A Multi-Cycle Digital High Power Microwave (MCD-HPM) source includes a microwave transmission line (MTL) to which a plurality electrically charged thin film transmission lines (TFTL's) are connected by switches. The switches are activated in sequence to generate a square wave at a microwave output frequency. The activation signal is controlled by a free space time delay, which can vary the timing and/or routing of the activation signal by modifying at least one free space element, thereby adjusting the switch activation timing and varying the output frequency. In embodiments, the switches are photo-conducting switches, the activation signal is a laser beam, and the switch timing is varied by reorienting and/or repositioning mirrors and/or other elements in the free space time delay. The elements can be manually adjusted, or mounted on motorized stages and automatically controlled. Optical amplifiers can be included to compensate for losses in the time delay elements.