No description available.

No description available.

No description available.

No description available.

Large phased array antennas are theoretically capable of delivering the directive gain of similarly sized aperture antennas while offering electronic beam scanning capabilities and greater operational flexibility. Unfortunately, the high cost associated with large phased antenna arrays has limited their use to highly specialized applications where no other antenna system configuration is possible. The recent development of less expensive microwave active devices has led to a renewed interest in large phased antenna arrays. These devices allow the amplification and signal processing required in phased antenna arrays to be distributed among many identical modules which combine the amplification, feed network, and radiating element sections of traditional antenna arrays. These modules can then be produced at a lower unit cost and result in an antenna system which is more easily integrated and repaired. The practicality of large phased antenna arrays is still limited by the great difficulty experienced in predicting their performance. Mutual coupling effects between the radiating elements produce significant variations from ideal array theory. The prediction and reduction of these effects requires a characterization approach which is computational rather than experimenta1. This document presents a new approach which allows the characterization of arbitrarily shaped planar radiating elements printed on a dielectric support slab backed by a perfect electric conductor ground plane. This analysis approach uses a Method of Moments technique to determine the electric current distribution over a set of bi-triangular sub-domain elements describing a single radiating element. The effects of mutual coupling in the fully active infinite antenna array are included in the analysis by a Floquet mode based Green's function used in the Method of Moments analysis. This characterization technique has been implemented in the computational electromagnetics code ASIA (Analysis Software for Infinite Arrays), The analysis approach presented here is validated by comparison with published input impedance data for two different radiating elements. Finally, preliminary analysis results are shown for a more complex radiating element. / Ph. D.

This dissertation considers the discrete-time control of a spacecraft consisting of a rigid-platform with retargetable flexible antennas. The mission consists of independent minimum-time maneuvers of each antenna to coincide with pre-determined lines of sight, while the platform is stabilized in an inertial space and elastic vibration of the antennas is suppressed. The system is governed by a set of linearized, time-varying equations of motion. A discrete-time approach permits consideration of the time-varying nature of the system in designing the control law. Both global and decentralized controls are proposed for a noise-free system with full-state feedback. Initially, a time-varying linear-quadratic regulator (LQR) is implemented, followed by two types of decentralized controllers. First, a collocated control law is devised in which actuator forces are based on the position and velocity at the actuator locations. Next, a new method called Substructure-Decentralized Control is proposed, where each flexible substructure is controlled based on state measurements associated with the substructure modes of the separately modeled appendages. In both global and decentralized cases, a linear control law is first implemented coupled with an open-loop disturbance-accommodating control based on the known inertial disturbances caused by the maneuver. Elastic motion is next controlled using nonlinear (on-off) antenna controllers for each decentralized case. For Substructure-Decentralized Control, the controls translate into quantized actual controls. Lastly, nonlinear (on-off) control laws are also used to control the rigid-body motion for each case. Next, the problem of controlling the time-varying system in the presence of noisy actuators and sensors is examined. It is assumed that only displacements, not velocities, are sensed for both rigid-body and elastic motion, making state reconstruction also necessary. A discrete-time, full-order Kalman filter is constructed for the time·varying system. A pseudo-decentralized control is proposed whereby feedback controls are based on system state estimates. As before, both linear and nonlinear controls are implemented. For each case mentioned, a numerical example is presented involving a spacecraft with a single flexible maneuvering antenna. / Ph. D.

