How to make a box that blocks RF signal? - Page 1
Forum section: electronics / beginners

author: huu_tri0101
author_rank: Contributor
timestamp: April 01, 2015, 08:59:42 am
content: I just want to make a box that block all RF signals (wifi, bluetooth,....) come from my smartphone, which put inside that box. And what tool I can measure to make sure doesn't have any leakage signal from the box? Do I need to build "a clean signal room" to make accurate measurement ?Thank you.

author: electr_peter
author_rank: Unknown
timestamp: April 01, 2015, 09:13:09 am
content: What exactly are you trying to do?Simplest solution - turn off smartphone and remove battery. Second solution - put smartphone in some metal/metal foil/metal mesh box - it will kill all wireless communications. However, if you want something serious, it starts getting difficult. You need spectrum analyser and some antennas as a bare minimum.

author: huu_tri0101
author_rank: Contributor
timestamp: April 01, 2015, 09:30:24 am
content: Quote from: electr_peter on April 01, 2015, 09:13:09 amCan you tell me "serious solution" please?I just want to make a box that isolated all the RF signals from the outside. Then I will put some test equipment and my smartphone to make some RF test.

author: electr_peter
author_rank: Unknown
timestamp: April 01, 2015, 09:43:36 am
content: Search for RF (or EMC) anechoic chamber.Some forum members have hands on experience:www.eevblog.com/forum/testgear/emc-chamber-build-log/

author: mcinque
author_rank: Unknown
timestamp: April 01, 2015, 11:45:48 am
content: I think he wants only a box and not an entire room; I guess it would be ok to have a metal box with proper metal mesh gasket shielding.Depènding on the frequency, even a small, microscopic aperture, will make the signal enter in the box. I'm talking of a small space, in GHz range, even a sheet of paper between the cover and the box it's an enter point.The matter is to minimize the reflections, because otherwise the signal you measure could be the signal + many reflections... rf adsorbing material are really expensive.

author: Simon
author_rank: Unknown
timestamp: April 01, 2015, 11:59:09 am
content: a complete metal box with no gaps will stop RFI, although aluminium lets RFI through a complete box of it with no gaps forms a "farraday cage" and will also work but steel will definitly block RF, if you need to stop high frequencies then yes you need to use a gasket on any lid, and RFI gasket as this is conductive, keep lid and box connected and so that the gasket itself dones not leak RF, you box and any lid/door also need to be as flat as possible to make a seal and you will need multiple fixings (bolts etc) reasonably close together so that the door or lid cannot bend and create too big a gap for the gasket to seal.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: April 01, 2015, 12:00:54 pm
content: Would an old microwave oven work?

author: Ian.M
author_rank: Super Contributor
timestamp: April 01, 2015, 12:26:03 pm
content: Not unless extensively modified.Most microwave ovens use a quarterwave choke designed into the door face or edge geometry to provide excellent attenuation at their operating frequency. At other frequencies it is rather ineffective. You would need to start with a stainless cabinet and interior to have conductive surfaces rather than a paint finish and to fit a conductive gasket, between the door and its facing surface. You'd also probably need to fully dismantle it to check the integrity of all joints and fit conductive gaskets and caulk any corners with bronze wool.You will also want to do something about the magetron waveguide opening, probably replace it with a filtered feedthrough plate to let you get power and data connections to any test equipment you have inside the oven and you may want to replace the turntable motor with a stepper or selsyn so you can rotate the device under test, or simply remove it and blank off its shaft hole.You will still have the problem that the interior is highly reflective with nothing to damp any resonances and is far too small for anything except near field testing.If the application is simply to effectively disable visitors' phones for confidentiality reasons, a steel ammo box with the gasket grooves and mating faces cleaned off to bare metal and tinned, and the gaskets fully covered with copper braid or flexible mesh is probably the most effective solution.

