# Connect 2mm copper wires electrically for 36kHz

I want to build a indoor non-moving Faraday cage against 36kHz electric fields with a 2mm diameter copper wire.

I will have several T-junctions where there is a running wire and an end of a wire comes from a 90° angle which has to be connected electrically to the running wire.

What is the best method to connect the wires for maximum conductivity for the 36kHz?

Soldering? With normal tin solder? Welding is out of scope in a normal room with no existing equipment I think. Wire wrapping? Cut the running wire and insert a 3-way terminal? Clamp?

• What I would do is wrap (by hand, no need for proper wire-wrap techniques) the incoming wire around the through wire (assuming this is stranded, good luck wrapping solid wire that thick!) and solder it. Whether you use leaded or lead-free solder doesn't matter; the solder conductivity shouldn't be a major concern. The main problem is that 36 kHz is a really low frequency to try to shield against, so your copper might not be thick enough. I haven't done the math. Commented Jun 29, 2021 at 18:20
• @Hansebenger: In each of your posts you have said you want to exclude 'electric' fields. Is this accurate or a poor choice of words? Do you know that the fields are predominately electric (E/H>377) or is your DUT mainly susceptible to E fields? As your chamber is electrically small, skin depth may be less relevant if you are simply trying to minimize E fields. Commented Jun 29, 2021 at 19:54
• @Tesla23 I really want to only shield against electric fields and I measured only electric fields in my first try (electronics.stackexchange.com/questions/573084/…). I got 4dB with it, the math of skin depth would say less than 1dB. Commented Jun 30, 2021 at 12:29
• @Hearth: It is 7 skin depths thick, skin depth with copper is 0,3mm at 36kHz. Commented Jun 30, 2021 at 12:30
• @Tesla23 Is material depth completely irrelevant for electic fields? Commented Jun 30, 2021 at 13:21

This is really an answer relating to all three of your recent questions.

The focus on skin depth, whilst a useful benchmark, is not the whole story for electrically small shielding. For fields at 36kHz, the wavelength is 8.3km, so your enclosure is electrically small. Also, unless the source of your interference is greater than several km away, it is likely that the fields may be predominately electric or magnetic, rather than have the normal wave impedance of free space.

Let's concentrate on electric fields for a moment. If I have a static electric field and I have a small aluminium shell (think ping-pong ball or matchbox), then the charges on the surface will arrange themselves so that the E-field inside is very small. Now imagine the fields changing at a 1Hz rate, the charges on the surface will rearrange themselves every second, currents will flow and the fields inside will likely increase slightly, and the slower the rate of change, the less they would increase. If you assume that you can estimate the internal fields using the skin depth, you find that you are predicting that the internal fields increase as you slow down the rate of change (as the skin depth increases as the frequency decreases) - this doesn't make sense. Clearly something breaks down in this approximation.

On the other hand, if there is a steady magnetic field present, you would expect it to penetrate the shell without loss, and as I slowly change it it still penetrates.

So how to get a handle on what is happening? Well there are some canonical problems that can be solved analytically that give insight. You can find the solution of a lossy conductive spherical shell here. Tesche calculates the shielding effectiveness of a 6 foot (1.83m) diameter aluminium shell 1/32 inch (0.79mm) thick under plane wave excitation. At 36kHz, the electric field is reduced by over 200dB, and the magnetic field by close to 70dB. Not bad for less than 2 skin depths!

Taking his figure 2(b), H-Field transfer function, I have added points calculated simply using the skin depth showing how far off they are:

So now let's apply this to your problem. It clearly suggests that there is a chance of shielding with a cheap aluminium shield. You have tried aluminium foil and failed. One possible issue is that you didn't pay enough attention to joining the foil, you just overlapped it relying on the capacitance. Whilst this is great for UHF and above, you can't rely on it at these frequencies. Even if you got 1nF of capacitance at each overlap (a lot), at 36kHz this represents over 4k of impedance - imagine laying a lot of aluminium squares and connecting them by 4k resistors - this would be a very poor approximation to a continuous aluminium shell.

So there is hope that your aluminium shell may work if it is properly constructed. It should reduce the electric fields inside. If your problem is magnetic fields and your DUT is sensitive to magnetic fields, then all bets are off.

• Thanks for this good answer. For all my 3 questions this is the least matching question to place this answer, I think it is best at electronics.stackexchange.com/questions/573284/… ;) Will accept it in a day independet of you moving your answer or not. Yes the field is only predominately electric and not 100% electric, but I focus on the electrical field. Commented Jul 1, 2021 at 12:51
• The concern with the connection of aluminium foil is another point for copper wire. Should buy a better multimeter, mine showed 5 Ohms Resistance (thought my Multimeter measures DC...) Commented Jul 1, 2021 at 12:52
• Just be aware that the effectiveness of the shield will be degraded by both connections between panels and any unfiltered wires in/out (including mains). I don't know what you are doing, but if it was possible I'd get a sheet metal shop to fabricate a big enough Al box (e.g. 2m x 2m x 2m), with bends instead of joins as much as possible, try to avoid cables in/out - make the box bigger and use batteries to power equipment, get mu-metal boxes to shield power inverters if necessary, optical fibre data connections for control / data IO. Find an EMC expert for guidance. Commented Jul 1, 2021 at 20:28
• If a forum organizer wants to move this to another thread that's fine by me. Commented Jul 1, 2021 at 20:29
• As stated in another question I only need 20dB. I have started to make models in boxes 65x35x37 cm. One with aluminium foil has performed well ;) Commented Jul 5, 2021 at 16:16