Georgios Atmatzakis

Experimental Demonstration of Degenerate Band Edge in Metallic Periodically Loaded Circular Waveguide

We experimentally demonstrate for the first time the degenerate band edge (DBE) condition, namely, the degeneracy of four Bloch modes, in loaded circular metallic waveguides. The four modes forming the DBE represent a degeneracy of the fourth order occurring in a periodic structure where four Bloch modes, two propagating and two evanescent, coalesce. The DBE is associated with four Bloch eigenmodes representing wave propagation in the periodic structure that coalesce in both wavenumbers and eigenvectors (i.e., polarizations), at a single frequency. It leads to a very flat wavenumber-frequency dispersion relation, and the finite-length structure’s quality factor scales as

A metamaterial pyramidal horn antenna for high power applications

N^{5}

A microwave antenna array with injection locked quantum dot laser sources

, where

RF power extraction from a quantum dot mode locked laser connected to an antenna

N

Experimental Verification of Degenerate Band Edge Dispersion in Metallic Waveguides

is the number of unit cells. The proposed waveguide in which DBE is observed here is designed by periodically loading a circular waveguide with misaligned elliptical metallic rings. We validate the existence of the DBE in such structure using measurements, and we report good agreement between full-wave simulation and the measured response of the waveguide near the DBE frequency; taking into account metallic losses. We correlate our finding to theoretical and simulation results utilizing various techniques, including dispersion synthesis, and scaling of the quality factor and group delay with length. Moreover, the reported geometry is only an example of metallic waveguide with DBE: DBE and its characteristics can also be designed in many other kinds of waveguides and various applications can be contemplated as high-power microwave generation in amplifiers and oscillators based on an electron beam interaction or solid-state devices, pulse compressors, and microwave sensors.

Relativistic vircator with an electromagnetic bandgap medium

Summary form only given. Horn antennas are widely used in high power microwave systems, mainly due to their high power handling properties. Different variations of the traditional horn antenna geometry have been introduced in the past years to improve the radiation characteristics. Some of these techniques are used to alter the boundary conditions on the walls of the horn antennas allowing for the propagation of hybrid modes (hybrid mode horns). As an example, corrugations can be used to create hard and soft horn antennas with reduced side lobes, increased radiation efficiency and low cross polarization levels. Similar to corrugations, metamaterials can be added to horn antennas to generate hybrid modes. The main objective of this work is to design a metamaterial horn antenna that operates in a hybrid HE11 mode, and to analyze its performance and power handling capability. The suggested structure is based on a traditional pyramidal horn antenna. Planar metamaterial structures are added to both of the E-plane walls of the antenna, which changes the boundary conditions at the surface. By properly choosing the metamaterial design parameters, and thus optimizing its effective surface impedance, an HE11 mode can be generated inside the horn. CST Microwave Studio is used for the antenna design. In the simulations, the E-plane walls of the antenna are coated with a homogeneous and isotropic material, the surface impedance of which can be set to any desired value. With this setup, the E-plane wall surface impedance can be optimized for HE11 mode operation and the sought radiation properties. The optimized antenna model, as well as the relationship between the boundary conditions on the E-plane walls and the radiation characteristics of the antenna, will be presented. Then, a metamaterial structure is designed to have the same surface impedance as the coating material, and is used in the simulation. In addition, the actual planar metamaterial structure will be fabricated and tested with an S-band horn antenna. The simulated and measured results of the optimized antenna will be reported and compared. The power handling capability of the antenna is mainly limited by air breakdown at its aperture, as well as the possible vacuum breakdown on the metamaterial surface. A magnetron capable of delivering up to 600MW will be used to feed the proposed antenna. The breakdown point can be assessed by changing the vacuum pressure inside the antenna and relating it to the simulated electric field values, or alternatively by tweaking the output power of the magnetron. The power rating of the antenna will then be indicated.

A SRR-Loaded sub-wavelength waveguide with H-plane-bend radiator and improved matching for high power application

The combining of power of multiple quantum dot mode locked lasers functioning as microwave sources is studied in detail. By injection locking two or more of these devices together, the lasers can feed different elements of the same antenna array radiating at a single frequency. The procedure to determine the injection locking properties of the lasers as microwave sources and to use these lasers to feed a microwave antenna arrays is discussed.

A soft corrugated pyramidal horn antenna for radial power extraction from an A6 magnetron

A new approach is presented for extracting and transmitting RF power from a Quantum-dot Mode-locked laser that acts as a source for a microstrip antenna. An SMA interface to the laser was created, the output impedance of the laser was measured and matched to 50 ohms and the frequency spectrum of the microwave signal was acquired. Then, a wireless link with microstrip patch antennas was set up to transmit this power.

A compact CSRR-loaded waveguide slot antenna

The degenerate band edge (DBE) is a fourth order degeneracy occurring in a periodic structure at which four Bloch modes, both propagation and evanescent, coalesce. We experimentally demonstrate here for the first time the DBE dispersion in loaded metallic waveguides at microwave frequencies. The proposed waveguide in which DBE is observed here is designed by periodically-loading a circular waveguide with misaligned elliptical metallic rings. We validate the existence of the DBE in the presented waveguide using measurements and we report good agreement with full-wave simulations of the waveguide near the DBE; taking into account metallic losses. We correlate our finding to theoretical and simulation results utilizing various techniques including dispersion synthesis. The proposed waveguide, and other metallic slow-wave structures supporting the DBE can be used for a variety of applications including pulse compression and high power microwave generation.

Characterization of a quantum dot mode locked laser functioning as a photonic microwave source

Summary form only given. The virtual cathode oscillator (vircator) can be one of the most promising high-power microwave sources due to its conceptual simplicity, high-output power capability, and potential tunability. Microwaves can be generated when the current of an intense relativistic electron beam exceeds the space-charge-limiting current of the structure. In this case, a virtual cathode comprising an electrostatic potential barrier is formed. Some of the electrons in the virtual cathode lose their kinetic energy and are then reflected back toward the anode. The energy conversion efficiency of the vircator is known to be very low (single digit). In this work, an electromagnetic band gap (EBG) material is investigated to increase the efficiency, to improve the tunability, and to extract the desired field structure at the output. The EBG structure follows a slot antenna concept, or more specifically, a leaky wave slot antenna. The slot array is cut at the waveguide wall to perturb the surface currents in a desired way. The fully electromagnetic, relativistic particle-in-cell (PIC) codes (MAGIC and CST-PS) and the fully electromagnetic software tool ANSYS-HFSS were used in this study.