Far field calculation

Hi
Far field for linear antennas can be calculated using formula:
R>2*D^2/lambda
but for a broadband reflector antenna that radiates at different frequencies and not have a linear dimension such as a linear antenna what should we do?

normally this is thumb rule but for big antennas u can use compact ranges or near field ranges where u measure near field data and transform it to far field data

I remember, for reflector antenna, the largest dimension can be taken as its diameter. You may need to check this in some book or document.

https://www.edaboard.com/viewtopic.php?t=290627