Would a giant Magnetron that is being focused, and attached to a nuclear reactor be a good weapon?
If you were to have a really big one in orbit would it be able to blow up ICBM’s being launched on Earth (from say China)?
Or if highly focused melt tanks on the the battleground? Or because Microwaves heat water… increase the temperature of Oceans causing severe storms?
PerfectDark
As with so many energy weapons the earth’s atmosphere attenuates the strength of the ‘beam’ fairly quickly. As a result, even with a huge laser (or magnetron or whatever) and a nuclear power plant to back it up you’d be lucky to get an effective range of a few miles. Add in the problems of aiming not to mention the hideous cost of dedicating a power plant to each weapon’s operation (and the cost of the weapon itself) and such a thing quickly becomes a bad idea.
Also, and I don’t know for sure, but I think the metal on a tank would effectively shield it from microwave energy.
As an aside there was an interesting take on getting a laser to shoot through the atmosphere. I am pulling this from a dim memory and don’t have a cite so take this with a large grain of salt.
Basically, as a laser shoots through the atmosphere the beam runs into jillions of atoms (and dust and smog and what not) along the way. Each hit deflects a tiny portion of the beam. Eventually enough of the original beam gets stripped away that you are left with nothing to do the work at the target.
Using an adapative optics mirror and something called atmospheric shift theory a laser would shine on a mirror composed of (say) 500 little mirrors that could all be oriented individually. The laser would break into 500 laser beams so small that when they hit an atom (or whatever) they simply bounce off instead of getting attenuated. At the top of the atmosphere, on your incoming missile, all 500 beams would ‘happen’ to recombine at the same point at the same time thuse recreating a coherent beam that still has its punch.
Obviously no one can ever know the position of every atom between ‘here’ and ‘there’ (Heisenberg I believe had something to say about that) but presumably the Atmospheric Shift Theory had a workaround for this so you didn’t have to know anything about what’s between you and the target. It just worked somehow. As I recall the downfall to this whole thing was the theory stipulated a ‘shift’ once every nanosecond or so. Coming up with a computer that could calculate 125 million (500[sup]3[/sup] for orienting 500 mirrors in the X, Y and Z axis) in less than a millionth of a second was the big stumbling block (at least it was in the 80’s or so when this stuff was supposedly being worked on).
I have no idea of the fate of this project if it did indeed ever exist in the first place. I’ll admit it sounds too crazy even for sci-fi but who knows? Some aspects of quantum mechanics are pretty crazy and seem to work well enough…maybe this could have as well.
Without some kind of waveguide, the output of a magnetron dissipates with the inverse-square of the distance. No matter how powerful the energy source (and what’s with all the reactor-powered this-and-that lately?), at any real range it would be nothing more than an annoying microwave noisemaker. If you’re looking for real power in a RF weapon, why not go with a bomb-pumped Maser? or better yet, skip right through the usual freqs, and go fo a bomb-pumped X-Ray Laser? Of course, those would be one-shot weapons, but at least you’d have enough power to make an impression.
The Pyramid Of Darkness.
The Weather Dominator.
The M.A.S.S. Device.
Giant Atomic Magentrons.
You used to be in charge of Cobra’s weapons developement program, didn’t ya, PerfectDark??? 
YO JOE!!! 
The microwaves wouldn’t be able to deliver any more heat to the ocean than the reactor was generating, and a nuclear reactor doesn’t generate enough heat to cause much of a storm. If it did, our nuclear submarines would have hurricanes following them everywhere.
The reactors in subs aren’t the big… I mean HUGE reactors for this thing. Anyway I will have to concede that the microwaves will be severely weakened by the atmosphere anyway. DARN. Well I’ll have to go with my huge Concave mirror in space that focuses the light to a really small point on Earth and you get your frying ants scenario.
PerfectDark
78 million watts, thermal. Not big enough? That’s a design from the sixties, the newer designs can go well above that. Aircraft carrier’s reactors can go over 500MW, and they weigh hundreds of tons, and occupy thousands of cubic feet. True, that’s not all that big by comparison to a civil plant, but big enough.
If you want God’s own power source, point your monstro-mirror at a heat exchanger. You’ll still have to figure out how to get all those joules to the business end of whatever mega-weapon you’re planning, but now you’ve a semi-practical power plant.
IIRC microwaves are used as anti satalite weapons. Think of what dammage stray electrical currents can cause to sensitive electronics.
Also on space.com they talked about satalites using plasma ‘sheilds’ to stop microwave attacks (but lasers are uneffected it seems.)
Two things:
The Army, IIRC, has developed a new riot-control device that uses something like a microwave gun. When aimed at an unruly crowd, it hits them with microwaves, and they get uncomfortably hot really quickly. This is via the Air Force Times, not the National Enquirer.
If you were able to get your own microwave gun, and somehow set the frequency at the same freqency of the molecular bonds in the metal of a tank, wouldn’t the bonds break apart, and the material melt? Maybe I’m a retard…but that seams feasable.
If you set it to those frequencies, it wouldn’t be a microwave weapon anymore. We’re talking mondo energies, here.
But still the might EMP is the best anti-electronics weapon availble.
Ahhh, right, right. Ok, then what am I talking about? I remember talking in a science class once about making a weapon where you tune some sort of EM wave to the same frequency of molecular bonds, and it would break apart all the molecules in the materiel. Seemed like a good idea.
Only on short distances, though. EMP strikes whole continents! 
Well yeah, but if you were holding a forward position just outsaid of said country, your troops and equipment would also get blasted by the EMP. The weapon I’m talking about is something mobile. It could be mounted on, say, a jeep, and fired at single targets.
Going with your idea, though…what’s a viable way to produce a huge EMP without detonating a nuclear weapon? I mean, I assume you’re just trying to knock out all the electrical equipment, and not kill all the military AND civilian population.
Simple: There is none. Gotta use a nuke.
Every now and again, some huckster sells a reporter on an EMP terror weapon, but it’s a lot of hot air.
As for microwaves and breaking molecular bnds, ya’ll still fergettin’ about that pesky inverse-square problem… I don’t care what beast you’re building, it’ll still have no range.