Coaxial Resonator Research Articles

Meissner Levitation of a Millimeter-Size Neodymium Magnet Within a Superconducting Radio Frequency Cavity

We report on the magnetic levitation of a millimeter-sized neodymium permanent magnet within the interior of a superconducting radio frequency (SRF) cavity. To the best of our knowledge, this is the first experimental work on levitating a magnet within an SRF cavity. The cavity is a coaxial quarter-wave microwave resonator made from 6061 aluminum, having a resonance frequency of 10 GHz and a loaded Q of 1400. The cylindrical magnet (N50) has a height of 1 mm, a diameter of 0.75 mm, a mass of 4 mg, and a remanence of 1.44 T. This produces a peak magnetic field 140 times greater than the critical field of aluminum. The magnet is placed either on the top of the coaxial portion of the cavity or on the cavity floor before cooling it below the superconducting transition temperature of aluminum. The coaxial mode's resonance frequency shifts as a function of the levitation height of the magnet and gives an idea of the magnet's position and mechanical motion. We observe a transition at a temperature of 650 mK where the Meissner effect levitates the magnet as the material beneath the magnet becomes superconducting. The magnet is levitated to a height of 2.5 mm above the surface of the cavity stub, which is a sufficient separation for the field strength of the magnet at the surface of the stub to be less than the critical field strength of the superconducting aluminum. We measure a 120 MHz upshift in the cavity resonance as the magnet is levitated from the top of the stub and 15 MHz downshift as it levitates from the floor of the cavity. Our measurements are consistent over several heating and cooling cycles. Our work provides a path towards a novel optomechanical system.

Dual microwave-excited atmospheric-pressure plasma jets with a single power source

In this paper, we propose dual microwave-excited atmospheric-pressure plasma jets (DME-APPJs) that can be generated with a single power source. A T-junction power divider that evenly distributes the input power to two ports is used, and two coaxial transmission line resonators are used as plasma generators for the DME-APPJs. The power divider distributes the incoming power in port 1 and supplies 46% of this power to port 2 and port 3, respectively. Surface treatments are conducted by exposing the multiple plasma jets to polypropylene (PP) specimens and then confirming the degree of hydrophilic change. Using the proposed DME-APPJs, it is possible to perform surface treatments on areas that are up to 50% larger compared to using two single ME-APPJs with the same power consumption. The DME-APPJs thus show more advanced performance for surface treatments by comparison with the single ME-APPJ.

Microwave filters on cavity resonators

Waveguide and cavity microwave filters continue to be the most important components of high-power radio electronic systems. Cavity microwave filters are of great interest as a separate group of devices of this type. The fields of application of such filters are constantly expanding, and the variety of their design is quite large. In addition, the requirements for their electromagnetic characteristics and reliability are currently increasing. In this regard, it becomes necessary to analyze the already created designs of these devices and study the main trends of their improvement. The objective of the present study is to review the known technical solutions of bandpass filters, made on cavity resonators of the microwave range. An overview of the designs of devices for filtering electromagnetic waves on cavity resonators with coaxial and waveguide coupling elements of various frequency ranges is carried out. It is proposed to use the configuration of basic resonators as the main criterion for the classification of these devices. In accordance with this criterion, five groups of filters are considered: on rectangular, cylindrical, spherical, coaxial resonators, as well as on resonators of complex shape. The review makes it possible to analyze the features of the implementation of different technical solutions for bandpass microwave filters of medium and high power levels and to identify the main trends in the development of this area of research.

Multi-conductor coaxial cavity resonators for miniaturization of UHF bandpass filters

In this paper, we present the compact bandpass cavity filters for UHF range. We investigate the properties of coaxial resonators with various numbers of tubular posts from one to four. We show that the resonant frequency can be four times smaller for the four-post resonator in comparison with the single post resonator with the same cavity dimensions. We synthesized and manufactured fifth-order bandpass cavity filter on four-post coaxial resonators for the 690–806 MHz frequency range with 20-dB return loss. The filter dimensions were 30 mm × 44 mm × 127 mm. We also synthesized and manufactured the fifth-order bandpass cavity filter on four-post coaxial resonators with inductive and capacitive cross-coupling between first and fifth resonators to provide transmission zeros in amplitude–frequency characteristic in lower- and upper-frequency band, respectively. Comparison with the standard double-post coaxial cavity filters demonstrates more than 30% volume reduction with similar transmission characteristics.

