About Balun filter

I measued air-substrate PIFA antenna for 2.4 GHz band in anechoic chamber before, but the antenna peak gain is higher than 5 dBi. Someone tell me, I shall use Balun filter to reduce the surface current on the coaxial cable. My question is, whare can find the informations and functions of Balun filter, or how to meause antenna pattern in accuracy?

If the return loss of your PIFA at antenna connector is ok, and if you are using a good shielded coax cable, your pattern measurement should not be affected.