Initial Results from a 2nd-generation Broadband VHF Interferometer Imaging System

Abstract:

A new 2nd-generation portable broadband VHF system has been designed
with improved sensitivity, broader bandwidth, higher time resolution,
2D-imaging for simultaneous sources and 3D-mapping capabilities. The
system utilizes a custom inverted-V antenna design which is sensitive to
one horizontal polarization with a broad and uniform gain pattern which
is peaked at the zenith. The antenna bandwidth extends from a few MHz
up to about 150 MHz. Raw data from 8 antennas can be simultaneously
sampled with 14-bit resolution at 360 MHz. A broadband threshold
trigger is used in software to capture entire flashes with precise GPS
absolute timing.

The system was initially deployed in October of 2018 at the Magdalena
Airport in New Mexico. A total of 7 VHF antennas were deployed,
providing 21 independent baselines. Over 1200 lightning flashes
triggered the system in the 16 days that it was operational between
October 13th and 29th. Lightning channels were mapped in great detail
up to high elevation angles where the older flat plate antenna design
had a sensitivity null. The increased sensitivity of the new antennas
was sufficient to even detect and map some positive leaders directly.
Furthermore, the availability of 2D imaging capabilities is providing
important insights into initial breakdown pulses and other phenomena.