OOps missed the part about need to be planar. Is it 300W cw or pulsed? Makes a diiferece. There are materials (ie ceramically loaded teflon from Rogers Corp and others). Teflon can handle the temperatures from the power but the copper trace may not. Ask Rogers Corp about thicker copper clad (i.e. typical 1oz copper, try 2 to 5 oz copper). The problems with frequency indendant antennas can be the matching balun (except for some patches, then it's the feed point). The trick is to get the heat out. If you have the deepth you can try loaded cross dipoles in a cavity. The dipoles can be designed to handle high power levels.