Gain Drop Research Articles

A Novel PRAMC-Based Ultralow-Profile Transmitarray Antenna by Using Ray Tracing Principle

In this paper, a novel ultralow-profile transmitarray (TA) is proposed by using multifolded ray tracing principle. The proposed TA unit cell would reflect the incident wave of x-polarization as a ground and transmit the incident wave of y-polarization with arbitrary phase shift in the operating frequency band. A broadband polarization rotation reflection surface based on artificial magnetic conductor with small phase difference in wide range of incidence angle is applied to transform the polarization of incident wave. Two kinds of TA antennas at 28 GHz are studied, where the distances between the source and the TA are, respectively, reduced to 1/3 and 1/4 of the focal length (named as TA antennas I and II), which are much less than that of the conventional TA antennas. The simulated results show that the proposed TA antennas can achieve −1 dB gain drop bandwidth of 5% and cross-polarization level of less than −30 dB, and the maximum aperture efficiencies can achieve 46% and 40%, respectively. For demonstration, the TA antenna I is fabricated and measured. Good agreement can be observed between the measurement and simulation.

Accommodation and pupil behaviour of binocularly viewing early presbyopes.

To study accommodation behaviour of early presbyopes with the full suite of accommodative stimuli, and to monitor changes in spherical aberration, pupil size and image quality that accompany the accommodative response. Using a high resolution Shack-Hartmann aberrometer, we measured refractive state as a binocularly viewed 0.30 logMAR (6/12 or 20/40) letter E was moved from 2m to 20cm and simultaneously monitored pupil diameter and spherical aberration for 19 subjects (mean age: 42±7.18years). Refractive state was defined using three standard criteria: minRMS, paraxial, and optimum image quality using the Visual Strehl ratio metric (VSOTF). Because of changes in spherical aberration, accommodative gain measured with the paraxial criterion is generally greater (on average by 34%) than the minRMS gain in early presbyopes. The slope of stimulus/response curve is relatively stable up to age 41 and then declines progressively in spite of a full binocular set of accommodative cues. Reduced accommodative gain appears once accommodative amplitude has dropped to below 3D, and gain drops to <0.5 as amplitude drops to 1.5D. This decline happens at different ages in different presbyopes. Prior to the early 40s, changes in accommodation are restricted to a reduction in amplitude, but during the 40s the continued loss of accommodative amplitude is accompanied by a concurrent drop in accommodative gain. Therefore, reduced near image quality in early presbyopes is caused by lower accommodative amplitudes and gains, which may explain the apparent acceleration in symptoms and near add prescriptions during the mid to late 40s.

Unsteady Performance of Rotating Detonation Engines with Different Exhaust Nozzles

In this paper, OpenFoam, an open source unsteady Reynolds-averaged Navier–Stokes software was carefully examined to analyze rotating detonation combustors. This tool was subsequently used to quantify the effect of the outlet conditions on five different exhaust nozzle geometries. Both detonation and deflagration of the hydrogen–air mixture were taken into account in the source terms of the species transport equation. The premixed –air mixture was injected at three different total pressures, ranging from 0.4 to 0.8 MPa. First, the rotating detonation combustor was described, and then five different nozzles were investigated: straight duct, conical, Bezier outer wall, Bezier inner wall, and two Bezier surfaces. All the nozzles were compared regarding their outlet total temperature, pressure gain, exit Mach number, and outlet flow angle. The use of a straight duct nozzle generated a noticeable drop in pressure gain of about 27% compared to the baseline combustor. The conical nozzle expanded the combustor outlet flow from sonic conditions into Mach 1.8 to 2.3. The Bezier outer wall underperformed compared to the conical nozzle at low combustor inlet pressures, while the situation was reversed at higher combustor inlet pressures. Using the inner wall as a divergent section showed no benefits in terms of Mach number and total pressure. The use of a nozzle with two Bezier surfaces did not provide any significant benefits in spite of the enhanced outlet-to-combustor-inlet-area ratio.

