[1] Reference antennas are built with particular care taken to make them simple, robust and repeatable.
These methods are only useful if the materials and dimensions of the antenna can be characterised very well, and the mathematics of the simulation or formulae used is known to be accurate.
In this case the reference antenna is built to have a high degree of repeatability in its radiation pattern and boresight gain.
A common practice is to measure the boresight gain of a reference antenna across its operational frequency band.
The International Telecommunication Union maintains a database of reference antenna radiation patterns.