It'l werk fine Rich, don't even have to bother to place it in attic.
Most communications in 800mHz(VHF-UHF) range and up are through very
elevated antennas and are actually "looking down" at your little dinky
'tenna. One of mine is a mag mount on top of one of my hf rigs.

73, Butch Magee KF5DE

REL wrote:
Who can fill me in on ground plane or effective ground plane.

A steel sign plank 18" X 12 " with a 800 Mhz Mobil in the middle is that
suitable for a hone antenna? In a roof joist.

Rich