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#61613 - 03/10/06 03:27 PM Faraday Cage
Duke Offline

Registered: 09/13/05
Posts: 53
Loc: Harlan KY
I know we have talked about this before relative to Electromagnetic Pulse, but I have a few specific questions and not much aptitude for this type of thing. I read just recently that the passenger compartment of a car is essentially a Faraday cage, so would a radio transceiver be protected if it were just lying on the seat, for instance? Also, would a metal trunk with a ground wire soldered to it suffice? I assume whatever was inside would need to be off the bottom on some sort of insulation. Any help of this variety would be appreciated. Thanks, Duke. I am particularly interested in keeping some Ham radio gear safe.

#61614 - 03/10/06 04:55 PM Re: Faraday Cage
M_a_x Offline
Old Hand

Registered: 08/16/02
Posts: 995
Loc: Germany
A transceiver would not be protected on the passenger seat. If you have a cell phone you could see that there still is some reception when you put it on a seat in the car. You could try the same with the ham gear but you may have to make sure that someone´s transmitting a signal. A metal box would to be HF proof if you want to be sure. A ground wire would help only if the car´s body is also grounded. I would at least put some foam in the box to protect the gear from shock.
If it isn´t broken, it doesn´t have enough features yet.

#61615 - 03/10/06 05:47 PM Re: Faraday Cage
Duke Offline

Registered: 09/13/05
Posts: 53
Loc: Harlan KY
What about metal boxes regardless of whether there is a car involved or not; for other objects that might be in basements, for instance. And should those be grounded?

#61616 - 03/12/06 03:42 AM Re: Faraday Cage
Nomad Offline

Registered: 05/04/02
Posts: 463
Loc: Just wandering around.
Like most things electronic, there is a simple answer and a complex one. A car is a faraday shield only under some circumstances. Lightning is one of them. But RF is not.

Various type of electromagnetic fields can be blocked by faraday shields, depending on the shield type and the field type.

What type shield works for what type field is a very complex subject. In general, if you put a transceiver in a metal box with no holes larger than about 1/10 wavelength of the field of interest, and ground the box to a suitable ground, the contents will be immune to the exterior field.

In practical terms, it is almost impossible to shield a radio from a large nuclear burst unless you are very far from the burst.

What exactly do you want to do? Nothing you can do within reason will keep ham gear safe near a nuclear blast. There is lots of stuff about this on the web, but mostly it is quite complex.

...........From Nomad.........Been "on the road" since '97

#61617 - 03/12/06 04:13 AM Re: Faraday Cage
desertrat1 Offline

Registered: 02/16/06
Posts: 144
Loc: Kingman AZ
Any thing that's metal, and completely covers and grounded will work. Metal window screen is highly effective provided it's grounded.
What you know isn't as important as knowing what you don't know

#61618 - 03/12/06 05:27 AM Re: Faraday Cage
Raspy Offline

Registered: 01/08/04
Posts: 351
Loc: Centre Hall Pa
EMP Electro Magnetic Pulse
By Rich “Raspy” Shawver

The way you make electricity require 3 things a conductor [Wire] a magnetic field and relative motion. A lightning strike generates an expanding [the moving] magnetic pulse or field that passes over power lines [the wire] generating a voltage spike in incoming services. The size of the spike is dependent on the strength of the field and the length of the wire involved. The resulting spike only has a duration of a very small fraction of a second but that is more than enough to damage electronic components. Circuit breakers are of no help. The spike is so short that it hits and does the damage long before the breaker has a chance to react and open. The spike resulting from a lightning strike can be 4 to 5 times as high the normal line voltage. The EMP of a nuclear weapon is exactly the same mechanism but with a much greater intensity. Depending on your distance from the detonation the field can be strong enough to generate damaging voltages in the internal wiring of the equipment.

If your equipment is hooked up but turned off, an incoming spike on the electrical power lines [and other attached lines, cable and phone] can arc over the switch if large enough. Also most electronic equipment now have circuits that have power running through them even when turned off for remote operation [even if it's not remote controlled]. Rather than building separate boards for each model companies have found it is cheaper to include many of the features on a common board. They just turn on the special features on the cheaper models.

