Would anybody know materials that have low loss tangent (below 0.01) above high frequency (3MHz) that can survive 300°C? (This exclude all polymers, such as PTFE; Fused quartz is an option, but I am looking for more machinable materials such as ceramics).
Dry air would probably fit those criteria and is easily machined, though getting consistent measurements with a micrometer can be a bit tough, and threaded fasteners tend to pull out rather easily…
Seriously though, it’s the sort of question that benefits from specificity and articulation. To what extent beyond 3 MHz? And what exactly do you mean by “survive”? It’s one thing if a person is asking for something that doesn’t melt at 300°C, quite another if asking that the dielectric properties be well-behaved between that point and -55.
That said, a bit of searching suggests that Corning makes a machinable ceramic they call MACOR that might be worth looking into.