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I'd also like to know what you'd use for waveguide at those power
levels. It's hard enough to keep moderate megawatts contained and
the waveguide in one piece.

I've definately seen waveguide with holes burned in it by RF.

Waveguide size is set by the operating frequency. For X-band, the smallest
dimension is less than a half of an inch.

If you put the right stuff in the waveguide, it still passes a signal well
and you raise the arc-over point dramatically.

Phased-array radars have very many small antennas, transmitters and
receivers. Each one handles only modest amounts of power.