Wideband Folded Reflectarray Using Novel Elements With High Orthogonal Polarization Isolation

A wideband folded reflectarray antenna (FRA) operating in a frequency band of 11–15 GHz is proposed in this communication. It consists of novel dual-polarization elements with high orthogonal polarization isolation. A two-layer element structure is used to control the reflection phase of two orthogonally polarized waves independently and the element presents a large phase variation of about 700° and approximately linear phase curves. Meanwhile, the multifrequency phase matching method is applied in the FRA design. A 30% bandwidth for a 1-dB gain drop, a peak aperture efficiency of 49.1%, and sidelobe levels below −14 dB are achieved. Factors with strong influences on the aperture efficiency are discussed. The simulated results are experimentally verified by an FRA prototype fed by an open-ended waveguide.

Analytical modeling and numerical simulation of the short-wave infrared electron-injection detectors

This paper describes comprehensive analytical and simulation models for the design and optimization of the electron-injection based detectors. The electron-injection detectors evaluated here operate in the short-wave infrared range and utilize a type-II band alignment in InP/GaAsSb/InGaAs material system. The unique geometry of detectors along with an inherent negative-feedback mechanism in the device allows for achieving high internal avalanche-free amplifications without any excess noise. Physics-based closed-form analytical models are derived for the detector rise time and dark current. Our optical gain model takes into account the drop in the optical gain at high optical power levels. Furthermore, numerical simulation studies of the electrical characteristics of the device show good agreement with our analytical models as well experimental data. Performance comparison between devices with different injector sizes shows that enhancement in the gain and speed is anticipated by reducing the injector size. Sensitivity analysis for the key detector parameters shows the relative importance of each parameter. The results of this study may provide useful information and guidelines for development of future electron-injection based detectors as well as other heterojunction photodetectors.

Energy and exergy analysis in double-pass solar air heater

In this study, an attempt is made to improve the energy and exergy performance of solar air heater by employing double pass with different absorber surface geometries (roughened, finned, and v-corrugated wire mesh) in the second pass, and also by mounting longitudinal fins in the back side of the absorber plate (first pass). The effect of varied mass flow rate and solar intensity on temperature rise of air, energy efficiency, exergy gain and pressure drop at steady state condition was determined for different types of solar air heaters utilizing an indoor solar simulator. The temperature rise of air, thermal efficiency and exergy gain depends on mass flow rate, surface geometries of absorber and solar intensity, whereas the pressure drop depends on mass flow rate and surface geometries of absorber.

Experimental verification of gain drop due to general ion recombination for a carbon-ion pencil beam.

Accurate dose measurement in radiotherapy is critically dependent on correction for gain drop, which is the difference of the measured current from the ideal saturation current due to general ion recombination. Although a correction method based on the Boag theory has been employed, the theory assumes that ionized charge density in an ionization chamber (IC) is spatially uniform throughout the irradiation volume. For particle pencil beam scanning, however, the charge density is not uniform, because the fluence distribution of a pencil beam is not uniform. The aim of this study was to verify the effect of the nonuniformity of ionized charge density on the gain drop due to general ion recombination. The authors measured the saturation curve, namely, the applied voltage versus measured current, using a large plane-parallel IC and 24-channel parallel-plate IC with concentric electrodes. To verify the effect of the nonuniform ionized charge density on the measured saturation curve, the authors calculated the saturation curve using a method which takes into account the nonuniform ionized charge density and compared it with the measured saturation curves. Measurement values of the different saturation curves in the different channels of the concentric electrodes differed and were consistent with the calculated values. The saturation curves measured by the large plane-parallel IC were also consistent with the calculation results, including the estimation error of beam size and of setup misalignment. Although the impact of the nonuniform ionized charge density on the gain drop was clinically negligible with the conventional beam intensity, it was expected that the impact would increase with higher ionized charge density. For pencil beam scanning, the assumption of the conventional Boag theory is not valid. Furthermore, the nonuniform ionized charge density affects the prediction accuracy of gain drop when the ionized charge density is increased by a higher dose rate and/or lower beam size.

