 50 kHz ... that's a big coil !
Sulaiman, Tue Mar 12 2019, 12:53PM
If you want to screen far-field electromagnetic waves then what your friend argued is close to correct.
But we operate TCs in near-field (scalar) mode
(e.g. at 50 kHz if you are 6m away that's only 1/1000th wavelength)
and
it is not an electromagnetic wave, it is a current carrying thread of plasma at varying potentials relative to 'earth'.
I GUESS that the maximum spacing is about the same as the distance of your body parts from the mesh
- but I would not bet my life on it.
===========================================
T Cs generally perform very porly as radiowave transmitting antennas due to their small dimensions relative to operating wavelengths,
so there is surprisingly little transmitted r.f. power. (I've checked)
(but more than enough r.f.i. to cause some problems)
(ask an amateur radio enthusiast how easy it is to launch 1W at 137kHz even.)
And operating wavelength is not (normally) determined by wire length or antenna height.
So a TC is mostly just an L.C resonator with essentially capacitive coupling of the topload and inductive coupling of the primary to nearby objects.
... no magic ... other than the awesome arcs ''n' sparks (and their physics) :)
P.S. ... 50 kHz ... that's a big coil !
Re: Proper faraday cage mesh size
johnf, Tue Mar 12 2019, 05:42PM
look up "shielding effectiveness" there are formulae for frequency vs mesh size /thickness/ material.
materials matter especially at the lower freqs where you do want magnetic type material as opposed to high freqs where copper ground planes rule
also look up the "rolling sphere" as it related to lightning protection
johnf, Tue Mar 12 2019, 05:42PM
look up "shielding effectiveness" there are formulae for frequency vs mesh size /thickness/ material.
materials matter especially at the lower freqs where you do want magnetic type material as opposed to high freqs where copper ground planes rule
also look up the "rolling sphere" as it related to lightning protection
Re: Proper faraday cage mesh size
Finn Hammer, Tue Mar 12 2019, 06:14PM
Sulaiman, JohnF
Thanks for your replies. I am not sure the coil qualifies the "big" criteria, at least not in the physical sense, and in the other senses, it is more like Great!.
50 kHz is more like the maximum comfortable frequency to run a 1kA switch at.
Anyway, here it is alongside myself, and I am 187 cm if i stood up straight.
Cannot seem to upload pictures to the site right now.
Cheers, Finn Hammer
Finn Hammer, Tue Mar 12 2019, 06:14PM
Sulaiman, JohnF
Thanks for your replies. I am not sure the coil qualifies the "big" criteria, at least not in the physical sense, and in the other senses, it is more like Great!.
50 kHz is more like the maximum comfortable frequency to run a 1kA switch at.
Anyway, here it is alongside myself, and I am 187 cm if i stood up straight.
Cannot seem to upload pictures to the site right now.
Cheers, Finn Hammer
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