Posted in the wrong board. Could't delete it.
Have a Diamond 2m/70cm dual band vertical antenna to be mounted in the attic. The antenna is 8+ft. in height. I have a very tall attic at the top ridge (10 ft.). There is a preformed rectangular box style heating/cooling duct which has a soft metallic covering running along the attic floor next to where I intend to mount the antenna.
How far above, if any, should I mount the antenna in order to keep the 3 radials above the duct? Or, does it even matter? Is farther the better or does it even matter?