Antenna radiation pattern

I am curious about how the antenna radiation pattern has been measured. Please explain the setup used. To my knowledge common mode current is an issue from a measurement point of view and common mode chokes does not work properly at +1GHz.

I’d imagine it’s the same way as they are always done.

The antenna under test is placed on a non metallic turntable (ideally a 2 axis one but that’s not required) and connected to a signal source (normally a pure carrier sine wave at the frequency to be measured).
A linearly polarized known antenna is placed in a fixed location a short distance away (normally a few m but as long as it’s far enough to avoid near field effects exact distance isn’t critical).
The signal level at the known antenna is measured as the antenna under test is rotated to different orientations.
The known antenna is then rotated 90 degrees to allow the other polarization to be measured then the process is repeated.

The results are plotted to give the antenna pattern.

That will give the relative gain levels, if absolute gain levels are needed then the antenna under test can be replaced with a known reference antenna and the signal level measured in order to measure the total gain of the test setup.