Dual-Band and Wide Stopband Coaxial Filters Using Open-Circuited-Stub-Loaded Resonators

Novel open-circuited-stub-loaded coaxial resonator (OCSLR) is introduced in this brief. This resonator consists of a traditional coaxial resonator, which is vertically loaded by an open-circuited stub. Based on the resonant properties, the first two resonant frequencies can be flexible controlled by properly choosing the impedance ratio and length ratio. The proposed dual-band filter is achieved by employing OCSLRs which act as dual-mode resonator. The proposed wide stopband filter is realized by two OCSLRs loaded at different heights together with a traditional quarter-wavelength coaxial resonator, resulting in the same fundamental frequencies and different harmonic frequencies. Two design examples are manufactured to validate the concept. The measure results show good agreement with simulations.

Triple-Mode Substrate Integrated Coaxial Resonator Based Bandpass Filter Featuring Flexible Transmission Zeros and Adjustable Bandwidth

This communication presents substrate integrated coaxial resonator (SICR) based triple-mode bandpass filters designed for K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> -band. A self-shielded SICR based third-order filter is realized using two planar coaxial resonators (SICR) embedded in a substrate integrated waveguide cavity (SIW). Merging the TEM based mode of two SICR's with TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">101</sub> mode of SIW significantly enhances the bandwidth in a compact size. A novel topology realized using short circuited metallic via connecting to the top conducting plane through a metal strip demonstrates high degree of flexibility in positioning the transmission zeros to improve the selectivity as well as extend the out-of-band rejection of proposed filter. The three experimental prototypes devised for K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> -band with fractional bandwidth more than 23% demonstrate multiple transmission zeros for good selectivity and out-of-band rejection with low insertion loss in a compact form factor.

Nd:GdVO4/Yb:YAG Intracavity Sum-Frequency CW Blue Laser Operating at 484 nm

We study a continuous-wave sum-frequency blue laser operating at 484 nm. The laser elaborated is based on the intracavity pumping technology. A 912 nm laser produced by an end-pumped Nd:GdVO4 crystal, using an 808 nm fiber-coupled laser diode, is employed to pump an Yb:YAG crystal in the cavity to achieve the simultaneous oscillations in 912 and 1030 nm lasers. An intracavity sum-frequency achieved in a type-I phase-matched BIBO crystal is used to obtain the 484 nm blue laser output. In our experiment, we apply a linear coaxial laser resonator structure. As a result, we obtain a sumfrequency blue laser output of 317 mW at a pump power of 25 W. The slope efficiency of the designed laser is 2.8%, and the beam quality factors are 1.34 in the X direction and 1.36 in the Y direction, respectively, with the power fluctuations about 3.8% within four hours.

Experimental observation of optical vortex array by intra-resonator modulated high-order laser resonator

A simple and new method is proposed to realize the vortex beam emitter containing the tunable optical vortex array. We studied the optical vortex array by the method of control the dual copper wire for the diffraction within the high-order resonator, that is the copper wires are inserted into the open coaxial spherical resonator to modulate the diffraction of the laser, and then one of the copper wires are controlled to obtain a vortex beam emitter of tunable optical vortex array, which is achieved simply by tuning the distance between the copper wires and the optical axis. The optical vortex array generated by the laser is the result of mode that is selected after different loss of diffraction occur in the open coaxial spherical resonator and we change the loss of diffraction by the adjustment of the distance between the copper wires and the optical axis in the laser resonator to realize the adjustment of the optical vortex array. In addition, we also established a detected experiment of vortex and analyzed the detection of the vortex by the lateral shear interferometry. The regulation of the optical vortex array is proved by the analysis of the interference pattern of vortex.

