MIMO reference antennas are proposed for Over The Air (OTA) measurement applications. The reference antennas could get rid of feeding cable interference and control Envelope Correlation Coefficients (ECC) continuously by only changing the length of an etched slot on a dual-feed PIFA. If only the ECC is investigated, the MIMO reference antenna is optimized to have a small variation of total efficiency from 70% to 50% when the ECC increases from 0.1 to 0.88. The prototypes are fabricated and measured in a Scattered Field Chamber (SFC). Measurements agree well with the simulations.