Impact of Trapping Effects on the Recovery Time of GaN Based Low Noise Amplifiers

This study investigates recovery time of the gain of AlGaN/GaN HEMT based low noise amplifiers (LNA) after an input overdrive pulse. Three LNAs, fabricated in two commercial MMIC processes and a Chalmers in-house process, are evaluated. The Chalmers process has an unintentionally doped buffer instead of the intentional Fe doping of the buffer which is standard in commercial GaN HEMT technologies. It is shown that the LNAs from the two commercial processes experience a severe drop in gain after input overdrive pulses higher than 28 dBm, recovering over a duration of around 20 ms. In contrast the LNA fabricated in-house at Chalmers experienced no visible effects up to an input power of 33 dBm. These results have impact for radar and electronic warfare receivers, which need to be operational immediately after an overdrive pulse. The long time constants suggest that these effects are due to trapping in the transistors with the Fe doped buffer playing an important role.

First in-beam studies of a Resistive-Plate WELL gaseous multiplier

We present the results of the first in-beam studies of a medium size (10 × 10 cm2) Resistive-Plate WELL (RPWELL): a single-sided THGEM coupled to a pad anode through a resistive layer of high bulk resistivity (∼109Ωcm). The 6.2 mm thick (excluding readout electronics) single-stage detector was studied with 150 GeV muons and pions. Signals were recorded from 1×1 cm2 square copper pads with APV25-SRS readout electronics. The single-element detector was operated in Ne/(5%CH4) at a gas gain of a few times 104, reaching 99% detection efficiency at average pad multiplicity of ∼1.2. Operation at particle fluxes up to ∼104 Hz/cm2 resulted in ∼23% gain drop leading to ∼5% efficiency loss. The striking feature was the discharge-free operation, also in intense pion beams. These results pave the way towards robust, efficient large-scale detectors for applications requiring economic solutions at moderate spatial and energy resolutions.

Search for photon bubble oscillations in V0332+53

We report results of our search for fast oscillations in lightcurve of one of the brightest accretion powered pulsars on the sky V0332+53 with the help of data of the PCA spectrometer of the RXTE observatory. In course of this search we have carefully explored complications appearing if one uses only sub-bands of the total bandpass of the PCA spectrometer. We show that lightcurves collected in the soft sub-band of the PCA spectrometer contains an additional instrumental noise, lightcurves of harder sub-bands lack some fraction of the anticipated Poisson noise. We show that this noise is caused by a cross-talk of energy bands, which lasts up to ~200usec. One hypothesis is that these effects are caused by temporarily drop of the PCA detector gain after any occurred event due to slowly moving ions in the detector volume. In order to avoid this effect we searched for fast oscillations in flux of V0332+53 only in the total bandpass of the PCA spectrometer 2-60 keV. We have not detected any quasi-periodic oscillations in lightcurve of the source with an upper limit at the level of 0.5% in the Fourier frequency range 200-1500 Hz.

Hydrodynamic Focusing of an Elastic Capsule in Stokes flow: An Exploratory Numerical Study

We study numerically the motion of an elastic capsule in a simple microfluidic device, a sheath flow focuser, designed to align (or focus) particles at the center of a microchannel. The geometry of the device is given, while the flow conditions are varied, and the outflux is constant. The efficiency is expressed in terms of the distance required to achieve focusing and the functioning is characterized by the deformations and stresses undergone by the capsule membrane. Calculations are performed for a ‘stiff’ and ‘soft’ capsule, corresponding to a capillary number equal to Ca = 0.05 and Ca = 0.3 based on the outflow rate. We report that as the sheath flow is increased the focusing efficiency grows and plateaus beyond a certain threshold, displaying a drop in the efficiency gain. Differently, the deformations and stresses undergone by the capsule membrane grow for the entire range of the flow parameters considered. This should be kept in mind if the stresses exerted on the membrane have to be minimized or need to be below certain characteristic thresholds typical of the specific application considered. Especially large initial offsets and stiff capsules benefit from focusing.

