How to stop WiFi in a tin box? - Page 1
Forum section: electronics / rf-microwave

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 07, 2025, 03:27:11 pm
content: Wi-Fi seems to be everywhere and impossible to block! I want to test Wi-Fi disconnects on a Raspberry Pi Pico2W, so I threw it in a tin box and shut the lid. No effect whatsoever!Yes, I understand — the cable leaks, the choke is ineffective, the lid isn’t airtight, and the paint might prevent good electrical contact. But really, what needs to be done to stop Wi-Fi from getting into a tin box, while still letting a USB cable go in?

author: BradC
author_rank: Super Contributor
timestamp: September 07, 2025, 03:44:03 pm
content: Is the box earthed to the Pi or just floating?

author: ledtester
author_rank: Super Contributor
timestamp: September 07, 2025, 03:46:16 pm
content: Somewhat recent thread:https://www.eevblog.com/forum/beginners/faraday-cage-to-block-wifi/One poster noted the cage had to be earthed in order to work:https://www.eevblog.com/forum/beginners/faraday-cage-to-block-wifi/msg3475570/#msg3475570

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2025, 03:56:18 pm
content: It probably leaked all around the slit between the lid and the walls, and yes, if you aren't careful with screening and filtering it will leak along the cable.As the Pico 2W's USB controller tops out at 12 Mbps, filtering out >99% of a signal over three orders of magnitude higher shouldn't be too tricky. A fully screened USB cable with the screen grounded to the box right where it enters will probably be good enough. If not, add a ferrite right next to the box wall inside it, covered in copper tape and soldered to the wall, and ground the cable screen again where it emerges from the ferrite. Repeat outside for even more attenuation.You'll have to either make a slotted plate to solder to the box to let you pass the cable with its plug on, or re-terminate the cable yourself so you can make the hole really small and get the gap round the cable and length of the screen ground bond to a small percentage of a quarterwave at 2.45 MHz*.You need to remove all the paint round the outside where the lid fits and check there is no lacquer on the inside of the lid as you need good continuity all the way round. If you still get too much leakage you may need to remove the paint from the lid edges and the sides below the lid and cover the lid seam with conductive adhesive copper foil tape, all the way round every time you close it 'in anger'!Yes its a PITA if you are in and out of the box a lot, but the alternative is to add a flange for a carefully engineered 1/4 wave choke between the lid and the box, with additional EMC gasketting to block residual leakage, and at that point, to do it right is so much work and cost, you might as well convert a bright stainless finish microwave oven by gutting it, adding a door gasket for additional band edge attenuation, and replacing the magnetron and waveguide cover with a feedthrough mounting plate to get desired signals in and out.Also check continuity between the sides and the bottom as that's a rolled seam so lacquer or paint may be insulating the bottom turning it into a plate antenna reradiator. If there are any doubts, fully solder the seam, and the side seam as well!If you need to see or measure inside the box use a USB hub and a webcam with LED illumination.* The Pico 2W only does 2.4 GHz WiFi, so a quarter-wavelength is 30.6 mm and you need to get any gaps down to at most a couple of mm, ideally a small fraction of a mm. Conductive adhesive copper foil tape is your friend!

author: radiolistener
author_rank: Super Contributor
timestamp: September 07, 2025, 04:16:14 pm
content: You are probably familiar with the concepts of the near and far field of an antenna, as well as inductive and capacitive coupling. The reason your “leakage” occurs is exactly this. The effectiveness of shielding strongly depends on the thickness and construction of the enclosure.For effective suppression of radiation, you need to:1) Provide a gap between the antenna and the shield - ideally on the order of one wavelength. For 2.4 GHz Wi-Fi, this is about 12.5 cm.2) Use a shield thick enough to provide the necessary attenuation.3) keep away from shield for at least wavelength distance.4) To prevent radiation from leaking through a cable exiting the shield, you need to install ferrite beads or cores on the cable to act as an RF choke, ideally at both sides, inside and outside shielding box. You probably seen such RF chokes on display or USB cables. Your cable have one, but I think this may be not enough, especially when there is 2 Watt transmitter. And keep that RF chokes as close as possible to shield border.As an example, a microwave oven effectively shields 2.4 GHz signals, typically providing around 60 dB of attenuation at 2.4 GHz.Regarding the Raspberry Pi: I have mine in a metal case for passive cooling with a plastic opening for Wi-Fi. This reduces the signal by 20 dB, making the connection noticeably weaker and unstable even within the same room.

