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I'm preparing to cut apart a non-trivial $ device and try to extend a 4-conductor cable, so I have been practicing my splicing skills. I figured a USB cable would be a good test.

I spliced, heat-shrink-insulated each solder splice and also soldered the ground wire. I wanted to see if the thing would actually transmit data once I was done, so I tried to get the original foil shield re-wrapped around the splices, but of course they wouldn't quite go.

I figured I could get a connected layer of aluminum around that spot fairly easily if I wrapped the works with some aluminum foil from the kitchen. I had foil on foil on ground wire, squeezed it tight, then heat-shrink wrapped the whole thing. It seems to charge fine right around 5 volts, and plugging my Anrdoid phone into my PC, the data flow seems good too. I ran some file operations on the phone from the PC and copied a few folders of music for a minor test.

I'm curious if that aluminum foil is working as a shield or if shielding not that big a deal in such conditions.

Is an aluminum foil wrap working as a shield in my spliced USB cable?

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  • \$\begingroup\$ It probably is helping, although not as much as a real shield would. Generally, it would work best to replace the shorter length of cable entirely with a longer length. In a pinch, consider conductive-adhesive copper foil tape for better shielding. \$\endgroup\$
    – rdtsc
    Commented Jan 27, 2021 at 18:50
  • \$\begingroup\$ It's better than nothing, but not anywhere near as good as a well designed, and terminated, shield. Even a small hole can play havoc with the shielding effectiveness, as can a pig-tailed shield termination. \$\endgroup\$
    – SteveSh
    Commented Jan 27, 2021 at 18:56
  • \$\begingroup\$ Keep in mind that USB cable has the signal wires (green/white) twisted. If your 4-wire interface has independent signaling, the tightly-coupled green-white wires will have substantial cross-talk, such that a heavy signal interference will occur, and the interface might fail or work unreliably. \$\endgroup\$ Commented Feb 1, 2021 at 0:07

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Aluminium foil can act as decent shield, many cables use coiled foil wraps around conductors for shields. The coiled foil wraps are typically not as good as a braid or solid conductor.

Aluminum's skindepth for 0.1mm goes to almost 1GHz so even a thin foil will conduct high frequencies.

However, on a digital signal, a shield probably isn't going to do much for a few cm of wire.

Also make sure you make a good connection between old and new shield.

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  • \$\begingroup\$ The foil is typically grounded by being in contact with a drain (ground) wire. \$\endgroup\$
    – Gil
    Commented Jan 27, 2021 at 20:58
  • \$\begingroup\$ Depends on the implementation by the OP, many shields are grounded only on one end \$\endgroup\$
    – Voltage Spike
    Commented Jan 27, 2021 at 21:30
  • \$\begingroup\$ Many shields? How 'bout some shields? I can't think of the last system I worked on that had a shield only grounded the one end, all of these being RF/GHz/high speed digital boxes and systems. \$\endgroup\$
    – SteveSh
    Commented Jan 28, 2021 at 2:52
  • \$\begingroup\$ For building scale fire alarm systems single ended termination is normal. Doesn't a double terminated shield require special treatment because of the loop created? \$\endgroup\$
    – K H
    Commented Jan 28, 2021 at 5:12
  • \$\begingroup\$ Yes, but in some applications a ground loop can be tolerated. \$\endgroup\$
    – Voltage Spike
    Commented Jan 28, 2021 at 5:48

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