A monolithically integrated Voltage Controlled Oscillator (VCO) transmitter (TX) for radiation sensor wireless readout is presented. The design is very well suited to wireless transmission of radiation sensor data in radiotherapy and to Extra Vehicular Activity Radiation Monitoring (EVARM) in space. The TX includes an on-chip loop antenna which also acts as the inductor for the VCO resonant tank circuit. The antenna dimensions of 0.5 mm2 are the smallest reported for an active antenna in the 5 GHz band. The VCO circuit is placed inside the loop antenna to optimize chip space. The complete TX chip size is only 1.5 mm2. The TX device consumes 3.3 mW of power and delivers -0.9 dBm of radiated power, sufficient to communicate with a low noise receiver connected to an off-chip patch antenna placed 1.38 m away.