Centre of pressure or centre of mass feedback in mediolateral balance assessment

The mediolateral balance assessment method (MELBA) consists of tracking a sinusoidal or multisine target with the center of pressure as feedback (CoPfb). The aim of the CoP trajectory is to elicit weight-shifting, i.e. movement of the center of mass (CoM). However, it is not known whether CoPfb elicits consistent mediolateral displacements of the CoM, whether CoM feedback (CoMfb) is required to achieve this and whether CoPfb or CoMfb elicit different kinematic strategies.The aims of this study were to determine (1) the extent to which CoP imposes CoM displacements (CoMd) during CoPfb, (2) whether larger CoMd are elicited by CoMfb and (3) whether different kinematic strategies arise when using CoPfb or CoMfb. Nineteen young adults performed MELBA with CoPfb and CoMfb from which coherence, gain and phase-shift between CoP–CoM and leg–trunk kinematics were calculated. CoMd and CoPd and leg and trunk excursions were also calculated.Results show that for CoPfb tasks, CoP–CoM coherence was high, while the gain dropped with increasing frequency. The drop in gain was highly consistent between subjects. Reasonable trunk–leg coherence (≈.6) was found over all frequencies and tasks. The leg–trunk angle gain increased with frequency in all tasks and was significantly higher in the CoMfb compared to the CoPfb. Significant interaction indicated that this difference increased with frequency.CoPfb in MELBA elicits consistent CoMd. However, different kinematics are employed in CoMfb with more trunk movement and an ankle-to-hip shift as frequency increases. Hence CoMfb may be preferable over CoPfb despite the larger measurement effort involved.

A novel solution to the gated x-ray detector gain droop problem.

Microchannel plate (MCP), microstrip transmission line based, gated x-ray detectors used at the premier ICF laser facilities have a drop in gain as a function of mircostrip length that can be greater than 50% over 40 mm. These losses are due to ohmic losses in a microstrip coating that is less than the optimum electrical skin depth. The electrical skin depth for a copper transmission line at 3 GHz is 1.2 μm while the standard microstrip coating thickness is roughly half a single skin depth. Simply increasing the copper coating thickness would begin filling the MCP pores and limit the number of secondary electrons created in the MCP. The current coating thickness represents a compromise between gain and ohmic loss. We suggest a novel solution to the loss problem by overcoating the copper transmission line with five electrical skin depths (∼6 μm) of Beryllium. Beryllium is reasonably transparent to x-rays above 800 eV and would improve the carrier current on the transmission line. The net result should be an optically flat photocathode response with almost no measurable loss in voltage along the transmission line.

A Folded Reflectarray Antenna With a Planar SIW Slot Array Antenna as the Primary Source

An integrated design of a planar source-fed folded reflectarray antenna (FRA) with low profile is proposed to achieve high gain for Q-band millimeter wave high data rate wireless communication applications. A planar substrate integrated waveguide (SIW) slot array antenna is adopted as the primary source to illuminate the proposed FRA. Considerations of the off-focus loss and the asymmetry E- and H-plane beamwidths loss of the planar source are thoroughly investigated to improve the aperture efficiency of the reflectarray antenna. A prototype is implemented and the experimental results are in good agreement with the predictions, which exhibit a measured |S 11 | less than -10 dB over the frequency band of 41.6-44 GHz, a maximum boresight gain of 31.9 dBi at 44 GHz with the aperture efficiency of 49% and a -3 dB gain drop bandwidth of 41.8-44.9 GHz. By integrating with the planar feed, the proposed folded reflectarray antenna benefits from its low-profile, low cost, and the ability of coplanar integration with millimeter wave planar circuits while maintaining the good performance in terms of gain and efficiency.

