Hello,
You can measure the correlation between two antennas in two ways:
1) By means of radiation patterns. This solutions is costly, because you have to measure full radiation patterns of the antenna system in the anechoic chamber for every frequency in the frequency range you want to have the correlation results.
2) By means of S-parameters. In some case, you can apply an approximation , just by going from S-parameters to Correlation or C-parameters. This solutions is very easy, and you only need a VNA. From the measured S-parameters (for any frequency range you can measure with the VNA) you can obtain the C-parameters and the correlation envelope (after a simple computation). You can find some examples in the attached files.
I hope this post will help you. Bye,
Pere