How to measure ip3 of an active antenna ?

How to measure ip3 of an active antenna (say 20-3000 MHz)?Equipment available:spectrum analyzer, N/work analyzer, Signal Generator,test range, etc.

SA as receiver
Signal Generator combined with N/work Ana (CW) as generators

In my opinion the IP3 measurement don't require high degree of accuracy, so the accuracy of SA may be enough.
IP3 change a little vs frequency, so you don't need many freq. points.

for what purpose do you need to use active antenna's IP3?
usually it's IP3 is very high and doesnt require any measurements