antenna factor (for different distances)

Hello....
We ve got a biconic-log periodic antenna...for emc measurements...i need to know what is the right antenna factor to take in considerationn..because in the calibration document we have different antenna factors for different distances..for exemple ..there are three differents cases: free space data, short measuring distance tip and short measuring distance marking ...
What are these? ..thnx

Presume you know why near field measurements needs a correction factor?

The distance between antenna and test object can either refer to the antenna tip or the antenna center (respectively a specific reference marking). Gives different effective distance and thus different correction factors.