Correct antenna measurement w/R10 lever extension for OPUS
OPUS assumes ARP and makes you manually enter its height above the mark (not slant). There are drawings on the NGS site (when it is available) showing where the ARP is for most models. It is usually the bottom where an antenna screws onto a pole, but I am not familiar with that model.
The phase center is a point in an electric field. So that's not something you can measure. Not with a tape measure, anyway. You want the ARP, which is always the mounting point.
"Convention is like the shell to the chick, a protection till he is strong enough to break it through." Learned Hand
Travis Nelson, CST