Calibration network for a base station antenna why ?

Hello all

I'm planning a base station antenna at 5GHz with 8 culoms ,and i need to have a calibration port to calibrate the antenna.
the calibaration network is made from directional coupler and the isolation level
as to be -30dB.

Does anyone know why do i need to use the calibration network?

thanks in advance

Does anyone has a clue?

Can you add more details? such as application etc.
calibration is usually done to avoid errors.
The base station usually has antenna matching network I suppose.
When you will use directional coupler you will get some signal at coupled port which will tell you whether your signal power level or any signal information is correct.
This is my guess.