An antenna interacts electromagnetically with its surroundings. Ground-plane dimensions, mast or vehicle structure, enclosure, fasteners, cable route and nearby conductors can change current distribution and scattering. The result can be different return loss, efficiency, beam pointing, sidelobes, nulls, cross-polarization and phase center from a free-space or reference-ground-plane measurement.
A radome can add insertion loss, reflection, beam shift and polarization error. Those effects depend on material, thickness, seams, spacing, incidence angle, moisture, ice, temperature and frequency. If the product is used with a radome, test or correlate the declared radome configuration rather than treating it as cosmetic hardware.
Freeze the installed assembly by drawing and revision. Use a representative platform or a documented correlation method, then retain both baseline range data and installed check data so future mounting or material changes can be assessed.