author: TimFox
author_rank: Super Contributor
timestamp: April 01, 2015, 12:34:23 pm
content: Building a shield box requires a good gasket to seal the gap when the box closes, as stated above.The simplest gasket is to get copper tape that is conductive on the glue side and totally cover the gaps where the cover meets the rest of the box.However, it becomes trickier if you need "pass-throughs" for power, signals, etc.

author: Ian.M
author_rank: Super Contributor
timestamp: April 01, 2015, 12:46:13 pm
content: That's a good way of EMP proofing a metal container for backup comms and navigation equipment. If you sail or drive off-road in remote areas that have frequent thunderstorms with a high probability of lightning strikes, keeping your backup radio, GPS and compass in a tin sealed with copper tape is a wise precaution.

author: Marco
author_rank: Super Contributor
timestamp: April 01, 2015, 01:07:41 pm
content: Just for reference ... gaps only leak because they are not far smaller than the wavelength along all dimension right? (ie. a 1000 pinholes of 1 mm diameter don't leak much at 900 MHz, a meter long 1 mm gap does.)PS. nevermind, searching a bit I see a 1 mm mesh is way too coarse to stop 900 MHz either way.

author: DmitryL
author_rank: Regular Contributor
timestamp: April 01, 2015, 01:20:36 pm
content: Quote from: Ian.M on April 01, 2015, 12:26:03 pmErr.. AFAIK ovens operate at 2.4GHz, which gives us 125mm wavelength. 1/4 of it is ~31mm. I have never seen 31mm gaps in their doors..

author: Ian.M
author_rank: Super Contributor
timestamp: April 01, 2015, 01:31:47 pm
content: Ever noticed how chunky the door surround round the window is? That's where they normally hide the ~31mm linear dimensions cavity behind a plastic cover strip that keeps steam and dirt out so the cavity Q doesn't get degraded. It would also be possible to put the cavity in the body face if a slimline door is a design requirement. Seehttp://diagramas.diagramasde.com/otros/Microwave(Sharp)%20Training.pdf, page 27.

author: TimFox
author_rank: Super Contributor
timestamp: April 01, 2015, 01:41:08 pm
content: Quote from: Marco on April 01, 2015, 01:07:41 pmA narrow slot in thin sheet metal forms an antenna. If the length of the slot is short, it's not a good antenna.When adding ventilation to a shielded box, it is common to use an array of small-aperture holes as "waveguides beyond cutoff", where the signal attenuates exponentially down the length of each hole since the waveguide formed by the hole is too small in area to support the frequency as a traveling wave. However, the holes must have sufficient length to achieve the required attenuation. The shielded rooms that I am familiar with used panels filled with honeycomb patterns (maybe 2 mm holes 20 mm deep) to give a large total area for airflow, but each hole was a cutoff waveguide. Instead of a "traveling wave", the penetration down the hole is an "evanescent wave".See this link for more details:http://formfactors.org/developer%5Cspecs%5Cwg_overview_098.pdf

author: Macinpup
author_rank: Newbie
timestamp: January 27, 2023, 05:21:23 pm
content: Nothing works because the rf rides not just the electric grid. You can be in the woods, underground, underwater, and an RF signal will reach you with or without the electric grid. Go watch YT channel LookoutfaCharlie

author: jonpaul
author_rank: Super Contributor
timestamp: January 27, 2023, 05:26:29 pm
content: Check Fartaday cage.Build a screen room, cost ~ 10..30k$j

author: Bud
author_rank: Super Contributor
timestamp: January 27, 2023, 05:53:23 pm
content: So much overengineering adviceDo it simple: take your bluetooth earphones paired with the phone, start playing an audio on the phone, put the phone in a tin cookie box, see if audio stops playing in the earphones, meaning your phone lost bluetooth connection and bluetooth devices cannot discover it anymore.Then call your phone number from another phone, listen if the phone in the tin cookie box not ringing, verify that you get "the number you are calling cannot be reached" voice prompt or that the call gets transferred to the voicemail box.Done.

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 27, 2023, 05:59:35 pm
content: A thread from 2015 resurrected by a new member with a youtube video.