There are vendors on ebay that will sell you an Atari - Composite Video or Atari - S-Video cable for not very much money - I bought one for my 800XL to use with a Phillips CM8833 monitor.
I haven't tried it with a more modern TV but I expect it would work perfectly. You really need to eliminate RF altogether if you can.
Mine came from the UK (to the UK) though so check postage / customs.