A 250KV-300PS-350HZ Marx generator as source for an UWB radiation system

Abstract

This paper aims at presenting the design and realisation of an autonomous, ultra wideband (UWB) radiation source consisting of a high gain broadband antenna driven by a subnanosecond pulsed power source. The high voltage pulse source is a single ten stage subnanosecond Marx generator which delivers pulses in the range of 250 kV/1.5 J, with a minimum 300 ps risetime, subnanosecond pulse duration at a maximum pulse repetition frequency of 350 Hz. The main particularity of our system is to integrate the pulse forming device (a peaking stage and a crowbar switch), directly on the last stage of the Marx generator. This adjustment avoids the loss of output pulse amplitude due to a classical pulse forming line. The development of the Marx generator combined with its own pulse forming device is explained and discussed in the paper. Dedicated home made probes based on capacitive line divider are realized to measure both the temporal characteristics and the high voltage amplitude of the pulses delivered by the pulsed power source. These probes allow to observe voltage pulses without perturbation in any place of the circuit and to measure their main characteristics. Its 2.3 GHz high cut off frequency and its sufficient division ratio permits to measure the rise-time, fall-time, pulse width and the amplitude of the output signal of the pulser. Calibration tests in the frequency and time domain are also performed and detailed in the paper. Another major factor in UWB radiation systems is the radiating element. The pulsed source is combined with a travelling wave antenna called Valentine antenna. Some mechanical modifications were made to improve the dielectric strength of this radiating element. A 3-D model of its structure, on a time domain electromagnetic software, was first performed to study the influence of these modifications on the main radiating characteristics of the antenna. Then, this antenna is tested with HV pulses in order to avoid any undesirable breakdown. Various tests on the whole source (battery-DC/DC converter-Marx generator-pulse forming device-antenna) were investigated in order to evaluate the figure-of-merit of our system. A novel method to measure high level electromagnetic fields, called MICHELSON method, is used. The incident field scattering on a target permits to move the field measurement toward a novel location, where a simple equipment can measure the scattered field without breakdown risk. The results obtained with this method are compared to ones measured with a classical derivative field sensor. Moreover, the high gain and capability of the Valentine antenna to radiate short pulses without dispersion allow achieving a high measured figure-of-merit. Finally in this paper, the maximum figure-of-merit obtained is 450 kV with our compact source.