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Additive manufacturing (AM) has become more important and common in recent years. Advantages of AM include the ability to rapidly design and fabricate samples much faster than traditional manufacturing processes and to create complex internal geometries. Materials are crucial components of microwave systems and proper and accurate measurement of their dielectric properties is important to aid a high level of accuracy in design. There are numerous measurement techniques and finding the most appropriate method is important and requires consideration of all different factors and limitations. One limitation of sample preparation is that the sample size needs to fit in the measurement method. By utilizing the advantage of additive manufacturing, the material can be characterized using different measurement methods. In this paper, the additive manufacturing process and dielectric measurement methods have been critically reviewed. The test specimens for measuring dielectric properties were fabricated using fused filament fabrication (FFF)-based additive manufacturing and were measured using four different commercial dielectric properties measurement instruments including split post dielectric resonator (SPDR), rectangular waveguide, TE01δ cavity resonator, and open resonator. The measured results from the four techniques have been compared and have shown reasonable agreement with measurements within a 10 percent range.

A modified wideband bandpass filter based on a bandstop filter is realised using half-wavelength and one-wavelength open-circuited resonators. Six transmission zeros are utilised to obtain the sharp rejection characteristics over a broad stopband. The passband bandwidth can be varied by changing the impedance of the configuration. A compact prototype filter with 27% bandwidth is demonstrated. Without coupling gaps between resonators, the new filter shows low insertion loss and is easy for fabrication. To illustrate the concept, a bandpass filter is designed, fabricated and measured. Simulated and measured results are found in good agreement with each other.

Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity due to signal leakage at the resonator opening. We have recently demonstrated three different approaches for modelling the photoacoustic signal of open resonators. In this work, one of the approaches is applied for the optimization of the geometry of the T-shaped resonator for improved signal strength and thus sensitivity. The results from the numerical optimization show an increase in the photoacoustic signal by a factor of approximately 7.23. They are confirmed using numerical methods other than the one applied for the optimization and by experimental measurement. The measurement shows an increase in the photoacoustic signal by a factor of approximately 2.34.

An interesting new physical phenomenon is uncovered-an open resonator array excited by an electron beam and able to generate a special kind of Smith-Purcell radiation (SPR). Although the frequency and direction satisfy the SPR relation, this is a single frequency radiation in a specific direction that is essentially different from ordinary SPR. The spectral density of this special radiation is also much higher than that of ordinary SPR. By means of theoretical analysis and digital simulations, the radiation mechanism together with its requirements are explored. This radiation may have great influence in modern physics and optics as it offers new ways to carry out coherent radiation generation and beam diagnostics.

This paper was stimulated by the experimental studies of solid-state lasers initiated by N. G. Basov and carried out at the Laboratory of Luminescence of the P. N. Lebedev Physical Institute under the direction of M. D. Galanin and A. M. Leontovich in the 1960-ies. Here, the classical parabolic equation method is extended in order to calculate complex eigenfrequencies of optical oscillations in a dielectric-filled open resonator. Accurate estimates confirm a high quality factor of ruby lasers. The developed approach can be used to find complex eigenfrequencies of other dielectric optical objects in laser systems of current interest.

An experimental study has been carried out to model and to measure the resonance frequency shift introduced by inserting a small microwire segment in an open resonator. A hybrid frequency synthesizer based on a Gunn diode was used to perform measurements. The open resonator was excited using this hybrid frequency synthesizer together with a Schottky diode detector used to measure a resonance curve. The results have shown a small difference between the model and the measured frequency shifts of the small microwire segment using the hybrid frequency synthesizer and the open resonator in the 3 mm wavelength band.

A microstrip highly sensitive differential sensor for complex permittivity characterization of urine samples was designed, fabricated and tested. The sensing area contains two pairs of open-stub resonators, and the working frequency of the unloaded sensor is 1.25 GHz. The sensor is easily implemented on an affordable substrate FR-4 Epoxy with a thickness of 1.6 mm. A Teflon beaker is mounted on the sensor without affecting the measurements. Numerically, liquid mixtures of water and urine at different percentages were introduced to the proposed sensor to evaluate the frequency variation. The percentage of water content in the mixture varied from 0% (100% urine) to 100% (0% urine) with a step of 3.226%, thus giving 32 data groups of the simulated results. Experimentally, the mixtures of: 0% urine (100% water), 20% urine (80% water), 33% urine (66% water), 50% urine (50% water), 66% urine (33% water), and 100% urine (0% water) were considered for validation. The complex permittivity of the considered samples was evaluated using a nonlinear least square curve fitting in MATLAB in order to realize a sensing sensitivity of about 3%.

The metamaterial inspired novel CPW fed MIMO antenna design works based on the principle of non bianisotropic- split-ring resonator and amalgamation of hexagonal open ring resonator (Hex-ORR) thereby resulting in a miniaturized antenna with dimensions of 47.4 × 31.7 × 1.6 mm 3 . The two NB-SRR antennas are placed in opposite directions with an edge distance of 0.022 λ 0 . To eliminate the bianisotropic property of the split ring resonator which produces the effect of anisotropy and cross polarization. the NB-SRR is proposed in which the rings are aligned together from end to end of the metal strip, which helps in improvising the bandwidth to a higher frequency. This antenna holds decent for the Sub-6 GHz 5G application covering bandwidth of 983.5 MHz (3.7887–2.8052 GHz) and 551.6 MHz (6.3834–5.3818 GHz) with center frequency of 3 GHz and 6 GHz, respectively. The lower frequency band is produced using hex-ORR, and a higher frequency band is provided using NB-SRR. The size of the antenna is optimized by considering the Non Bianisotropic-SRR size minor than the resonant wavelength. The average isolation loss between the antenna elements is - 25 dB, the radiation gain is 5 dBi, and the efficiency is 97%. The proposed MIMO antenna parameters such as ECC, CCL, and TARC are also examined, and the results indicate that the proposed antenna design is a good candidate for Sub-6 GHz 5G applications.

A novel, high surface encoding capacity compact planar multiresonator tailored for Chipless RFID tag applications is discussed in this article. The tag consists of three hexagonal open loop resonators that are etched on the ground plane of a 50Ω microstrip transmission line. It operates within the frequency range of 2.12 GHz to 5.45 GHz, with a bandwidth of 3.33 GHz. Frequency Shift Coding is employed to record the tag's identification in the spectral domain. A maximum of 343 distinct code words can be generated utilizing three resonators. A notable feature of this tag is its capability to achieve distinct resonating frequencies by adjusting the overall dimensions of the slot. The tag prototype is designed and fabricated on an RT5880 lossy substrate, characterized by loss tangent of 0.0009 and dielectric constant of 2.2. Experimental data from actual prototypes are presented to verify the dependability of the suggested design.

For gyrotron applications in plasma installations, one of the most important factors is the gyrotron efficiency. To maximize the interaction efficiency, it is necessary not only to optimize such operating parameters as the magnetic field, beam voltage, and current but also the axial profile of the electromagnetic (EM) field in the interaction space. The present paper describes a study of the effect of the profile of an irregular waveguide serving as a resonator on the axial structure of the EM field. Specific attention is paid to the profile of the uptaper connecting the regular part of a resonator to the output waveguide. Conditions of applicability of the nonuniform string equation, which is widely used in gyrotron designs for finding the axial structure of the EM field, are discussed. Also discussed are the occurrence of reflections from a smooth uptaper and the analogy between the nonuniform string equation and the stationary Schrodinger equation.