EMP Danger: 1m Distance, Concrete Walls & Directional Effects

[wimms]

What is considered dangerous energy of EMP to blow away electronics equipment at 1m distance?
How does concrete wall change that? And if concrete wall has metal rods in it?
Can EMP be directional?

ps. Any pointers?

 

[russ_watters]

EMP stands for "electromagnetic pulse." "Electromagnetic" is a broad term which covers the entire range of the electromagnetic spectrum. Usually the term is used when referring to the radio and microwave range. That includes radios, cell phones, microwave ovens, and radar. Clearly electromagnetic energy can be directional - all you need is a dish.

As for how much it takes to fry components, it depends entirely on the device you are trying to fry. It could be kilowatts or megawatts. Concrete won't do anything to stop it. Steel will.

 

[LURCH]

I'm not sure how much amplitude it would take, but I do know that the H-bomb test in which we discovered the effects of EMP put out electronics hundreds of miles away, if that gives any sense of proportion.

This site gives some good info, but not any data on the amount of energy required (unless you count the explosive energy needed to power the e-bomb).

 

[wimms]

hmm, for some reason I thought EMP is more like flashlight, single superfast discharge of energy through electric magnet, sort of monopulse. I have understood, that sharp step of magnetic field induces electricity in wires that kills stuff.
As Russ put it, basically even morse code is EMP in action, let alone pulsed laser in CD-writers.
I'm not sure, but isn't there some kind of distinction between EMP of harmonic oscillators and EMP of single half-period pulse, like discharging single highvolt capacitor into electromagnet?

By devices I meant home appliances like radios, TVsets, audio equipment.

H-bombs aren't of interest, because I'm interested about safety limits of playing with EMP at home.

Russ, concrete won't stop it, but concrete walls have metal rods in them, sort of faraday cage or someth. How would you try to account for it?

 

[Chi Meson]

In general a rule of thumb is that for a Faraday cage to work, the "holes" in the conducting material must be smaller than the wavelength of the E-M radiation in order to block them. Radio waves are from meters to kilometers long, so rebar in concrete structures would absorb some of that energy.

More energetic photons, like microwaves, can be shielded only if the holes are on the order of millimeters in size. (This is why you can see into your microwave oven without boiling the blood in your face: those waves are about 1 to 2 cm in wavelength)

But I know that in submarines, electronic instruments are protected from EMPs by a metallic mesh that is finer that a centimeter (that's as specific as my source will get: my Dad still thinks the cold war is on). THis leads me to believe that the E-M waves that fry electronic equipment are of the microwave to infrared variety.

Iron reinforced concrete would be essentially transparent to such E-M waves.

Of course I could have research this before speaking up, but if I'm wrong, I'm sure someone will tell me.

 

[wimms]

ok, how well is protected audio-video equipment that has metal body?
Russ estimated kilowatts to megawatts would damage it.
Also, suppose iron reinforcement in concrete wall is a barrier for EMP. Where does EMP energy go then? Reflect off the walls? But if its generated within the walls, will it bounce back-n-forth until it just dies out?