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Protection against EMP ... Answered

Hi !

Data :

Let's say that I want to protect some important electronics devices against EMP.

Let's say that they need to work 24/7 and that they are, thus, connected to an external source of power (electrical socket before EMP, and solar panel after EMP).

Let's say that they need to emit and receive RF signals.

Questions :

1) As there are electrical connections with the exterior, a Faraday cage would be pointless, wouldn't it ?

2) Would a solar panel survive to an EMP ?

3) Is this possible to use the Faraday cage as an antenna for RF communication ? (I guess it's a stupid question as connecting the cage to the device would make the cage part of the device ...)

4) Are thin and transparent gold foils enough to act as a Faraday cage ?

Edit : Interesting readings found thanks to the links Nachomahma provided :
Edit2 : an article about EMP protection.
The first part is about EMP from nuclear bombs. The last part gives some clue about HMP.


1) No. There are ways to greatly attenuate the pulse coming down the power wires and you can use optical for data.. 2) Not sure, but probably not. 3) Since most Faraday cages are well grounded, no. 4) Not sure. Probably depends on the amount of current.

3) Since most Faraday cages are well grounded, no.

Actually I'm not sure a Faraday cage requires to be grounded.
If I understood the theory correctly, a simple ungrounded casserole dish with its metalic cap should act as a Faraday cage : during an EMP, only the external surface of the box is influenced, and the inside remain unchanged.

. A Faraday cage does not have to be grounded, but it's hard to build one as large as what you are looking at (equipment + adequate space for cooling + room to work on it >= small closet) without it being grounded.

Ideally, (and unless I'm underestimating it), I think everything (except the panel) could fit in half of a micro-ATX box ...

In theory, the system would not emit that much heat (few watts), reducing, thus, the space for cooling.
What would takes most space would be the batteries ...

About using optical fibers instead of wires for data communication, my problem is that it could let enter microwaves pulses throught the holes. =o/
(That's why I was thinking about using transparent gold-foils windows).

However, maybe I could use "audio" communication instead ? I think audio signals could be sent through the shield ... =o)

> half of a micro-ATX box . OK. We weren't thinking on the same scale. . > let enter microwaves pulses throught the holes . My uW oven has holes in the door and last time I check it wasn't leaking. Good engineering and careful construction should keep you safe. . > gold-foils windows . With the small area you are talking about, the voltage differential should be low enough to keep the current from vaporizing the metal. . > use "audio" communication . Ultrasonics should work well, but it won't have near the bandwidth of optical. . . From what I've read, it's next to impossible to shield from a "direct hit." All you can do is make the sweet spot smaller.

. BTW, I've asked several ppl about using a Faraday cage as an an antenna. I've gotten a lot of weird looks, but no definitive answer. The consensus is that it would probably work. Having the transmitter inside of the antenna seems to throw everyone for a loop.

Having the transmitter inside of the antenna seems to throw everyone for a loop.

That sounds like a recipe for interference doesn't it?

. That's one of the proposed problems ("but the EM is supposed to go out, isn't it? hmmmm I'm just not sure"), but no one I've talked to has ever heard of using a FC for an antenna. Everyone seems to agree that it's probably just a really fat wire, but they don't have a clue as to how to determine what the xmiter would see (effective length, &c). If I get curious enough, I'll find somebody that knows what they're talking about. Probably start at ARRL.

Well, solid or hollow, a wire carries most "charge" on the outside or skin, so my assumption at first would be to say a round wire will work like a round antenna; omnidirectionally.

Although antennas don't always receive the same way the send, it may be hard to determine without testing, but because a Faraday cage prevents both the "entrance" of EMI, and any stray EMI from escaping, it shows it can "receive" in both directions (in order to drain to ground).

if you made the cage the antenna, wouldnt that defeat the purpose of the cage? and its only the amps that travel on the outside of the wire, not the volts.

if you made the cage the antenna, wouldnt that defeat the purpose of the cage?

That's what I'm wondering too.
'cause the cage would become part of the device ... and if the cage could carry RF signals to the device, it could also carry the destructive EMP ...

So I guess that using a FC as an antenna would be a bad idea ...

it would defeat the purpose. and if you put an antenna in the cage, you might get interference, or loss a lot of signal strengh.

putting an antenna inside an efficient cage (functioning as a cage and not an antenna) should effectively neutralize the signal, or shall we say, reroute it to ground. They are used to protect in both directions, if I am not mistaken.

> if you made the cage the antenna, wouldnt that defeat the purpose of the cage? . Yeah. I'm just trying to find out if it would work as an antenna or not.

If I understand correctly, if a wire (hollow or not) carries most charges outside, and if the inside remains neutral, then, connecting a receiver inside of the hollow box should be useless, because the external RF signals would only move the charges which are outside of the wire ... that would, thus, be like connecting the receiver to a ground.


My uW oven has holes in the door and last time I check it wasn't leaking. Good engineering and careful construction should keep you safe.

Yes you are right.
I wrongfully thought that uW could be smaller than 1 mm of wavelength, and that the doors of uW ovens were designed specially for the frequency of the magnetron ...
In fact, (according to wikipedia) uW range from 1mm to 300mm.
So, this means that smaller wavelength are in the IR range.

And now, I'm wondering if I should consider strong low IR pulse as potential treat for my electronics devices ... it's still electro-magnetic after all ...

An HPM e-bomb is supposed to be designed to emits only a pulse in a specific range of frequency, and should not, thus, emits IR or light.
However, a nuclear bomb generates strong EMP and HMP as well as light ...


So, wouldn't the ideal protection be a thick metals box without any hole ?
Anyway, unless I find a mean to transmit energy through the box, I'd still need at least ONE hole for the external source of power ....... =o/

The ideal protection seems hard to imagine ....

. IR should be pretty easy to block. A simple labyrinth would probably work and might be overkill.
. IEEE's Spectrum magazine has had several good articles on EMP production and protection. I believe that's where I saw a simplified schematic for incoming power protection.

Thanks for the links NachoMahma =o)
I'm going to explore those sites.

1a) If your RF transceiver is cheap (eg, WiFi, small ham), you may be better off sacrificing it to the pulse and keeping a spare inside the cage. 2a) The panel will survive if stored inside the cage.

Actually, I would like the system autonomous : it should not require human intervention after the EMP.

We could imagine that it would deploy the panel and a new transceiver automatically just after the EMP, but it would be vulnerable to an eventual second EMP ... =o/

. Who do you mean we, kemo sabe? heehee

. I think you're getting out of the range (primarily money) of most DIYers.
That's precisely why I said "could imagine" lol