Only materials which form a low magnetic flux path surrounding a source of a magentic field or a target to be protectect can shield magnetic fields.
The principles of magnetic fields and how to shield them have been known since the 19th century when Maxwell's laws were published. Volume II John, the one with the blue cover you misplaced.
Gee,
I just wound two coils using 2 feet of 22 gauge wire wrapped around a finger. I set them up an inch apart and drove one with a small oscillator driven amplifier and hooked the other up to a Fluke model 85 DVM. I set the level for 5 millivolts of signal on the output. the magnetic coupling worked nicely. Inserting into the gap a .062" thick piece of 5052 alloy aluminum the voltage dropped to 3.5 millivolts!
I guess that is what happens when you actually test a hypothesis, reality sets in. Yes I know why the level dropped and it is the same reason aluminum can be used to reduce magnetic field interference!
Quote all the sources you don't understand all you want.
The decoupling was due to shielding the electrical field, not the magnetic field. You got it wrong. Now go back and measure magnetic field with a gauss meter and you'll not only get the right answer, you'll learn something. What you changed was capacitive coupling, not mutual transconductance due to magnetic coupling.
It doesn't need magnetic properties to shield against magnetic fields.
No, it won't shield against a static magnetic field, but it will shield against time varying magnetic fields.
The time varying magnetic fields induce eddy currents in the aluminum (or copper for that matter). Because of the aluminum's resistance, the energy that goes into creating the eddy currents is dissipated as heat. Which is energy which doesn't get passed through to the circuit being shielded.
Basically shielding works in two ways, reflection loss and absorption loss. Aluminum being able to shield against magnetic fields is an example of absorption loss. It's not nearly as effective as steel and other ferromagnetic materials of the same thickness, but if it's thick enough it can be just as effective as a thinner sheet of steel.
Here's a graph of magnetic field absorption loss for steel and copper.
An externally hosted image should be here but it was not working when we last tested it.
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I hope that everyone, who is somewhat technical, looks more deeply into aluminum shielding. IF the aluminum shielding is thick enough, let's say 1/4'' or more, then there is some measurable shielding of the magnetic component of EMI even down to 50 Hz. This is called: ABSORPTION LOSS. It is not remarkable, or cheap, but the shielding effectiveness gets MUCH more effective at more audible frequencies such as 200 Hz or more, therefore a potential 'buzz' would change to more of a low hum. A wooden or plastic cabinet, unless made conductive throughout the thickness of the material, would not have this property. This is where the confusion lies.
We tend to avoid steel, because it is potentially a distortion source. This also can be measured.
We tend to avoid steel, because it is potentially a distortion source. This also can be measured.
I produce instruments for high power testing laboratory, short circuit tests of HV and UHV circuit breakers. Some 4 years ago we had a set of instruments working in a close vicinity of conductors leading short circuit currents. I proposed double box made of thick aluminum. The customer insisted to make a version with smaller box inside made from thinner steel. We made both versions, and compared both by measurements, induced interference voltage as a function of short circuit current and box orientation. The steel version measured WORSE, the induced interference had non-linear dependence and was unpredictable with switching phenomena. We put all the instruments into double boxes made from thick aluminum.
They do. Godfrey's comment is also my understanding, but perhaps FEA leads to better results from multiple rings in different locations. One example of multiple aluminium rings is the BMS 18N850
Or foam nowadays, porosity/shrinking can be limited by applying a slight overpressure (~0.5 bar) during the cast.
For an example, google how the Porsche V8 aluminum block is manufactured.
Funny as sheit one : YouTube - Engine block casting , identical to what i did (less clumsy) for ~1 year at 17/18, while awaiting navy admission test/screening.
The company i worked for made 3-1/2ft diameter aluminum police station emblems/logos at the time, ~350lbs lift with 2 guys for a single shot poor at +40C ambient.
Welding and both side machining would seem simpler and cheaper.
Engine stringers of a +100ft planing aluminum yacht are 1" thick, handle the various forces/torques of a +20K lbs weight diesel engine, not extremeley hard to weld. (for those who know how to weld aluminum that is, some learned regular welding at age 10 and argon-arc welding at 18)
Or as the saying goes in the yacht business : It ain't much, if it ain't Dutch.
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