This paper aims to design a 2x2 antenna array testbed to serve as a complete tower-top calibration system that is able to eliminate the effect of transmitter differences of an array elements. The testbed architecture is based on the advanced tower-top interlinear reference calibration scheme where a non-radiative reference sensor is located in the centre of the array and four transmitter elements are coupled to its. The prototype system was built using commercial off the shelf components and it operates in the 2.4 GHz ISM band. Software control capability supported with PC parallel port interface makes it flexible for various calibration algorithm implementation. A continuously updated (real time) calibration is also possible and easy to apply. We presents an experimental results of tower-top calibration accuracy in transmit mode operation. The measured performance of prototype system calibration shows amplitude and phase imbalance better than 0.15 dB and 1.1º. Keywords: array calibration, antenna array, base-station transmitter architecture