Steven A. Lane

Reconfigurable Antennas for Wireless and Space Applications

Reconfigurable antennas, with the ability to radiate more than one pattern at different frequencies and polarizations, are necessary in modern telecommunication systems. The requirements for increased functionality (e.g., direction finding, beam steering, radar, control, and command) within a confined volume place a greater burden on todays transmitting and receiving systems. Reconfigurable antennas are a solution to this problem. This paper discusses the different reconfigurable components that can be used in an antenna to modify its structure and function. These reconfiguration techniques are either based on the integration of radio-frequency microelectromechanical systems (RF-MEMS), PIN diodes, varactors, photoconductive elements, or on the physical alteration of the antenna radiating structure, or on the use of smart materials such as ferrites and liquid crystals. Various activation mechanisms that can be used in each different reconfigurable implementation to achieve optimum performance are presented and discussed. Several examples of reconfigurable antennas for both terrestrial and space applications are highlighted, such as cognitive radio, multiple-input-multiple-output (MIMO) systems, and satellite communication.

Active Structural-Acoustic Control of a Rocket Fairing Using Proof-Mass Actuators

The feasibility of using proof-mass actuators to control noise transmission actively into a small rocket fairing, given practical constraints on actuator power and mass, is explored. The modal-interaction approach was used to develop a fully coupled structural-acoustic state-space model that relates the out-of-plane structural modal velocities to the spatially varying pressure response in the cavity. The dynamics of the proof-mass actuators were included in the structural-acoustic model. The modal-interaction approach also allowed the decomposition of the acoustic response into radiation modes, which proved essential for determining the optimal locations for sensors and actuators. Numerical simulations using linear quadratic Gaussian controllers with collocated proof-mass actuators and displacementsensorsdemonstrated approximately 4.2 dB of attenuation overthe300-Hz bandwidth forthegivenactuatorconstraints.However,thiswasonly slightly morethantheattenuationprovidedby thepassive effects of the proof-mass actuators, which was approximately 3.5 dB.

CubeSat Deployable Antenna Using Bistable Composite Tape-Springs

In this letter, a new conductive composite tape-spring is proposed for CubeSat deployable antennas that is constructed using a glass fiber reinforced epoxy with an embedded copper alloy conductor. The tape-spring is bistable enabling the antenna to be elastically stable in both the deployed and stowed states. A dipole antenna is designed, simulated, and tested to prove the viability of the electrical properties of this material.

Dynamic Modeling and Experimental Validation of a Cable- Loaded Panel.

Power and signal cable harnesses on spacecraft are often at 10% of the total mass and can be as much as 30%. These cable harnesses can impact the structural dynamics of spacecraft significantly, specifically by damping the response. Past efforts have lookedat how to calculate cable properties and the validation of these cablemodels on onedimensional beam structures with uniform cable lengths. This paper looks at how to extend that process to twodimensional spacecraftlike panels with nonuniform cable lengths. A shear beam model is used for cable properties. Twomethods of calculating the tiedown stiffness are compared.Of particular interest is whether or not handbooks of cable properties can be created ahead of time and appliedwith confidence. There are three frequency bands in which cable effects can be described. Before any cables become resonant, the cable effects are dominated bymass and static stiffness. After all the cables become resonant, the effect is dominated by increased damping in the structure. In between these two frequency cutoff points, there is a transition zone. Thedynamic cablemodelingmethod is validated as a distinct improvement over the lumped-mass characterization of cables commonly used today.

Modeling and Experimental Validation of Space Structures with Wiring Harnesses

Powerand data-handling cables, which can account for up to 30% of a satellite’s dry mass, couple with the spacecraft structure and impact dynamic response. Structural dynamicmeasurements suggest that amore complete representation of cable effects is needed to improvemodel predictive accuracy. To that end, a studywas performed to characterize cable harness impacts on dynamic response. From this study, a finite element modeling method supported by empirically determined cable properties and structural behaviorwas developed. Themodelingmethod was validatedwith a considerable amount ofmodel simulation and experimental data for a variety of cables attached to a free–free beam. At low frequencies, the cable effect was dominated bymass and stiffness, changing the apparent stiffness; damping was a secondary effect. At higher frequencies, where the cables themselves were resonant, the cable effect was dissipative, increasing the apparent damping in addition to affecting the overall frequency response. Tiedown stiffness was found to be an important, but difficult to measure, parameter. Finite element models of a cabled beamwere shown to be valid for all cable families studied.As a result, thefinite elementmodelingmethod itself was validated.

