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Help With Building A Faraday Cage

 
victor43
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05/23/2012 04:32 PM
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Help With Building A Faraday Cage
Hello everyone.

Here is my interest and please feel free to ask questions. What I want is to build a Faraday Cage using tightly woven aluminum window screen mesh to keep all Radio Frequencies (1 MHz to 300+ GHz) out of this enclosure so that there is no RF signals entering or exiting the cage. What I would like to know is a few things. First is it possible to bring in power i.e. via a power bar power cable and a lan cable into this RF protected screen room without compromising (leakage) the room ? Secondly are there any places out there (online including) that I can purchase or download a design so that a general contractor can construct this enclosure. I need to construct a room, including a door of about 6' x 6' x 5' (L x W x H) which will house certain computerized items. Lastly will aluminum (window screen) mesh with spacing of a maximum 1 mm work for the entire frequency range ? This project is simply an experiment of mine and there is no other purpose of doing so. I also need to be able to disassemble the cage in the future in the event I decide to move out and move to a different house. Unfortunately I am not a contractor however I will give it a go in case I cannot find anyone who might be able to construct this room. Does anyone know of any architects or engineers that can layout the design in order to construct this room ? It does not matter where these professionals are located inside North America even though I am in Ottawa, Canada.

One final note does anyone know if the special paint that is mentioned on the net that keeps out RF Radiation actually works ? Would applying this paint be as effective as the above described enclosure in keeping out all RF signals ? Would it work as a compliment to the enclosure ?

Thanks for taking the time to read this post.

Vm
victor43
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05/23/2012 04:35 PM
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Re: Help With Building A Faraday Cage
An old refrigerator should get you started. Google is your buddy.
Anonymous Coward
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05/23/2012 05:00 PM
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Re: Help With Building A Faraday Cage
I do not think auminum is suitable - it is not magnetic. In esoterics it is not used at all, you cannot programme it.
Anonymous Coward
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05/23/2012 05:04 PM
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Re: Help With Building A Faraday Cage
I do not think auminum is suitable - it is not magnetic. In esoterics it is not used at all, you cannot programme it.
 Quoting: Anonymous Coward 1175381


Don't post if you have no idea what you're talking about. Aluminum is conductive and works just fine as a Faraday cage.
Anonymous Coward
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05/23/2012 05:07 PM
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Re: Help With Building A Faraday Cage
The thing about a Faraday cage for high frequencies is you pretty much have to close all the gaps. Conductive paint will probably not do it. GLP is not the place to ask, as some of the disinformation that's already been posted proves.

Ask this in a forum where electrical engineers post. Not a lunatic fringe site.
Anonymous Coward
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05/23/2012 05:10 PM
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Re: Help With Building A Faraday Cage
Check out this stuff ;)

[link to www.amazon.com]
Bullwinkle
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05/23/2012 05:16 PM
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Re: Help With Building A Faraday Cage
I use a metal garbage can.
I have bonded the lid to the can.
I have lined it with chip board so noting inside touches the metal.

I have also used ammo cans.
I have removed the rubber seal and replaced it with the braid from large coax cable.

Full protection is only with storage.
There is little you can do if in use.
Anonymous Coward
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05/23/2012 05:26 PM
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Re: Help With Building A Faraday Cage
I use a metal garbage can.
I have bonded the lid to the can.
I have lined it with chip board so noting inside touches the metal.

I have also used ammo cans.
I have removed the rubber seal and replaced it with the braid from large coax cable.

Full protection is only with storage.
There is little you can do if in use.
 Quoting: Bullwinkle 16570247


Another great example of the disinformation you'll find on a site like GLP.

An ammo can isn't going to do jack shit against an EMP from a nuke, which is what I assume everyone is worried about. The gaps in the can are far too wide to keep the ultra high RF out.

GLP IS NOT AN ADVICE SITE! THIS IS A RECREATION SITE.

You people get what you deserve if you believe anything posted here as fact.
THE Professor !
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05/23/2012 05:30 PM
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Re: Help With Building A Faraday Cage
Build one of scrap 2x4, etc. and used aluminum screen wire. ONLY problem was, I built it about 1 inch too small, as I didn't allow for the wire sticking out on all sides. USE IT FOR FLASHLIGHT BATTERIES, FLASHLIGHTS, etc.

Had a professional metalworking company make an all steel box, and told the guy ahead of time, if a cell phone or radio was placed inside, it couldn't ring OR play when the lid was placed on top. Man placed his cell phone inside the day it was done, called it and NOTHING happened. However, cost me $700, but my generator DOES fit inside, on top of some bottom/side shielding rubber mats from WalMart ... another $50 !

