In this contribution, we derive expressions for carrier concentration and conductance in a Terahertz Photomixing Antenna (PMA) using a basic 2-ray model. We introduce a modulating signal into the analysis and assuming a basic ON/OFF keyed scheme for the modulating signal, we derive expressions for the carrier concentration and conductance. We investigate the effects of input power, angle of incidences of the two laser beams and the beat frequency on the carrier concentration in the PMA. © 2017 IEEE.