[Antennas] Full legal Limit cable

[Jim Isbell]

What coaxial cables are capable of feeding my antenna at the full legal 
(Ham) limit?  Can RG58 handle the power at SWR under 2:1?  Or do I need 
to go to RG8 or RG11?

I am constructing an inverted "L", end fed at about 50 to 75 ohms.  I 
want to make sure it can handle the output of my 4-1000A linear at the 
full legal limit on SSB.

I have a large coil of solid outer conductor (corrugated aluminum, I 
think) coax that is just slightly larger in OD than RG58 but it has no 
markings on it.  Is there an easy way to measure the impedance?