" Be warned by my lot, which I KNOW you will NOT." ( Omar Kyam, Rubiad )
Anonymous Coward
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05/23/2012 05:34 PM
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Re: Help With Building A Faraday Cage
IF you can secure the room or space from the pulse
you still have to block any electro waves via network and electrical

Network would need fiber option that passes between the wall or mesh.

Electrical is tricky and I think impossible as it needs to be Isolated electrical with an isolation unit but I wouldnt trust it and besides if everything outside the box is fried whats the point. You can always have a UPS with extra batteries charged Inside the space.

Really depends on what your doing.
Is this a small space like a box or a room
Life and Love

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05/23/2012 05:38 PM
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Re: Help With Building A Faraday Cage
Being so broadband (1 MHz - 300 GHz) is the problem.

Guidelines:

(1) Low frequency coupling is dominated by wires and cables, e.g., power and internet you bring in.

(2) High frequency coupling is dominated by apertures, seams, and joints.

(3) You could bring signals in via fiber optics through an aperture with a tube-line extension to act as waveguide below cutoff.

(4) Power is tough, but at least you can filter the heck out of it, unlike signals.

(5) If any energy does get inside, the electric fields will be enhanced by the high Q of the interior.

(6) A wavelength at 300 GHz is 1 mm. You'll need a mesh finer that one tenth of this to achieve decent attenuation.

(7) Skin depth of copper at 1 MHz is about 0.065 mm. Five skin depths would be 0.325 mm.

Hope this helps (and that my math is correct!)
We become like that to which we are devoted. - Choose wisely.
Anonymous Coward
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05/23/2012 05:45 PM
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Re: Help With Building A Faraday Cage
Wondering "lead board" sheetrock (lined with lead and used in the walls of xray rooms) would help with his project?
Anonymous Coward
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05/23/2012 05:48 PM
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Re: Help With Building A Faraday Cage
The gaps in the can are far too wide to keep the ultra high RF out.
 Quoting: Anonymous Coward 16528092


[link to www.amazon.com]
Anonymous Coward
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05/23/2012 06:03 PM
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Re: Help With Building A Faraday Cage
lessemf.com

the wallet seems ok value not sure about the rest but if it floats your boat!
Anonymous Coward
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05/23/2012 06:09 PM
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Re: Help With Building A Faraday Cage
Anonymous Coward (OP)
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05/23/2012 06:43 PM
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Re: Help With Building A Faraday Cage
IF you can secure the room or space from the pulse
you still have to block any electro waves via network and electrical

Network would need fiber option that passes between the wall or mesh.

Electrical is tricky and I think impossible as it needs to be Isolated electrical with an isolation unit but I wouldnt trust it and besides if everything outside the box is fried whats the point. You can always have a UPS with extra batteries charged Inside the space.

Really depends on what your doing.
Is this a small space like a box or a room
 Quoting: Anonymous Coward 14981250



Thanks for the reply. Yes this enclosure would be like a small room about 6'x6'x5' (LxWxH) with a door to enter/exit the room. Think of it like a clean room. UPS won't work because I need power for an extended period of time. My question to you is, can RF signals (i.e. AM/FM Radio, cell phone and other wireless devices) piggy back through the power or network cable and penetrate the enclosure ? I am not concerned with EM Radiation caused by the power cable of the power bar or by the Rj45 LAN cable. I guess in my post I should have mentioned that I am not concerned with the emissions or radiation caused by the power cable and the lan cable inside the faraday cage.

Many thanks for the reply.
WindyMind

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05/23/2012 06:47 PM
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Re: Help With Building A Faraday Cage
copper, I think you need copper and may have to soder it so that it is like a screen or maybe not so much but aluminum ....well it is an experiment

I would make a little copper thing and a little aluminum thing just to compare and see what works.
Anonymous Coward (OP)
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05/23/2012 08:06 PM
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Re: Help With Building A Faraday Cage
Being so broadband (1 MHz - 300 GHz) is the problem.

Guidelines:

(1) Low frequency coupling is dominated by wires and cables, e.g., power and internet you bring in.

(2) High frequency coupling is dominated by apertures, seams, and joints.

(3) You could bring signals in via fiber optics through an aperture with a tube-line extension to act as waveguide below cutoff.

(4) Power is tough, but at least you can filter the heck out of it, unlike signals.

(5) If any energy does get inside, the electric fields will be enhanced by the high Q of the interior.

(6) A wavelength at 300 GHz is 1 mm. You'll need a mesh finer that one tenth of this to achieve decent attenuation.

(7) Skin depth of copper at 1 MHz is about 0.065 mm. Five skin depths would be 0.325 mm.

