Metal mesh to make 2.4ghz faraday cage for BLE/WIFI

[Psi]

Hi,

i'm looking at the option of making a small (500mm x 500mm x 500mm) faraday cage for testing BLE/Wifi.

Can anyone answer the following questions.

1) Is making your own faraday cage easy? just a enclosed metal mesh box, or does it get quite complicated?  (I put together a small grounded enclosed box out of alu/tinfoil (5min job) and it didn't seem to block BLE at all.

2) Is there anywhere i can get rolls of metal mesh that will work for 2.4ghz blocking at a good price.

3) Are there any pre-made cheap faraday cages i could just buy?, like under $150.  I know you can get nice ones for $1000

Thanks

[swingbyte1]

You could look at using an old microwave oven if large enough. Remove the magnetron and add pass through.

Sent from my Nexus 5 using Tapatalk

[Ian.M]

1:  Aluminium forms an insulating oxide layer so all your seams probably made poor electrical contact. The resulting slots, if over 6cm long would be very good at transmitting 2.4GHz band RF.   To be reasonably effective you cant have any holes greater than 10% of a quarterwave, which is 3mm for the 2.4Ghz band.   The cheap DIY option is tin plated steel, with all seams soldered or taped with copper foil with conductive adhesive.   For very small enclosures unetched FR4 PCB  with all seams soldered is a reasonably good option.  You can get a bit more screening with double sided PCB with the edges bevelled, soldered inside and out so its effectively a double Faraday cage, insulated from each other and separated by the PCB thickness.  Whatever you do, you'll need a door gasket as well - a copper braid crush gasket is cheap but a total PITA from a maintenance point of view.

2: You can get expanded fine meshes in brass and copper down to about 1mm hole size, and that would be your best option for a viewing port for a DIY Faraday cage, but it doesn't come cheap so you'd probably want to use five solid sides.

3: The door  on a microwave is typically a tuned quarterwave choke RF trap designed for 2.45GHz, so it will be less effective at the limits of the 2.4Ghz ISM band.  Depending on how sensitive what you are doing is to low level leakage, you may need to clean the paint off and supplement it with a thin highly conductive elastomer crush gasket.

[ahbushnell]

Hi,

i'm looking at the option of making a small (500mm x 500mm x 500mm) faraday cage for testing BLE/Wifi.

Can anyone answer the following questions.

1) Is making your own faraday cage easy? just a enclosed metal mesh box, or does it get quite complicated?  (I put together a small grounded enclosed box out of alu/tinfoil (5min job) and it didn't seem to block BLE at all.

2) Is there anywhere i can get rolls of metal mesh that will work for 2.4ghz blocking at a good price.

3) Are there any pre-made cheap faraday cages i could just buy?, like under $150.  I know you can get nice ones for $1000

Thanks

The best is just copper sheet/foil. 
Also bronze screen is used for screen rooms.  I used to use them in grad school. 

You need to seal the joints.  Soldering works well.  If you need a door use EMI gasket material.  You can use the bronze screen for the door if you make sure there is uniform pressure on the seal. 

[ahbushnell]

Hi,

i'm looking at the option of making a small (500mm x 500mm x 500mm) faraday cage for testing BLE/Wifi.

Can anyone answer the following questions.

1) Is making your own faraday cage easy? just a enclosed metal mesh box, or does it get quite complicated?  (I put together a small grounded enclosed box out of alu/tinfoil (5min job) and it didn't seem to block BLE at all.

2) Is there anywhere i can get rolls of metal mesh that will work for 2.4ghz blocking at a good price.

3) Are there any pre-made cheap faraday cages i could just buy?, like under $150.  I know you can get nice ones for $1000

Thanks

The best is just copper sheet/foil. 
Also bronze screen is used for screen rooms.  I used to use them in grad school. 

You need to seal the joints.  Soldering works well.  If you need a door use EMI gasket material.  You can use the bronze screen for the door if you make sure there is uniform pressure on the seal.

Here is a source of brass screen.
https://www.twpinc.com/wire-mesh-material/brass

[Ian.M]

Woven mesh is less than ideal.  When the surface is nice and new and bright, all the strands are well connected, but as it tarnishes and especially if it has been significantly flexed, they are likely to develop high resistance contacts which will compromise its screening performance.  Short of heavily plating it with tin or copper to bond the strands when its new, there's little that can be done to alleviate the problem.    If you are serious about screening use a mesh that's produced from sheet metal not wire.

[Psi]

Thanks guys

[rhb]

Aluminum duct tape might be a solution.  This is metal foil with a pressure sensitive adhesive.   I measured 260 pF/sq inch for two pieces stuck together yesterday.  Small piece on face of larger piece. It's fairly cheap, e.g. $10 per roll.   With even 1/2" of overlap the reactance at 2.4 GHz should be very low. It comes in several widths.  For the size of enclosure that you want, you might want to try using the duct tape to bond regular aluminum foil.  Spray F77 or similar contact adhesive on a close fitting pasteboard box and use the tape to cover the seams with 1" overlap on each side.  Cut the box from a piece of flat sheet, cover both sides with foil and then fold and tape in place with the duct tape on the inside and outside.

