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My personal experience with the ATU finds that elevation does improve coverage. Some will argue that a counterpoise is needed and when ground mounted will offset the difference between elevated and ground mounting.
I tried the ATU at ground level with 12-20 foot radials. I also tried it mounted about 5 feet above the roof of a wood frame 1.5 story house. The roof top installation far out performed the ground mount with radials.
The roof top ATU install was not grounded whereas the ground mount ATU was grounded to the radial system.
Common theory would dictate the results should be reversed but its hard to argue with real world results.