Hope this helps (and that my math is correct!)
 Quoting: Life and Love


Thanks for the reply. I apologize for asking you to repeat the excellent points you have made but would you mind explaining in *layman* terms the following points please: (1), (2) and (5).

I don't know if its possible in point (4) to ignore the RF Radiation caused by the electrical wire and the RJ45 LAN cable inside the enclosure ? The only concern that I have with point (4) is that by allowing these two and only two cables (one for power and the other LAN cable) into the enclosure that it does not allow inside UHF(and other bands) signals to exit the enclosure through the same opening/path and vice versa in that it will not allow these same type of RF device signals to enter into the enclosure again through the same pathway that these two wires use to enter the enclosure ?

I understand point (6) perfectly. I will likely need to have the woven mesh aluminum specially made if I cannot find the 1/10 mm distance width you have described off the shelf.

In point (7) how is skin depth related with attenuation of RF signals ?

Thanks again
Life and Love

User ID: 9710640
United States
05/23/2012 08:48 PM
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Re: Help With Building A Faraday Cage
Being so broadband (1 MHz - 300 GHz) is the problem.

Guidelines:

(1) Low frequency coupling is dominated by wires and cables, e.g., power and internet you bring in.

(2) High frequency coupling is dominated by apertures, seams, and joints.

(3) You could bring signals in via fiber optics through an aperture with a tube-line extension to act as waveguide below cutoff.

(4) Power is tough, but at least you can filter the heck out of it, unlike signals.

(5) If any energy does get inside, the electric fields will be enhanced by the high Q of the interior.

(6) A wavelength at 300 GHz is 1 mm. You'll need a mesh finer that one tenth of this to achieve decent attenuation.

(7) Skin depth of copper at 1 MHz is about 0.065 mm. Five skin depths would be 0.325 mm.

Hope this helps (and that my math is correct!)
 Quoting: Life and Love


Thanks for the reply. I apologize for asking you to repeat the excellent points you have made but would you mind explaining in *layman* terms the following points please: (1), (2) and (5).

I don't know if its possible in point (4) to ignore the RF Radiation caused by the electrical wire and the RJ45 LAN cable inside the enclosure ? The only concern that I have with point (4) is that by allowing these two and only two cables (one for power and the other LAN cable) into the enclosure that it does not allow inside UHF(and other bands) signals to exit the enclosure through the same opening/path and vice versa in that it will not allow these same type of RF device signals to enter into the enclosure again through the same pathway that these two wires use to enter the enclosure ?

I understand point (6) perfectly. I will likely need to have the woven mesh aluminum specially made if I cannot find the 1/10 mm distance width you have described off the shelf.

In point (7) how is skin depth related with attenuation of RF signals ?

Thanks again
 Quoting: Anonymous Coward 3118042


No problem. Glad to help out.

(1) Work in actual metallic enclosures has shown that RF energy penetrates into the space more from cables (including all wire-like metallic penetrations) than from apertures, because the shielding effectiveness of apertures is very high (good) at low frequencies. The cables conduct energy into the room on their conductors or shields unless you spend work grounding and otherwise protecting them.

(2) At higher frequencies the cable coupling effects drop off and aperture coupling (holes, seams, joints) dominates. Actually, both effects are always present, but they vary with the frequency of the source signal.

(5) You are essentially building a "microwave oven" of sorts when you build a box out of highly conducting material. That means that any energy that gets inside will bounce around a lot, and that means that the peak field strengths in the room will be a lot higher than you would have in a "normal" room. We've measured the Q (quality factor) in actual metallic spaces and have seen Q as high as 800.

(4) Your cables have to carry currents (power or internet) or they wouldn't be of any use. But if any energy couples to these cables outside the box at other frequencies, then those currents will get inside the box just like the power or internet currents. And power/internet is connected to a VERY large antenna!

As far as radiation is concerned, if any of your internal signals happen to couple currents to those cables, they are going to get outside the box and perhaps radiate to the rest of the world.

(7) Skin depth is defined (mathematically) as the distance into a conductor that the surface currents are attenuated by 36.1% (1/e). And each skin depth attenuates equally well, so three skin depths of thickness will attenuate by 0.361 x 0.361 x 0.361.

Feel free to ask again if you need to. I'd be glad to continue the conversation.
We become like that to which we are devoted. - Choose wisely.
Burt Gummer

User ID: 7702124
United States
05/23/2012 09:05 PM
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Re: Help With Building A Faraday Cage
Being so broadband (1 MHz - 300 GHz) is the problem.

Guidelines:

(1) Low frequency coupling is dominated by wires and cables, e.g., power and internet you bring in.

