Jiheon Ryu

A High Directive Paraboloidal Reflector Antenna for High Far Voltage in an Ultra Wideband Source System

This paper proposes a paraboloidal reflector-based integrated antenna-source (IAS) system for increasing the strength of radiated electric fields in ultrawideband (UWB) pulses. With the transient analysis of a paraboloidal structure, the reflector for an omnidirectional radiation source is designed. The paraboloidal reflector-based IAS system is structurally uncomplicated and compact. The proposed high power wideband radiator has a peak value of 42.9 kV/m at a distance of 40 m and a center frequency of 1.1 GHz. Thus, the strength of electric field rises by 8.9 times compared with that of the system without reflector. It is numerically and experimentally proved that both the far voltage and the UWB gain are significantly increased.

An Integrated Antenna-Source System of Very High Ultra Wide-Band Gain for Radiating High-Power Wide-Band Pulses

Integrated antenna-source (IAS) systems depends on techniques that reduce the complexity of traditional separated antenna-source systems. The IAS has a number of advantages such as compactness, simplicity, and efficiency. However, the ultra wide-band (UWB) gain of IAS is low since the integrated system employs omni-directional antennas. Here, we propose a new IAS system. We increase the size of capacitance and implement a smooth transition structure in the new system. As a result, the proposed IAS shows a UWB gain of 0.68. It is more than double compared to that of conventional IAS systems. The radiated electric field measured at 4 m from the system has a peak value of 80.8 kV/m and a center frequency of 350 MHz. The radiated power density decreases by half when the radiation angle is changed from 0° to 60°.

Modified TEM Horn for Enhanced Radiation Characteristics at Low Frequency

This paper presents a modified TEM horn that improves radiation characteristics at a low frequency region. The proposed antenna consists of an asymmetric TEM (ATEM) horn and a loop structure with an elliptical shape. The bandwidth and gain at low frequency region can be enhanced by using the ATEM horn configuration and adding a loop structure with an elliptical shape to the ATEM horn. The bandwidth of the proposed antenna is from 2.14 to over 20 GHz, whereas that of the conventional TEM horn is from 2.7 to over 20 GHz, where the dimensions of both antennas are the same except for the thickness of the loop structure. The physical and electrical dimensions of the proposed antenna are 60 mm × 62.5 mm × 64 mm (width × height × length) and 0.428 λ L × 0.445 λ L × 0.456 λ L, where λ L corresponds to the lowest frequency of the bandwidth. The realized gain of the proposed antenna is improved by 0.802 dB on average at the low frequency region (2 to 8 GHz), where the maximum gain increase is 2.932 dB when compared to a conventional TEM horn.

Electromagnetic Transient Simulation of Spark-Gap Switched Pulse Generators for Predicting Pulse Waveforms

This paper presents an electromagnetic simulation method for spark-gap switched pulse generators (SGSPGs). Electromagnetic simulation programs can be useful for designing nanosecond or subnanosecond SGSPGs; however, these programs do not support simulation models of spark-gap switches. This paper describes a simulation method that determines electric field at an observation point in the SGSPG by replacing the spark-gap switch with a step voltage source. In this simulation, the discharge properties of the spark-gap switches, including the breakdown voltage and rise time, are determined using the results of the studies on electrical breakdown. A 1-ns single pulse-forming line was fabricated to validate the proposed method. The simulated waveform was in good agreement with the measured waveform.

Modified low-frequency compensated TEM (LFCTEM) horn antenna to improve the radiation performance

A Low Frequency Compensated TEM (LFCTEM) horn antenna realizes a poor gain in the lower part of operating bandwidth. This paper presents the Modified LFCTEM (MLFCTEM) antenna which has elliptical cutting shapes on LFCTEM to improve performance at the low frequency. To verify the improvement of the proposed antenna performance, MLFCTEM is compared with the LFCTEM antenna. Simulated results show that the MLFCTEM has return loss less than -10 dB in the frequencies from 2 to 18 GHz. In the case of being compared to the LFCTEM, it also has had an improved gain value at the low frequency which increases up to about 2.5 dB and has more directive radiation pattern.