Design Procedure for Bandpass Filters Based on Integrated Coaxial and Rectangular Waveguide Resonators

In this article, we present a novel bandpass filter based on dual-mode resonators. The dual-mode operation is based on integrated coaxial and rectangular waveguide cavities. Furthermore, we also discuss a systematic design procedure for the selection of the dimensions of the various constituent elements of the filter, which is based on the well-known aggressive space mapping technique. The design of a five-pole bandpass filter operating in the X-band is discussed in detail, including the measured data. The comparison between simulations and measurement shows very good agreement, thereby fully validating both the new filter structure and the design procedure.

3D printed coaxial microwave resonator sensor for dielectric measurements of liquid

AbstractThis paper presents a coaxial resonator for dielectric measurements of low and medium loss liquids at multiple resonance frequencies up to 8 GHz with the fundamental transverse electromagnetic (TEM) resonant mode at 2 GHz. The measurement is by filling the entire resonator cavity with the material under test and the permittivity of the material is readily extracted using simple equations. This technique provides an easy and accurate extraction method of dielectric properties without any analytical approximation and dedicated software algorithm as used in broadband open coaxial probes or the complex perturbation formula in resonator‐based methods. This significantly reduces the risk of systematic errors from the model approximation. The measured quality factor of the resonator is 2650 to 3500 depending on the resonant mode. This allows for the measurement of samples with loss tangents up to 0.05 (Acetone taken as a reference). The device was made by 3D printing and verified by measurements of several common solvents at all four resonance frequencies. The results obtained agree well with values reported in literature. Further measurements of crude oil samples were carried out and results confirmed with values obtained using other techniques.

Ultrasensitive Open-Ended Coaxial Cable-Based Microwave Resonator Learns to Sense Impacts

This article presents an ultrasensitive open-ended hollow coaxial cable resonator (OE-HCCR) that was trained to detect impact events that occurred in the surrounding space through machine learning (ML) techniques. First, inspired by the well-known mass-based accelerometer, for the first time, we propose an OE-HCCR-based accelerometer with the addition of an inertial mass block as the element responsive to acceleration. The OE-HCCR is formed by a metal shorting post welded at the RF input end and the open end of the cable. The instantaneous displacements of the mass block mounted at the endface of the cable due to accelerations lead to variations of the gap distance between the mass block and the endface of the coaxial line, which can be accurately metered by the coaxial resonating structure. Impact events that occur in the surrounding space of the OE-HCCR result in vibrations transmitted to the OE-HCCR system and thereby measured by the OE-HCCR. Second, we demonstrated that employing ML algorithms to analyze the transient responses from the OE-HCCR makes unambiguous mappings between the transient responses of the OE-HCCR and the sources of vibrations possible, due to the ultrahigh sensitivity of the OE-HCCR and the spatial anisotropy of matter in the surrounding space. The preliminary results demonstrate the feasibility of using ML-assisted OE-HCCR system for locating impact events and recognizing different impactor implements that caused the impact events.

Compact Substrate-Integrated Bandstop Filters Using Double-Resonant Coaxial Resonators

This letter reports on the RF design and practical development of miniaturized substrate-integrated-(SI)-coaxial-resonator-based bandstop filters (BSFs). Size compactness is achieved through the use of double-resonant coaxial resonators that are shaped by two coupled capacitively loaded posts within the volume of a circular cavity. The operating principles of the double-resonant SI-coaxial-resonator-based BSF concept are demonstrated by means of coupled-resonator diagrams and synthesized examples of both single-stage and multi-stage higher order BSF configurations. For proof-of-concept validation, various single- and two-stage coaxial-resonator-based BSF prototypes were designed, manufactured, and measured at $S$ -band.

