M. Lalande

High Pulsed Power Sources for Broadband Radiation

This paper explains the design and production of two autonomous ultrawideband (UWB) radiation sources. These sources consist of a high-gain broadband antenna that is driven by one of two subnanosecond pulsed power sources. Each source is made up of a Marx generator and a pulse-forming device based on the use of a gaseous spark gap. The first source combines a four-stage 200-kV/34-J Marx generator with a coaxial pulse-forming line. Its main characteristics are an output voltage of 100 kV, a 250-ps rise time, a subnanosecond pulse duration, and a repetition rate of about 40 Hz. The second pulsed source is a ten-stage subnanosecond Marx generator that delivers pulses in the 250-kV/1.5-J range, with a 300-ps rise time and a subnanosecond pulse duration at a pulse repetition rate of 350 Hz. Probes were produced based on capacitive line dividers to measure both the temporal characteristics and the high-voltage (HV) amplitude of the pulses delivered by the pulsed power sources. The antenna, combined with these two pulsed sources, is a traveling-wave antenna called the Valentine antenna. Some mechanical modifications were made to the antenna to improve its dielectric strength. First, a 3-D model of the antenna was created on time-domain electromagnetic software to study the influence of these modifications on its main radiating characteristics. Its high gain and its capability to radiate short pulses without dispersion allow us to achieve a high measured figure of merit (the maximum value of far-field peak-to-peak electric field strength multiplied by the distance). A new method called the Instantaneous Electromagnetic Field Measurement by Signature of a Neutral Object (MICHELSON) method is used to measure the very intense electromagnetic fields that are radiated. The incident field is diffracted by a special small-dimension target. The diffracted field is measured by a conventional low-power UWB antenna. The target that is used has small dimensions, and no cables are used in the field region; thus, the electromagnetic interference that is generated and undergone by the measurement device is considerably limited. The figure of merit that is measured is 436 kV.

A Novel Antenna for Transient Applications in the Frequency Band 300 MHz – 3 GHz: The Valentine Antenna

We propose a novel ultrawideband (UWB) antenna designed specially for transient UWB radar applications. This work is a part of a new project concerning an optoelectronic UWB radar demonstrator with an array of four antennas. This project required the development of a new UWB antenna: the Valentine antenna. This antenna must be lighter and more compact in H-plane to allow the assembly of the array. This array must have a volume lower than 1 m3. This antenna, which is composed of curved metallic strips, radiates ultrashort pulses in the frequency band 300 MHz - 3 GHz with very low dispersion, a high gain and a low cross-polarization in the axial direction. The Valentine antenna must support 10kV of peak voltage. This paper describes the Valentine antenna and its main radiation characteristics

Utilization of Target Scattering to Measure High-Level Electromagnetic Fields: The MICHELSON Method

The intention of this paper is to present the instantaneous electromagnetic field measurement by signature of a neutral object (MICHELSON) method. The MICHELSON method is relatively new and enables the measurement of high-level electromagnetic fields from the utilization of a target. The incident field scattering on the target allows us to move the field measurement to a new location where the scattered field can then be measured using simple equipment and without breakdown risk. This method does away with the necessity for cables in the test zone. When using small targets, the induced disturbances in the incident field are limited. The MICHELSON method is implemented to characterize high-level electromagnetic sources whereby the signals radiated can be harmonic signals as well as transient pulses.

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

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.

Optoelectronic ultra-wide band radar system: RUGBI

We propose an ultra wide band radar system based on a coherent emission of an ultra-wide band antenna array using photoconductive switching devices. The triggering process is obtained by excitation of semiconductor samples in linear mode using a picosecond laser source. The emitting antenna system and the receiving antenna developed by our research laboratory, present some specific qualities suitable for radiation and measurement of ultra-short pulses

UWB antenna array with autonomous scanning capability using integrated opto-electronic feeding device

The originality of the radiation source presented here is localized in its ultrafast speed and its autonomy of scanning a large spatial area. That behavior is obtained by means of a trigger mode of radiation perfectly controlled by a single optical source. The transmitter consists of a powerful picoseconds laser, an optical power divider and n optoelectronic ultra-wideband radiation sources which produce several electric pulse trains with a repetition rate slightly different one with each other. Each train is then addressed toward a single antenna. When transmitted the electromagnetic waves interact to produce an ultrafast orientation of the array emission lobe allowing scanning of large spatial field in less than 1μs.

