Will a Faraday Cage protect a Magnetic Compass from electrical deflection?

[MonniotC]

A couple of years ago I mounted a new Magnetic compass in the bulkhead of my boat, an Achilles 24. I had peviously mounted a small GPS unit (inside the bulkhead), and a GPS repeater (thru-bulkhead) within about 6 inches of the new compass. I found (not surprising really) that when I switch on power to the GPS & repeater, the compass deflects by 5-10 degrees. There's no other suitable location for the compass. I was wondering if surrounding the compass with a Faraday cage made of wire mesh might mitigate the effects. Anybody know?

 

[ex-Gladys]

Faraday cage is really for electric field protection, not electromagnetic... and if it did work, it'd probably isolate the compass from the earth's magnetic field. Not desirable

 

[AntarcticPilot]

A couple of years ago I mounted a new Magnetic compass in the bulkhead of my boat, an Achilles 24. I had peviously mounted a small GPS unit (inside the bulkhead), and a GPS repeater (thru-bulkhead) within about 6 inches of the new compass. I found (not surprising really) that when I switch on power to the GPS & repeater, the compass deflects by 5-10 degrees. There's no other suitable location for the compass. I was wondering if surrounding the compass with a Faraday cage made of wire mesh might mitigate the effects. Anybody know?

No, it won't. It's the magnetic field created by the electrical circuits in the other equipment that affects the compass, and there is no way of shielding that; at least, not one that will leave the compass working as a compass (an iron box would shield the compass, but it would also shield it from the Earth's magnetic field, making the compass useless)! A Faraday cage will protect from electrical fields, but that's not what affects your compass. The only way of dealing with it is to separate the two pieces of equipment.

 

[billcole]

One thing which might help is using twisted cable to feed the power to the GPS and repeater.
Having the conductors twisted together rather than parallel to each other cancels out the magnetic field induced by the current that flows in each conductor

 

[lw395]

One thing which might help is using twisted cable to feed the power to the GPS and repeater.
Having the conductors twisted together rather than parallel to each other cancels out the magnetic field induced by the current that flows in each conductor

That and making sure the data link to the repeater does not form a big loop.
There is probably current flowing supply> GPS > repeater > supply.

So careful placement of the data wiring might limit the area enlosed by that circuit.
Taking all the wires to the repeater direct from the GPS and twisting them may be worth trying.

 

[alan]

You might try putting a shield around the GPS units rather than the compass and see if that helps. You can experiment with tin foil or metallic flyscreen material to experiment before going for something more expensive (copper or ... ). Just wrapping the rear of the GPS units with aluminium foil may help. As has been said: wrapping the compass with foil (or whatever) will isolate it, but alas it will not work either!!!

Alan.

 

[JumbleDuck]

You might try putting a shield around the GPS units rather than the compass and see if that helps. You can experiment with tin foil or metallic flyscreen material to experiment before going for something more expensive (copper or ... ). Just wrapping the rear of the GPS units with aluminium foil may help. As has been said: wrapping the compass with foil (or whatever) will isolate it, but alas it will not work either!!!

Aluminium and copper will shield from electric fields, but not magnetic fields. For that you need proper magnetic shielding foil - something like mu metal. Searching for "magnetic shielding" on Amazon produces a few sellers, like https://www.amazon.co.uk/YSHIELD®-Mumetall-shielding-Width-meter/dp/B017Y3K5HI. It's not cheap, but make sure you buy from a supplier for industrial or electronic uses and not from someone selling to the swivel eyed EMF-sensitivity loons, because they just ask to be ripped off.

 

[KellysEye]

You could tape a small piece of iron to the bulkhead to stop the deflection you would need to move it around to find where it works. You said there is no other option but normally there is a bulkhead each side of the hatch, if there is one move the GPS repeater to the other bulkhead and fill in the the hole it leaves.

 

[MonniotC]

Thanks for the ideas everyone. Moving the repeater to the opposite bulkhead is not possible as the main switch/fuse panel is bonded to it on the inside. I can move the GPS receiver away from the area, though, and that will remove one of the power supply pairs of wires from the vicinity. I'll twist the power wires for the repeater & try screening it off with aluminium as well.

 

[AntarcticPilot]

Aluminium and copper will shield from electric fields, but not magnetic fields. For that you need proper magnetic shielding foil - something like mu metal. Searching for "magnetic shielding" on Amazon produces a few sellers, like https://www.amazon.co.uk/YSHIELD®-Mumetall-shielding-Width-meter/dp/B017Y3K5HI. It's not cheap, but make sure you buy from a supplier for industrial or electronic uses and not from someone selling to the swivel eyed EMF-sensitivity loons, because they just ask to be ripped off.

And surely putting a conductor round a GPS unit will block the extremely weak GPS signals, and stop it working?

