Shielded coils - dumb question that left me wondering.

shorted

Yes and no. The shield can is much further from the core than the windings, which reduces the effect quite a bit--slug cores aren't like toroids or E-cores, where the inductance is almost independent of the wire position, as long as it threads the core.

But an unshielded coil will in general have higher inductance, higher Q, and lower loss than one in a can. If there's a ferrite shield inside the can, the effect is less.

Cheers

Phil Hobbs

THAT is the key.

However, one can take advantage of the shield conductivity at high frequencies and turn the device into a resonant cavity with slow-wave structure (the coil). The net result is a miniaturized tank with admittedly lower Q, but the compactness may be the desired result. Once made a tune able FM cavity that way; maybe 3-4 inched diameter and maybe 6 inches long (been over 40 years ago). Got idea from a yet older IEEE article. here is a special name for this but i do not remember..

Helical resonator

Chris

No it's not, that's another issue in a different type of core..

The important thing regarding losses in the shield is the relative density of the flux that encloses the shield. As Phil mentioned, the shield is further away than the windings so most of the flux passes inside it, around the windings. You might say that the windings thread the main flux path, but the core is only a fraction of that path.

-- Joe

shorted

Yes, definitely. That's how it keeps the magnetic field inside the can. :-)

Maybe you've seen BMF power transformers with a copper strap around the outside - that's a shorted turn, specifically put there to short out the leakage inductance.

Hope This Helps! Rich

For a "regular" coil I would say "yes, always, but often very weakly coupled". This is only a semantic difference from Phil's "yes and no", however -- choose the explanation that makes the most sense to you and run with it.

In general the advantages in isolation from other components makes up for the loss of Q, which is why you see shielded coils...

Thanks!