Make sure that the hdmi leads are not in the same cable run as the aerial RF lead . Keep the 2 at least 9 " apart .
Makes no difference- there aren't any concerns with running HDMI and aerial wires together - unless of course you're using crappy aerial wires and 'HDMI' cables that don't meet the HDMI specifications.
I've seen hundreds working perfectly OK, and fitted loads as well.
I suspect it's yet another of these 'urban myths' which seen to appear from nowhere?.