(2) High frequency coupling is dominated by apertures, seams, and joints.

(3) You could bring signals in via fiber optics through an aperture with a tube-line extension to act as waveguide below cutoff.

(4) Power is tough, but at least you can filter the heck out of it, unlike signals.

(5) If any energy does get inside, the electric fields will be enhanced by the high Q of the interior.

(6) A wavelength at 300 GHz is 1 mm. You'll need a mesh finer that one tenth of this to achieve decent attenuation.

(7) Skin depth of copper at 1 MHz is about 0.065 mm. Five skin depths would be 0.325 mm.

Hope this helps (and that my math is correct!)
 Quoting: Life and Love


Thanks for the reply. I apologize for asking you to repeat the excellent points you have made but would you mind explaining in *layman* terms the following points please: (1), (2) and (5).

I don't know if its possible in point (4) to ignore the RF Radiation caused by the electrical wire and the RJ45 LAN cable inside the enclosure ? The only concern that I have with point (4) is that by allowing these two and only two cables (one for power and the other LAN cable) into the enclosure that it does not allow inside UHF(and other bands) signals to exit the enclosure through the same opening/path and vice versa in that it will not allow these same type of RF device signals to enter into the enclosure again through the same pathway that these two wires use to enter the enclosure ?

I understand point (6) perfectly. I will likely need to have the woven mesh aluminum specially made if I cannot find the 1/10 mm distance width you have described off the shelf.

In point (7) how is skin depth related with attenuation of RF signals ?

Thanks again
 Quoting: Anonymous Coward 3118042


No problem. Glad to help out.

(1) Work in actual metallic enclosures has shown that RF energy penetrates into the space more from cables (including all wire-like metallic penetrations) than from apertures, because the shielding effectiveness of apertures is very high (good) at low frequencies. The cables conduct energy into the room on their conductors or shields unless you spend work grounding and otherwise protecting them.

(2) At higher frequencies the cable coupling effects drop off and aperture coupling (holes, seams, joints) dominates. Actually, both effects are always present, but they vary with the frequency of the source signal.

(5) You are essentially building a "microwave oven" of sorts when you build a box out of highly conducting material. That means that any energy that gets inside will bounce around a lot, and that means that the peak field strengths in the room will be a lot higher than you would have in a "normal" room. We've measured the Q (quality factor) in actual metallic spaces and have seen Q as high as 800.

(4) Your cables have to carry currents (power or internet) or they wouldn't be of any use. But if any energy couples to these cables outside the box at other frequencies, then those currents will get inside the box just like the power or internet currents. And power/internet is connected to a VERY large antenna!

As far as radiation is concerned, if any of your internal signals happen to couple currents to those cables, they are going to get outside the box and perhaps radiate to the rest of the world.

(7) Skin depth is defined (mathematically) as the distance into a conductor that the surface currents are attenuated by 36.1% (1/e). And each skin depth attenuates equally well, so three skin depths of thickness will attenuate by 0.361 x 0.361 x 0.361.

Feel free to ask again if you need to. I'd be glad to continue the conversation.
 Quoting: Life and Love


Suggestion related to #4......

Expand the size of your room to incorporate an enclosed insulated boxed off generator. Same principle as on boats and RV's.
Since it is INSIDE the whole cage.....you are not bringing in a line from the outside for the genset.

Just a thought.
Life and Love

User ID: 9710640
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05/23/2012 09:09 PM
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Re: Help With Building A Faraday Cage
Suggestion related to #4......

Expand the size of your room to incorporate an enclosed insulated boxed off generator. Same principle as on boats and RV's.
Since it is INSIDE the whole cage.....you are not bringing in a line from the outside for the genset.

Just a thought.
 Quoting: Burt Gummer


Yep, that's a good idea. You can handle the exhaust ok with waveguide below cutoff.

What we did (running lightning and EMP tests) was to charge a bunch of 12-volt car batteries outside the room, take them into the room, and then power the equipment with them.
We become like that to which we are devoted. - Choose wisely.
Burt Gummer

User ID: 7702124
United States
05/23/2012 09:12 PM
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Re: Help With Building A Faraday Cage
Suggestion related to #4......

Expand the size of your room to incorporate an enclosed insulated boxed off generator. Same principle as on boats and RV's.
Since it is INSIDE the whole cage.....you are not bringing in a line from the outside for the genset.

Just a thought.
 Quoting: Burt Gummer


Yep, that's a good idea. You can handle the exhaust ok with waveguide below cutoff.

