Anti Air Microwave Weapon
Dave replied the topic: Anti Air Microwave Weapon
On frequencies: for most power at the given range, you'll be at the lowest frequency. That's due to atmospheric propagation losses, which increase with frequency, and the difficulty of generating high power as the frequency increases.
There are different levels of vulnerability:
First, your receiver is vulnerable to interference. Even a relatively low signal at close range can introduce errors in a digital receiver. And if you've ever listened to AM radio during a thunderstorm, the shot of static every time lightning strikes is another example of interference. It doesn't damage anything, but information is not getting through.
Moving up a little, higher level signals can overwhelm your receiver input. A good receiver will have some protection against that, so that the outcome is similar to the first scenario - nothing's damaged, but information's not flowing. At a certain level though, the components will be damaged. This won't cause the aircraft to fall out of the sky necessarily, but they might lose a critical capability. When you talk about changing frequencies, this is one area where it makes sense - you might have your antagonists tune their emitters to the right frequencies to black out the protagonist's entire communications system.
The next step up is the big one. At this level, you're not entering the system through the radio front end, but through any and every crack and crevice in the vehicle skin. A couple of loose screws is enough to let these signals through and cause untold damage. Because once they're through the skin, you're talking about voltages that far exceed the breakdown voltage of the components on the board. All of a sudden the board is fried. And because we're no longer limited to entering through an antenna, it could be any board, like an engine management computer or a flight control computer. Now you're talking about the aircraft falling out of the sky.
Of course to do that, you need to hit the aircraft with a whole lot of broadband energy, like the electromagnetic pulse from a high altitude nuclear detonation. I would guess that the Chinese system you referred to is probably doing a smaller-scale version of that. As you've noted, aircraft are designed to withstand that sort of thing in the form of lightning strikes. But this is the region you'd need to be in for your story.
My guess (and it's just a guess) is that to create a plasma channel you'd be up at gigawatt power levels. So if your aliens have mastered lightweight, controlled, compact, fusion reactors, it's plausible.
Happy writing :)