A well-written article discussing the potential threat of wideband noise sources to Wi-Fi networks concludes that it is not as easy as some people assume to jam Wi-Fi. The use of frequency-hopping and spread-spectrum techniques (which are different, by the way) in the microwave bands makes Wi-Fi substantially less vulnerable (though admittedly not totally immune) to interference than it might appear.
The article systematically dismantles naive claims that a "simple 100W broadband noise generator" would knock out Wi-Fi networks within a couple of miles. The main argument is that the 100W of energy would be spread across 0-2.4GHz if the noise generator were truly simple (i.e. presumably untuned), resulting in a low energy density in the Wi-Fi band/s. In practice, I suspect a jammer would probably design his system to produce most of the 100W in the specific microwave frequencies used by Wi-Fi.
A 'proof of concept' noise generator should not be too difficult to construct although getting 100W at microwave frequencies is a technical challenge unless you have the $$$ to buy commercial microwave amplifiers ... or the technical nouse perhaps to adapt a Klystron from, say, a microwave oven.
Don't try this at home folks. High power microwaves are used in ovens because they cook things - your cornea, retina and brain, for example.