Rongguo Zhou

A Varactor Based 90

In this paper, a symmetric four-port microwave varactor based 90 ° directional coupler with tunable coupling ratios and reconfigurable responses is presented. The proposed coupler is designed based on the modified structure of a crossover, where varactors are loaded. By applying suitable biasing voltages to the varactors, the power-dividing ratios between the two output ports (i.e., port 2 and port 3) of the coupler can be easily controlled. Moreover, it is found that the realizable power ratio using the proposed structure is very flexible (it could be extremely large or small). Therefore, under the special case when the coupling ratio is tuned to be 1, the proposed coupler is reconfigured to be a crossover. Good isolation and return-loss performance have been maintained for different power-dividing ratios. To theoretically analyze the proposed device, closed-form design equations are derived using the even-odd mode method. Based on these analytical equations, an experimental prototype working at 1 GHz is designed, fabricated, and characterized. The measurement results match well with the simulation and theoretical results, validating the proposed design theory.

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Two-dimensional (2-D) metallic wire arrays are studied as effective media with an index of refraction less than unity (n eff < 1). The effective medium parameters (permittivity ¿eff, permeability ¿eff and neff) of a wire array are extracted from the finite-element simulated scattering parameters and verified through a 2-D electromagnetic band gap (EBG) structure case study. A simple design methodology for directive monopole antennas is introduced by embedding a monopole within a metallic wire array with (n eff < 1) at the antenna operating frequencies. The narrow beam effect of the monopole antenna is demonstrated in both simulation and experiment at X-band (8-12 GHz). Measured antenna properties including reflection coefficient and radiation patterns are in good agreement with simulation results. Parametric studies of the antenna system are performed. The physical principles and interpretations of the directive monopole antenna embedded in the wire array medium are also discussed.

Directional Coupler With Tunable Coupling Ratios and Reconfigurable Responses

This letter presents a modified Doherty power amplifier (DPA) architecture to release bandwidth limitation of the conventional DPA. The proposed DPA structure eliminates two quarter wavelength impedance inverters used in the conventional DPAs. Instead, both the carrier and peak amplifiers in the proposed DPA are matched to 70 Ω at the output ports, which enables an easier implementation of broadband matching networks. Broadband input matching network (IMN) and output matching network (OMN) are then designed to achieve wideband DPA with enhanced performance. To verify the design concept, a broadband DPA is designed, fabricated, and measured on a RT/Duroid 5880 substrate with 2.2 dielectric constant and 0.787 mm substrate thickness. In the working frequency bands (0.8 to 1.2 GHz), the designed DPA provides 50.8% to 78.5% power-added efficiency (PAE) at full output power, 30.3% to 40.1% PAE at 6 dB of output power back-off (OBO), 10.8 to 14.8 dB gain (the gain variation is within 2.6 dB over different input power levels at each specific frequency), and maximum output power between 40.2 and 42.9 dBm.

Metallic Wire Array as Low-Effective Index of Refraction Medium for Directive Antenna Application

This paper presents an improved two-antenna microwave passive direction finding system inspired by the human auditory system. By incorporating a head-like scatterer between two monopole antennas, both phase and magnitude information can be utilized to estimate the direction of arrival (DOA) of a microwave signal, thus eliminating ambiguities associated with phase wrapping at high frequency, just like human ears. In addition, better DOA sensitivity is demonstrated with the incorporation of the head-like scatterer. Both numerical modeling and experimental results of a simple X-band two-monopole configuration with symmetric and asymmetric scatterers are presented.

Design of GaN Doherty Power Amplifiers for Broadband Applications

A new wideband balun is presented. The designed balun has a symmetrical four-port structure with one port open-ended. It consists of a wideband impedance transformer and a broadband phase inverter. To design the wideband impedance transformer, an S-plane high-pass prototype has been exactly synthesized with third order Chebyshev response. A wideband parallel strip balun operating from 0.72 to 2.05 GHz is fabricated and tested to verify the design concept. The measured results match well with the design theory.

Improved Two-Antenna Direction Finding Inspired by Human Ears

We designed, fabricated and measured a rectangular DR antenna made of distilled water with high dielectric constant and low loss. The resonant modes and resonant frequencies of the DR antenna are analyzed using the dielectric resonator model, which are verified via both simulation and experiment. The measured antenna return loss agrees well with the simulation result when both the air gap and the feeding probe offset are taken into account.

Design of a Wideband Balun Using Parallel Strips

Gradient index (GRIN) structures have attracted great interests since their invention. Especially, the recent advance in the fields of transformation optics, plasmonics, and nanofabrication techniques has opened new directions for the applications of GRIN structures in nano-photonic devices. In this paper, we apply Luneburg lens and its transformed counterpart to realize efficient coupling to plasmonic nano-waveguides. We first briefly present the general structures of Luneburg lens and generalized Luneburg lens, as well as the design process of flattened Luneburg lens applying quasi-conformal mapping techniques. After that, we study the performance of these lenses for coupling electromagnetic signals to nano-waveguides (the metal-insulator-metal nano-waveguide). Different coupling schemes are investigated. It is found that the proposed Luneburg lens based optical couplers can be used to provide broadband light couplings to plasmonic nano-waveguides under wide incident angles.

A compact water based dielectric resonator antenna

In this paper, we propose a novel design of broadband monopole optical nano-antennas. It is consisted of a corrugated half elliptical patch inside an elliptical aperture. By adjusting the dimensions of the elliptical patch and the elliptical aperture, the overall performance of the proposed monopole nano-antenna can be made remarkable broadband. Full wave electromagnetic simulations have been used to investigate the effects of different parameters on the nano-antenna performance. Moreover, the proposed broadband nano-antenna can support light waves with different polarizations. It is expected that the new optical antenna will pave the way towards the development of high performance optical antennas and optical systems.

Design of wide-angle broadband Luneburg lens based optical couplers for plasmonic slot nano-waveguides

In this paper, we report novel designs of tunable THz plasmonic devices based on liquid metals. The designed devices will be able to dynamically control and change the spectrum responses of extraordinary THz wave transmissions. Different THz device configurations are investigated, and numerical simulations have been conducted to theoretically verify the performance of the proposed structures. Moreover, an equivalent circuit model has been developed to describe the operating principle of the proposed THz devices. Good agreement has been achieved between the theoretical models and the numerical results. These new THz devices are expected to be applied in various areas of sensing, communication, and imaging.

Design of a new broadband monopole optical nano-antenna

Microwave direction finding has attracted a lot of attention due to its wide applications in the military and commercial areas, such as electronic warfare [1], wireless communications [2], etc. Various direction of arrival (DOA) estimation techniques including the estimation of signal parameters via rotational invariance techniques (ESPRIT) [3] and multiple signal classification (MUSIC) [4] have been developed to achieve high degree of accuracy. All of these techniques are based on arrays with multiple antenna elements. However, the power consumption and cost of the RF circuits associated to the arrays increase with the number of antenna elements. To reduce the size, decrease the power consumption and lower the cost, an accurate DOA estimation technique with reduced number of antenna elements is highly desirable.