The only safe way to keep equipment from being damaged is to enclose it in a metal structure that is grounded. The equipment has to be totally isolated from the cage and any outside connections. A structure of this type is called a Faraday Cage. A metal container [such as an ammo box] in which equipment can be stored if insulated from the container and the container is grounded will act as one. The way a Faraday cage works is the moving magnetic field strikes the cage. This starts to generate an electrical charge. As the electrical charge builds this in turn starts generating its own magnetic field that pushes back or repulses the original magnetic field. This then causes the original field to flow around the cage rather than through it. This is what keeps the EMP pulse from effecting the equipment inside the cage. While the best design for a cage is solid sheet it can be a fine mesh grid. The size of the openings in the grid is somewhat critical. If the openings or mesh size is too large the field the wires counter generate does not build up fast enough to block the field pulse. Something the size of chicken wire would be so large that there would be some leakage. The field on the cage has to build up fast enough to block the holes in the mesh. The cage does need to be grounded to dissipate the resulting electrical charge.

There are devices called lightning arresters among other names. That are sold on the market that are designed to stop or at least minimize the effects of such spikes. Many are good devices. But they are limited to the amount of protection they can supply. A large enough spike can overwhelm their capacity. While you can not shield wiring outside your own home. Inside it can and should be shielded. Some wiring is designed with shielding. This is a metallic or braided sheath surrounding the wiring. Another way is to run the wiring through EMT conduits. Both methods can be combined for better results. This shielding needs to be grounded. Just remember that this wiring is attached to wiring that is outside your control and probably not shielded.

The best way to build your shelter is to line the inside or outside with sheet metal. [Door included] The metal of the door must make electrical connection with the rest of the sheeting. For more reliability Line both inside and outside independently. A double Faraday cage. You do not have to do this for your entire house but should be done for a shelter, panic room, communication shack or a room that is a combination of any of the three. The best metal for making the cage is copper because it conducts better but any metal will work. [Gold and platinum would actually work even better but who could afford it.]

If you do run electrical or electronic conducting lines into your structure have them shielded but do not have your equipment connected to them except during actual use. I do not mean disconnected by a switch but physical distance such as pulling the plug out of the wall socket.

Remember if the moving magnetic field is strong enough the internal wires of the equipment itself can cause a voltage spike that will damage it. That is why most cars will be knocked out because the voltages would knock out any electronics. [Most cars are totally controlled by chips] That is why older vehicles [pre chip] are a better BOV. The voltages can be high enough to actually burn out the wiring itself.

I can see the question in your eyes. A car is basically a metal box wouldn’t that act like a faraday cage? If that is so how can a car be effected? Simple the box that is a car has some huge holes in it electrically speaking. That and many modern vehicles have gone to composite material mostly plastic for sheet material rather than steel to reduce weight. The engine compartment, that has much of the wiring running hither and yon to operate the engine. Plus you have the main power supply cables from the battery and alternator and they either hook into everything that uses electricity or that item does not have power. Look under the hood sometime even in the tight compartments today there are places you can see the ground. The passenger compartment has windows some real big holes in the box. Yeah, but the wires are under the dash. How much of the dash is plastic? Look at all the holes for gages and dials. In most cases the entire underside is open. Out your car in the bottom of your swimming pool. Just kidding. But if you did would the water be able to get to the wiring? If so then so could the pulse. While you could close off the engine area covering the windows with plate or mesh just wouldn’t be a workable solution. It would be difficult to say the least to see out and drive. But the local constabulary might have a comment or two.

If your only choice for a BOV does rely on electronics there is a remedy. On any car you could pull the entire wiring harness and up grade it. This would be with heavier gage wiring and using shielded wire. Get a spare set of the control modules needed for you car. Not just the computer control boxes but also the sensors and the items that are controlled by the computer. [They are also susceptible to pulse damage.] Wrap them in insulation [Electrical not temperature type I.E. rubber] and store them in a metal box. Attach the box to the frame so that it makes good electrical contact. Then do as they do to movie stunt cars. Attach a grounding strap to the frame. The strap is usually a braided copper cable that then drags on the ground. This is to dissipate and charge that builds up on the vehicle. The strap needs to be checked regularly and replaced as needed.

