Welded mesh for Faraday cage

Of course, there is always this material, which could be substituted for cage wire mesh.

Given it is fairly stiff, I was thinking it could be bent into an oval shape and two ends added. That would eliminate the corners, thereby reducing seams to seal.

Any thoughts please?

Mark Miller

Wavelength of 4GHz is 7.5cm. Mesh spacing has to be several times smaller than that to have decent (E.g. 3 to 10 dB) attenuation.

For VHF instruments (wavelength 1 or 2 meters) we commonly use mesh of

2mm. It can be effective at the 40dB level but door gaps and hinges are big leakage sources. BeCu spring contacts are standard but still most of the leakage at that point is coming from the seams.

Die-cast aluminum boxes (even with overlapping lips on the seams) are markedly inferior (due to leakage at the seams) to a box made out of printed circuit board material and soldered shut.

At some point you'll give up with the mesh and hinges and BeCu springs, and just build a box out of unetched printed circuit board soldered shut at all the seams. Printed circuit board is cheap. Use fiber optics and/or good quality feedthrough caps for all input/ output. Really, do skip all the intermediate screwing around with mesh if you need serious (as opposed to cosmetic) attenuation.

Tim.

It would need to be of a finer "mesh size" than 2" x 3" rectangular holes.

You'll need to line it with another mesh of copper. Think "macrome".

Then conductive epoxy at each node to attach it to the cage frame.

Then, you're a little closer.

The smaller the hole opening, the more opaque it will be to RF. "Real" screen rooms use gaskets on the doors, like little fingers to grab and seal.

Hello Mark,

I don't know what attenuation you expect, so just a number

Screening Efficiency for (mesh size)/lambda =3D 0.05 (so 20 bars per wavelength) and (bar thickness)/(mesh size) =3D 0.1, will give you about

19 dB attenuation for plane wave excitation. Half the mesh size, gives 6 dB increase in attenuation. Of course metal thickness >> skin depth.

(bar thickness)/(mesh size) follows an "ln" relation, so increasing the bar size does help, but not as much as halving the mesh size.

If you don't have absorbing materials in your cage, you will get strong standing wave patterns that will reduce the effective screening significantly.

With kind regards,

Wim PA3DJS

without abc, PM will reach me.

Rummage around on the internet and download Mil-hdbk-419.. It is on grounding, bonding, and shielding. And has a chapter on building a screen room.

Dan

Here's something I've been wondering about for a long time:

I put my cellphone inside of a steel cookie tin, sealed the lid with aluminum tape, and put the cellphone inside of the cookie tin into the microwave.

When I called my cellphone, it still rang.

Why didn't the steel cookie tin + microwave oven block the signal?

Jon

I don't know, but I would throw the microwave oven away. You probably already have cancer if it leaks that badly.

Does it make your smoke alarm chirp too (for a split second)?

Not a complete hijacking though. Call it a tangent.

You should power off the cell phone, and power it on while it is in the tin, and in the mw cavity. Turn it on and drop it in the tin quickly and seal it, then the oven door.

THEN try to call it.

Previously, the phone had recently been pinged by the node it was currently hooked in with. After going in the box, it may have lost that hook, but still got the ring packet from the tower.

How it made it in is a different story.

Yeah. And you have no clue about how. Jeez, wot a dolt.

I was working with magnetics before you knew what a magnet was.

No doubt. That steel plate in your head must have had a real impact on you. When they had the top of your skull open did they dig out the atrophied brain and replace it with feces?

You have to be careful when doing that--it's easy to solder the wrong side of the copper someplace if you aren't paying attention, which will result in a very leaky shield. I sometimes put copper tape round the cut edges of the board, particularly when making lids. That makes it more convenient to get full shielding.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

I haven't done the experiment, but I find it hard to believe a die cast box leaks a lot of RF. Are you referring to those Hammond boxes?

Surplus shops usually have assorted RF shields. Just soldering one of those to the PCB would save some effort.

BTW, BeCu makes decent battery contacts if you have to home brew a fixture.

Oooops. Looks like someone's fetish is peeking out. Nice job, loser.

e

nd

t

If its a new phone (CDMA, UMTS, etc..) it will drop the call if it loses more than 6 or 7 sequential packets from the towers. Yes, towerS, plural. Or in other words, a couple milliseconds. That's all it takes. Things were different back in the IS-54 days.

More likley, either the shielding wasn't as perfect as originally thought, or there was some electromagnetic coupling of the container to the cell phone antenna. Phone can (but often dont') work in the bowels of big container ships, for example. Same concept here.

The glue isn't conductive and the microwave is defective.

--
You can't fix stupid. You can't even put a Band-Aid? on it, because it's
Teflon coated.

More like the tin isn't mu metal and the microwave is defective.

The microwave isn't necessarily defective - the door seals are tuned to the freq. of the maggie, and cell phones aren't.

You're probably right on the cookie tin - I wonder if the phone would still ring if the lid were soldered on. :-)

Speaking of cell phones, has anybody not seen this gag yet?

Cheers! Rich

Yer an idiot. The lid seals fine to the level it needs to. Far smaller than a 4mm mesh, which was already referred to.