EMR min. mesh aperature

Rule of thumb is to use Lamba/20 for antenna effect of slots or holes. so 1Ghz yields 1.5cm hole. 10ghz- 1.5mm. I've seen old screen rooms made of copper screen, maybe 3-5mm square. The problem is making sure the mesh is electrically connected at every cross.

For a real 3 meter chamber the price isnt that awful.

EMC pre-screening?

Cheers

Hmm ... every cross. I didn't consider that. So woven galvanized mesh would not work? I thought all that was required was a path to ground from every strand. Or, in some cases, I have seen it in use just floating and OK for screening higher frequencies.

Can you please clarify, if possible?

Welded mesh is not common in small aperatures. Otherwise, maybe one of the expanded steel meshes would be better. But difficult to electrically join the seams.

Robert Davis

Hello Robert,

What is the frequency range of interest and how much attenuation do you require? I assume that you did some measurements and discovered that interference from outside came in.

For reflectors (antenna systems), mesh size of lambda/20 is used as rule of thumb. The goal of a reflector is to get reasonable amount of reflectivity. For your application attenuation between inside and outside the cage is of importance.

Obtaining significant attenuation is completely different. When you have a mesh material that gives, for example, 30 dB transmissive attenuation for plane wave excitation, the cage attenuation can be 10 dB (or less) at certain frequencies. This is because of constructive internal reflection on the walls inside the cage.

Many years ago I needed about 20 dB attenuation at about 1 GHz (almost walk-in size). The equipment could not be transported to an EMC test house, so we had to try something. I used rectangular mesh material (11mm mesh size). Overlaps were soldered. From memory, attenuation was about 10 dB, insufficient for our application.

Though these internal resonance effects can be reduced with RF absorber material, building a faraday cage for RF is not an easy task. You also need to consider all feed through of power, signal and other conducting lines as these may degrade the best cage to useless.

Products like fine chicken mesh do a bad job when it comes to attenuating low impedance fields (magnetic fields) and UHF RF fields. Looking to "external power and communications systems" (kHz to GHz), I don't think cheap galvanized mesh will work for your application.

Best regards,

Wim PA3DJS

If the galvanising is applied after weaving the mesh, which sounds likely, then all crosses would effectively be 'soldered' together in any case. I would use it in multilayer though, with offset. Could an industrial equivalent to kitchen foil or thin ali cu sheet be any use for this, assuming that you could solve the bonding problem ?.

One problem that may not be trivial is the power entry to the box. My guess is that you would need multistage frame bonded filters with screened dead space between them. It's quite amazing how rf leaks through just about anywhere there is space...

Regards,

Chris

When I was in college I worked in a fast food place. I had a manager that was a senior in a EE program. We frequently discussed how our roughly 100 mW drive through headsets at 162Mhz, would work just fine in a huge double walled, doubled insulated steel freezer with a thick layer of ice on the walls. yes, you could hear the unit straining at zero AGC, and it was not 100% reliable , depending on battery status, but they worked. The lighting power feed into said freezer was all in conduit, and the walls were single point grounded on each size as well to a plumbing ground, as well as a the freon piping being grounded. The Air ducts had to be well below waveguide cutoff for 162 and had metal grids on them. And we checked, there was no antenna inside connected to the store "repeater".

We actually thought the leakage came in around a double lapped door seal, with a 1/8th inch thick rubber seal and a inch of metal overlap. The door would show continuity with a ohmmeter,

So I would agree to the advice that you use a bonded, soldered, wall.

Steve

You can use sheet metal for infinitely small holes in the "mesh", and you can solder the seams if you use galvanized steel or copper - but you will have the common problems that keep commercial screen rooms expensive - doors (getting a good electrical connection for the door is difficult) and power (and/or signal, if any) feeds leaking. You can use a steel door - but the door-to-doorframe gap is going to be a problem you'll have to solve. If you can run all your measurements from battery power, that would solve the power feedthrough issues.

Forgot to add: a good estimate of whether your screen room is working effectively can be obtained by taking a broadcast radio receiver into the room and closing the door - if you can still pick up radio stations, it needs work. I suppose a cell phone or wireless network connection would allow evaluating the effectiveness at higher frequencies these days.

Fingerstock is quite usable in many situations.

If the Fingerstock is destroyed, you may have to replace both the door and as well as the door frame.

Paul