Beam Tilting Antenna Using Integrated Metamaterial Loading

This communication presents a technique to re-direct the radiation beam from a planar antenna in a specific direction with the inclusion of metamaterial loading. The beam-tilting approach described here uses the phenomenon based on phase change resulting from an EM wave entering a medium of different refractive index. The metamaterial H-shaped unit-cell structure is configured to provide a high refractive index which was used to implement beam tilting in a bow-tie antenna. The fabricated unit-cell was first characterized by measuring its S-parameters. Hence, a two dimensional array was constructed using the proposed unit-cell to create a region of high refractive index which was implemented in the vicinity bow-tie structure to realize beam-tilting. The simulation and experimental results show that the main beam of the antenna in the E-plane is tilted by 17 degrees with respect to the end-fire direction at 7.3, 7.5, and 7.7 GHz. Results also show unlike conventional beam-tilting antennas, no gain drop is observed when the beam is tilted; in fact there is a gain enhancement of 2.73 dB compared to the original bow-tie antenna at 7.5 GHz. The reflection-coeflicient of the antenna remains <; - 10 dB in the frequency range of operation.

(Invited) (ECS Electronics & Photonics Division Award )Wafer-Level Step-Stressing of InGaP/GaAs HBTs

Since InGaP/GaAs heterojunction bipolar transistors (HBTs) are utilized in a wide variety of RF and other applications, a great deal has been learned about their reliability. Nevertheless, this knowledge is limited by the fact that most reliability studies heavily emphasize the stress-related evolution of a single parameter, the DC current gain, beta. We have found that interrupted stressing experiments, with complete characterization of HBTs during the interruption, can give a more complete picture of the degradation that occurs during bias stressing of HBTs. We have previously correlated electrical signatures with degradation in step-stressing of packaged InGaP/GaAs HBTs [1]. In this work we will present some results of step-stress experiments which have been conducted at the wafer level for InGaP/GaAs HBTs. Wafer-level evaluation is advantageous due to the potential for greater automation and flexibility in the testing as well as the ability to identify problems at an earlier stage in the process.The HBTs are stressed at room temperature at a constant collector voltage. Stressing is initiated at a given base current, held for a fixed time, which is typically 10 s. The applied stress is stepped up to a higher level of base current and the process is iterated until a failure is recorded. Full DC parametric electrical characterization of the HBT gets carried out initially and after certain predefined stress steps. Although the stress is typically carried near room temperature, it is understood that the junction of the HBT is effectively self-heated and can reach extremely high temperatures.A certain set of degradation processes are responsible for stress-induced failures and these occur singly or in combinations. In addition, many HBTs within a wafer and among duplicate wafers have not only a number of common failure modes, but a tendency towards a consistently realized destructive power limit. The consistency of these results and the utility of electrical signature analysis lead us to conclude that wafer-level step-stressing is an excellent tool for quickly monitoring the HBT quality. We have also found many commonalities in the wafer-level and package level results for the ceramic packages used in a previous study [1].The degradation of HBTs by these stressing methods leads eventually to a large drop in current gain. Thus, the interrupted step stressing method arrives at the same end state as continuous, single-parameter monitoring, but with much more clarity about the degradation processes that occur on the way to the end state. The interrupted step-stress process in combination with electrical signature analysis is very likely to find the weak link in an HBT process. We will discuss the utility of the technique in terms of the opportunities to ruggedize the process. It is also possible that the identified weak link is fundamental to the technology or inconsequential in terms of meeting reliability requirements. In that case, wafer-level step-stressing is still an excellent tool for early identification of quality issues that may affect reliability through monitoring the consistency (or lack thereof) in degradation observed across different wafers over different periods of time. A.G. Baca, A.J. Scruggs, A. Gorenz, T.R. Fortune, J.F. Klem, R.D. Briggs, J.B. Clevenger, G.A. Patrizi, and C.T. Sullivan, “A Survey of Electrical Signatures Characteristic of Step-Stressed InGaP/GaAs HBTS,” ECS Transactions vol. 50 (6), pp. 273-282 (2012). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Single-layer reflectarray with novel elements for wideband applications

