Standard gain horn antenna for antenna pattern measurement

I am thinking about equipment required for microwave sensor radiation pattern measurements. Setup is: (1)room with microwave absorber, (2)microwave sensor is placed on a (3)rotating platform. (4)Receiving antenna connected to (5)spectrum analyzer. Rotating platform and spectrum analyzer are connected to (6)personal computer, controlled using (7)software. My main concern is receiving passive antenna. Software just reports angle of rotating platform and peak level from spectrum analyzer.

Which kind of antennas are suitable for antenna pattern measurement? Could we use patch antenna instead of horn? If it is known that frequency is within the range.
Why there are standard gain horn antennas which are linearly polarized? Should not it be circular polarized horn?

Main questions:
1) what are requirements for radiation pattern of "standard horn". Which antenna is best? For example -3dB beam width of 30 degree, or wider/narrower?
How radiation pattern of passive antenna will affect measures pattern?

2) which polarization must be used for measurements? I guess linear is OK if measured device have linear polarized antenna too. What if measured device contains circular polarized patches array?

3) should I do some calibration? I need only radiation patterns, not real power levels. So I guess when all data converted to dB, then results will be the same regardless of passive antennas gain? So no calibration is OK?

its nice to have TWO calibrated gain horns. That way you can substitute the DUT with a know gain horn antenna to calibrate the entire system as a given distance, and then vary the azimuth to get a pattern with a DUT

Ideally for best antenna pattern measurements is to use the narrowest beamwidth antenna as possible.
The most used antenna for pattern measurements is horn antenna with vertical polarization and a beamwidth less than 20 degrees.

If use a very low reflections anechoic chamber, you can use any other antenna type for pattern measurements, even a vertical dipole will do the job.