 

[JumbleDuck]

Thanks for the ideas everyone. Moving the repeater to the opposite bulkhead is not possible as the main switch/fuse panel is bonded to it on the inside. I can move the GPS receiver away from the area, though, and that will remove one of the power supply pairs of wires from the vicinity. I'll twist the power wires for the repeater & try screening it off with aluminium as well.

As above, screening with aluminium won't do a blind bit of good. It's not magnetic and therefore does not interact in any way with the magnetic field which is causing your problems. I have just verified this, by the way, with a Brio wagon, a hand bearing compass and a Kit-Kat wrapper, so it's not a purely theoretical claim. Some mu metal (it's a nickel-iron alloy) shielding may help, but you might not be able to wrap it around enough of the repeater.

 

[JumbleDuck]

And surely putting a conductor round a GPS unit will block the extremely weak GPS signals, and stop it working?

Good point, though of course not a problem if the OP has an external antenna.

 

[lw395]

And surely putting a conductor round a GPS unit will block the extremely weak GPS signals, and stop it working?

One might like to assume the GPS aerial is remote from the box.

It's only DC or slowly varying current that will affect a magnetic compass.
The data lines tend to have a DC component.
An opto isolator is another way to break the current loop.

Shielding such as braid or foil will not have any effect, unless it has the side effect of carrying the return current, reducing the current loop to (near) zero area.
Shielding is good for stopping the alternating part of the signal radiating to interfere with VHF or whatever.

 

[prv]

Look for the MAIB report on "Water Rail" for someone who tried to shield a compass with a lead box and then was lost at sea.

(Mostly because he'd also mounted the compass back-to-front on the boat rather than the attempt at shielding, but it certainly didn't help.)

Pete

 

[Len Ingalls]

Just a thot- if GPS is running,in slack waters,it will display correct course .
Make up a deviation card for ,at least, N,S,E,W and more points if you wish. (do this while motoring)
Now you know how much to add/subtract from your compass.
If GPS quits,or you kill power to,revert to reading compass directly.
Easier than re-locating stuff.
/ Len

 

[blackfish]

Ah, yes, compasses. I remember them.

 

[Len Ingalls]

Ah, yes, compasses. I remember them.

Fuses blow. Electronics die. Compasses still work.
/ Len

 

[William_H]

A faraday cage is simply any cage that is made of electrically conductive material. Copper iron, ali foil or lead.
Radiation from a conductor called electromagnetic radiation comes in 2 forms electromagnetic radiation and electrostatic radiation. The radiation has different characteristics depending on if it is from direct or alternating or varying current. Now electrostatic radiation (effectively like a capacitor) is easily stopped by a conducting material. Electromagnetic radiation when changing is stopped because the conductor shield becomes like a transformer secondary inducing current from the passing and changing magnetic field. But magnetic radiation from static not changing current is just a steady magnetism which can only be stopped or guided around your (compass) by a magnetic material. (ie iron). But as said it is the earth's magnetic field we seek to use so we can't stop static magnetism cos we need it. Of course the changing magnetic field part of interfering radiation will not affect our compass anyway.
So as said a magnetic screen around the wires or more simply twisting the wires (which cancels out any magnetic field) is the way to go.
Just as an aside I saw a picture of a guy standing in the open doorway of a helicopter washing, with a power hose the insulators on high voltage power lines. He wore a "Faraday suit." ie a metal screen covered all parts of his body and was connected to the body of the helicopter. Now you might think that a hovering a helicopter made no connection to the ground so no way he could get electrocuted even though his hose is touching 132kv. Not completely so. The helicopter body made a capacitor to ground allow the AC to pass through the hose stream through the helicopter body to ground.
The suit therefore bypassed any (small but lethal) current to the helicopter body. Still a bit scary to me. olewill

 

[Lon nan Gruagach]

Just a wee point here.. twisting wires isnt perfect, its good, but not 100%
The problem is due to the fact that the cores are not in exactly the same place as each other, close, but no cigar. An improved solution is a different style of cable, namely star-quad where the cores are laid at the corners of a square and then twisted. Each diagonal pair forms 1 conductor, the average location of which is at the centre of the bundle, same as the other conductor. Yet another improvement is to add a 5th core in the middle of the square and connect this to the cable screen.

 

[Lon nan Gruagach]

Just as an aside I saw a picture of a guy standing in the open doorway of a helicopter washing, with a power hose the insulators on high voltage power lines. He wore a "Faraday suit." ie a metal screen covered all parts of his body and was connected to the body of the helicopter. Now you might think that a hovering a helicopter made no connection to the ground so no way he could get electrocuted even though his hose is touching 132kv. Not completely so. The helicopter body made a capacitor to ground allow the AC to pass through the hose stream through the helicopter body to ground.
The suit therefore bypassed any (small but lethal) current to the helicopter body. Still a bit scary to me. olewill

I have seen a video of that, nutters!