The offset between the antenna phase center and the reference point (usually bottom of antenna) for Trimble receivers is stored in a configuration file and used by field apps to more correctly calculate elevations.
For TDC150 with the internal antenna, two APC offset measurement methods are supported. Measure from bottom of antenna, and Measure from Bottom of antenna mount.
When the monopole and antenna mount is used, the antenna height (H) you need to specify in your application is the length of the monopole (read the graduation on the monopole; see below picture). The field application will automatically add to H the distance (d) from the base of the monopole adapter to the correct antenna phase center (APC) offset so that the application uses the real height of the GNSS antenna phase center above the ground.
The Trimble monopole (a.k.a. Half pole) has three adjustable heights. 1.00 m (3.281 ft), 1.10 m (3.609 ft), and 1.20 m (3.937 ft).
This information is documented in the TDC150 User Guide.