But be aware that there are two types.  One with just a thin adhesive and another with a thick mastic.  The latter is far better for HVAC work, but the former looks as if it would be pretty good for EMI shielding.  I plan some testing.

You'll probably need to go to an HVAC supplier or online to find it.

[Ian.M]

That's a lot of work to avoid a little tin-smithing.   Cut up some large rectangular cooking oil tins for materials.   It doesn't come cheaper than that, and 0.5mm thickness with all soldered seams will be lifely to give better results than 0.024mm heavy aluminum foil and aluminum duct tape covered cardboard box.   Also a flat door frame for a tin-plate box with a good surface for a conductive gasket  can easily be built by soft soldering small brass angle strips together, mitered at the ends while they are clamped in place to a heavy sheet of MDF, then reinforcing the corners with tin-plate strips and attached by tack soldering in the center of each side to the deep upturned lip (to allow it to flex to permit thermal expansion), of the hole in the tin plate  and working outwards then going back and filling in to avoid heat related buckling.  There's no such easy option for a cardboard box.

[rhb]

I scavenged a bunch of scrap galvanized sheet from an HVAC shop which I plan to use for durable small enclosures.  It's easy to form and solder.

What I described is about 10-15 minutes work most of it measuring and lay out.  Not as durable.  My main point was you don't need a DC bond.  The capacitance of the tape takes care of the RF.

For ready made, a steel file cabinet suitably modified or a large cookie or biscuit tin are also good.  The file cabinet would need a high conductivity lining to block 2.4 GHz.  Cooking lard is often sold in a tin which is roughly the size the OP seeks. Those are available for free from restaurants for the asking.

Mt personal enclosure is a pair of 2" x 5" x 7" Bud chassis with piano hinge, a hook latch and conductive cloth EMI gaskets.  Still to come are steel baffles over the seams, high conductivity shield and a temperature controlled box using Peltier devices.  I'm hoping to cover 0-70 C or better.

[Ian.M]

I scavenged a bunch of scrap galvanized sheet from an HVAC shop which I plan to use for durable small enclosures.  It's easy to form and solder.

Galvanised steel/iron is prone to shedding highly conductive zinc whiskers.  Its far more common than the plague of tin whiskers and has caused several major data center failures due to whiskers from galvanised cable trays being blown into equipment by the air conditioning.   If you are keeping a galvanised EMI enclosure long term, either hoover it out regularly paying particular attention to dislodging loose whiskers in mechanical joints, or paint the interior to attempt to control whisker growth and prevent any from separating from the surface

What I described is about 10-15 minutes work most of it measuring and lay out.  Not as durable.  My main point was you don't need a DC bond.  The capacitance of the tape takes care of the RF.

...unless you are really really unlucky and the dimensions of a separate piece of foil + capacitance of the glue layer happen to form a resonant patch antennae within the band you are interested in. It can then re-radiate interference in or out.

Some smears of cheap graphite based conductive paint in the overlaps then taping them down while its still wet would avoid the issue, but if you've got or can improvise the equipment to do so, its probably easier to test for leakage before putting the D.U.T. inside.  If you *are* that unlucky, fixing the issue afterwards isn't going to be easy short of a new box build - you could try the paint as is dissipative, let it dry then add another layer of foil on top.

[Psi]

I've sourced some nice copper RF mesh for the cage, so that's sorted

Now i need to make a frame to construct the shield around.

Not too sure what i should use, one idea is those kids toys that connect together, see below.
But i would like something a bit more professional.
Does anyone have an idea for something similar to below but black/white and maybe a bit better constructed.
Something i can clip together to make a frame and then solder the copper mesh around.

https://www.aliexpress.com/item/300PCS-Straw-Building-Blocks-Children-Constructor-Interlocking-Enginner-Toys-Kids-Connectors-Straws-Blocks-Set-Creative-Gift/32853257406.html

[tautech]

I've sourced some nice copper RF mesh for the cage, so that's sorted

Now i need to make a frame to construct the shield around.

Not too sure what i should use, one idea is those kids toys that connect together, see below.
But i would like something a bit more professional.
Does anyone have an idea for something similar to below but black/white and maybe a bit better constructed.
Something i can clip together to make a frame and then solder the copper mesh around.

Do you have access to a propane or oxy torch......I'm thinking 1/8" brazing rod. Or maybe jump to 1/4" for something more sturdy. If you have some plumber or aircon mates......copper tubing. They'll have some sort of torch too and can probably knock you something together. Go too big and sweating the copper mesh to it without a torch could be a problem.

[Ian.M]

Does your local DIY  stock brass angle strip?  If so, it will make a neater frame than brazing rod.  Copper rivet the corners so it holds together while soldering it, and you should be able to get away with a very small pencil blowtorch or even a really heavy duty soldering gun for assembly using soft solder.

