There is an interesting question in QST this month from a ham that wants to
put an antenna in the attic and re-use some TV coax already existing. The
answer was that it was probably ok, depending on a few factors like the feed
point impedence of the antenna, the design and band coverage of the antenna,
use of a tuner, and transmitter power (cable TV coax may not be suited to
high power). I'd be more inclined to think that dB loss in coax related to
frequency is more critical than 50 ohm vs. 70 ohm mismatch. That is, I
wouldn't use RG58 for UHF applications, but would use RG 6 for HF. This is a
subject about which I am curious, so I'll be interested in other repsones,
too.

Good DX!