Use of Slotted Waveguide Antenna Stiffened Structures (SWASS) in future commercial and military aircraft calls for the development of an airworthiness certification procedure. The first step of this procedure is to provide a computationally low-cost method for modeling waveguide antenna arrays on the scale of an aircraft skin panel using a multi-fidelity model. Weather detection radar for the Northrop Grumman X-47 unmanned air system is considered as a case study. COMSOL Multiphysics is used for creating high-fidelity waveguide models that are imported into the MATLAB Phased Array Toolbox for large-scale array calculations using a superposition method. Verification test cases show that this method is viable for relatively accurate modeling of large SWASS arrays with low computational effort. Additionally, realistic material properties for carbon fiber reinforced plastic (CFRP) are used to create a more accurate model. Optimization is performed on a 12-slot CFRP waveguide to determine the waveguide dimensions for the maximum far-field gain and separately for the maximum critical buckling load. Using the two separate optima as utopia points, a multi-objective optimization for the peak far-field gain and critical buckling load is performed, to obtain a balance between EM performance and structural strength. This optimized waveguide is then used to create a SWASS array of approximately the same size as an aircraft wing panel using the multi-fidelity modeling method that is proposed. This model is compared to a typical conventional weather radar system, and found to be well above the minimum mission requirements. / Master of Science / Antennas used in military and commercial aircraft have traditionally been designed independently from the aircraft structure. Increasingly, e↵ort has been made to integrate these processes, in order to create more efficient, dual-purpose structures. Slotted waveguide antennas, hollow rectangular tubes with slots cut in one face, are commonly used to create arrays for aircraft on-board weather radar. A type of structurally embedded antenna, slotted waveguide antenna stiffened structures (SWASS), consists of slotted waveguides that are sandwiched between two layers of a composite material. This sandwich structure can be used in place of the conventional structure used for aircraft skin, allowing the slotted waveguides to function not only as antennas, but also as part of the aircraft’s load-bearing structure. Because of the geometric complexity of the slotted waveguides, generating accurate models of the antenna performance can be difficult and requires a great deal of computational power. This thesis presents and validates a method for reducing the complexity of modeling the antenna performance of SWASS arrays. Additionally, optimizations are performed to improve both the waveguide’s performance as an antenna and as a load-bearing part of the aircraft structure. Finally, the optimized SWASS array is compared to the actual mission requirements of the Northrop Grumman X-47 unmanned aircraft, and is found to perform above the required levels.

Helices are broadband antennas that provide moderate gain, largely real input impedance and circular polarization when operating in the axial mode regime. A modified form of the helix, the spherical helix, has been shown to yield similar polarization and gain characteristics over a narrow bandwidth but a much larger beamwidth. This investigation examines multifilar hemispherical helices and produces two specific designs with some desirable radiation characteristics. The side-fed quadrifilar helix and the top-fed bifilar helix are the new hemispherical designs which are studied in detail both numerically and experimentally. The Numerical Electromagnetics Code 4 is used to analyze the radiation characteristics of the proposed multifilar hemispherical helices. Directivity, E- and H-plane radiation patterns, axial ratio and input impedance of a few example designs are calculated. Measurements, taken in the anechoic chamber at the Virginia Tech antenna range, generally confirmed the simulation results. It is shown that the bifilar hemispherical helix provides a flat gain curve over the measured bandwidth (~14%) and generally elliptical polarization with near circular polarization in limited case. / Master of Science

This master thesis aimed to gain knowledge about the field of "adaptive array antenna systems for mobile satellite communication" and develop algorithms to control hardware equipment. We were able to set up and configure a beamformer to receive radio frequency (RF) signals using an array antenna. To do this we tested several development environments before settling on Python script programming using the Thonny editor. We created several plots of antenna patterns using both rectangular and polar coordinate systems, and developed Python code to generate animations and vector figures of the results. We also used different algorithms to control the pointing direction of the array antenna and examine the RF signal strength under various conditions. The results of our work can be used in future research to increase the robustness and flexibility of satellite communication and help provide redundancy against communication disruptions. To apply the findings of this master thesis more closely to a real-life application, we suggest using an field-programmable gate array (FPGA) to calculate and control beamforming, as the Raspberry Pi 4 computer used in this project is too slow for real-time processing. We also recommend using 2D-arrays instead of the 1D-array used in this project, which would allow for control of the beam lobe in both azimuth and elevation. Another suggestion is to build smaller blocks of 2D-array antennas connected with synchronized clocks into larger antenna arrays, which would result in improved performance. This was a challenging and rewarding project that provided valuable insights into a topic with many potential future applications within e.g. satellite communications, radar technology and mobile telephony.

Antenna arrays offer notable superiority over a single antenna element. By weighting the signals before combining them, antenna arrays offer several enhanced features such as beam steering or beam switching without physically moving the aperture. While in general the array platform can take any given shape, cylindrical arrays offer many advantages compared to linear and planar arrays due to their azimuthal symmetry, and as such find numerous applications in radar, sonar, etc.. This research proposes a Wi-Fi band cylindrical switched beam array that is capable of full azimuthal beam switching for direction finding applications. Six microstrip patch antennas are arranged in a hexagonal platform to create a cylindrical array. The antenna beam is switched electronically using a SP6T RF switch providing azimuthal coverage with 60-degree resolution. Multiple antennas and arrays were designed, analyzed, and fabricated. The devices and the full array system were measured, and their RF performance was characterized. The experimental results validate the feasibility and practicality of the proposed design, and demonstrate a high-speed array platform for direction finding that can be used for search and rescue operations in emergency situations.