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An exponential spacing and sinusoidal folded helical (ESSFH) antenna backed with a cavity is developed in this paper. Compared with the conventional helical (CH) antenna, the proposed antenna not only has smaller dimension but also exhibits a wider working bandwidth, a higher gain, and a better circular polarization (CP) characteristic. To reduce the dimension of the helical antenna, a sinusoidal structure is adopted along the circumference of the helix. However, it deteriorates the CP characteristic of the antenna. Therefore, the structure of the exponential helix spacing is introduced into the sinusoidal folded helical (SFH) antenna. Then, to further improve the gain of the ESSFH antenna, its ground plane is replaced by an optimized cavity. Compared with the CH antenna, the helix diameter of the ESSFH antenna Dλ is reduced from 0.32 to 0.23, and its volume is reduced to 53%. The ESSFH antenna backed with a cavity has an impedance bandwidth of 0.43–1.02 GHz, which is much wider than 0.48–0.60 GHz of the CH antenna. Moreover, it has an axial ratio of 1.77, while the axial ratio of the CH antenna is 2.61. In addition, its effective potential gain is 0.56, which is 22% higher than that of the CH antenna. [ABSTRACT FROM AUTHOR]

In this paper, a high-power ultra-wideband radiation system, composed of multiply radiation modules, is developed based on the space-synthesis method. The radiation module mainly consists of a transistorized pulser, a 2 × 2 combined antenna array, and a power divider. To improve the out parameters [the amplitude, the pulse repetition frequency (PRF), and the rise time] of the transistorized pulser based on the Marx circuit, the influence of the traveling wave process on the output pulse must be concerned. Based on the theoretical analysis, the printed circuit board circuit parameters and the circuit structure of the pulser are optimized. To improve the power synthesis efficiency, the pulse jitter characteristic of the pulser is improved by replacing the conventional base triggering mode with the collector voltage ramp triggering mode for the first-stage avalanche transistor in the pulser. The previous optimized antenna array is utilized in this radiation system, which has a better radiation performance in the prescribed aperture area. In addition, based on the gradient microstrip structure, the power divider integrated with the pulser is designed, which has the advantages of wide bandwidth, low loss, and light weight. Experimental results show that the peak effective potential rEp of the radiation system of 20 radiation modules is 221.8 kV, the maximum PRF could reach 10 kHz, and the half-power radiation angles of its radiation field are about 5° in both the E plane and the H plane. More radiation modules could be integrated into the system to achieve a higher electric field in the future. [ABSTRACT FROM AUTHOR]

Output power of a transistorized pulser is usually limited by the power capacity of avalanche transistors. To improve the total output power, the power synthesis method is widely used, in which a single pulser with high output power and high time base stability is required. However, the time base stability tends to deteriorate as the output power increases. To improve the output power under the premise of high time base stability, from the perspective of carrier movement, the mechanisms of pulse jitter and pulse drift are investigated. It is found that the pulse jitter is caused by time dispersion of the ionization process in the collector depletion region, while the pulse drift is due to the decrement of the diffusion coefficient Dn and the electron mobility μn, which are both temperature-dependent. Based on the microscopic theoretical study, some macroscopic improvements on the time base stability are made. Some parameters of the trigger pulse and the circuit (e.g., charging capacitance) are optimized experimentally. Consequently, we achieved a pulser with an amplitude of 1.8 kV, pulse jitter of 25 ps, pulse drift of 100 ps/min at a pulse repetition frequency (PRF) of 100 kHz. Additionally, a new parameter k, the product of the highest PRF f and the peak power Ep, is defined to evaluate the output power. With almost the same time base stability, the proposed pulser has a k of 6.48 GHz W, which is improved significantly. Finally, a synthesized pulser with an amplitude of 2.5 kV and highest PRF of 100 kHz is achieved. [ABSTRACT FROM AUTHOR]

In this article, a Narrow-Width Combined Antenna (NWCA) is proposed for the compact design of high-power ultra-wideband (UWB) systems. The dependence of performances on three dimensions of the combined antenna is investigated so as to minimize its size with a given excitation. It indicates that the working process of the combined antenna can be divided into two stages: (1) energy transmitted from the feeding point to the aperture by the TEM horn structure, and during this stage, the passband is determined by the effect of the impedance taper, which is related to the length and aperture impedance of the antenna, and (2) energy radiated to the free space from the aperture, during which the height of the aperture is the dominant factor. Therefore, the three dimensions of the combined antenna can be appropriately adjusted to make the antenna more compact. Thus, the NWCA is designed by reducing the width and making a slight compensation in height and/or length. Compared with the conventional cubic antenna, the aperture area of the developed NWCA is reduced by 47%, whereas the amplitude of the radiating field only reduces by 2.5% with the given pulsed excitation at the cost of a slight decrease in the pulse duration. It demonstrates that the NWCA is an effectively compact design for the combined antenna in the application of the radiation of the high-power UWB pulse. [ABSTRACT FROM AUTHOR]

This paper introduces an application of the time-domain gating technique in water content measurement of gas–liquid two-phase flow. According to the principle that water and gas have different absorption effects on microwaves, a sensor with transmitting and receiving antennas is designed. Combined with the back propagation neural network, the water content of two-phase fluids can be obtained from the complex transmission coefficient (S21) of antennas. In order to reduce the influence of the sensor structure on antenna measurement, the time-domain gating technique can be used to filter out the interference components in S21 so that the water content can be more easily predicted. Built on the above, an automatic test system is established. In the range of 0%–78%, the absolute test error of the water content is lower than 3%. [ABSTRACT FROM AUTHOR]

In this Note, a type of Differential Switched Oscillator (DSWO) system is developed and compared with the conventional single-ended switched oscillator; the power capacity of the DSWO is twice with the same insulation level and twice total length. The DSWO system consists of a differential high-voltage pulsed source, a DSWO, and a pair of differential helical antennas. The differential pulsed source is based on the hydrogen thyratron and pulsed transformer whose peak voltage can theoretically reach ±100 kV to break down the high-pressure switch, whose limiting gas pressure is 25 atm; the DSWO is designed to generate a damped oscillation pulse with a central frequency of 300 MHz, which is also the central frequency of the differential helical antennas. Thus, a damped oscillation pulse can be produced and radiated to generate high-power mesoband circularly polarized electromagnetic fields, and the axial ratio is 1.98. According to the measured results, the central frequency of the developed DSWO is 284 MHz, the percent bandwidth of the radiating field is 11%, and the amplitude of the far-field effective potential is 105 kV. [ABSTRACT FROM AUTHOR]

In this note, a portable ultrawideband (UWB) electromagnetic radiator is developed based on a transistorized pulser with the peak power of 1.4 MW, the rise time less than 150 ps, and the repetition frequency of 50 kHz. To generate high-amplitude pulses, a 100-stage Marx circuit with parallel connection of multiple transistors is proposed. To improve the pulse repetition rate, the parallel charging Marx circuit is adopted with ferrite beads connected in series between stages for high isolation of pulses. In order to radiate the UWB electromagnetic pulse directionally, a compact combined antenna array is fabricated and connected with the pulser via a coaxial feeding module. The effective potential of the UWB radiator reaches 10.5 kV with the band range (−10 dB) from 173 MHz to 2.32 GHz. [ABSTRACT FROM AUTHOR]