Abstract: A compacted frequency reconfigurable antenna working at X and Ka bands has been designed by combing RF switches with Koch fractal dipole antenna. The length of the designed antenna is about eighty percent of the common dipole at the same resonant frequency. When working at Ka band, the other parts of antenna disconnected by RF switches affect the radiation performance seriously. So, we tried to improve and optimize the antenna. The final modified antenna is composed of a Koch fractal dipole and common dipole added in front of the Koch fractal dipole. The performance of the modified antenna at X and Ka bands are determined by the Koch fractal dipole and the common dipole respectively. Both simulation and experiment show that, the modified antenna has good performance at X and Ka bands.