Hualiang Zhang

Accurate Respiration Measurement Using DC-Coupled Continuous-Wave Radar Sensor for Motion-Adaptive Cancer Radiotherapy

Accurate respiration measurement is crucial in motion-adaptive cancer radiotherapy. Conventional methods for respiration measurement are undesirable because they are either invasive to the patient or do not have sufficient accuracy. In addition, measurement of external respiration signal based on conventional approaches requires close patient contact to the physical device which often causes patient discomfort and undesirable motion during radiation dose delivery. In this paper, a dc-coupled continuous-wave radar sensor was presented to provide a noncontact and noninvasive approach for respiration measurement. The radar sensor was designed with dc-coupled adaptive tuning architectures that include RF coarse-tuning and baseband fine-tuning, which allows the radar sensor to precisely measure movement with stationary moment and always work with the maximum dynamic range. The accuracy of respiration measurement with the proposed radar sensor was experimentally evaluated using a physical phantom, human subject, and moving plate in a radiotherapy environment. It was shown that respiration measurement with radar sensor while the radiation beam is on is feasible and the measurement has a submillimeter accuracy when compared with a commercial respiration monitoring system which requires patient contact. The proposed radar sensor provides accurate, noninvasive, and noncontact respiration measurement and therefore has a great potential in motion-adaptive radiotherapy.

A Tunable Bandstop Resonator Based on a Compact Slotted Ground Structure

A new slotted ground structure featuring compact size and spurious-free passband in the context of microstrip lines is proposed. The compactness of the new structure originates from the lengthened coupling gap in the ground. The spurious-free response in a wide passband is a result of combining both narrow and wide slots in the new structure, which effectively forms a stepped-impedance slot resonator. Compared to the previously proposed dumbbell- and H-shaped defected ground structure, over 40% size reduction can be realized in the new structure. The new structure also provides flexibilities in introducing additional functionalities to the bandstop resonators. By embedding varac- tors in the slotted ground structure, tunable bandstop resonators can be implemented and 13% tuning range centered at 2.36 or 2.67 GHz have been obtained.

A tri-section stepped-impedance resonator for cross-coupled bandpass filters

We report a new tri-section stepped-impedance resonator (SIR) that features a short and low-impedance section inserted in a two-section SIR. The inserted section not only makes the resonator more compact, but also enables the flexibility of introducing cross coupling in a filter configuration. Using the tri-section SIR, a cascaded triplet bandpass filter is demonstrated, featuring sharp roll-off at the high edge of the passband.

Tuneable complementary metamaterial structures based on graphene for single and multiple transparency windows

Novel graphene-based tunable plasmonic metamaterials featuring single and multiple transparency windows are numerically studied in this paper. The designed structures consist of a graphene layer perforated with quadrupole slot structures and dolmen-like slot structures printed on a substrate. Specifically, the graphene-based quadrupole slot structure can realize a single transparency window, which is achieved without breaking the structure symmetry. Further investigations have shown that the single transparency window in the proposed quadrupole slot structure is more likely originated from the quantum effect of Autler-Townes splitting. Then, by introducing a dipole slot to the quadrupole slot structure to form the dolmen-like slot structure, an additional transmission dip could occur in the transmission spectrum, thus, a multiple-transparency-window system can be achieved (for the first time for graphene-based devices). More importantly, the transparency windows for both the quadrupole slot and the dolmen-like slot structures can be dynamically controlled over a broad frequency range by varying the Fermi energy levels of the graphene layer (through electrostatic gating). The proposed slot metamaterial structures with tunable single and multiple transparency windows could find potential applications in many areas such as multiple-wavelength slow-light devices, active plasmonic switching, and optical sensing.

A Fully Symmetrical Crossover and Its Dual-Frequency Application

In this paper, a new fully symmetrical four-port microstrip crossover and its dual-frequency application are presented. The proposed single-band crossover has a simple structure and an easily controlled bandwidth; and its dual-frequency application can provide flexible frequency ratio between the two working frequencies. Moreover, analytical design equations are derived using the even-odd-mode method. The final explicit design equations are concise. To verify the design concepts, both a single-band crossover working at 1 GHz and a dual-frequency crossover working at 1/2.3 GHz are fabricated and tested. The measurement results agree well with the design theory.

Miniaturized coplanar waveguide bandpass filters using multisection stepped-impedance resonators

We report several types of coplanar waveguide tri-section stepped-impedance resonator (SIR) structures that offer further size reduction compared to the conventional two-section SIRs. In addition, the tri-section SIRs provide flexibility of introducing effective capacitive coupling to the ground lines for realizing slow-wave structures. The slow-wave effect can be used to implement more compact bandpass filters. The principles of achieving size reductions using tri-section SIRs are described and confirmed by simulation results. To demonstrate the effectiveness of the proposed tri-section SIRs in size reduction, the tri-section SIRs with slow-wave effects were implemented in a two-pole directly coupled bandpass filter and a fourth-order quasi-elliptic bandpass filter with reduced size. The measured results matched well with the theoretical prediction.

A Novel Ultra-Wideband Differential Filter Based on Microstrip Line Structures

A novel differential filter with ultra-wideband (UWB) response is proposed in this letter based on microstrip line structures. 180° UWB phase shifters and 360° transmission lines are employed in the design to get the 180° phase shift over broad bandwidth. In this way, the broadband conversion between in- and out-of-phase signals can be realized. Utilizing this characteristic, the undesired common-mode noises will be cancelled out at the center of the filter, while the input differential-mode signals can still propagate well. The proposed new differential filter was calculated by transmission line model, simulated by full-wave electromagnetic simulator, and validated by the measurement. The simulation and measurement results verify its good performance. It is validated that, in the proposed filter, the differential-mode signals can propagate well with UWB frequency response, while the common-mode noises are well suppressed with more than 10 dB suppression in the concerned frequency band.

Designs of dual-band Wilkinson power dividers with flexible frequency ratios

This paper presents new designs of Wilkinson power dividers for dual-band operations. Two types of dual-band quarter-wavelength (λ/4) transmission lines are proposed first, where both open- and short-ended stubs are applied. ABCD-matrix is then employed to derive the design equations of these dual-band lines. Based on these equations, it is found that the proposed structures feature a large frequency ratio range (from 2.16 to 4.9). By replacing the conventional λ/4-line with these dual-band lines, the desired dual-band Wilkinson power dividers can be realized. A power divider working at 1.8 / 5.8 GHz is designed using this method. Its measured performance verifies the design concepts.

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.

^{\circ}

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.