As part of hunting a nasty EMC problem I embarked on trying to create a shielded environment that would allow me to make some kind of measurements on a device that fails the radiated emissions testing. There is just too much interference around to make a measurement without any form of shielding. A problem is that the emissions come mostly from the wires and these can't be shielded or filtered so I need quite a big setup to include the wiring as part of the measurement.
I did find conductive cloth on Aliexpress but making a huge tent with it would be costly an no guarantees on whether it would work. I thought about different solutions and came up with the idea of using wire mesh that is used for fences or cages. At least this would be self-supporting and simplify the design because it shouldn't need a frame. So I went to the hardware store and bought some bird cage wire mesh and started folding it into a cage:
This is the nearly finished 'product'; some initial testing looked promising enough to go forward. After I added the wooden edge frame I was quite content with how sturdy the cage felt. But that feeling didn't last long. I put some weight on top to keep the mesh for the lid flat and then it decided to collapse in on itself at the left side. Curses!
I managed to straighten it out but it will need some reinforcement later on.
The size is 60x80 by 90 cm high. This is a size I have a storage space for.
I used aluminium tape (real aluminium so it is conductive) to get rid of the sharp points of the wire mesh. I laced the bottom and the side together with steel wire so there is a lot of contact between the mesh.
The shielded chamber 101 says that the shield of signals coming in needs to be fully connected to the wall of the chamber so I made this with some bulkhead connectors and a piece of copper plate:
For other signals a copper tube to feed them in is the recommended option so I added a piece of 22mm copper pipe. I choose 15cm as a reasonable length. Last but not least I soldered the copper plate into the mesh.
Ofcourse I also needed some kind of antenna. I opted to make a simple dipole and found a website saying that making a dipole wide, makes it broadband. Just slap something together and see what it does. I found some phenol copper clad in my stash with PCB material and made an antenna which is nearly 90cm high. The plates are 10cm wide.
Let's hook things up:
Meanwhile I made a lid as well that fits nicely around the cage. I used a piece of PVC pipe to hang cables from. The mesh allows to tie cables down as well.
The cage provides a shielding of around 25dB at relatively low frequencies:
The yellow trace is with the lid open, the blue trace is with the lid closed. The blue trace is at the noise floor of the spectrum analyser in most places. The FM radio stations remain clearly visible though but for the measurements I wanted to make most urgently, these are not in the way.
Working close to the noise floor of the spectrum analyser did cause some challenges. Even with the preamplifier on, I just missed a bit of dynamic range. So I bought a simple amplifier from Amazon.
I used a USB cable and a ferrite core (with only the power wires wrapped around it) to power it.
The amplifier itself is build around a SPF5189 which turns out to be a very low noise amplifier.
Now this amplifier has a noise floor itself but it seems to give an extra 5dB to 10dB of dynamic range. Enough to pull the signals a bit more out of the noise.
Same signal measured with (top trace) and without (bottom trace) amplifier:
All in all I'm not dissapointed. Unfortunately the cage isn't big enough to do real far field measurements from a few meters away. And the antenna leaves much to be desired for sure but this cage did allow me to try a few modifications and gather some data to justify going to a real EMC lab or needing to do more work on the circuit.
A future enhancement could be adding a finer mesh or conductive cloth to improve shielding. I have found a report that indicates a finer mesh leads to better shielding at lower frequencies. Or maybe I opt to make a large tent from conductive cloth which I can put up in the living room temporarily