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Help with Weaponized Microwaves

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6 months 5 days ago #639 by Mason
Hello, new to the forum. I am writing a science fiction story featuring microwave weapons, and wondered if someone might speak with me on the subject.

There are two primary functions for the microwave weapon. One is destroying electronics, possibly from orbit. The other is creating a radiation field that works together with a spark coil. Incoming mortars, missiles or the like are to be ionized by the microwave field, to effectively build up enough charge to arc an electrical current to them and destroy them. This would be to protect a spaceship, operating in Earth's atmosphere.

With help from a friend, I was estimating a few 10kw microwave emitters might be enough (more likely eight 5kw emitters). And that one second of exposure under a 10kw microwave laser would be enough to effectively build up enough charge to arc an electrical current to it.


The main question we have is what sort of effective range we'd be looking at. And whether 10kw is a good number for these.

Thank you for any advice you can give. I apologize if this question isn't appropriate to the board.

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6 months 4 days ago #643 by Dave
Hi Mason,

A couple of useful pages to get you started:
www.microwaves101.com/encyclopedias/power-handling
www.microwaves101.com/encyclopedias/atmospheric-breakdown

10 kW will cook some electronics, but for plausible arcing (e.g. the field is so strong that incoming weapons self-destruct as soon as they get close) consider bumping up your sources to MW levels. By way of comparison, using some example figures from the textbook Microwave Engineering, by D Pozar, coaxial cable breaks down at 520 kW and waveguide breaks down at around 2.3 MW.

From the charts on the power handling page linked above, it looks like you need a field strength of about 3 MV/m to cover a 100 mm gap at sea level (1 atmosphere). This drops with altitude, if the pressure was only 0.01 atmospheres then 30 kV/m would probably do it.

In terms of range, lets assume your shield is spherical around the spaceship. The relationship between field strength, power, and distance is:

E = sqrt(30*P)/r

Rearrange to solve for distance

r = sqrt(30*P)/E

So, to get 30 kV/m from a 10 kW source, you've got an effective range of 18 mm. Hmm, not much.

Bump up the source to 10 MW, the effective range is 577 mm. Still not great.

At 1 GW, the effective range is still only 5.77 m.

If it fits your story arc, it might be more plausible to suggest a smart system (more like your microwave laser) that sends a focused beam to each target. Give it a 50 dB gain antenna, which has around a degree beamwidth, and you can get that same 5.77 m range from a 10 kW source. A still plausible 10 MW source now gets you 180 m of range. 100 MW pushes that out to 577 m, and a 1 GW source with a 50 dB antenna will happily fry things at 1.8 km range.

Hope this is helpful, good luck with the story!

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6 months 4 days ago #646 by Mason
Thank you Dave! That is perfect! I see that the microwave radiation shield doesn't work out quite as hoped. You'd have to use something like 100GW to get any effective range. Even if you only covered half the area, things don't improve by much.

My friend and I came to the same conclusion as yourself. Better to go with more of a maser. It could adjust its power and spot size for a few purposes. Radar, jamming, maser against flesh and electronics, and an ionizing maser. The latter not being intended to destroy the target, but to make it vulnerable to an electro-laser or spark-coil.


If I may, I'd like to ask you about one other thing. The best frequencies for such a tool. And whether it is feasible to have one microwave emitter capable of producing all of them.

Thank you again, Dave. This has really helped with the story. Microwaves are quite interesting, so I hope to get them accurately.

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6 months 3 days ago #647 by Dave
Hi Mason,

One point to note: the field strength I picked is roughly enough that it might cause arcing. Most microwave components are far more sensitive than that, and will be blinded or destroyed at much lower power levels. So if your antagonists are only using "smart" weapons with guidance packages, that's a plausible solution.

Best frequency is a great question. Low frequency propagates better, and is easier to generate the power, but high frequency is better for penetrating the inevitable gaps in the structure.

You could, of course, go for some pseudo-scientific mumbo jumbo like this:
The pi-ratio was first discovered by Australian stealth researchers in the late 20th century. When the target is coherently illuminated with pure sinewaves at the two pi-frequencies (3.1415926 GHz and 31.4159265 GHz), it induces a high-Q resonant mode in any structure with radiused components. The result is to increase the effective power by a factor of 1000, instantly destroying any nearby electronics and causing the structure itself to disintegrate.

That's all pidooma, but might be plausible to some... that said, I'll stick with engineering and let the authors do the writing :)

Is it feasible to generate that much power at those frequencies in a single emitter? Honestly, probably not. Not with today's technology anyway. If it was, Boeing, Lockheed Martin, Raytheon etc would love to know about it.

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6 months 3 days ago - 6 months 3 days ago #648 by Mason
That is a good point on the arcing. I think you'd make a pretty good scifi writer.

I would like to ask, as unfortunately I couldn't find the information. Do microwaves build up charge in metal, outside of a microwave oven? More specifically, I was hoping to find a formula to calculate the buildup of electric charge, so I can calculate how what sort of a tool, power and timeframe would be necessary to build up the charge.

Sorry, these details will no doubt be on the site. If the engineering challenges are steep or impossible today, it should be all right. The characters using these weapons are from an advanced alien civilization.

Thank you for your great help, Dave.
Last Edit: 6 months 3 days ago by Mason.

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6 months 2 days ago #651 by Dave
Hi Mason,

Short version: yes.

Long version: it's strongly dependent on parameters like the target's shape, range, electrical size, and you may need Maxwell's equations to work it out. At this point, I bow to a number of people on this site whose knowledge on this topic far outstrips my own.

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