Thesis (MTech (Electrical Engineering))--Cape Peninsula University of Technology, 2009 / This thesis presents the design, analysis and implementation of an eight-element phased array antenna for wideband X-band applications. The microstrip phased array antenna is designed using eight quasi-Yagi antennas in a linear configuration and is printed on RT/Duroid 6010LM substrate made by Rogers Corporation. The feeding network entails a uniform beamforming network as well as a non-uniform -25 dB Dolph-Tschebyscheff beamforming network, each with and without 45° delay lines, generating a squinted beam 14° from boresight. Antenna parameters such as gain, radiation patterns and impedance bandwidth (BW) are investigated in the single element as well as the array environment. Mutual coupling between the elements in the array is also predicted. The quasi-Yagi radiator employed as radiating element in the array measured an exceptional impedance bandwidth (BW) of 50% for a S11 < -10 dB from 6 GHz to 14 GHz, with 3 dB to 5 dB of absolute gain in the frequency range from 8 GHz to 11.5 GHz. The uniform broadside array measured an impedance BW of 20% over the frequency band and a gain between 9 dB to 11 dB, whereas the non-uniform broadside array measured a gain of 9 dB to 11 dB and an impedance BW of 14.5%. Radiation patterns are stable across the X-band. Beam scanning is illustrated in the E-plane for the uniform array as well as for the non-uniform array.

The Mini-Numerical Electromagnetic Code (MININEC) program, a PC-Compatible version of the powerful NEC program, is used to design a new type of reduced-size antenna. The validity of the program to model simple well-known antennas, such as dipoles and monopoles, is first shown. More complex geometries such as folded dipoles, and meander dipole antennas are also analysed using the program. The final design geometry of a meander folded dipole is characterized with MININEC, yielding results that serve as the basis for the practical construction of the antenna. Finally, the laboratory work with a prototype antenna is described, and practical results are presented.

The electromagnetic interaction between antenna elements is referred to as mutual coupling. This phenomenon can cause undesirable effects in an antenna array such as distortion of the radiation pattern and loss of efficiency. This report models mutual coupling in a scattering parameter context and utilizes microwave network theory to establish a set of nonlinear simultaneous equations describing a compensation network. Iterative numerical techniques are then used to solve for the parameters of the compensation network. Three specific networks are investigated. Compensation examples are presented for two and eight element arrays. / M.S.

In this thesis a mathematical analysis is made of the input impedance to a cylinder antenna with an axial slot. It is excited by a parallel-wire line connected across the center of the slot. This causes standing waves along the slot. The analysis is then based on an analogy between the slot and a transmission line. Following this analogy equations for the transmission-line parameters are developed for the slotted cylinder. The slot distributed inductance and capacitance are determined by assuming an infinite slot length, while the conductance is obtained for a finite slot. The phase constant, which is contained in the expressions for the line parameters, is a function of the line parameters. Thus, a final answer requires the solution of simultaneous equations. This is done on a digital computer. This analysis applies to the case where the wavelength of excitation is of the same order of magnitude as the diameter of the cylinder. Possible frequencies for which this antenna might be used are in the microwave range. / Master of Science

The uses for wideband antennas include wideband measurement systems, spread spectrum communications systems, feeds for reflectors and elements in wideband arrays. Here, wideband antennas are discussed and Archimedean spiral antennas are found to be appropriate elements for use in a new type of wideband array referred to as the wideband switched array. The design of an Archimedean spiral and a necessary wideband balun transformer feed structure are presented. To aid in the design, the Electromagnetic Surface Patch Code (ESP) developed by Ohio State University is used. The spiral and feed structure are constructed and measured results are compared to predictions obtained by ESP. / Master of Science

