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Microwave detection techniques based on the theory of perturbation of cavity resonators are commonly used to measure the permittivity and permeability of objects of dielectric and ferrite materials at microwave frequencies. When a small object is introduced into a microwave cavity resonator, the resonant frequency is perturbed. Since it is possible to measure the change in frequency with high accuracy, this provides a valuable method for measuring the electric and magnetic properties of the object. Likewise, these microwave resonators can be used as sensors for sorting dielectric objects. Techniques based upon this principle are in common use for measuring the dielectric and magnetic properties of materials at microwave frequencies for variety of applications. This thesis presents an approach of using Artificial Neural Networks to detect material change in a rectangular cavity. The method is based on the theory of the perturbation of cavity resonators where a change in the resonant frequencies of the cavity is directly proportional to the dielectric constant of the inserted objects. A rectangular cavity test fixture was built and excited with a monopole antenna. The cavity was filled with different materials, and the reflection coefficient of each material was measured over a wide range of frequencies. An intelligent systems approach using an artificial neural network (ANN) methodology was implemented for the automatic material change detection. To develop an automatic detection model, a multi-layer perceptron (MLP) was designed with one hidden layer and gradient descent back-propagation (BP) learning algorithm was used for the ANN training. The network training process was performed in an off-line mode, and after the training process was accomplished, the model was able to learn the rules without knowing any algorithm for automatic detection.

Microwave detection techniques based on the theory of perturbation of cavity resonators are commonly used to measure the permittivity and permeability of objects of dielectric and ferrite materials at microwave frequencies. When a small object is introduced into a microwave cavity resonator, the resonant frequency is perturbed. Since it is possible to measure the change in frequency with high accuracy, this provides a valuable method for measuring the electric and magnetic properties of the object. Likewise, these microwave resonators can be used as sensors for sorting dielectric objects. Techniques based upon this principle are in common use for measuring the dielectric and magnetic properties of materials at microwave frequencies for variety of applications. This thesis presents an approach of using Artificial Neural Networks to detect material change in a rectangular cavity. The method is based on the theory of the perturbation of cavity resonators where a change in the resonant frequencies of the cavity is directly proportional to the dielectric constant of the inserted objects. A rectangular cavity test fixture was built and excited with a monopole antenna. The cavity was filled with different materials, and the reflection coefficient of each material was measured over a wide range of frequencies. An intelligent systems approach using an artificial neural network (ANN) methodology was implemented for the automatic material change detection. To develop an automatic detection model, a multi-layer perceptron (MLP) was designed with one hidden layer and gradient descent back-propagation (BP) learning algorithm was used for the ANN training. The network training process was performed in an off-line mode, and after the training process was accomplished, the model was able to learn the rules without knowing any algorithm for automatic detection.

Milimetrinių bangų sritis yra perspektyvi daugelyje sričių, nuo kasdienio vartojimo įtaisų tokių, kaip telekomunikacijų tinklų įranga iki specifinių taikymų kariuomenėje, bei diagnostinėje medicinoje. Milimetrinių bangų imtuvuose dažniausiai naudojamas netiesinis elementas yra Šotkio (Schottky) diodas. Nors jo veikimo dažnių ruožas yra iki terahercų srities, tokie trūkumai kaip jautrio priklausomybė nuo temperatūros, neatsparumas elektrinėms perkrovoms, bei jo parametrų blogėjimas laikui bėgant verčia ieškoti naujų alternatyvų. Disertacijoje pristatomi dviejų tipų mikrobangų diodai, pagaminti naudojant įvairiatarpius GaAs/AlGaAs darinius. Puslaidininkinių darinių sluoksnių kokybė buvo eksperimentiškai įvertinta naudojant nuostoviosios fotoliuminescencijos ir laike koreliuoto fotonų skaičiavimo metodus. Aprašytas mikrobangų diodų gamybos procesas. Elektrinės savybės buvo įvertintos išmatavus mikrobangų diodų voltamperines charakteristikas, o detekcinės jų savybės ištirtos plačiame dažnių ruože. / Spectrum region of millimeter wave is extensively used in various areas: from consumer devices in telecommunication networks, to specific applications in military and diagnostic medicine. Schottky diode is the most commonly used two terminal device in microwave receivers. Although the operational frequency of Schottky diode is reaching the terahertz frequency range it has several drawbacks such as sensitivity dependence on temperature, long-term instability and sensitivity to overloads. These drawbacks impelled one to search new type of devices. Two types of microwave diodes fabricated using GaAs/AlGaAs heterostructures are presented in the thesis. The quality of semiconductor epitaxial layers was experimentally estimated using photoluminescence and time correlated single photon counting techniques. The process of microwave diode fabrication is described. Electrical properties of microwave diodes were estimated from current-voltage characteristics and properties of microwaves detection were investigated in wide frequency range.

Spectrum region of millimeter wave is extensively used in various areas: from consumer devices in telecommunication networks, to specific applications in military and diagnostic medicine. Schottky diode is the most commonly used two terminal device in microwave receivers. Although the operational frequency of Schottky diode is reaching the terahertz frequency range it has several drawbacks such as sensitivity dependence on temperature, long-term instability and sensitivity to overloads. These drawbacks impelled one to search new type of devices. Two types of microwave diodes fabricated using GaAs/AlGaAs heterostructures are presented in the thesis. The quality of semiconductor epitaxial layers was experimentally estimated using photoluminescence and time correlated single photon counting techniques. The process of microwave diode fabrication is described. Electrical properties of microwave diodes were estimated from current-voltage characteristics and properties of microwaves detection were investigated in wide frequency range. / Milimetrinių bangų sritis yra perspektyvi daugelyje sričių, nuo kasdienio vartojimo įtaisų tokių, kaip telekomunikacijų tinklų įranga iki specifinių taikymų kariuomenėje, bei diagnostinėje medicinoje. Milimetrinių bangų imtuvuose dažniausiai naudojamas netiesinis elementas yra Šotkio (Schottky) diodas. Nors jo veikimo dažnių ruožas yra iki terahercų srities, tokie trūkumai kaip jautrio priklausomybė nuo temperatūros, neatsparumas elektrinėms perkrovoms, bei jo parametrų blogėjimas laikui bėgant verčia ieškoti naujų alternatyvų. Disertacijoje pristatomi dviejų tipų mikrobangų diodai, pagaminti naudojant įvairiatarpius GaAs/AlGaAs darinius. Puslaidininkinių darinių sluoksnių kokybė buvo eksperimentiškai įvertinta naudojant nuostoviosios fotoliuminescencijos ir laike koreliuoto fotonų skaičiavimo metodus. Aprašytas mikrobangų diodų gamybos procesas. Elektrinės savybės buvo įvertintos išmatavus mikrobangų diodų voltamperines charakteristikas, o detekcinės jų savybės ištirtos plačiame dažnių ruože.