To the accuracy needed when planning observations, the primary beam of the VLA antennas can be taken to be a Gaussian with FWHM equal to arcsec or arcmin. If you must image a source, or a field of view, that is significantly larger than the FWHM of the VLA primary beam, then you should plan to make a mosaic from a number of pointings (see Lecture 14).
In standard VLA software, the primary beam correction ( , Lecture 2) is modeled by a power law expansion of the form where and the coefficients are , , , , and (Napier and Rots 1982). This correction is usually not applied at small values of X for which the Napier-Rots expansion gives 1. It is not well determined at large values of X for which 0.15.