A History of the VHF Broadband Digital Interferometer of Lightning Research Group Osaka University

The development of the Broadband VHF Digital Interferometer (Broadband VHF DITF) has been an ongoing endeavor of the Lightning Research Group of Osaka University (LRGOU) for approximately these 30 years. Over that time, the system has reached what members considered a “completed form” multiple times, only for further improvements to be identified, prompting continued refinements. The first such milestone occurred shortly after the turn of the 21st century in Darwin, Australia, which was LRGOU's primary research site at the time. Using the first-generation analog to digital converter (ADC)-then referred to as an event-triggered system-the group was able to display two-dimensional images of lightning discharge paths on a computer screen even before the thunder was heard. The next major breakthrough came around 2010, during a collaborative research project with the New Mexico Institute of Mining and Technology. By continuously recording digital data, the team was able to visualize positive breakdowns, which had been nearly impossible to capture with the eventtriggered system. However, one limitation remained: due to the masking effect caused by intensity differences between positive and negative VHF pulses, it was still impossible to fully visualize positive breakdowns in bidirectional leaders at the onset of discharges. To overcome this issue, recent efforts have incorporated continuous digital data recording while eliminating the use of any attenuators. As a result, it is now possible to visualize both positive and negative breakdowns propagating simultaneously in opposite directions.