A novel single-layer reflectarray element with three resonances, showing a linear phase curve and a large phase variation range over 800° is proposed for a broadband operation. Different size of elements are selected by using a simple guideline to achieve the required phase distribution in a given frequency band. Two offset-fed 10 × 10 reflectarrays with different scattering angles are designed. Simulations and measurements show the bandwidth of 28% for a 3-dB gain drop and 44% for −10-dB side lobe level inspite of the large F/D of 2.1. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:950–954, 2014

A 2-D Multibeam Half Maxwell Fish-Eye Lens Antenna Using High Impedance Surfaces

This letter describes the design and measurement of a planar multibeam half Maxwell fish-eye (HMFE) lens antenna with seven beams in azimuth based on variable high impedance surfaces (HISs). The desired refractive index profile is controlled by using slotted square mushroom metallic cells of different sizes on a grounded dielectric slab. The lens antenna, 6.2λ×3.1λ in lens plane and 0.65 λ total height, is fabricated and characterized in TM mode at 13 GHz for both single- and multiple-beam configurations. The optimized lens for single-beam operation achieves 14.1 dBi measured gain, 67 ° and 12 ° half-power beamwidth for E-plane and H-plane, respectively, and 55.3% impedance bandwidth for -10-dB return loss. This lens can be used to launch multiple beams by implementing an arc array of seven-element short monopoles at the periphery of the lens. The measured overall scan coverage is up to 45 ° with gain drop less than 2.4 dB. Its low profile, light weight (only 136 g), and ease of manufacturing makes it well suited for Local Multipoint Communication Systems (LMCS).

Driver's Arms' Time-Variant Neuromuscular Admittance During Real Car Test-Track Driving

Attempts to measure and model driver steering behavior have been so far mainly performed with driving simulators and time-invariant techniques. The goal of this paper was to quantify the driver's arms' time-variant admittance in real driving and to provide a range of parametrically fitted values on the estimated frequency response functions. The human arms' neuromuscular (NMS) admittance was estimated by applying torque disturbances on the steering wheel during real car test-track driving. To capture the time-variant behavior, the admittance was estimated using a 1.28-s sliding time window. The results showed that drivers adapt their admittance while cornering, exposing a variant behavior during different corners and driving speeds. The frequency response function (FRF) of the admittance while cornering has the properties of a second-order system. During cornering, drivers have increased stiffness values, whilst in straight driving, the FRFs resemble a second-order system ( -40 dB/decade gain drop; double pole at low frequencies) for low frequencies, with a zero for frequencies above 6 Hz (on average). The FRFs during cornering were parametrically fitted to a second-order inertia-spring-damper model. The fitted parameter values can be used for NMS driver models and motivate the stability analysis of the combined closed-loop driver steering system.

Mid-infrared Raman amplification and wavelength conversion in dispersion engineered silicon-on-sapphire waveguides

Raman amplification based on stimulated Stokes Raman scattering (SSRS) and wavelength conversion based on coherent anti-Stokes Raman scattering (CARS) are theoretically investigated in silicon-on-sapphire (SOS) waveguides in the mid-infrared (IR) region. When the linear phase mismatch Δk is close to zero, the Stokes gain and conversion efficiency drop down quickly due to the effect of parametric gain suppression when the Stokes-pump input ratio is sufficiently large. The Stokes gain increases with the increase of Δk, whereas efficient wavelength conversion needs appropriate Δk under different pump intensities. The conversion efficiency at exact linear phase matching (Δk = 0) is smaller than that at optimal linear phase mismatch by a factor of about 28 dB when the pump intensity is 2 GW cm−2.