What we did (running lightning and EMP tests) was to charge a bunch of 12-volt car batteries outside the room, take them into the room, and then power the equipment with them.
 Quoting: Life and Love


IMO....easier to store diesel fuel underground...and a 500 gallon tank lasts for a LONG time with a genset.

Last Edited by Useless Cookie Eater on 05/23/2012 09:13 PM
Life and Love

User ID: 9710640
United States
05/23/2012 09:15 PM
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Re: Help With Building A Faraday Cage
Suggestion related to #4......

Expand the size of your room to incorporate an enclosed insulated boxed off generator. Same principle as on boats and RV's.
Since it is INSIDE the whole cage.....you are not bringing in a line from the outside for the genset.

Just a thought.
 Quoting: Burt Gummer


Yep, that's a good idea. You can handle the exhaust ok with waveguide below cutoff.

What we did (running lightning and EMP tests) was to charge a bunch of 12-volt car batteries outside the room, take them into the room, and then power the equipment with them.
 Quoting: Life and Love


IMO....easier to store diesel fuel underground...and a 500 gallon tank lasts for a LONG time with a genset.
 Quoting: Burt Gummer


Very true - I bet it would last a VERY long time.

(We were doing short run testing, so we could use batteries for a few hours at a time.)
We become like that to which we are devoted. - Choose wisely.
Burt Gummer

User ID: 7702124
United States
05/23/2012 09:22 PM
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Re: Help With Building A Faraday Cage
Suggestion related to #4......

Expand the size of your room to incorporate an enclosed insulated boxed off generator. Same principle as on boats and RV's.
Since it is INSIDE the whole cage.....you are not bringing in a line from the outside for the genset.

Just a thought.
 Quoting: Burt Gummer


Yep, that's a good idea. You can handle the exhaust ok with waveguide below cutoff.

What we did (running lightning and EMP tests) was to charge a bunch of 12-volt car batteries outside the room, take them into the room, and then power the equipment with them.
 Quoting: Life and Love


IMO....easier to store diesel fuel underground...and a 500 gallon tank lasts for a LONG time with a genset.
 Quoting: Burt Gummer


Very true - I bet it would last a VERY long time.

(We were doing short run testing, so we could use batteries for a few hours at a time.)
 Quoting: Life and Love


Yep. I understand your thought process for testing.
Again...IMO....it's best to engineer and create real world tests for LONG RUNS and not short outages / issues.

zombies

Last Edited by Useless Cookie Eater on 05/23/2012 09:31 PM
Anonymous Coward
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New Zealand
05/23/2012 09:27 PM
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Re: Help With Building A Faraday Cage
good microwave oven
Life and Love

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05/23/2012 09:33 PM
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Re: Help With Building A Faraday Cage
good microwave oven
 Quoting: Anonymous Coward 1089603


^^THIS^^ would work in the microwave frequency range but not for low frequencies (1 MHz) or very high frequencies (300 GHz) like the OP is looking for.
We become like that to which we are devoted. - Choose wisely.
Anonymous Coward (OP)
User ID: 3118042
Canada
05/25/2012 06:23 AM
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Re: Help With Building A Faraday Cage
Being so broadband (1 MHz - 300 GHz) is the problem.

Guidelines:

(1) Low frequency coupling is dominated by wires and cables, e.g., power and internet you bring in.

(2) High frequency coupling is dominated by apertures, seams, and joints.

(3) You could bring signals in via fiber optics through an aperture with a tube-line extension to act as waveguide below cutoff.

(4) Power is tough, but at least you can filter the heck out of it, unlike signals.

(5) If any energy does get inside, the electric fields will be enhanced by the high Q of the interior.

(6) A wavelength at 300 GHz is 1 mm. You'll need a mesh finer that one tenth of this to achieve decent attenuation.

(7) Skin depth of copper at 1 MHz is about 0.065 mm. Five skin depths would be 0.325 mm.

Hope this helps (and that my math is correct!)
 Quoting: Life and Love


Thanks for the reply. I apologize for asking you to repeat the excellent points you have made but would you mind explaining in *layman* terms the following points please: (1), (2) and (5).

I don't know if its possible in point (4) to ignore the RF Radiation caused by the electrical wire and the RJ45 LAN cable inside the enclosure ? The only concern that I have with point (4) is that by allowing these two and only two cables (one for power and the other LAN cable) into the enclosure that it does not allow inside UHF(and other bands) signals to exit the enclosure through the same opening/path and vice versa in that it will not allow these same type of RF device signals to enter into the enclosure again through the same pathway that these two wires use to enter the enclosure ?

I understand point (6) perfectly. I will likely need to have the woven mesh aluminum specially made if I cannot find the 1/10 mm distance width you have described off the shelf.

