Ammo can Faraday Cage?

[Ozman]

Been looking on line, but thought I'd see what this group had to say.

I'd like to use a few of my larger ammo cans as Faraday cages for ham equipment and other radio gear. Questions:

1. Do I need to remove the rubber gasket from the lid?

2. Can I use foam padding as the "insulator" beween the contents and the can structure?

3. Any other thoughts?

Side note: would a regular "Stack On " gun cabinet also work as a Farday cage as long as the electronics inside were insulated from the metal walls and floor(s) and shelves? I've seen some people mention using good old fashioned filing cabinets this way but dont know if it works.


As always, thanks,

Steve

 

[Bob J]

For the cage to be efficient you need to make sure that there are no significant gaps/holes that are larger than 1/4 the wave length of the highest frequency you want to shield against....

This means that depending on the wavelength you may need to replace the gaskets.... You will also need to make sure that the lid is solidly grounded to the main case.... Strips of copper braid typically do a pretty good job of this... Also make sure you have a solid ground for the cage.... Usually this means a good grounding rod.... It used to be that you could just tie to copper pipes in the house but since the advent of PVC/CPVC etc that has proven to be pretty unreliable.... Tying to mains ground is ok as long as what you are working with will not be generating much hash unless you use an inline hash filter to keep the noise out of some of your more sensitive household electronics..... As for the gun cabinet, no problem as long as the same detail listed for the ammo box are met.... Basically any well grounded conductive enclosure with no gaps greater the 1/4 wavelength should work fine....

Foam padding is not a problem inside the case but you will need to ensure that you have a plan to keep the temperature down as much as possible... The general rule of thumb for component life/associated failure rate is that the failure rate will double with each 10C increase in the ambient temperature within the case....

Best of luck with your project.....

Been looking on line, but thought I'd see what this group had to say.

I'd like to use a few of my larger ammo cans as Faraday cages for ham equipment and other radio gear. Questions:

1. Do I need to remove the rubber gasket from the lid?

2. Can I use foam padding as the "insulator" beween the contents and the can structure?

3. Any other thoughts?

Side note: would a regular "Stack On " gun cabinet also work as a Farday cage as long as the electronics inside were insulated from the metal walls and floor(s) and shelves? I've seen some people mention using good old fashioned filing cabinets this way but dont know if it works.


As always, thanks,

Steve

 

[47stages]

I believe that all of the current created in a Faraday cage due to changing EM fields travels on the outboard side due to the skin effect, therefore electronics can be touching the inner wall and still be safe. The people in the Science Museum use that effect to touch the inner side of the of the cage in the lightning exhibit and not be shocked. Grounding is important though as Bob J mentioned. Best.

 

[Bob J]

Very true..... Check this out as an extreme example....

http://tesladownunder.com/tesla_coil_sparks.htm#Dalek cage

I have a much smaller tesla coil that is powered by 2 microwave oven transformers mounted in an ammo can..... Makes a nice portable HV power source.... 4KV @ half an amp is no joke....

I believe that all of the current created in a Faraday cage due to changing EM fields travels on the outboard side due to the skin effect, therefore electronics can be touching the inner wall and still be safe. The people in the Science Museum use that effect to touch the inner side of the of the cage in the lightning exhibit and not be shocked. Grounding is important though as Bob J mentioned. Best.

 

[matt]

I believe that all of the current created in a Faraday cage due to changing EM fields travels on the outboard side due to the skin effect, therefore electronics can be touching the inner wall and still be safe. The people in the Science Museum use that effect to touch the inner side of the of the cage in the lightning exhibit and not be shocked. Grounding is important though as Bob J mentioned. Best.

This is correct. Also, no need to ground the box.

Connect the lid of the can to the body with a copper strap (wider is better), not wire. Make sure all the paint is removed under the connection points. Replace the water seal with EM gasket.

 

[OCB]

You can also get some copper foil (may be referred to as copper tape) with conductive adhesive from McMaster-Carr to cover any small gaps, etc. We used it on a small Faraday cage at work with pretty good results, but as a disclaimer, I'm not an EE, nor do I play one on TV.

 

[ShadeWPI]

Electrical Engineer here with a few comments:

1) A true farraday cage works on both changing currents (AC) and constant currents (DC) effects. The theory is that if you create a hollow space within a perfect conductor, the surface of the conductor will have a uniform electrical potential (Voltage) since any difference in the electrical potential between two points would create an infinite current.

Unfortunately we don't have perfect conductors, we're stuck with metal, so there will always be some potential difference across the conductor, though it will be significantly less than free air. Adding insulation to the inside prevents this potential difference from conducting current through the sensitive electronics inside.

2) For electrical fields, any conductive material will work as a gasketing material, the more conductive the better. Magnetic fields are another story though. While materials like Copper, Tin and Silver are great electrical conductors, their permissivity (ability to conduct magnetic fields) is near that of air. For magnetic fields you want iron, steel, nickle, manganese, etc. Materials with good magnetic properties. If you can find copper clad steel, it's one of the best EM (Electro-Magnetic) gasketing materials availale. If you'd like to maintain close to a water tight seal, pick up some metallic braid (steel if possible) cut the water gasket, slide the braid over the gasket, glue the gasket back together with neoprene cement, then over-lap the braid. The braid may allow some leakage, but it's a lot better than removing the gasket completely.

3) For long-term storage that you don't need to access repeatled, leave the water-proof gasket in place (water is more of an enemy to long-term storage than possiblem EM issues) remove the paint from the cover and base and apply a strip of 1" copper foil tape to the joint. Next, apply a striip of aluminum ducting tape over the copper foil tape (aluminum has slightly better magnetic properties than copper). If you want to be additionally sure, pick up some "Anti-Oxide Grease" this is designed to prevent fire hazzards with alumnum wiring in homes. It is a conductive grease that prevents alumimum wires from oxidizing. Give the water-seal a liberal coat in the grease, then replace the seal. The grease will provide some conduction around the gasket while still providing a water-tight seal.

4) The problem with most cabinats and gun-safes from an EM perspective is they are coated with a protective layer that is not conductive. If you want to turn your safe into a EM shield, pick up some very wide conductive braid (copper braid should be fine, there's a lot of iron in a good safe) then solder or braise the braid to the inside of the full length of the hinge joint. Make sure the braid is wide enough not to interfere with the openning or closing of the safe.
Next, add an EM gasketing material to the entire interface between the body of the safe and the door. One my favorates is a strip of curved "fingers" the mounts to the body and the door compresses the fingers. The best ones are gold plated spring steel, but there are much cheaper ones available (nickle or tin plated copper are the most common)
Finally, mount a very heavy wire to the safe and connect this wire to your water pipes (likely the best ground in your house) to make sure the whole safe is grounded.
Line the safe with neoprene foam rubber padding.

5) The "Skin Effect" was described earlier. AC currents (wall power, lightning strikes, EM pulses) travel on the surface of conductors more easily than on the interior. The "Skin Depth" or the region of a conductor that carries the majority of the current depends on the frequency (higher frequency, lower skin depth) This has lead to copper clad wires (copper layer over aluminum) for lower cost, "lizen" wire - wires made up a dozens to hundreds of very tiny thinly insulated wires woven together and even silver clad typically copper wire with a heavy silver layer on the outside.
While this will help with an ammo-can ferraday cage, the electrical potential fields will be extremely high before you'd even worry about damaging electrical equipment, so the skin-depth effect may be minimal.