Multilayer magnetic shielding

On a hot summer night, would you offer your throat to the wolf with the red roses...

But he won't know that.

None of it any more worthy of attention.

Phil's idea of "reality" is decidedly strange.

He has moved from being Roman Catholic to Greek Orthodox, but only as a parishioner, rather than a prelate.

Probably not. But a text-only group doesn't offer a lot of reliable clues, not that Phil Allison would worry about that.

--
Bill Sloman, Sydney

==================

** My bench scope is an Australian made model by a company from Melbourne who made them by the thousands and many other pieces of test gear too. Thei r name was BWD Instruments. Their biggest markets were educational institutions, various labs and the m ilitary. They were also happy to do special orders based on existing models.

One of the senior engineers ( with a thick Scottish accent ) told me about a requirement for CRT scopes for use inside coal fired power stations. The engineers wanted an unusually high degree of magnetic shielding, regular sc opes had proved unsatisfactory in this respect.

So, BWD had a batch of mu-metal shields made a few mm bigger overall than n ormal and fitted them over the usual one.

..... Phil

;)

I really shouldn't tease Phil A. like that when he's off his meds.

Cheers

Phil Hobbs

Interesting. I've heard of using steel around mu-metal in tough situations, e.g. PMTs running near NMR magnets. I could see where two layers of mu metal could be better than that.

I've never needed to do it myself.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Nah I mean the induced current flowing over the shield -- it causes a voltage drop across that small length of shield. Small like I said, and a small fraction of 30m like you said.

Also, RG-174 is a little leaky, which...

...you noted here. :) But again, not much.

How deep, like the -40dB territory (which, I guess the input inductors are open field, solenoids or something?), or even better than that?

But yes, also a good sign. :)

It's more of a mismatch thing, innit? A layer of copper opposes some field, then re-radiates it, but the distance between layers provides some attenuation -- like leakage in a transformer, it doesn't couple perfectly between layers. Something like a crude dielectric mirror, except the dielectric has some crazy imaginary epsilon?

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/

Assuming he had any wits worthy of a f*ck!

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/

Ah, saw the pictures in the other reply -- that explains the inductors and such!

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:

Phil Hobbs is a Massive Asshole: =========================

** Stop kidding yourself - you are not teasing me.

However you *are* inadvertently revealing your true persona to the NG and the world. The picture that emerges is of a spoil rotten brat who grew into life long narcissist and bully.

Naturally you are well up on the ASD ladder, which is entirely consistent. Zero honesty, devoid of insight and no empathy with anyone.

Exactly like JL and the Slowman scumbags. All thriving on being complete assholes.

Now I see you are a religious crackpot as well. May you burn long in hell .

..... Phil

Only to Phil Allison.

Phil Allison sees a lot ASD

A whole lot more than is remotely credible, for a syndrome that affects perhaps 2% of the population. His diagnostic system seems to be somewhat disordered.

Not actually true, but once Phil gets started on one of his tantrums he does make a lot of silly claims.

The Greek and Russian Orthodox churches are long established, and - for people who take religion seriously - perfectly respectable.

They are less enthusiastic about clerical celibacy than the Roman Catholic church, which makes them less of a menace to their parishioners.

Phil Allison's ideas about who might burn in hell don't seem to be based on any firm grip of theology.

--
Bill Sloman, Sydney

I could make all the peaks disappear completely, so >50 dB.

Don't think so. Six millimeters is way inside the near-field region, so E and H act more or less independently, and besides, you wouldn't get additive attenuation that way, I don't think. It acts more like a shorted turn, which just eats up the field.

A bit does get round the edge, as you'd expect.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

I got a piece of mu metal from Amuneal at an APS show. I rolled it up into a tube and it worked great as a zero B-field space... We used it to set zero field on some cheap Hall probes.

This has some nice pics. (including the effect of dropping)

What strikes me as weird is the temperature dependence. My simple model of ferromagnetism suggests things should stay constant once you are below the Curie temperature. Any material science types no what's up?

George H.

Phil,

I appreciate what you're trying to do, but unless you're trying to just shield the probe, why do you not look at other locations around the noise source?

You mention that measurements degrade when the lid goes on. Just what are you trying to protect from the source's noise? Surely that would be the most sensible detector?

RL

I know a guy who claims to know how to anneal mu-metal to u=1e6.

--

John Larkin      Highland Technology, Inc 

The best designs are necessarily accidental.

Most of our houses were built after bathrooms were invented. We are rude but clean.

--

John Larkin      Highland Technology, Inc 

The best designs are necessarily accidental.

I've met Hobbs and he is obviously a large, hyperintelligent alien life form. It must be hard on him to pretend to like beer and coffee and roast beef, and to lounge on a deck under an alien sun with only gin and tonic for nutrition.

I'm empathic, I just don't like being around people a lot. They are too noisy.

--

John Larkin      Highland Technology, Inc 

The best designs are necessarily accidental.

Curie temperature applies only to the microscopic level of magnetic properties, and at the macroscopic level, flux can be pinned (inhibited from moving) at any impurity or internal strain. The difference with cold-worked mu-metal is that it exhibits some hysteresis (AC heating, and low-frequency loss of susceptibility) which is largely absent after annealing (as I understand it, that is best done in a hydrogen atmosphere).

Below Curie temperature, there's still some thermal scattering (phonons) as we see in electrical resistivity changes. That affects the spin of the coupled electrons that make materials magnetic, and a rise in magnetization at low temperatures is expected. Materials that don't have such a rise, might be exhibiting antiferromagnetism (still ordered, but not high-susceptibility).

I'm trying to attenuate the solenoidal field so it doesn't get into the wiring and screw everything up. I've got the kickout down to a few microvolts at full output, and the output ripple only a little more than that, so with a cap multiplier it'll be good for powering front ends.

Ultrasensitive measurement gizmos of one sort or another. We're always needing a couple of switchers on each board, so I want to factor them out into a small, electrically unobtrusive board that we can use many times.

We do a lot of proof-of-concept systems, so it would be nice to have a switcher board that's actually instrument-quiet and not just switcher-quiet. That would let us forget power supplies and concentrate on what we're good at.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Two things occur to me. I've read a lot about using nested magnetic-alloy shileds to reduce internal fields. This was discussed extensively down-thread.

The other thing is a bit of shielding theory I learned from the original MIT RadLab chapter on shielding, Shelkunoff. The basic point was that shileding resulted from the severe impedance mismatches when transitionf from metal to air and then back to air. This was alluded to down thread once.

. . .

Joe Gwinn