vhf airband antenna gain

Hi, I have two antennae. One is a reference, and the other is DUT. I have to use Gain Transfer Method to find AUT gain. For this, i have used a reference horn antenna in the chamber and measured s21. then I put DUT on and measured S21. Now my question is, "How can I convert this S21 into...

I contacted Uniden a few days ago and got a response yesterday, the only problem is that the antenna will apparently give you "around 3db" of gain. The question is what that is measured in, and how much better a 1/2λ dipole would be. Full question & response quoted below.