It sounds as if you are considering 'coax with messenger' which provides both support and grounding functions. The common size of the messenger is 17 AWG. It's usually cooper clad steel which is specifically listed in the NEC
Quote:
810.21
...
(H) Size. The grounding conductor shall not be smaller
than 10 AWG copper, 8 AWG aluminum, or 17 AWG
copper-clad steel or bronze.
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My personal preference is to run #10 AWG copper. I find it's easier to work with and find clamps and connectors that last when exposed to the elements.
An exception is when I run an overhead span between buildings or up into a tree. In either case, I need the support function of the messenger wire so why not use the grounding capability as well.