In point (7) how is skin depth related with attenuation of RF signals ?

Thanks again
 Quoting: Anonymous Coward 3118042


No problem. Glad to help out.

(1) Work in actual metallic enclosures has shown that RF energy penetrates into the space more from cables (including all wire-like metallic penetrations) than from apertures, because the shielding effectiveness of apertures is very high (good) at low frequencies. The cables conduct energy into the room on their conductors or shields unless you spend work grounding and otherwise protecting them.

(2) At higher frequencies the cable coupling effects drop off and aperture coupling (holes, seams, joints) dominates. Actually, both effects are always present, but they vary with the frequency of the source signal.

(5) You are essentially building a "microwave oven" of sorts when you build a box out of highly conducting material. That means that any energy that gets inside will bounce around a lot, and that means that the peak field strengths in the room will be a lot higher than you would have in a "normal" room. We've measured the Q (quality factor) in actual metallic spaces and have seen Q as high as 800.

(4) Your cables have to carry currents (power or internet) or they wouldn't be of any use. But if any energy couples to these cables outside the box at other frequencies, then those currents will get inside the box just like the power or internet currents. And power/internet is connected to a VERY large antenna!

As far as radiation is concerned, if any of your internal signals happen to couple currents to those cables, they are going to get outside the box and perhaps radiate to the rest of the world.

(7) Skin depth is defined (mathematically) as the distance into a conductor that the surface currents are attenuated by 36.1% (1/e). And each skin depth attenuates equally well, so three skin depths of thickness will attenuate by 0.361 x 0.361 x 0.361.

Feel free to ask again if you need to. I'd be glad to continue the conversation.
 Quoting: Life and Love


Thank you again for your reply.

Point (1) could you describe the kind of energy that can penetrate the box through the two cables I plan on bringing into the room ? Could this energy be a High Frequency signal or ELF ? Also could you possibly describe how would you bring the cables into the enclosure so that this energy you have described is minimized ?

Point (2) do mean that if the box is trying to prevent a High Frequency signal from getting in/out I should be more concerned with the aperture coupling effects which would allow this this same high frequency signal through (in/out) the cage's seams, joints and holes ?

Point (5) how does the effect of a high peak field strength have on a human body ? Is there any effects (interference) inside the box to computer equipment of having such a high peak strength ?

Point (4) This one is an important one so please be patient with me. From what I understand you are saying that other frequencies that originate outside the box such as UHF for example could possibly get inside the box via the coupling effect of the two cables ? Will the signal entering the
cage/room via the coupling effect of these two cables be a clean signal or could the coupling effect in any way degrade or amplify (interfere) the RF signal in any way ? In other words could a cell phone signal @ 2.4GHz be drawn into the cage via the coupling effect of either cable so that this cell phone if placed inside the box and if called will ring ? How can I filter or reduce or prevent entirely the effects of coupling that would allow RF signals from entering or exiting the box ? I understand that these cables act as an antenna. So then how do I filter or shield out all signals that enter or exit the box via the coupling effects of these two cables leaving aside the EMF caused
by the cables ? What would you recommend ?

You also have said that the internally generated signals that couple onto the cables then you say they will radiate outside the box ? Then my question is how can internally generated signals be prevented from getting outside the box ? I would think that the same solution preventing signals from coupling onto the cables and getting into the box will be the same solution for the signals that are trying to get ?

In your original post you said "(3) You could bring signals in via fiber optics through an aperture with a tube-line extension to act as waveguide below cutoff." Now does this tube like extension apply in preventing the coupling effect from happening ? If yes can you provide a little more details of what your trying to describe here and thanks ?

Point (7) Ok I think I understand that this point is about attenuation of signals depending on the skin depth of the current on a conductor. My only question is how does it affect either the type of material used for the perimeter or the box or the two cables coming into the box ?

Thank you again

Vm
Anonymous Coward (OP)
User ID: 3118042
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05/25/2012 06:27 AM
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Re: Help With Building A Faraday Cage
Being so broadband (1 MHz - 300 GHz) is the problem.

Guidelines:

(1) Low frequency coupling is dominated by wires and cables, e.g., power and internet you bring in.

(2) High frequency coupling is dominated by apertures, seams, and joints.

(3) You could bring signals in via fiber optics through an aperture with a tube-line extension to act as waveguide below cutoff.

(4) Power is tough, but at least you can filter the heck out of it, unlike signals.

(5) If any energy does get inside, the electric fields will be enhanced by the high Q of the interior.

(6) A wavelength at 300 GHz is 1 mm. You'll need a mesh finer that one tenth of this to achieve decent attenuation.

