room size Faraday Cage with ambient 100milligauss magnetic field

[loop123]

Hi,

There is a room in my cousins that I plan to build a room size Faraday Cage but the building has ambient 100 milligauss magnetic field from high tension 60 Hz power lines just outside. The Faraday Cage may not shield against it. Can 60 Hz 100 milligauss magnetic field affect circuits that deal with 10 microvolt and below causing interference? If not, then it is ok to build Faraday Cage amidst such strong magnetic field building?

Please show the room size Faraday Cage you built. Thanks.

[ejeffrey]

A faraday cage will definitely not screen 60 Hz magnetic fields.   If you are doing measurement sensitive to low frequency magnetic fields you need magnetic shielding like mu-metal.  Be prepared to open your pocket book.

100 milligaiss seems pretty high for just a power line unless you are right under it.  Generally the currents are balanced between multiple conductors run in parallel so the field drops off rapidly with distance.  Fields like that are more likely to come from something like an AC induction motor or power transformer close to or inside your building.   

[loop123]

Yes my cousins office are right under hundreds of kilovolts of high tension power line. What kinds of circuits can be affected by 100 milligauss magnetic field? can it affect Instrumentation Amplifiers?  its the only place I can put up a room shielded from interference currents.

[Terry Bites]

Do you need to exclude the powerline field from an entire room or just the bench area?
How much of a pain the external field is depends on your measurement set up, in particular its common mode rejection and sensor type/ or signal source.

Solution: always go for differential measurements. A transformer can convert single eneded to differential for AC measurements. If you can't do that, then galvanically isolate the input stage.

Magnetic fields have little effect on semiconductors, their cross sectional area (loop dimension) is very low for one thing. Wiring/pcb traces will be the issue.
Put your mesurement electronics in a mu-metal or steel box. Electrostatic sheilding is still needed.

One alternative to heavy magnetic shielding is to add indcutors on the room's sides.
You then use a pickup coil or other mains locked 60Hz (50Hz) and generators with gain phase shift adjustment to drive the coils null out the field. Use Amperes law for a flat solenoid.
B=μ0nI at the centre. A solenoid like this does not produce a uniform field but it may be good enough. If a real uniform field is needed, then use a pair of Helmholtz coils.
www.ck12.org/c/physics/lorentz-force/rwa/Creating-a-Uniform-Magnetic-Field/
You can add negative feeback. Use a servo loop (analog or digital) to adjust the amplitude and phase to null out the field. The servo negative fb is driven from a field sensor.
A lo-fi power amp is going to be ok as a coil driver though a power transformer may be needed to adapt the coil impedance to the amp's abiltiy to drive very low impedances.

This has been sucessfully appled to AC/DC scope current probes where the measurement relies on nulling the flux in the probe core.
I read this article on a MEG shielding technique (https://en.wikipedia.org/wiki/Magnetoencephalography) rooms and that has 9allegedly) nulled external fields down to pico Teslas. The AC flux sensor doesnt need to be ultra linear. A hall device may be ok here. Its also possible to take an average from an array of mag sensors.

The powerline field will have a strong be directional component Start out with one big coil or Helmholtz pair on the box roof and see how it goes. Cheap

[jonpaul]

Hello: What's your goal and objectives??

Why do you need this?

A very small screen room cost ~ $20,000 in 1970s.

A normal room sized screen room will coat >100K.



Suggest that you first Study the difference between E field,  and H fields and electromagnetic waves,

Determine the susceptibility of the DUT, to  E, H, RF  over the spectrum.

HV line >20kV will  generate RF noise from corona.

Enjoy,

Jon

[Andy Chee]

Hello: What's your goal and objectives??

Why do you need this?

From the content of his other threads, the purpose of the faraday cage is to perform EEG/EMG experiments inside the cage.  He is attempting to replicate a previously published EEG/EMG experiment.

[loop123]

What do you mean "Wiring/pcb traces will be the issue."? what can he affected by 100 milligauss magnetic field for instance? like can it affect the reference? Pls give examples of worse case scenerio.

Do you need to exclude the powerline field from an entire room or just the bench area?
How much of a pain the external field is depends on your measurement set up, in particular its common mode rejection and sensor type/ or signal source.

Solution: always go for differential measurements. A transformer can convert single eneded to differential for AC measurements.If you can't do that, then galvanically isolate the input stage.

Magnetic fields have little effect on semiconductors, their cross sectional area (loop dimension) is very low for one thing. Wiring/pcb traces will be the issue.
Put your mesurement electronics in a mu-metal or steel box. Electrostatic sheilding is still needed.

