Lostgallifreyan wrote:

Conversely, I found some nice coax in a skip once that had two heavy
braids amounting to almost complete coverage around a single fine stranded
core. (Found outside a telephone exchange, but I don't know what frequency
they were intended for, though I used some for an outdoor VHF receiving
quarter wave dipole with good results, and I suspect it will do for a SW
longwire once I get a matching transformer for it).

If it was a UK (BT) telephone exchange, then it probably was "Cable
coaxial 2003".
Used for critical video and general HF use. I don't know what it's
officially spec'd to, but it would work well up to several hundred MHz.
Characteristic impedance of 75ohms, and easily capable of 100W into a
decent match.