(7) Skin depth of copper at 1 MHz is about 0.065 mm. Five skin depths would be 0.325 mm.

Hope this helps (and that my math is correct!)
 Quoting: Life and Love


Thanks for the reply. I apologize for asking you to repeat the excellent points you have made but would you mind explaining in *layman* terms the following points please: (1), (2) and (5).

I don't know if its possible in point (4) to ignore the RF Radiation caused by the electrical wire and the RJ45 LAN cable inside the enclosure ? The only concern that I have with point (4) is that by allowing these two and only two cables (one for power and the other LAN cable) into the enclosure that it does not allow inside UHF(and other bands) signals to exit the enclosure through the same opening/path and vice versa in that it will not allow these same type of RF device signals to enter into the enclosure again through the same pathway that these two wires use to enter the enclosure ?

I understand point (6) perfectly. I will likely need to have the woven mesh aluminum specially made if I cannot find the 1/10 mm distance width you have described off the shelf.

In point (7) how is skin depth related with attenuation of RF signals ?

Thanks again
 Quoting: Anonymous Coward 3118042


No problem. Glad to help out.

(1) Work in actual metallic enclosures has shown that RF energy penetrates into the space more from cables (including all wire-like metallic penetrations) than from apertures, because the shielding effectiveness of apertures is very high (good) at low frequencies. The cables conduct energy into the room on their conductors or shields unless you spend work grounding and otherwise protecting them.

(2) At higher frequencies the cable coupling effects drop off and aperture coupling (holes, seams, joints) dominates. Actually, both effects are always present, but they vary with the frequency of the source signal.

(5) You are essentially building a "microwave oven" of sorts when you build a box out of highly conducting material. That means that any energy that gets inside will bounce around a lot, and that means that the peak field strengths in the room will be a lot higher than you would have in a "normal" room. We've measured the Q (quality factor) in actual metallic spaces and have seen Q as high as 800.

(4) Your cables have to carry currents (power or internet) or they wouldn't be of any use. But if any energy couples to these cables outside the box at other frequencies, then those currents will get inside the box just like the power or internet currents. And power/internet is connected to a VERY large antenna!

As far as radiation is concerned, if any of your internal signals happen to couple currents to those cables, they are going to get outside the box and perhaps radiate to the rest of the world.

(7) Skin depth is defined (mathematically) as the distance into a conductor that the surface currents are attenuated by 36.1% (1/e). And each skin depth attenuates equally well, so three skin depths of thickness will attenuate by 0.361 x 0.361 x 0.361.

Feel free to ask again if you need to. I'd be glad to continue the conversation.
 Quoting: Life and Love


Suggestion related to #4......

Expand the size of your room to incorporate an enclosed insulated boxed off generator. Same principle as on boats and RV's.
Since it is INSIDE the whole cage.....you are not bringing in a line from the outside for the genset.

Just a thought.
 Quoting: Burt Gummer


The idea of a generator is a good one but it would need gasoline and then the fumes created by the generator would have to be addressed and not to mention the noise factor. But an idea none the less.

Thanks
Anonymous Coward (OP)
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05/25/2012 06:30 AM
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Re: Help With Building A Faraday Cage
Suggestion related to #4......

Expand the size of your room to incorporate an enclosed insulated boxed off generator. Same principle as on boats and RV's.
Since it is INSIDE the whole cage.....you are not bringing in a line from the outside for the genset.

Just a thought.
 Quoting: Burt Gummer


Yep, that's a good idea. You can handle the exhaust ok with waveguide below cutoff.

What we did (running lightning and EMP tests) was to charge a bunch of 12-volt car batteries outside the room, take them into the room, and then power the equipment with them.
 Quoting: Life and Love


An excellent idea of using car batteries but I still need to get a LAN cable into the box.

Thanks
Life and Love

User ID: 1164971
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05/25/2012 10:29 AM
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Re: Help With Building A Faraday Cage
Thank you again for your reply.
 Quoting: Anonymous Coward 3118042


I'll take it one paragraph at a time...

Point (1) could you describe the kind of energy that can penetrate the box through the two cables I plan on bringing into the room ? Could this energy be a High Frequency signal or ELF ? Also could you possibly describe how would you bring the cables into the enclosure so that this energy you have described is minimized ?
 Quoting: Anonymous Coward 3118042


Any metallic object will pick up energy if place in an electric field. (That's how a radio or TV antenna works.) What happens is that the fields induce a current on the wires and that current flows right into your box through the hole you made for the wire. This could be quite signification, say for EMP.

It is more of a low frequency phenomenon because those wire currents will also reradiate before they get into the box, which will diminish the currents you will be concerned about.

