SiC microwave power technologies

Two SiC transistors that are investigated for microwave power applications are the 4H-SiC static induction transistor (SIT) and the 4H-SiC metal-semiconductor field-effect transistor (MESFET). Ultrahigh frequency 4H-SiC SITs have demonstrated record-breaking pulsed power per package (900 W) with excellent associated power-added efficiency (PAE) of 78%. S band 4H-SiC MESFETs have shown a record power-density of 5.6 W/mm and 36% PAE, as well as 80 W continuous-wave (CW) power (1.6 W/mm), with an associated PAE of 38%. X-band MESFET power density of 4.3 W/mm was obtained for exploratory CW devices. These performance gains are afforded by the advantageous material properties of silicon carbide. SiC SIT technology offers many military system advantages including lower cost, lower weight, higher power and high temperature of operation and higher efficiency transmitters with minimal cooling requirements. SiC RF MESFET's and circuits are candidates for use in efficient linear transmitters for commercial and military communications.     

On the Continuity of High Efficiency Modes in Linear RF Power Amplifiers

A novel formulation for the voltage waveforms in high efficiency linear power amplifiers is described. This formulation demonstrates that a constant optimum efficiency and output power can be obtained over a continuum of solutions by utilizing appropriate harmonic reactive impedance terminations. A specific example is confirmed experimentally. This new formulation has some important implications for the possibility of realizing broadband >10% high efficiency linear RF power amplifiers.     

Millimeter wave imaging lens antenna

The factors which influence the design of lenses suitable for imaging at millimetre wavelengths are discussed. Examples are given of optical designs for a military tracking application, firstly for use with an open ended waveguide array, and secondly for use with monolithic arrays, each covering a field of view of ±20°.     

CONTINUOUS FERMENTATION OF GLUCOSE SOLUTIONS

Use of a continuous-flow plug fermentor to ferment glucose solutions was found to be possible only with unrefined commercial glucose and not with pure glucose. Even when the former was used problems were encountered due to a gradual increase in back pressure across the yeast bed. With the commencement of glucose metabolism in the yeast bed, ‘shock’ excretion of potassium and magnesium ions as well as of low molecular weight nitrogenous material was observed. The product from the fermentor contained an abnormally high level of α-diketones viz., 1–3 mg/litre.     

Probabilistic Discovery of Semantically Diverse Content in MANETs

Mobile ad hoc networks rely on the opportunistic interaction of autonomous nodes to form networks without the use of infrastructure. Given the radically decentralized nature of such networks, their potential for autonomous communication is significantly improved when the need for a priori consensus among the nodes is kept to a minimum. This paper addresses an issue within the domain of semantic content discovery, namely, its current reliance on the preexisting agreement between the schema of content providers and consumers. We present OntoMobil, a semantic discovery model for ad hoc networks that removes the assumption of a globally known schema and allows nodes to publish information autonomously. The model relies on the randomized dissemination and replication of metadata through a gossip protocol. Given schemas with partial similarities, the randomized metadata dissemination mechanism facilitates eventual semantic agreement and provides a substrate for the scalable discovery of content. A discovery protocol can then utilize the replicated metadata to identify content within a predictable number of hops using semantic queries. A stochastic analysis of the gossip protocol presents the different trade-offs between discoverability and replication. We evaluate the proposed model by comparing OntoMobil against a broadcast-based protocol and demonstrate that semantic discovery with proactive replication provides good scalability properties, resulting in a high discovery ratio with less overhead than a reactive nonreplicated discovery approach.     

Dielectric material modeling in the MoM-based code FEKO

The majority of structures of electromagnetic interest contain regions of non-metallic materials. The applicability of a numerical analysis tool is greatly reduced by an incapability or limitation in treating dielectric regions. The treatment of propagation medium properties is inherent in the formulation of field-based analysis tools. In the method of moments (MoM), dielectric regions require extended formulations and additional resources to model. A number of techniques for the optimum representation of these regions has been implemented in FEKO, a commercial MoM-based analysis tool. Each of the available techniques within the MoM have certain applications to which they are better suited. The optimum technique to use is often immediately evident to the inexperienced user. This paper briefly introduces each of the available techniques, and then demonstrate, by way of an example, how the choice of technique can influence the efficiency and accuracy of simulation.     

Printing Reconfigurable Bundles of Dielectric Elastomer Fibers

Active soft materials that change shape on demand are of interest for a myriad of applications, including soft robotics, biomedical devices, and adaptive systems. Despite recent advances, the ability to rapidly design and fabricate active matter in complex, reconfigurable layouts remains challenging. Here, the 3D printing of core-sheath-shell dielectric elastomer fibers (DEF) and fiber bundles with programmable actuation is reported. Complex shape morphing responses are achieved by printing individually addressable fibers within 3D architectures, including vertical coils and fiber bundles. These DEF devices exhibit resonance frequencies up to 700 Hz and lifetimes exceeding 2.6 million cycles. The multimaterial, multicore-shell 3D printing method opens new avenues for creating active soft matter with fast programable actuation.     

Experimental Study of Capture Effect for Medium Access Control with ALOHA

In this paper, we investigate the capture effect through experiments conducted with Iris nodes equipped with AT86RF230 radio transceivers. It is shown that the first arriving packet in a collision can capture the radio channel for equal power transmissions and may be decoded depending on the amount of overlap. A new 3‐packet‐capture scenario is introduced and implemented. To be able to understand the impact of capture on the throughput performance of wireless sensor networks, we present an analysis of the capture coefficient using our practical results. For real‐world implementations, the throughput of pure ALOHA considering a finite number of users is presented in analytical form. The capture coefficient is then applied to pure ALOHA as a case study. Using analytical and practical implementations of the capture effect on ALOHA, a very good match in channel throughput performance enhancement is demonstrated over the non‐capture effect case. TinyOS‐2.x is used to program the nodes and to observe data exchange on a computer through a base station.     

Quantitative ECT: Comparison of Recovery Coefficient and Linearity of Detector Response for Single Photon and Coincidence Detection (F-18, B+ 511 keV)

The response characteristics of two tomographic systems were compared for imaging of positron emitters: a) a SPECT system with a 3/8 in crystal and 511 keV detector shielding, equipped with a specially designed 511 keV collimator, and b) a PET V system using coincidence detection. SPECT transverse plane resolution was 19 mm FWHM and 35 mm FWTM for a radius of rotation of 16 cm. Corresponding resolution for PET was 14 mm FWHM and 28 mm FWTM. Transverse images through a phantom containing cylindrical sources of various cross sections and uniform activity were obtained for each detector. The measured count density or recovery coefficient was found to decrease with source size, the dependence being similar for both systems. The theoretical values for recovery coefficients were calculated by convolution of a Gaussian fit to the SPECT resolution (FWHM, FWTM) values with the uniform cross section of each source. This simple mathematical model confirmed that the recovery coefficient dependence on source size was primarily related to the limited resolution of the detector. Experimental measurements demonstrated that the SPECT resolution for volume sources was sufficient for quantitation, although some limitations exist with respect to source sizes smaller than the detector resolution.