# Selective EMP weapon

According to wikipedia, EMPs have a number of different waveforms. Radio transmitters are carefully designed with a near-perfect sine waveform so that their signals can later be filtered out by the receiver. A receiver's antenna is carefully designed with a specific length to receive signals within a particular frequency band. Standard AM/FM Radio frequencies are too close together to need to adjust the length of the antenna per station but in concept, only antennas of a certain length will pick up a radio signal of a particular frequency.

So then, could this idea work in reverse for a selective EMP weapon? Rather than designing an antenna to receive signals of a particular frequency, could an EMP be designed to only be received by devices containing circuits with a particular length? Or might there be some other method for designing an EMP's particular signal to affect only certain devices?

Hard science preferred but others are welcome in case there is no hard solution. Also open to variations on the EMP concept, like maybe this can be done with a continuous wave or etc.

• When you say particular length, do you mean that length and that length alone or do you mean a range? Commented Apr 10, 2017 at 19:26
• @JoeKissling I suppose question is just a matter of how small the range is, so either will work. Commented Apr 10, 2017 at 19:45
• Generally speaking, An EMP will kill anything with conductors in it that are larger than the wavelength(s) in it. Commented Apr 10, 2017 at 19:50
• @JoeKissling I think this would make a good answer, with some more detail and sources. Commented Apr 13, 2017 at 15:09
• I'll see about getting one together then Commented Apr 13, 2017 at 15:13

This, depending on your situation, could be much easier than specially crafting an EMP for your enemy. At the time of detonation, put your things in a Faraday Cage.

Another possible work around would be to keep your electronics our of the blast range. I'm spitballing here, but if you had numerous small EMP's you could effectively wipe out enemy tech without hitting yourself. This is made slightly less feasible by our relative inability to create small EMP's.

• Unless the enemy read your answer and protected their electronics - Just sayin. Commented Apr 11, 2017 at 19:15
• No need to say it, @Enigma. Everybody tries to do exactly that. Every military since we got to know that nuclear blast causes emp. Commented Apr 12, 2017 at 16:13

Each signal has frequency band like this. Target equipment has work frequency f which correcponds to first harmonic (high peak in the middle). It also receives subharmonics (minor peaks) because the world is not ideal. Subharmonics in fact are overtones of f (2*f, 3*f etc) and are unwanted. Q factor (wikipedia.org/wiki/Q_factor) describes how good any circuit. With high Q factor spectre has higher middle peak and less and lower minor peaks but you can't escape subharmonics in equipment at all. Any circuit will receive signals with wavelength less than its dimension. Well designed antenna could supress them but some high subharmonics (f.e. 27*f) will stay.

On the other hand radars etc has special counter-measures such as wikipedia.org/wiki/Frequency-hopping_spread_spectrum. In fact FHSS has been used almost century and you could find it even in simple walkie-talkie. Noise suppression is so good now that someone could filter useful signal 10^6 times less than noise.

For signals, spread is wider when impulse is shorter and EMI has a lot of energy to make sure target equipment (maybe with some defence) will be damaged. So single EMI has wide sideband and energy of its subharmonic could be powerful enough to damage common staff (f.e. computer or cellphone) near taget equipment.

Modern EMI producers are not only en.wikipedia.org/wiki/Nuclear_pumped_laser but also en.wikipedia.org/wiki/Gas_dynamic_laser and en.wikipedia.org/wiki/Explosively_pumped_flux_compression_generator. Note they all able to produce EMI with mega-watts/ampers/joules in contrast with kilo-watts for typical radar, watts for computers and milli-watts/ampers for gadgets/phones.

So if you want to damage equipment then use short-term super powerful EMI (given from denotation) and damage anything in specific area. If you want to suppress some translation then try to supress everything in specific direction but most likely you will fail.

You see the point? Enemy's equipment could easily survive and you could only try to damage all from the area.

• False. Nuclear EMP is a narrow peak and its spectrum is close to uniform. Put a peak into a FFT yourself if you don't believe it. Discharge EMP is really similar, as seen on the same image. Current emp technology is closer to nuclear than to radar. Commented Apr 12, 2017 at 16:18
• @Molot It's true that EMP is a narrow peak and its spectrum is uniform with sideband. According theory with more time-limited signal you get more unlimited band. In fact it will increase number of sidebands you could observe. In the limit case, Dirac function has endless spectrum