Abstract: A compact ultra wide spectrum high power microwave radiation system made up of 100 kV nanosecond pulse generator, subnanosecond gas switch and TEM horn antenna is designed. The output voltage waveform of the pulse generator is a pulse with a rising time of 280 ps, a decaying time of 180 ps and a peak amplitude of 100 kV. The system can work at 100 Hz repetition rate and the peak radiating field on boresight is about 10 kV/m at the distance of 7.5 m. The -3 dB bandwidth of the radiated field is from 230 MHz to 810 MHz and -20 dB bandwidth is from 100 MHz to 1.3 GHz. The system with small volume of 80 cm50 cm26 cm and the weight of 45 kg is expected to find many applications in areas of high power microwave.