Gone in a Picosecond

T he technology for generating and detecting electromagnetic waves has evolved significantly over the last 120 years. In the early 1890s, Guglielmo Marconi used a spark-gap transmitter to build a wireless telegraphy system. In his design, he charged a capacitor to a high dc voltage and connected it to a parallel combination of an inductor, a second capacitor, and an antenna through an air gap. In this configuration, when the dc voltage of the first capacitor reaches the breakdown voltage of the gap, the air in the gap ionizes and reduces the resistance across the gap. This results in a large step voltage applied to the parallel combination of the inductor, second capacitor, and the antenna and converts the dc energy stored in the first capacitor to a damped oscillation at a low megahertz range that is radiated from the antenna. Marconi’s design used the spark gap as a fast high-voltage switch. The technology for generating electromagnetic waves then evolved further with the invention of vacuum tubes in the mid1920s. Vacuum tubes enabled oscillatory signals to be amplified in the megahertz range and provided enough bandwidth for transferring audio signals. At the start of World War II, the radar application of electromagnetic waves received significant attention. Major resources were allocated for developing pulsed radars in the microwave range to locate war planes by measuring the round-trip travel time of the electromagnetic pulses. In addition, the time-domain signature of the reflected pulses was analyzed to identify different types of aircraft. In this application, shorter pulses are preferred because the reflection of a short pulse from different parts of an aircraft arrives in different times, making it easier to identify the object.