 A member of the Signal Corps, Armstrong was building radio direction finding equipment to track German military signals at the then-very high frequencies of 500 to 3500 kHz.The triode vacuum tube amplifiers of the day would not amplify stably above 500 kHz, however, it was easy to get them to oscillate above that frequency.Armstrong's solution was to set up an oscillator tube that would create a frequency near the incoming signal, and mix it with the incoming signal in a 'mixer' tube, creating a 'heterodyne' or signal at the lower difference frequency, where it could be amplified easily.