Low insertion-loss single-layer and bilayer metamaterial filters in terahertz (THz) frequency regime were demonstrated on top of low cost flexible Scotch tape by utilizing pattern transfer method. The transmittance of the flexible 51-μm-thick Scotch tape was found out to be higher than 0.85 in the range of 0.2 to 3 THz, which is excellent for the substrate materials for THz applications. Free standing filters exhibited record low insertion loss of 0.6 dB and band rejection ratio as high as 30 dB. The resonance reflection characteristics of the bilayer filters were maintained when they were attached on top of curved PET bottle or metallic surfaces, providing promising application in THz identifications.

In this study, a rectenna is adopted in a microwave-based wireless power transfer system to rectify the received radio frequency (RF) to DC power. Each rectenna comprises a DC-DC converter at the DC output port to ensure a stable power supply and a Schottky diode for the DC-combining network. The results indicate that the RF-DC conversion efficiencies of the rectifier circuit and 4×1 rectenna were 71% and 71.2% when the unit-cell RF input levels were 19 dBm and 19.6 dBm, respectively. Furthermore, the DC output levels of a 4×8 rectenna with and without the DC-DC converter were 28 V and 30 V, respectively, when the unit-cell RF input level was 20.2 dBm. Additionally, the RF-DC conversion efficiencies were 52.6% and 59.9% with and without the DC-DC converter, respectively. Thus, we experimentally verified that the Schottky diode combined with the DC-DC converter at the DC output port can increase the DC power combining level, making it suitable as an extendable array rectenna system.

A slot structure was combined with a discharge electrode to limit incident high-power microwaves via the integration of plasma discharge. At the target resonating frequency of 9.45 GHz, the surface current was concentrated at an electrode, and the electric field was enhanced by the proposed design to lower the response power level of the incident signal. When a low-power signal is injected, plasma is not generated, and the incident wave travels without insertion loss. Double-stage slot structures were utilized to broaden the band-pass characteristics in the frequency domain, and the demonstrated plasma limiter exhibited an insertion loss of 1.01 dB at 9.45 GHz. The xenon gas pressure was optimized with the shortest distance of 100 µm between the upper and lower electrodes to reduce the discharge power of the plasma. In the case of a high-power signal input, as xenon-gas breakdown occurred, the transmitted signal was close to zero, and most of the high-power signal was reflected with a blocking efficiency of 40.55 dB. The demonstrated result will be useful to protect the receiver of a radio detection and ranging system from the high power microwave.

The audible noise (AN) generated by corona discharge from a full-scale, high-voltage direct-current (HVDC) transmission line was evaluated over a one-year period from December 2017 to November 2018. The double bipolar HVDC transmission line using a metallic return conductor was demonstrated using a full-scale tower having a height of 21 m in Gochang, Korea. The measured AN in an outdoor environment was statistically evaluated and compared in different seasons and weather conditions. The A-weighted spectrum of the HVDC corona noise exhibited a greater distribution than the background noise above 1.0 kHz in the frequency domain. Various background noises were considered and eliminated to improve the measurement accuracy. The highest AN levels were exhibited during the spring season. Individuals in the age range of 18–60 years were randomly selected to conduct a human-perception test. The results indicated that 15% of the participants became aware of the AN from the corona discharge when it was >5 dBA greater than the background noise. The AN was <36 dBA when the conductor surface gradient was <18 kV/cm with a six-bundle sub-conductor.

A microwave power limiter with an H-type slot structure is proposed as a waveguide structure that blocks high-power microwaves through plasma discharge. At an operating frequency of 5.75 GHz, the electric field is concentrated in the discharging electrode gap; when high power is applied, the plasma limiter discharges, and low power is transmitted. By contrast, when low power is applied, it is transmitted without loss. An H-type resonance structure was used to increase the electric field concentration and selectivity within the waveguide. At 5.75 GHz, the insertion loss of the H-type slot structure was 1.85 dB. The electric field concentration increased to 49492 V/m, and the 3 dB bandwidth was 535 MHz. In addition, the plasma discharge onset level in the xenon gas was lower because of the electrostatic potential. When an electrostatic potential of 300 V was applied, the plasma discharge onset level decreased from 310 to 180 W.