A Pulsed Modulator Combined With Very High PRF Photoconductive Switches to Build a Self-Scanning UWB Radiation Source

The development and construction of self-scanning capability short-range ultrawideband (UWB) radar is forthcoming for the French Military for Defense. This paper is devoted to the presentation of the self-scanning radar principle and of the development, construction, and tests of the different blocks needed to implement the elementary part of the forthcoming radar. The elementary radiation source is composed of a microstrip line with photoconductive switches, an optical system to trigger the switches, a pulsed modulator used as bias voltage to load the line, and a UWB antenna to radiate high electric fields in a broad frequency band. The main proposed innovations are devoted to the integration of the photoconductive switches within the antenna and to the use of the pulsed modulator as bias source instead of a dc voltage source. This modulator allows considering the increase in the yield of the whole source as well as the increase in the radiation bandwidth. Experimental results are presented in order to compare the switching efficiency with a dc bias source against this pulsed modulator bias source. Finally, photoswitches triggered by an 80-ps/20-μJ laser source are able to generate monopolar or bipolar pulses in the kilovolt range with subnanosecond duration and a 33-MHz pulse repetition frequency. First radiated electric field results of the whole device are also reported.

High-PRF UWB Optoelectronic Radar System: A CLEAN-Type Algorithm to Overcome Depth Limitation

The ultrafast speed and the large spatial area scanning autonomy are the two main advantages of the new radar presented in this paper. Indeed, the autonomous beam scanning capability with high pulse repetition frequency (PRF) is obtained by coherent combining of electrical fields emitted from active antennas array integrating photoconductive switching devices. An original signal processing associated with this new radar system is presented. We show a hybrid version of a CLEAN algorithm able to overcome the depth limitation induced by high PRF with standard imaging signal processing. Simulations and measurements are performed to validate the proper operation of the algorithm.

The NeTTUN project: Development of a ground prediction sensor

The NeTTUN (New Technologies for Tunnelling and Underground Works) Project is run by a consortium of 21 Industrial, Research & Development laboratories and Small and Medium Enterprises organizations across 9 countries in Europe; the ultimate goal is to enable groundbreaking change in the construction, management and maintenance of tunnels. NeTTUN aims at developing a fully automated system, that installed on Tunnel Boring Machine (TBM) provides identification of large obstacles that can obstruct the digging (e.g. other tunnels, cavities, boulders, foundations, archaeological remains, etc) as well as soil changes (e.g. from gravel to fractured rock).

A pulsed power source to drive an innovative autonomous scanning UWB radiation system

Ultra Wide Band (UWB) antenna arrays offer the possibility to generate a wide band signal in a sharp direction, which can drastically improve the detection sensitivity. With a configuration including as many antennas as generators, an UWB array presents the advantage of increasing the radiation power on one hand and offering the agility to the array on the other hand. Indeed, the application of time delays between the feeding pulses of the antennas permits to steer the radiated fields in each wanted direction and to realize a coherent sum of each initial power. However, a major difficulty with such a configuration is the minimization of the radiation source jitters, to obtain the best synchronization possible. A solution consists of using pulsed optoelectronic devices, operating in linear switching regime, which permits to bypass this difficulty and to obtain ultra-short electrical waveforms with small temporal jitters (2ps typically). Many applications such as transient radar cross section (RCS) measurements, UWB synthetic aperture radar (SAR) and high power UWB radiation source can be developed by using such systems. In this paper, the design of an innovative autonomous scanning radiation system will be approached. The first experimental results will be also presented. These results are essentially focused on the high voltage pulsed power source which is previously described. This source aims at driving the pulsed optoelectronic devices. It consists of a series-connected IGBTs component.