Coaxial Cavity Research Articles

Imaging nano-defects of metal waveguides using the microwave cavity interference enhancement method

Here, we demonstrate a microwave (MW) cavity interference enhancement method to image nano-defects on the surface of metal waveguide. The MW cavity interference system mainly consisted of a MW coaxial resonant cavity with a nano-probe. The MW signals have been evenly divided into two channels. One was the reference signal inputted into the MW waveguide and coupled into the MW cavity via the probe. Also, the coupling strength depends on the distance between the probe and the MW waveguide. Another one was directly inputted the MW cavity to interfere with the reference signal, and was enhanced in the cavity. Then, the surface topography of the metal waveguide was mapped by calculating the enhanced signals. In our experiment, a weak signal of ∼1 pW coupled from the waveguide can be detected by a MW cavity with the quality factor of ∼209. As a proof of application, the topography of nano-defects on the surface of metal waveguide in an MW chip has been mapped with a resolution of ∼15 nm. We have proved that this is a high-resolution, easy-to-manufacture, low-cost, and real-time online monitoring approach for online assessment and screening chips. This potentially has broad applications in the fields of chip manufacturing, chip inspection, nano-structure detection, and so on.

Simulation of Multimode Competition of a Coaxial Cavity Gyrotron With Multiple Electron Beams

In this article, the time-dependent multimode competition equations, the dependence of the starting current on the static magnetic field, and the current ratio of a coaxial cavity gyrotron with multiple electron beams are investigated according to the linear and nonlinear theories. By numerical calculation, the mode competition of a 170-GHz coaxial cavity gyrotron is studied when it has one, two, and three electron beams. Researches demonstrate that under the given magnetic field $B$ , electron beam voltage $U$ , and pitch factor ${\alpha }$ , a gyrotron with different electron beam numbers has different startup scenarios, which show that the gyrotron with three electron beams has potential in the selection of mode, the depression of mode competition, and the increase of output power. Results find that the coaxial cavity gyrotron with three electron beams can operate steadily in the ${\text {TE}_{38,18-}}$ mode at 170.39 GHz and it has an output power of 5.02 MW and a beam–wave interaction efficiency of 35.42% when $B=6.82\text{T}$ , $U=88$ kV, $\alpha =1.3$ , $I=158$ A and the three electron beams locate at 11.14, 12.93, and 14.32 mm. Three electron beams in the gyrotron have different beam–wave interaction efficiencies, and the closer the electron beam is to the inner conductor, the higher its beam–wave interaction efficiency is.

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.

Improved Mode Selection in Coaxial Cavities for Subterahertz Second-Harmonic Gyrotrons

A coaxial metal rod with partial dielectric coating is considered as a means for efficient suppression of all volume competing modes in cavities for second-harmonic gyrotrons operated in whispering gallery modes. The rod radius is selected small enough to have only a slight effect on operating mode, which therefore remains insensitive to fabrication tolerances and a misalignment of the coaxial insert. By contrast, for the competing modes such a rod is shown to reduce the effective cavity length, thereby greatly increasing the starting currents. Such a method of mode selection is demonstrated to be more versatile, when compared to that provided by a tapered coaxial conductor. The advantage of a dielectric-coated coaxial insert is illustrated by the example of a cavity for a 100-kW 300-GHz pulsed gyrotron operated in the second-harmonic mode.

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.

A Compact Reflector Antenna Fed by a Composite S/Ka-Band Feed for 5G Wireless Communications

A novel dual-band coaperture reflector antenna which sustains the 3.5 GHz band and the 28 GHz band simultaneously is proposed in this article. It allows a switch working mode or dual-band working modes to accommodate different application scenarios. To achieve high efficiencies simultaneously at dual bands, a shared aperture strategy is proposed for the reflector antenna with a large frequency ratio and small aperture. A very compact composite feed consisting of a pair of dipoles and a conical horn is designed to illuminate a shaped axially displaced ellipse (ADE) reflector antenna. At the higher frequency band, the antenna is a classical ADE design. But at the lower frequency band, it is actually a single reflector. The main reflector acts as a back cavity for the lower band dipole pair which is fed by a coaxial cavity with a length of only 5/8 wavelength and the subreflector works like a director of the low-band feed. The measured antenna with aperture diameters of $2.4\lambda $ at 3.6 GHz and $18.7\lambda $ at 28 GHz achieves total efficiencies of 42% and 47%, respectively.

Optimal Design of an X-Band, Fully-Coaxial, Easily-Tunable Broadband Power Equalizer for a Microwave Power Module

A microwave power module (MPM), which is a hybrid combination of a solid-state power amplifier (SSPA) as a driver and a traveling-wave tube amplifier (TWT) as the final high power stage, is a high-power device largely used for radar applications. A gain equalizer is often required to flatten the TWT output power gain owing to its big gain fluctuations over the operating frequency range. In this paper, the design of an X-band, fully-coaxial, easily-tunable broadband power equalizer for an MPM is presented. The structure is composed of a coaxial waveguide as the main transmission line and a coaxial cavity loaded with absorbing material as a resonant unit. Sensitivity analyses of the attenuation amplitude and resonant frequency of the equalizer in terms of coaxial cavity length, thickness of the absorbing disc, and insertion depth of the probe were carried out. The measured results were in good agreement with the simulated ones, showing that the equalization curve met the requirements well and proved that this optimal structure has the advantages of a large power capacity, a wide operating frequency band, is easily tunable, and good transmission performance.

