Westflex W-103 low loss airspaced 50 ohm coax.
Popular with some amateurs because of it's low loss, the only disadvantage is that the coax is very stiff and tends to remain in whatever shape it is bent.
Has a single solid copper centre rather than the usual multi-stranded.
Slightly thicker than RG213, and ideally you need to use special 10mm plugs (which are about £6 each!).