Mitigation: For power cables, filter the heck out of them. All you need is 60 Hz, so use an LC filter (or something more complex) to cutoff current above that.

For LAN cables, don't use a cable. Rather, convert the signal to fiber optics (non-conducting), then run the fiber through the wall and re-convert back to a LAN cable. You'll still have to drill a hole in your wall (problem for higher freqs) but you can mitigate that by putting a cylindrical sleeve on the inside of the hole as that makes for a waveguide below cutoff frequency. But you can't put metal stuff through a hole like this, only non-conducting stuff.

Point (2) do mean that if the box is trying to prevent a High Frequency signal from getting in/out I should be more concerned with the aperture coupling effects which would allow this this same high frequency signal through (in/out) the cage's seams, joints and holes ?
 Quoting: Anonymous Coward 3118042


Once you have your wires/cables protected (see above), you should turn your attention to seams, joints, holes, etc. Smaller is better. Copper tape is a good approach up through microwave freqs... not sure about higher. You will have to minimize your joints, else the room will leak in/out. (At low freqs something called the Bethe small hole theory applies and you won't have much leakage)

Here's a chart of attenuation vs. frequency for a slot aperture of different lengths. [link to www2.evaluationengineering.com]

Point (5) how does the effect of a high peak field strength have on a human body ? Is there any effects (interference) inside the box to computer equipment of having such a high peak strength ?
 Quoting: Anonymous Coward 3118042


You'll have interference effects before you have health effects to your body or damage effects to equipment. To illustrate, talk on your cell phone or cordless phone while holding it next to your computer or TV set... you'll probably hear all sorts of interference.

Point (4) This one is an important one so please be patient with me. From what I understand you are saying that other frequencies that originate outside the box such as UHF for example could possibly get inside the box via the coupling effect of the two cables ? Will the signal entering the
cage/room via the coupling effect of these two cables be a clean signal or could the coupling effect in any way degrade or amplify (interfere) the RF signal in any way ? In other words could a cell phone signal @ 2.4GHz be drawn into the cage via the coupling effect of either cable so that this cell phone if placed inside the box and if called will ring ? How can I filter or reduce or prevent entirely the effects of coupling that would allow RF signals from entering or exiting the box ? I understand that these cables act as an antenna. So then how do I filter or shield out all signals that enter or exit the box via the coupling effects of these two cables leaving aside the EMF caused
by the cables ? What would you recommend ?
 Quoting: Anonymous Coward 3118042


Using the cell phone example, take a look at the graph I linked to above. You'll see how aperture attenuation works at 2.4 GHz.

I talked about mitigating cable effects above. Power = filter. LAN = fiber.


You also have said that the internally generated signals that couple onto the cables then you say they will radiate outside the box ? Then my question is how can internally generated signals be prevented from getting outside the box ? I would think that the same solution preventing signals from coupling onto the cables and getting into the box will be the same solution for the signals that are trying to get ?
 Quoting: Anonymous Coward 3118042


Yes, you're right. The power line filters and fiber approach will take care of this problem, too.

In your original post you said "(3) You could bring signals in via fiber optics through an aperture with a tube-line extension to act as waveguide below cutoff." Now does this tube like extension apply in preventing the coupling effect from happening ? If yes can you provide a little more details of what your trying to describe here and thanks ?
 Quoting: Anonymous Coward 3118042


Let's start with a hollow cylinder of some small radius and maybe 3-4" long. Attach that cylinder to the inside of your box where you've drilled a hole. A signal that would penetrate through the hole by itself will be attenuated significantly by the cylinder if the cylinder diameter is small enough.

Here's a reference to get you started: [link to www.formfactors.org]


Point (7) Ok I think I understand that this point is about attenuation of signals depending on the skin depth of the current on a conductor. My only question is how does it affect either the type of material used for the perimeter or the box or the two cables coming into the box ?
 Quoting: Anonymous Coward 3118042


This is a surface quantity, so it won't affect your cables at all. The thickness of material and the type of material you use for your surfaces determines the shielding effectiveness of the perfectly tight enclosure. The higher the conductivity, the better (or, the lower the resistivity the better). (That is, copper is better than steel.)

Here's a link to a skin depth calculator: [link to www.microwaves101.com]
We become like that to which we are devoted. - Choose wisely.
Anonymous Coward (OP)
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05/26/2012 11:52 AM
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Re: Help With Building A Faraday Cage
Many thanks "Life and Love" for your helpful replies. It has been a learning experience no doubt. Should I run into some more questions I will post back again.

Thanks to everyone for their helpful comments and suggestions.





GLP