This dissertation is concerned with the dynamics and control of spacecraft consisting of a rigid platform and a given number of retargeting flexible antennas. The mission consists of maneuvering the antennas so as to coincide with preselected lines of sight while stabilizing the platform in an inertial space and suppressing the elastic vibration of the antennas. The dissertation contains the derivation of the equations of motion by a Lagrangian approach using quasi-coordinates, as well as a procedure for designing the feedback controls. Assuming that antennas are flexible, distributed parameter members, the state equations of motion are hybrid. Moreover, they are nonlinear. Following spatial discretization and truncation, these equations yield a system of nonlinear discretized state equations, which are more practical for numerical calculations and controller design. Linearization is carried out based on the assumption that the inertia of the rigid body is large relative to that of flexible body. The equations of motion for a two-dimensional model are also given. The feedback controls are designed in several ways. Disturbance-minimization control plus regulation is considered by using constant gains obtained on the basis of the premaneuver configuration of the otherwise time-varying system. ln the case of unknown constant disturbance, proportional-plus integral (PI) control has proven very effective. Pl control is used to control the perturbed motions of the platform with multi-targeted flexible appendages. A new control law is obtained for the system with small time-varying configuration during a specified time period by applying a perturbation method to the Riccati equation obtained for Pl control. According to the the proposed perturbation method, the control gains consist of zero-order time-invariant gains obtained from the solution of the matrix algebraic Riccati equation (MARE) for the post-maneuver state and first order time-varying gains obtained from the solution of the matrix differential Lyapunov equation (MDLE). The solution of the MDLE has an integral form, which can be approximated by a matrix difference equation. The adiabatic approximation, which freezes the matrix differential Riccati equation or Lyapunov equation is also discussed. Comparisons are made based on system stability by Lyapunov’s second method. A spacecraft consisting of a rigid platform and a single flexible antenna is used to illustrate disturbance-minimization control, and a spacecraft consisting of a rigid platform and two flexible antennas reorienting into different directions is used to demonstrate the effectiveness of the disturbance-accommodating control. A time-varying spring-mass-damper and a two-dimensional model, representing a reduced version of the original spacecraft model, are considered to demonstrate the perturbation and adiabatic approximation methods. To illustrate the effect of nonlinearity on the dynamic response during reorientation, a numerical example of the spacecraft having a membrane-type antenna ls presented. / Ph. D.

No description available.

No description available.

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.

Thesis (PhD) -- Stellenbosch University, 2001. / Some digitised pages may appear illegible due to the condition of the original hard copy. / ENGLISH ABSTRACT: A new technique for the design of multimode devices in overmoded waveguide is presented. The technique applies the principle of growth-based design and uses a conceptual functional block representation of the design structure to provide necessary flexibility to the design algorithms. Two growth based design strategies are proposed and evaluated. The first uses a generalized synthesis-oriented scanning technique, and the second uses an evolutionary strategy. The techniques provide reliable solutions to a variety of multimode design problems. In order to facilitate sufficiently fast numerical analysis, novel enhancements of the mode matching technique are developed and the use of surrogate models is investigated. In addition, to allow physical evaluation of the finished devices, original techniques of measuring multimode devices are formulated and utilised. Two practical problems are used to evaluate the performance of the design procedures. The first is the design of overmoded waveguide chokes for microwave heating facilities, and the second is the design of multimode horns for antenna and spatial power combining applications. Various examples of each type of problem are presented with measurements of manufactured solutions. / AFRIKAANSE OPSOMMING: ’n Nuwe tegniek vir die ontwerp van multimodusstelsels binne multimodus golfleier word voorgestel. Die tegniek maak gebruik van die beginsel van groei-georienteerde ontwerp en ontgin ’n konsepsuele funksionele module-voorstelling van die ontwerpstruktuur om die nodige buigsaamheid aan die ontwerpsalgoritmes te verleen. Twee groei-georienteerde ontwerpstrategiee word aangebied en geevalueer. Die eerste is gebasseer op ’n veralgemeende sintese-georienteerde skandeertegniek, en die tweede maak gebruik van ’n evolusie-strategie. Die tegniek verskaf betroubare oplossings vir ’n verskeidenheid van multimodusontwerpsprobleme. Ten einde ’n numeriese analise-tegniek daar te stel wat vinnig genoeg is, word oorspronklike verbeterings van die modal-pas metode ontwikkel en surrogaatmodelle is ook ondersoek. Verder, vir fisiese evaluasie, word oorspronklike meettegnieke vir multimodusstelsels geformuleer en gebruik. Twee praktiese probleme word gebruik om die ontwerpprosedures te evalueer. Die eerste is die ontwerp van multimodus golfleierdrywingsdempers vir mikrogolfverhitting, en die tweede is die ontwerp van multimodus horings vir antenna- en ruimtelike drywingskombineerdertoepasings. Verskeie voorbeelde van elke tipe probleem word gegee met metings van gei'mplementeerde oplossings.