Suite: I shield with the aluminum tube
The result is catastrophic! The amplitude of the interference signal now exceeds the digital oscilloscope display!
With the tube, the facing surfaces increase, increasing the parasitic capacity and thus the amplitude of the disturbance!
My hand attenuates ... We'll see why :-)
By referencing the shielding to ground, I very attenuate the parasite! I short-circuited the parasitic capacitor
I install a braid to see the impact ...
Its attenuating effect, due to its low inductance, is very significant!
It is verified that even this new braid of cross section identical to the green / yellow wire, ie 2.5 mm², is much more efficient!
Continuation of the experiment: braid in place, I remove and add in parallel the wire of 2.5 mm²: one validates its total uselessness compared to the braid of the same section!
This time a somewhat special shielded cable
A weak spurious signal remains on the screen
Why ? Two holes are observed in the shield: at both ends
We will use for testing simple aluminum adhesive, available in DIY store
One side covered: one cosntate already the best!
Both sides: and here is the result! It's almost perfect!
The smallest hole in a shield allows a sufficiently short wavelength to enter!
The antenna of a GSM is very short, so a hole of the same length allows to return all the parasitic waves as short!
On a microwave the holes in the grille behind the glass are very thin: to block the dangerous short waves!
As soon as I remove the adhesive, the parasite comes back!
I approach the disruptor of the hole in the shield as much as possible!
By cons, shielding again hermetic, the proximity of the disturber no longer has significant importance ...
The shield must be waterproof!
I reinstall the tube not referenced ...
I will use the 2 wires, seen at the beginning of the course, to connect mass of the tube and mass of my devices
I begin with the inductive wire, which is wound in turns
Transitions retain a high amplitude: about 50% of the vertical amplitude of the screen
With the wire rolled in differential: non-inductive, the improvement is blatant!
We will now compare 3 armored boxes ... A little different!
Here 2 boxes connected with quality shielded cable
To the left a hole too wide lets the waves pass, bad sealing! Right, good shielding!
This one resumes the shielding correctly but with a low-end shielded cable
With this low-end wire box: 0.1 V peak-to-peak! The catastrophe !
With good quality box and high-end shielded cable: nothing to say, it's almost perfect!
The 2 boxes with the right cable give good results! The too wide hole does not leave enough room to let in the fairly low parasitic frequency I use here