Yes the entire national power grid would be knocked out. While the most of the grid components them selves would be able to withstand such a pulse all the electronic control equipment would be fried. The means of operating and monitoring the system. Local power may be restorable but an inter linked grid would take a long, long time to restore.

Will your car or home equipment be affected? Mainly it will depend on luck. How far you are from the source and intervening obstacles. Even the angle that the local grid or house wiring is to the source can have an effect.

Sorry to be so long winded but it is not a simple subject and needs some explanation on the hows and whys.

Here are a couple of web sites that have a more technical explanation of how the field is generated.
When in danger or in doubt
run in circles scream and shout

And always remember TANSTAAFL

#61619 - 03/12/06 02:17 PM Re: Faraday Cage
aardvark Offline

Registered: 03/11/06
Posts: 109
Loc: So. California
As a former radio engineer, i feel i should point out a couple of things. First, most of the information presented is correct, shielding is provided by a Faraday cage and the openings in it have to be much smaller than the wavelength of radio you are interested in shielding.

But the devil is in the details. The cage, or box has to be electrically continuous all the way around, otherwise it will leak. This means that any opening (door, lid etc) has to have an electrically conducting lip in order for the cage to be continuous. So ammo cans may not be a good choice, they only have a rubber seal. Shiny new paint cans might be better. If you look in radio gear, especially old HF rigs, you will notice that some sections of the chassis are bolted on with screws every one inch or less. This is for electrical continuity, not strength. Some may even have beryllium-copper finger stock around the openings to improve the conductivity.

Second, i'm pretty sure that the cage itself doesn't need to be grounded in order for shielding to be effective. As mentioned in earlier posts, the charge redistribution in the conducting cage is what provides the shielding by cancelling the field internally. One does not need to add or remove charge to make this happen (the reason for the ground).

What might be an interesting experiment is to take your cell phone and wrap it completely in aluminum foil, making sure that the seams are folded tight. Then try calling it to see if the RF from cell site can penetrate the shield. I would bet that this works regardless of whether the aluminum foil is grounded or not. Then you could poke holes of various sizes to see how big or small it has to be to let the RF in. It's also probably not a good idea to leave the phone in there for too long, it's probably trying to transmit at full power to contact a cell site...

As far as communicating after a nuclear bomb EMP, i would use tin cans and a length of string. <img src="/images/graemlins/smile.gif" alt="" />

#61620 - 03/12/06 09:19 PM Re: Faraday Cage
Duke Offline

Registered: 09/13/05
Posts: 53
Loc: Harlan KY
Basically, what I am interested in doing is keeping my radio(s) up if possible should we be attacked by an EMP. I would like to be able to communicate. My thinking is that since detonation could be 30km or higher in the atmosphere, there would be no destructive force of the type we converntionally contemplate. If one were near ground zero of a blast, then that's that. However, in the event of survivability I would like to protect the devices from destruction from the pulse itself. So that's what I was contemplating. If I could save a hand-held, or protect a base station or mobile station through some pre-emptive effort then it would be worthwhile. These posts at least give me something to do re the most imminent threat, in my opinion. Thanks.

#61621 - 03/12/06 10:35 PM Re: Faraday Cage

Several years ago the Tempest computer spying topic came up in the media. Later, I got an ad in the mail advertising rubber paint. You were supposed to paint your room with it and this would prevent any rf emissions from your computer from going outside of your room to an "enemy receiving device". I thought about writing to my state attorney general about the scam, but got sidetracked and never got around to it.

#61622 - 03/13/06 04:26 PM Re: Faraday Cage
JimJr Offline

Registered: 05/03/05
Posts: 133
Loc: Central Mississippi
Starting back in 1986, QST ran a series of articles on EMP and amateur radio. Here a link the the articles: Electromagnetic Pule and the Radio Amateur- Part 1. Links to subsequent articles are provided in the article.




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