Improved and simplified design of the relativistic helitron resonator

One of the main trends in microwave electronics is the ultra-large power production. The electron stream energy is converted inside vacuum systems, where the key moment is increasing output power of microwave devices, which is possible only when using more and more powerful electron streams. Increasing electron stream power is possible due to either enhancing the carried currents or as a result of increasing the electron energy. Given the law that connects currents and voltages in electronic systems operating when the current is limited by a spatial charge, the production of ultra-high-power electron flows is associated with the usage of relativistic velocity electrons, i. e. approaching the light speed. Likewise, at present, relativistic electrovacuum devices (traveling-wave lamps and backward-wave lamps) use magnetic focusing for linear relativistic streams, which prevents the implementation of simple superconducting electrodynamic systems, because highfrequency metal superconductivity disappears in constant magnetic fields. Meanwhile, simplified ultra-highpower superconducting device structures can significantly increase the device energy due to the strong ohmic loss reduction, which just limits the device energy, destroying the working electrodynamic system surface by increasing power or pulse duration of the generator. The article outlines the modernized design of a new-type microwave generator – the relativistic helitron. The paper considers a simpler coaxial resonator design, obtained by using the supercritical narrowing of the inner conductor radius by the Hn1l mode of the electromagnetic field, rather than a coaxial resonator with notch filters.

Microwave Device Inspired by Fiber-Optic Extrinsic Fabry-Perot Interferometer: A Novel Ultra-Sensitive Sensing Platform

The fiber-optic extrinsic Fabry-Perot interferometer (EFPI) is one of the simplest sensing configurations and is widely used in various applications. Inspired by the EFPI, we report a novel and universal ultra-sensitive microwave sensing platform based on an open-ended hollow coaxial cable resonator. Two highly-reflective microwave reflectors were fabricated in a coaxial cable to form a microwave Fabry-Perot etalon. Although the operating wavelength of the proposed device is increased by five orders of magnitude compared to the fiber-optic EFPI (e.g., from 1500 nm to 150 mm), the resolution regarding the "pseudo cavity length" of the proposed device is as high as 0.6 nanometer, which is comparable to that of the EFPI. The resolution can be further increased by high-precision machining of the device. Due to its low cost, high sensitivity, all-metal structure, robustness, and ease of signal demodulation, it is envisioned that the proposed device will revolutionize the sensing field and enable many important sensing applications that take place in harsh environments.

Numerical Computation of Resonant Frequency and Field Distribution Based on S-Parameters in the Open Coaxial Resonator

As the core components, the open coaxial resonators are widely used in microwave oscillators and amplifiers recently. In this article, a method using the inverse matrix of S-parameter to solve the resonant frequency and field distributions of open resonators is proposed and verified by simulation software for the first time, which can avoid the calculation errors caused by the boundary setting of simulation software. This method is convenient, accurate, and less time-consuming for the arbitrary complex coaxial resonators. It is promising for the design of the transit-time oscillators and triaxial klystron amplifiers in the future.

Non-destructive monitoring of food freezing process by microwave resonance spectroscopy with an open-ended coaxial resonator

Abstract In this study, microwave resonance spectroscopy was applied to monitor the freezing process of food product using an open-ended coaxial resonator. The resonator was designed to detect a signal corresponding to the dielectric property of a product placed in space over a probe. Frozen sucrose solutions with various concentrations and temperatures were employed, and the spectra patterns obtained and their variations upon freezing were investigated. The peak intensities of the spectra were significantly dependent on the concentrations and temperatures of the sucrose solutions, whereas the intensities obtained from pure water (i.e., ice) did not significantly depend on temperature. The dependence of the peak intensities on temperature was found to comprehensively represent the temperature of the core part of the products. The proposed monitoring technique can be employed to detect the status of frozen products in terms of temperature, as demonstrated in the freezing process of a sucrose-agar solution.