One alternative to heavy magnetic shielding is to create rectangular wound indcutors on all the room's sides.
You then use a pickup coil or other mains locked 60Hz (50Hz) and generators with phase shift adjustment to drive the coils null out the field. Use Amperes law for a flat solenoid.
B=μ0nI at the centre. A solenoid like this does not produce a uniform field but it may be good enough. If a real uniform field is needed, then use a pair of Helmholtz coils.
www.ck12.org/c/physics/lorentz-force/rwa/Creating-a-Uniform-Magnetic-Field/
You can automate all this of course. Use a servo loops (analog or digital) to adjust the amplitude and phase to null out the field. The servo negaive fb is driven from a field sensor.
A lo-fi power amp is going to be ok as a coil driver though a power transformer may be needed to adapt the coil impedance to the amp's abiltiy to drive very low impedances.
Giving all indcutors a common ground creates a farday cage against electostatic fileds too.

This has been sucessfully appled to MEG (https://en.wikipedia.org/wiki/Magnetoencephalography) rooms and has nulled external fields down to pico Teslas. The AC flux sensor doesnt need to be ultra linear. A hall device may be ok here. Its also possible to take an average from an array of mag sensors.

The powerline field will have a strong be directional component Start out with one big coil or Helmholtz pair on the box roof and see how it goes. Cheap

[rhb]

Make a box of galvanized sheet steel HVAC duct material and measure the field inside.  Low carbon steel is not as good as nickel aka mu metal, but it does help.

I'm budgeting $6-7k USD for an 11' x 16' room.  Not a cheap project.

Have Fun!
Reg

[CaptDon]

Every conductor passing through a magnetic field generates current flow and therefore voltage! This is the basics of electrical generation. And every current flow generates a magnetic field. Wow, you had 79 interactions in your last post. You may be on the trail of EEG studies but to proceed with the electrical aspects of your post and finally move along from point A to point B in your endeavors would require you to hire a competent engineer as part of your staff. It is impossible to get all of the relevant information you require to complete your task here on the forum. I suspect you will have un-anticipated short comings we can't even foresee. You really need a local highly experienced engineer by your side to guide you. You wanted someone to build an amplifier for you. Again, you need local talent to work side by side with you who can on a daily basis modify your designs by looking over your shoulder and anticipating the best possible solutions. To attempt to get you to where you want to go would require you having a B.S.E.E. or a million posts on the forum. Network with your local ham radio guys or university students in electrical and mechanical sciences. Anything less will just be sliding forever on ice.

[loop123]

Make a box of galvanized sheet steel HVAC duct material and measure the field inside.  Low carbon steel is not as good as nickel aka mu metal, but it does help.

I'm budgeting $6-7k USD for an 11' x 16' room.  Not a cheap project.

Have Fun!
Reg

Not room size then but only human size. I saw a bed canopy Faraday Cage, will this work?

https://youtu.be/8dd5IET6t1A

What brand is guaranteed to work 100%?

CaptDon. I already have the equipment including those used by R&D institutes like the $16750 g.USBamp. No need to build from scratch.

[Terry Bites]

The larger the loop area the greater the induced voltage.
Faradays Law http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html
100miligauss@60Hz will induce about 3.0uV rms in a 1cm radius loop. V0 = ωB0πR2N/squrt2

PCB traces and wiring will present far greater loop areas than the internals of an ic.
Keeping loop areas small is something that you'd endevour to minimise.
You so often see twisted pair wiring, it presents very small area vs length.
Minimising the area reduces EM induced voltages and EM radiation.

Lets say you made a single Helmholtz coil wound 10 from turns on with a radius of 1m, you'd only need 2.5A to reach 10uT. H≈ 0.716 NI/R  Finally a use for that crappy subwoofer.

[rhb]

Make a box of galvanized sheet steel HVAC duct material and measure the field inside.  Low carbon steel is not as good as nickel aka mu metal, but it does help.

I'm budgeting $6-7k USD for an 11' x 16' room.  Not a cheap project.

Have Fun!
Reg

Not room size then but only human size. I saw a bed canopy Faraday Cage, will this work?

https://youtu.be/8dd5IET6t1A

What brand is guaranteed to work 100%?

CaptDon. I already have the equipment including those used by R&D institutes like the $16750 g.USBamp. No need to build from scratch.

This should work very well.  I'm quite tempted to buy one.  I had no idea you could get a screen room so cheaply.

https://www.amazon.com/RYUMD-Shielding-Breathable-Radiation-220220220cm/dp/B09M8MSQVT/ref=sr_1_32

https://www.amazon.com/Chang-Shielding-Breathable-RadiationEMFWiFi-120220220cm/dp/B099F7FZS2

https://www.amazon.com/Chang-Shielding-Breathable-RadiationEMFWiFi-120220220cm/dp/B099F7FZS2/ref=sr_1_31

The 2nd one is enough to screen a shallow workbench if turned on its side.

However, neither provide any magnetic shielding which steel does.

Have Fun!
Reg