For the copper rivets you need really heavy gauge bare copper wire several mm dia.   Stretch it straight, anneal in a gas flame and quench from red hot in water so its dead soft.  Clean off any scale with a scouring pad.   As tooling to form the rivets, you'll need a small steel block with a hole drilled in it that closely fits the wire of depth equal to the desired rivet shank length.  Start by drilling right through at no more than 1/2 the wire diameter to have a hole you can use to push the formed rivet out of the block then drill to the desired depth.  Ideally the hole should either be reamed or polished with lapping compound on a softwood lap, but if the drill bit is sharp and you stone the corners slightly it will probably be clean enough as is.  Lightly oil the hole.

To make the rivets, insert the wire till it bottoms out. Snip off about 1 dia above the block surface.  Upset and form the head with a light hammer.  Use a pin in the through hole to push the formed rivet out.   

When riveting you'll need a dolly - take a square section steel rod, drill the center of the end to form a shallow dimple to take the rivet head then taper the end till its thin enough to give you good access in the corners of the brass angle frame.  Whenever possible clamp the dolly in a vice, or put the other end against a scrap-wood block on the bench.  If you have to hand hold it, it needs as much mass as possible or it will be a royal PITA to control.

[Psi]

I have a tig welder, i'm just trying to avoid spending a lot of time on this.
I'm making it for work but i'm spending personal time doing so.

Something that just 'clips' together would be quicker.

[rhb]

I have a tig welder, i'm just trying to avoid spending a lot of time on this.
I'm making it for work but i'm spending personal time doing so.

Something that just 'clips' together would be quicker.

If you've bought copper mesh, why would you not use soldered rigid copper pipe for the frame? The mesh can be soft soldered directly to the frame.  It will also have the virtue of making repairs to the screen easier.

[tautech]

I have a tig welder, i'm just trying to avoid spending a lot of time on this.
I'm making it for work but i'm spending personal time doing so.

Something that just 'clips' together would be quicker.

Never used a TIG as just a heat source for brazing or sweating type joints but I don't know any reason why you can't.
A roll of 1/4" aircon copper tubing and fold most of the frame up around a suitable radius former leaving just the interconnects between the 2 end squares would be how I'd attack it. Some plumbers Silfos or Easyflo is very easy to use on copper and you'd probably need no flux if everything's bright and shiny.

Alternatively if you have a powerco linesman mate some 1/4" solid copper rod that they use for 11KV droppers would make a good frame too. With a little bit of ingenuity some 1/4" ID tubing could be used to manufacture corner fittings. 

[Psi]

The goal is to get away from anything that takes time,
i want to snap something together in 5min and wrap it in copper shielding with a couple of solder tacks to hold the mesh to itself at each corner.

That would be doable using those aliexpress building toys. but i'd rather find something a bit more professional that just snaps together.

[rhb]

You don't have to solder the copper pipe.  You can just slip it together, but it won't work as well if you don't.

[jeremy]

If you don’t need the frame to be conductive, basically every aluminium supplier (incl bigger hardware stores) sells some sort of aluminium box section frame kits. They sell plastic corner connectors and you just cut the box section to length. Certainly could snap together in less than 5 min. For example: https://www.actionaluminium.com.au/products/aculok-system/

[Smokey]

I am officially not an RF guy, but I've been pretending recently.  So keep in mind I don't really know what I'm talking about.  But....

What good would a faraday cage be if you didn't have absorbing material to knock out all the reflections inside the cage? 

[buta]

1) Is making your own faraday cage easy? just a enclosed metal mesh box, or does it get quite complicated?  (I put together a small grounded enclosed box out of alu/tinfoil (5min job) and it didn't seem to block BLE at all.

Want to know the requirement and acceptance test procedure.
What is the expected attenuation from the cage?
what is the test procedure to find "it (a small grounded enclosed box) didn't seem to block BLE at all"?
Have you identified the BLE source?

[jonpaul]

see screen rooms
use only copper small mesh screen
seal all joins and gaps with RF Shielded finger stock or 3M copper Shielded tape.
Solder all connections.

In 1970s Dick Sequerra built screening rooms in the Marantz and Sequerra labs, size accommodation for the tech/ eng, workbench and equipment 2m HX 2.5m d x 3mw cost ~10..25k$

Jon

[ahbushnell]

Woven mesh is less than ideal.  When the surface is nice and new and bright, all the strands are well connected, but as it tarnishes and especially if it has been significantly flexed, they are likely to develop high resistance contacts which will compromise its screening performance.  Short of heavily plating it with tin or copper to bond the strands when its new, there's little that can be done to alleviate the problem.    If you are serious about screening use a mesh that's produced from sheet metal not wire.

I would solder the joints.  It's nice to see inside what you are testing.  But I assume you will need to bring in power and signals.  Power can go through EMI filters.  Signals are trickier. 

[Ian.M]

Soldering the edge joints of a woven metal mesh doesn't prevent bad contact between crossing  wires in the weave, hence my comments above.  I *really* wouldn't fancy trying to get a solder joint on every wire crossing in a fairly fine weave mesh - as, assuming a 3mm wire pitch, that's close to 4500 crossings to solder in a 20 cm square 'window'!