Highly sensitive open-ended coaxial cable-based microwave resonator for humidity sensing

A novel portable open-ended hollow coaxial cable resonator (OE-HCCR) is proposed and demonstrated for direct relative humidity (RH) measurements without the assistance of any humidity-sensitive material. The sensor is fabricated from a homemade hollow coaxial cable platform that provides ease of modification and improved robustness. The microwave cavity resonator is a metal post-coupled open-ended coaxial resonator, which includes two highly-reflective reflectors, i.e., the metal post and the metal plate-backed open end of the coaxial line. The gap distance between the metal plate and the open end is defined by a circular spacer. By simple one-step adjustments of the thickness of the spacer, the RH measurement sensitivity of the OE-HCCR can be greatly enhanced. The device physics is discussed; numerical calculations and experimental investigations are conducted to verify the sensitivity enhancement of the OE-HCCR for RH sensing. Due to the increased sensitivity, this is the first report of an open-ended coaxial line that is able to directly sense gaseous analyte, to the best of our knowledge. The proposed OE-HCCR with the sensitivity enhancement has the potential for highly sensitive chemical sensing combined with functional materials.

Metal-organic framework portable chemical sensor

Combining the chemical-specific adsorption properties of metal-organic framework (MOF) materials with the dielectric sensitivity of a novel open-ended hollow coaxial cable resonator (OE-HCCR), a mechanically-robust and portable gas sensor device (OE-HCCR-MOF) with high chemical selectivity and sensitivity is proposed and experimentally demonstrated. The operating principle of the device is based on changes in the dielectric property of the host MOF layer in response to variations in the types and concentrations of guest molecules. The changes in the dielectric property of the MOF layer modify the phase-matching condition of the microwave resonator, causing shifts in the resonance frequency of the device. By monitoring the resonance frequency shift, the adsorptions of guest molecules can be monitored in real-time and accurately quantified. In proof-of-concept demonstrations, a 200-μm layer of MOF (HKUST-1) was placed within an OE-HCCR to develop a prototype OE-HCCR-MOF sensor. The novel sensor showed high sensitivity to variations in the concentrations of carbon dioxide with good reversibility. The chemical selectivity of the prototype sensor for carbon dioxide compared to methane was also investigated.

Analysis of Coaxial Gyrotron Cavity With a Misaligned Insert

The coaxial cavity can rarefy the mode spectrum and restrain the mode competition, which is beneficial to the mode selection of high-frequency and high-power gyrotron. However, the misaligned insert would result in frequency shift and a decrease in interaction efficiency. Besides, the ohmic loss on the cavity walls would become uneven, which deteriorates the long pulse operation of the gyrotron. In this article, the effects of the insert structural parameters on the characteristics of the cavity, the beam-wave interaction, and the ohmic loss distribution on the cavity walls in a coaxial cavity with a small misalignment of insert are discussed. An optimal design of coaxial resonator is presented to reduce the influence of a slightly misaligned insert on the output characteristics of the resonator.

Microwave characteristics analysis of photosensitive material InP under weak optical irradiation

In this paper, the microwave characteristic of photosensitive material InP irradiated by different optical signals is studied based on the coaxial resonant cavity. The coaxial resonant cavity sensor has an operating frequency range from 1 to 4 GHz, an operating mode of TEMn/4, and a quality factor of 8000 or more. According to the theory of semiconductor and electromagnetic field, the equation describing the relationship between the irradiation power and the quality factor of the coaxial resonator is derived. Additionally, in the short-time irradiation experiment, the variation of microwave characteristics of InP with irradiation power of 10mW, 50mW, 100mW, and 250mW is studied by frequency-domain and time-domain scanning methods, respectively. The measurement results indicate that the microwave characteristic variation of the InP under short-time weak power irradiation is mainly caused by the non-thermal effect, while the thermal effect is not obvious. In order to investigate the influence of thermal effect on the microwave characteristic of InP under continuous illumination, taking the irradiation power of 100mW as an example, the irradiation experiment with a duration of 1 min is performed on the InP. The thermal and non-thermal effects at the moment of opening and closing of the monochrome light source and during irradiation are analyzed. Furthermore, the stability and response speed of the system are analyzed and tested, and the measurement results are in good agreement with the theoretical analysis.