The polarization conversion of electromagnetic (EM) waves, especially linear-to-circular (LTC) polarization conversion, is of great significance in practical applications. In this study, we propose an ultra-wideband high-efficiency reflective LTC polarization converter based on a metasurface in the terahertz regime. It consists of periodic unit cells, each cell of which is formed by a double split resonant square ring, dielectric layer, and fully reflective gold mirror. In the frequency range of 0.60 - 1.41 THz, the magnitudes of the reflection coefficients reach approximately 0.7, and the phase difference between the two orthogonal electric field components of the reflected wave is close to 90° or -270°. The results indicate that the relative bandwidth reaches 80% and the efficiency is greater than 88%, thus, ultra-wideband high-efficiency LTC polarization conversion has been realized. Finally, the physical mechanism of the polarization conversion is revealed. This converter has potential applications in antenna design, EM measurement, and stealth technology.
We propose an ultra-broadband and polarization independent planar absorber comprising multilayered graphene. The bandwidth of the proposed absorber is extended by increasing the number of layers of graphene, and it is polarization independent due to its symmetrical unit structure. The full wave simulation results show that an absorber with three graphenebased layers can efficiently harvest an electromagnetic wave with random polarization from 17.9 GHz to 188.7 GHz (i.e., covering frequency regimes from K to D bands and relative bandwidth of ∼ 165%). The physical absorption mechanism of ultra-broadband absorption has been elaborated upon using the destructive interference method and multiple resonances approach in a multilayered medium. The proposed absorber can be used in many applications such as medical treatment, electromagnetic compatibility, and stealth technique.
This paper presents high-power broadband graphene non-Foster circuit (NFC) enabled Class-J GaN HEMT power amplifier (PA). A graphene resonant-tunneling diode (RTD) NFC is proposed to provide negative differential resistance characteristics and to create an effective negative capacitance. The NFC is integrated in the input matching network of the Class-J GaN HEMT PA to cancel out the transistor input parasitic capacitance so as to enhance PA bandwidth, efficiency, output power, and gain. The PA design is based on Cree's packaged GaN HEMT CGHV40030F biased with drain supply voltage of 50 V at quiescent drain-to-source current of 44 mA. The PA operates from 3.4 to 4.0 GHz with center frequency of 3.7 GHz. The effective negative capacitance of the NFC from 3.4 to 4.0 GHz stands at −3.3 to −6.0 pF. An effective capacitance of −3.7 pF has been obtained at 3.7 GHz. The PA has small signal gain of 16.1 dB at 3.7 GHz. Large signal simulation input power sweep from 1 to 33 dBm at 3.7 GHz, indicates that the PA has 57.8% drain efficiency, 54.7% power added efficiency (PAE), 44.6 dBm (28.8 W) output power and 11.6 dB transducer power gain at input power of 33 dBm. K E Y W O R D S broadband, class-J, GaN HEMT power amplifier, graphene non-Foster circuit, high power
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