In one embodiment, a high frequency quasi optical oscillator power source is provided which includes an array of amplifier devices each having an input antenna tuned for a fundamental frequency. A amplifier coupled to the input antenna amplifies fundamental frequency signals. A multiplier coupled to the amplifier provides signals at a multiple of the fundamental frequency and at the fundamental frequency. The output antenna coupled to the multiplier is tuned at the fundamental frequency and at a multiple of the fundamental frequency. A grating lobe-to-main lobe converter located above the array of amplifier devices converts grating lobe power to main lobe power at the multiple of the fundamental frequency. A frequency selective reflector opposes the amplifier array to reflect signals at the fundamental frequency and to pass signals at the multiple of the fundamental frequency.