Reflector Impulse Antenna of High Electrodynamic Potential

A few versions of impulse antennas are studied with the aim of suggesting an optimal design of the feed line and radiator for the reflector-based antenna and ensuring operation of the facility at high voltages and under field conditions. The high-voltage system described as the final choice is intended for shaping and radiating electromagnetic pulses of ultrashort duration, while maintaining the proper electric strength of its components. It is characterized by an electrodynamic potential (field-range product) W = 1.2 MV. The system is a variant of the half-reflector impulse antenna design, involving a half-circular parabolic reflector that is excited through a nonuniform dual-line feeder of simple geometry.

A Square Patch Capacitive Voltage Divider for Measuring High-Voltage Ultrawideband Pulses in a Coaxial Pulse Forming Line

This paper presents a square patch capacitive voltage divider (CVD) for measuring high-voltage ultrawideband pulses in a coaxial pulse forming line. The dimension parameters and the sensitivity factor of the CVD are expressed as functions of the low-frequency cutoff and the high-frequency cutoff of the pulse waveform via a theoretical analysis in frequency domain. A coaxial line integrated with a square patch CVD is designed, manufactured, and tested. A 1.3-ns bipolar pulse is measured by the CVD. The experimental results demonstrated that the waveform measured by the CVD was nearly identical to the reference waveform and the error of the maximum amplitude was 2.1%.

Optimum Focal Point of a Paraboloidal Reflector Antenna With an Omnidirectional Radiative Source

A paraboloidal reflector antenna is useful for enhancing the directivity of a feed antenna with a low directivity. In this letter, we investigate a paraboloidal reflector antenna that has an ultrawideband (UWB) omnidirectional radiative source as the focal feed. To obtain the optimized focal length f for a given diameter D of the paraboloidal reflector antenna, we employ a simple relation between f and D, which can reduce the spreading loss of the source field while maintaining the aperture field uniformity. Simulation results show that the f values obtained from the above relation can maximize the far voltage-a well-known figure of merit for UWB applications.

Explosively Driven Ferroelectric Generator for Compact Pulsed Power Systems

Recently, the characteristics of the explosively driven magnetohydrodynamic generators and ferromagnetic generators that can be used as initial sources of compact power systems were presented by the authors. In this paper, we examined the electro‐hydrodynamic properties of the PZT 56/44 ceramics that were depoled by the explosively driven shock wave in a ferroelectric generator (FEG). Small‐sized FEGs had been manufactured and the electric currents and load voltages for the input stress of 6.5 GPa and 12 GPa and load resistances from 10 to 1,080 Ω were measured. Maximum values of power (0.7 MW) and total energy (3.7 J) were obtained at the load resistance of 1,080 Ω and 125 Ω, respectively when the input stress was 12 GPa. For evaluating the experimental current waveforms two‐dimensional hydrodynamic simulation which shows shock wave propagation into the PZT ceramics and a simple semi‐empirical theoretical calculation have been performed.

Output characteristics of GaAs photoconductive semiconductor switch at high bias voltages

A photoconductive semiconductor switch (PCSS) with a gap of 2 mm was fabricated on a semi-insulating GaAs substrate. The PCSS was excited by a 1064 nm wavelength pulse laser which has 700 ps pulse width (FWHM) and energy of 135 μJ. Output pulses of the PCSS were recorded at bias voltages from 100 V to 1.4 kV in fluorinert ambience which has high dielectric strength (Eb > 160 kV/cm). At bias voltages below 1 kV, the PCSS exhibited the linear operational mode, where the output pulse amplitude linearly increases with the increased bias voltage. When the bias voltage gets higher than 1 kV, the output pulse amplitude begin to increase more rapidly due to nonlinear optical process. The pulse width and fall time of the output pulses increased correspondingly.