As I mentioned in my previous update, my Vizio TV does not like the UAV's composite output: the 5200's output is very jumpy. The 2600's output is not solid either, but it acts differently from the 5200.
Somewhere in the last month, I learned about RetroTINK products, which are designed to solve just this problem: they correctly interpret the analog output from the game system, and convert it to HDMI output that the TV then correctly upscales to full size.
I purchased a RetroTINK 2X-MINI, which converts composite and S-video to 480p, and wowee! The image is solid and beautiful, with crisper lines and brighter, more vibrant colors, probably just about the best you can get from an analog Atari output.
I am using composite today, but I have decided that I will modify my systems for S-video, since the 2X-MINI supports S-video input as well as composite. The combination of UAV+RetroTINK is probably the best it can get from an Atari with analog outputs. Perhaps the only better image can be had via HDMI, such as with a RetroN 77 or Sophia chip for the 5200.
Pictures show this better than words can describe it, so...
Here's my 5200 over RF:
Here's the same 5200 using my TV's composite input:
Here's the same 5200 using composite to the RetroTINK 2X-Mini: