Something I was curious about, but not sure where to start - suppose you wanted to make something that at 400-500K, would emit radio waves from which the temper

Something I was curious about, but not sure where to start - suppose you wanted to make something that at 400-500K, would emit radio waves from which the temperature could be derived. And as small, simple and durable as possible, so it didn't break down. I mean, there's going to inherently be infrared, so how can you convert that to radio of roughly a desired frequency range without complex machinery?