Faraday cage emissions hole size vs distance - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: August 28, 2023, 11:38:56 pm
content: Hi,what is the logic behind the hole size being less than 1/10th of the wavelength of the highest frequency to be blocked? More precisely what is the attenuation at what distance for that rule of thumb?I assumed that rule of thumb would block almost all the electric field emissions. But when I enclosed a main foil-type heater panel (obviously running at 220V AC @ 50Hz) with mesh with 3mm holes, I still get more than 50V a 10-20 cm in front of it. That is whether I ground it or not. Why is that?How can I estimate the electric field strength at a distance based on the mesh hole size?Thank you

author: ricko_uk
author_rank: Super Contributor
timestamp: September 02, 2023, 12:08:57 am
content: Hi,any comments anybody? I could not find any info apart from various forums threads mentioning that transmission line effects come into play. But nothing about electric field intensity correlating hole size, distance, amplitude (or attenuation) and frequency.Thank you

author: Dundarave
author_rank: Regular Contributor
timestamp: September 02, 2023, 01:20:40 am
content: I’d suggest that you try completely sealing it temporarily, with tin foil for example, to ensure that you’re not measuring something else, like the power cable or other ambient source.If your readings drop to zero, then you at least know that your measurement set-up is accurate. If the reading doesn’t change, then you need to look elsewhere to fix your measurement set up.

author: KE5FX
author_rank: Super Contributor
timestamp: September 02, 2023, 01:45:07 am
content: For RF shielding, the opening with the largest dimension dominates the behavior. You might have a 1-meter-wide mesh with 1x1 mm apertures, but if there is a 0.01-mm crack along its edge, it will form a 1-meter slot antenna.If you are trying to shield 50/60 Hz magnetic fields from an AC line-powered device, though, you've got a different sort of problem. A couple of layers of expensive mu metal may be needed for that, or several layers of another metal. Ultimately there is no good substitute for distance.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 02, 2023, 01:45:11 am
content: The wavelength over 10 is really a rule for far field radiation of E&M travelling waves. So measuring several wavelengths away. So not applicable for 50 Hz where everything building sized is near field.That said, a grounded mesh should screen low frequency electric fields very well, but it won't screen low frequency magnetic fields at all. The screen needs to be grounded to work properly and if there is a ground potential between the screen and your meter you will still pick that up.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 03, 2023, 02:22:55 am
content: Thank you all

author: AnalogTodd
author_rank: Regular Contributor
timestamp: September 03, 2023, 01:47:43 pm
content: Quote from: ejeffrey on September 02, 2023, 01:45:11 amVery true. A good read on this isNoise Reduction Techniques in Electronic Circuitsby Henry Ott. Low frequency magnetic fields can't be blocked easily with copper or aluminum mesh like a typical Faraday cage uses. You need a material with high magnetic permeability like Mu Metal.

author: jonpaul
author_rank: Super Contributor
timestamp: September 03, 2023, 03:28:01 pm
content: See a univ or grad level book on classic electromagnetic theory.The attenuation of a TEM wave thru a conductive plan with a hole is well documented.Look at commercial radio TX/RX and microwave/mm wave TX and RX for good shield/enclosuer/RF seal designsJ

author: EPAIII
author_rank: Super Contributor
timestamp: September 05, 2023, 02:56:33 am
content: The big problem with 50/60 Hz power line frequency is the radiation is everywhere. It literally comes from the wiring in any building and the transmission lines from the generators and transformer stations to your location. This can be easily observed on an oscilloscope by connecting a 6" ground wire from the probe's shield to it's tip. You will pick up a lot of 50/60 Hz with that "grounded" scope probe.As a first check I would leave your measuring pick-up at the same place and turn your heater off. In fact, completely disconnect the power cable that goes to it. And see what you measure at that same 10 to 20 cm distance.I suspect you need to enclose your entire experiment in an overall Faraday cage and also shield any AC power cables that run from outside that overall cage to the smaller cage you have around your heater. Only when you have totally eliminated that external 50/60 Hz radiation will you be able to observe any radiation coming from your device under test.PS: Jonpaul meant "conductive plane" not "conductive plan". The internet does not seem to have much on this other than to say microwave ovens have a Faraday cage for protection against leakage. You will need to go to an actual E&M text book. Try a public library with actual paper and ink books. Or, better yet, a library at a university where physics or electrical engineering is taught.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 07, 2023, 08:17:31 pm
content: Thank you all, more research to do and more things to try now