author: Ian.M
author_rank: Super Contributor
timestamp: September 07, 2025, 04:31:05 pm
content: Poor man's quick and dirty alternative: Use a USB A-A extender cable with an exposed shell at the socket end. Check it has screen continuity. Use a very short A to micro-B adapter cable to connect the Pico. Wrap the pico in tape for insulation then wrap it and the adapter cable right back to the extender A socket in lots of turns of kitchen foil, folding the end every few wraps. Put a few rubber bands where the foil covers the A socket to keep it in close contact and the inside should be reasonably WiFi free. However unlike the box which can be tested with a USB SDR inside it or a WiFi USB stick transmitting at max. power and a spectrum analyser and suitable antenna outside it, the closely wrapped foil screen is 'hope and pray'!Radiolistener makes a good point that an imperfectly conductive faraday cage can re-radiate due to eddy currents in its walls, so you may well need a box-in-a-box as he suggests. However as you don't need metrology grade attenuation, only enough to prevent it handshaking with nearby access points, you are in with a good chance with a single box. As thin tinned steel isn't the best for thickness or conductivity, I'd try removing the paint and covering the outside with copper foil tape before I tried double-boxing.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 07, 2025, 06:18:36 pm
content: I did post something about this in the past. But here it is again.We had a big problem with RF leakage in a HF transmitter amplifier. We could not get the feedback down to anything reasonable and the thing would feedback and cause the whole thing to oscillate, this includes the transmitter and then the amplifier. Eventually someone wrapped the amp in padded aluminum tape. This cured the problem. I never thought it would work but it did. The aluminum part of the tape was facing out and was not connected to anything. Perhaps wrapping the metal box in this sort of tape may help? The tape was 2 inch wide and the padding part was about 1/4 inch of spongy rubber stuff. available at hardware store. There were openings for the fan assisted ventilation so the tape was not entirely continuous. I suppose this is a quick and dirty way of double boxing.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2025, 06:38:27 pm
content: The insulated lip between the lid and box forms a slot antenna which will allow high enough frequencies, like WiFi, to pass through.Clear the paint and add finger stock.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 07, 2025, 06:39:49 pm
content: This is getting ridiculous. Bud box, screwed shut, edges sealed with aluminum foil tape. The only gap is just the thickness of the adhesive. The feedthrough capacitors are 1.5 nF — and WiFi still works perfectly.

author: tggzzz
author_rank: Super Contributor
timestamp: September 07, 2025, 06:59:54 pm
content: Is the tape designed for EMI purposes with conductive adhesive?If you lightly touch the box with DMM probes, what is the indicated resistance at various points?Is any part of the PCB in contact with the box?Consider using a diecast aluminium box, and/or putting a battery inside the box and having no external connections.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 07, 2025, 07:16:44 pm
content: The box is a die cast box. The pico PCB is not in contact, the only electrical contact is via the USB GND.https://www.budind.com/product/general-use-boxes/econobox-series-aluminum-boxes-electronics/cu-124

author: Bud
author_rank: Super Contributor
timestamp: September 07, 2025, 07:30:18 pm
content: Did you try connecting USB shield with USB ground at the box ?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 07, 2025, 07:45:25 pm
content: Yes, USB GND, cable shield are connected to the box on the outside. Only USB D+/- and Vcc passes through the caps. I had to admit, these are old stock through caps, probably not ever meant to be for GHz environments. But still…

