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by knorker 1966 days ago
The gain of a Yagi (even these ridiculously long ones) I don't think are anywhere near precise enough. Keep in mind that even a 20dB gain antenna (simplified, since radiation patterns are complicated) only focuses the radiation pattern into 3.6 degrees. That's over 7 times the diameter of the moon in the sky.

I think even at GNSS frequencies you may need Arecibo-sized antennas to get useful directivities. E.g. check this diagram: http://www.coseti.org/9006-013.htm

No, I think a phased array is a better bet, but if it's possible to steer that tightly, you'd need a shitload of antennas. Like, a shitload. E.g. US PAVE PAWS active phased array has 2677 antennas to create a 2.2 degree beam. "Only" ~4.5 moons.

I don't know the maths, but that probably means millions or billions of antennas to beamform this right.

So yeah, I'm staying with "not feasible", probably even for a superpower.
1 comments
londons_explore 1966 days ago
You don't need to make a finely focussed beam... You simply need to be able to put nulls at each of the planes antennas.

If the attacker has two antennas on the ground, say 1km apart, and the plane is 1km up, then no real precision is required - there exists a phase offset between your two antennas where only one of the planes antennas picks up your signal, and the other antenna picks up nothing. If the plane where stationary, this could be found by a simple sweep of possible phase offsets.

If the plane is moving, it becomes harder to find and track the necessary offset, but if the plane is flying half way between the attackers ground based antennas, the offset is ~constant, so a sweep again starts to look doable...

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knorker 1966 days ago
So TX1 sends data headed for RX1, plus the TX2 signal phase-modulated to cancel out TX2 at RX1?

I don't know how much an aircraft shakes, but if I understand what you're saying then this is possibly even harder. You'd need to predict the positions in fractions of a wavelength, don't you? And atmospheric changes could possibly affect it too.

> If the plane is moving

In the air, they tend to. On the ground accurate positioning is less important to protect.

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