Experimental Techniques and Structural Parameter Estimation Studies of Spacecraft Cables

Signal and electrical power cables pose unique challenges to spacecraft structural design and are often poorly modeled or even neglected. The objective of this research was to develop test methods and analysis techniques to accurately model cable-loaded spacecraft, using linear finite element models. Test methods were developed to characterize cable extensional and bending properties when subjected to low-level lateral dynamic loads. Timoshenko beam theory, including shear and bending, was used to model cable lateral dynamics, and the model formulation applicability was validated through experiment. An algorithm was developed to estimate cable area moment of inertia and shear area factor, shear modulus product, from a single driving point mobility function. Test methods and the parameter estimation algorithm were validated, using metallic rod test specimens. Experiments were performed on cables of differing constructions and spans, to develop a database for finite element modeling validation experiments.

Optical Space-Time Division Multiple Access

This work presents an approach to multiple access for free-space laser communication (lasercom) called space-time division multiple access, which aggregates traffic from multiple users at the network edge. The objective is to share resources to lower the cost, size, weight, and power consumption per user, thereby making lasercom feasible for users that require only moderate average information rates. This concept relies on fast, agile electronic beam steering, which was implemented in this investigation using liquid crystal optical phased arrays. We designed and built an experimental terminal incorporating a bidirectional communication aperture that was shared among the users, and two independently operated acquisition and tracking apertures. Using two remote user terminals, experiments were conducted to measure access node performance for a variety of operating conditions traceable to anticipated applications. The transmit and receive directions of the downlink and uplink communications channels were rapidly hopped between the two users, and data were exchanged between the access node and a user while the optical channel dwelled on the latter. Results showed that the measured information throughput efficiency correlated well with model predictions and was high enough to realize the expected advantages in applications with many users. Throughput efficiencies, defined as the actual data throughput as a percentage of the throughput without multiple access, exceeded 85% for dwell times of 100 ms and greater. This translates into an average information rate of 400 Mb/s for as many as 20 simultaneous users. Current optical phased arrays are capable of providing fast transitions between remote users, with values measured in the range 10-18 ms. The use of persistent tracking links was a key factor in achieving fast transitions, and it was found that motion of the remote terminals had no significant impact on performance.

Chamber Core Structures for Fairing Acoustic Mitigation

Abstract : The U.S. Air Force Research Laboratory is pursuing an innovative composite structure design called chamber core for constructing launch vehicle payload fairings. A composite chamber core fairing consists of many axial tubes sandwiched between face sheets, tubes that can be used as acoustic dampers to reduce low-frequency interior noise with virtually no added mass. This paper presents the results of experimental studies of noise transmission through a 1.51 m diameter x 1.42 m tall chamber core cylinder. It was tested in a semireverberant acoustics laboratory using band-limited random noise at sound pressure levels up to 110 dB. The bare cylinder provided approximately 12.7 dB of attenuation over the 0-500 Hz bandwidth and 15.3 dB over 0-2000 Hz. The noise reduction increased to over 18 dB for both bandwidths with the axial tubes acting as acoustic dampers. Narrowband reductions in excess of 15 dB were measured around specific acoustic resonances. This was accomplished with virtually no added mass to the composite cylinder. Results were compared with the performance provided by a 2.5 cm acoustic blanket treatment. The acoustic dampers were as effective as the acoustic blanket at low frequency, but not at higher frequencies. The acoustic dampers were better able to couple with and damp the low-frequency acoustic modes. Together, the acoustic blanket and dampers provided over 10 dB more noise reduction over the 2000 Hz bandwidth than the bare cylinder.

Evacuated enclosure mounted acoustic actuator and passive attenuator

This invention presents a novel means to passively achieve a compact moving-coil actuator with a very low natural frequency. The diaphragm and voice coil are mounted in a sealed enclosure from which the air is partially or completely evacuated. This reduces the air spring effect. The diaphragm is supported by a non-linear, buckling, or collapsible support apparatus. By taking advantage of the non-linear stiffness properties of such structures, the stiffness of the actuator can be designed to be small at the operating point, which when combined with the moving mass, yields a low natural frequency.

Active Acoustic Control of a Rocket Fairing Using Spatially Weighted Transducer Arrays

Apreliminarystudy,includingexperimentalresultsforanovelactiveacousticcontrolapproachtoreducethelowfrequency modal response in a rocket fairing, is presented. The control method uses spatially weighted transducer arrays with H2 feedback control laws to attenuate globally the targeted acoustic modes. The nature of the fairing acoustic problem is described, the theory of the control approach is discussed, and important feasibility issues regarding the actual implementation of the control method are presented. Several controllers were implemented on a full-scale composite model of a small rocket fairing. The results demonstrate that the controller was able to reduce the response of the low-frequency modes by 6 ‐12 dB with very little spillover. In addition, a spatially averaged reduction of the acoustic response of the fairing interior in excess of 3 dB over the 20 ‐200-Hz bandwidth was demonstrated. Thefeasibility studies indicate that limitations on actuatorpowerand volumetric displacement under actual launch conditions are not necessarily prohibitive, but may be satise ed with continued development of actuator technology and placement optimization.