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As I understand it, the high price of helium is one of the major issues with airships. Could you use (yes, flammable) hydrogen instead, especially for unmanned floating-installation type applications?

If you want to bounce radio signals off something a mile up in the sky, would a hydrogen-filled balloon do the job? The safety issue is surely not a big one if it's not designed to regularly land: a Hindenburg situation seems unlikely if it's neither carrying people nor intended to land.