author: tszaboo
author_rank: Super Contributor
timestamp: September 07, 2025, 08:32:41 pm
content: You can also cover the antenna on the pi pico with a copper tape. Short it out, for the test if needed, it shouldn't matter for this test.Some RF receivers are very very sensitive. I was doing EMC precompliance testing, and we had the DUT in a proper anechoic chamber and it was still receiving from the outside.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2025, 08:59:27 pm
content: Quote from: dobsonr741 on September 07, 2025, 06:39:49 pmUnless the seams with the aluminum foil tape are conductive, they still form a slot antenna and let the 2.4 and 5 GHz right through. I guess they make special tapes with conductive adhesive, but I never used them. I liked making RF proof boxes out of double sided copper clad printed circuit board material, with a soldered seam on both sides for strength.

author: tggzzz
author_rank: Super Contributor
timestamp: September 07, 2025, 09:23:03 pm
content: Quote from: edavid on September 07, 2025, 09:17:05 pmFrom a dim 35yo memory, microwave ovens can leak up to 1W, i.e. -30dB from 1kW. No idea what a typical attenuation would be.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 07, 2025, 09:55:23 pm
content: Turns out the tape material really matters. It’s probably not the adhesive thickness so much as whether it’s conductive. With copper foil instead of aluminum, I can push the Pico’s reported RSSI down near –90 dBm, and the link finally dies in the –85 to –90 range. Ferrites on the USB cable or beads on this inside on the wires made no difference, which makes sense at 2.4 GHz. That restored my faith in the feedthrough caps. My best theory now is slot the antenna effects — it’s the length of the gap that matters most, not the width, and the conductivity of whatever is doing the blocking front of it.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 16, 2025, 11:59:22 am
content: Shielding with aluminum is tricky. The metal itself is fine, but the surface is aluminum oxide, which is an excellent insulator (unless you have a special finish on the aluminum, i.e. chromate conversion). The oxide forms almost instantly, sanding it before assembly won't work. Need some wiping action when mating pieces together to break through the oxide. This applies to aluminum tape, too.One trick I've used in the past to force good electrical contact between smooth aluminum parts is to put pieces thin copper screen between the pieces. The rough texture of the screen helps bite through the oxidation.For tapes, "conductive" adhesive is a misnomer, the adhesive itself is not conductive, it just has metal particles inside it which provide conduction through the adhesive. For shielding, I prefer tape with an embossed patternlike this, so the tape itself is able to cut through the adhesive. Both types of tape take a good amount of pressure to work properly.

author: David Hess
author_rank: Super Contributor
timestamp: September 16, 2025, 02:24:19 pm
content: Quote from: mtwieg on September 16, 2025, 11:59:22 amTektronix had an EMI or EMC option on their old oscilloscopes which included something like twisted finger stock with sharp edges sandwiched between aluminum members to make a continuous conducting surface. I have also seen something like copper coaxial shielding, but with a sharp "wool" like texture, used as a gasket between aluminum surfaces.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 16, 2025, 07:39:29 pm
content: Quote from: David Hess on September 16, 2025, 02:24:19 pmYeah there's a zoo of different gasket/fingerstock options out there, including different plating materials for better galvanic compatibility. I was just mentioning something more commonly found in a lab.

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2025, 01:37:44 pm
content: yes that tape is confusing you see if you just slap it on you might get some ohms of resistance. And that is copper tape. The conductive tape, lets say if you wanna use it as test leads to slap on something, expect 15 ohms or so, with no pressureHVAC tape will give you open circuit if you try the same thing.Altoids tin is laquered, bud box is anodized (maybe). Did you see if the bud box is conductive on the surface to glancing dmm probe?For aluminum to be electrically conductive like you want, it needs plating. Things like alodine make it conductive too, if its pressed good.