How do I measure the gain of CP antenna inside an anachoic chamber? What should be the reference antenna, I mean it should be LP or CP? What is the procedure of measurement?
You can measure the gain of a CP antenna with a CP antenna of the same hand, or using any two orthoganally polarised antennas, including measurement of the phase between them, and then reconstructing circular polarizatiion receive (or any polarisation you want) mathematically.
In case you want to use LP antenna, measure axis ratio. Then measure gain as mentioned above with axis ratio dependant correction factor. Example, if axis ratio is1:1, correction factor is 3 db
Part 5 of the third reference describes how to do the measurement. Use a circularly polarized antenna as the source and a circular (best choice) or linear antenna as the reference gain antenna.