An antenna's signal path contains 3 main fields. The boundaries that separate all 3 are described below, however for our purposes it is really only important to understand that the far field is the boundary beyond which radiative rather than "reactive" forces dominate, and the waves behave like "normal radiation".
If we consider the diagram below, where R is the distance from the antenna to the point in its signal path we're trying to consider, and D is the perpendicular size of the antenna, the near field can be described as the distance for which $R<0.62\sqrt{\frac{D^3}{\lambda}}$, where lambda is the wavelength of the transmitted wave.
Region for which $0.62\sqrt{\frac{D^3}{\lambda}}<R<\frac{2D^2}{\lambda}$.
The far field of an antenna is described by the region that satisfies the following 3 properties:
$$ R<\frac{2D^2}{\lambda} $$
$$ R>>D $$
$$ R>>\lambda $$
For the purposes of a cubesat, we will generally always be operating within this range, and thus all equations relevant to antenna far fields will apply.