Exponent presented Randwulf Technologies a requirement for a 500 MHz to 6000 MHz Vivaldi antenna design to be used for a Ground Penetrating Radar (GPR) system. The antenna should have less than 2:1 VSWR over the bandwidth and produce as little secondary reflection from the initial backscatter pulse as possible.
In order to produce as broadband a design as possible, the transition found in most textbooks was re-examined. It was realized that the use of a quarter-wave radial stub was acting to limit the transition bandwidth. It was further understood that the traditional circular open circuit was band-limiting at the low end of the bandwidth. The proverbial circular “open” was cut in half to produce a true open circuit, and terminations were placed across the open circuit to minimize any reflection from a wave traveling toward the open circuit from either driving the antenna or from backscatter. The details of this analysis and design are found in .
Slotlines were etched into the outer periphery of the antenna and loaded with matched resistances to improve the lower frequency performance of the antenna, and to aid in secondary backscatter reduction in the antenna. The HFSS model of the prototype antenna is given below:
The physical realization of the HFSS model is presented next:
A TDR analysis using HFSS was performed on the antenna. Here is the impedance plot:
The fabricated antenna was then evaluated with an HP 8753D Network Analyzer in TDR mode to determine the quality of the coax to stripline to bilateral slotline transitions. We first present the plot in terms of impedance:
The plot is also presented in terms of VSWR along the transition:
The measured VSWR from 400 MHz to just above 6000 MHz is presented here:
The slight deviation above 2:1 VSWR at around 700 MHz was deemed acceptable for the prototype antenna. The upper frequency limit of the antenna VSWR is not known, as the upper frequency of the VNA used was only around 6 GHz.
The radiation patterns predicted by HFSS are shown below:
 Bancroft, Randy & Chow, Ri-Chee “Vivaldi Antenna Impedance Bandwidth Dependence on Stripline to Bilateral Slotline Transition” Microwave and Optical Technology Letters, Vol. 55, No. 4, April 2013 pp 937-941