Dual-Band Coaxial Filter and Diplexer Using Stub-Loaded Resonators

This article presents a simple design of a coaxial dual-band cavity filter and diplexer using stub-loaded resonators (SLRs). Based on the resonance characteristics of a coaxial SLR, the first two resonant frequencies can be tuned over a wide range by adjusting their structural parameters. Controllable external quality factors can be acquired by using the modified tapping structure for the two designated passbands. A coupling window and a coupling pin are proposed to obtain the expected coupling coefficients in the two bands. To demonstrate this concept, a third-order dual-band bandpass filter and an eight-pole diplexer are designed and fabricated. The measured results agree well with the full-wave simulation results.

Research on a High-Order Mode Multibeam Extended-Interaction Oscillator With Coaxial Structure

The stable operation of the high-order mode multibeam extended-interaction oscillator (EIO) with coaxial structure can effectively overcome the size limitation of the fundamental mode operating circuit at high frequency. The research on the transverse mode shows that the high-order TM31 mode can establish a stronger axial electric field than the fundamental TM01 mode in a large size coaxial cavity. Although the design of a coaxial extended interaction cavity is straightforward, the operating stability is greatly affected by the mode competition. Based on the interaction characteristic between the multiple beams and the multigap cavity operating in the high-order mode, this article proposes a simplified analysis method based on the equivalent circuit theory to study the high-frequency characteristics of the complex structure. The space-charge wave theory is combined with the dispersion curve to analyze the stability of the five-gap coaxial EIO operating in the TM31 mode. Finally, the particle-in-cell (PIC) simulation has obtained a TM31 mode EIO operating at 35.3 GHz with an output power of 290.3 kW. The device uses six beams with a total current of 25.8 A (evenly divided among six beams) at a voltage of 51 kV. The design provides a promising approach for making a stable high-power RF circuit applied in high-frequency EIOs.

Particle-in-Cell Simulation and Analysis of 28-Vane Megawatt-Class Pulsed Power Coaxial Magnetron in X-Band

This article presents the design and analysis of a 28-vane megawatt-class pulsed power coaxial magnetron operating in $X$ -band for medical linear accelerator (LINAC) applications. The systematic design procedure of the coaxial magnetron is presented. The proposed coaxial magnetron is analyzed using particle-in-cell (PIC) simulations. The output is taken from the standard WR112 waveguide through a rectangular slot in the outer wall of the coaxial cavity. The maximum total efficiency of 62.04% is obtained at 9.2987 GHz, having a 2.12-MW peak output power for stable $\pi $ -mode operation. The $\pi $ -mode operation is determined by 14 spokes of electrons in the PIC simulation. Electronic efficiency and circuit efficiency of the designed coaxial magnetron are obtained as 73.86% and 84%, respectively. The output peak power can be enhanced by optimizing the applied magnetic field in the operating region of $\pi $ -mode. Total efficiency is improved by 2.74% by changing the shape of the cathode end hats from conventional to conical. The dielectric properties of the slot-mode suppressor are optimized based on the output performance of coaxial magnetron. The optimal values of dielectric constant and loss tangents are obtained as 21 and 0.08, respectively. The frequency-tuning characteristics of the magnetron are also analyzed, and the tuning range is obtained as 9.2987 GHz ± 20 MHz.

An S- and V-Band Dual-Polarized Antenna Based on Dual-Degenerate-Mode Feeder for Large Frequency Ratio Shared-Aperture Wireless Applications

An $S$ -/ $V$ -band dual-polarized antenna is proposed for large frequency ratio shared-aperture wireless applications, which includes a 2.4 GHz patch and a structural-reused 60 GHz substrate integrated waveguide (SIW) parallel-plate orthogonal long-slot array. A dual-degenerate-mode feeder including a circular waveguide and a coaxial cavity is designed to excite the developed $S$ -/ $V$ -band shared-aperture dual-polarized antenna. Two degenerate dominant modes of the circular waveguide and a 180° hybrid junction with five ports are used to realize the dual-polarized differential feeding at $V$ -band. By the use of this differential feeding method, the beam direction of the $V$ -band long-slot array antenna is unchanged with the frequency. Two degenerate high-order modes TE $^{1}_{1,1,0.5}$ and TE $^{2}_{1,1,0.5}$ of the coaxial cavity are used to realize the dual-polarized feeding at $S$ -band. Two SMA connectors for two orthogonal-polarized feeding are placed at their respective electric field minimum points to increase the isolation between input ports. Finally, the dual-band dual-polarized antenna is fabricated and measured.

Compact coaxial slotted‐cavity feed antenna with shaped‐beam pattern for high‐power microwave using high‐order modes and polarisation cancellation techniques

The coaxial slotted-cavity antenna is an easily-manufactured, novel high-power microwave antenna developed specifically for its compact and shaped-beam characteristics. It can directly cascade with the high-power microwave source, and radiate linear-polarisation electromagnetic wave without extra power dividers and mode converters, which obtains a low profile. In this study, a double-layer coaxial cavity operating in two high-order modes is introduced to eliminate the high-order interference modes caused by structural discontinuities, and three shaped beams on the compact aperture can be achieved by the polarisation cancellation technique. Unique proprieties of three shaped beams, namely the peak of gain enhancement beam, the ripple of Gaussian beam and the null depth of conical beam are investigated, all of which can be controlled by the rotation angle of three phase adjusted slot pairs. This study demonstrates the design process of such antenna, and verifies it against simulation and experimental results. The measured return-loss of all three shaped beams are higher than 13 dB, while the corresponding radiation efficiencies are not lower than 98.9%. Furthermore, the estimated minimum power capacity of such antenna exceeds 1.26 GW at 9.375 GHz, making it a highly potential candidate in application towards high-power microwave.

Slot antenna excited by novel substrate integrated coaxial line cavity for millimetre wave application

In this Letter, a novel compact slot antenna has been realised using substrate integrated coaxial line (SICL) cavity. SICL technology helps to achieve cavity size of 0.41 λ 0 × 0.48 λ 0 while maintaining its inherent advantage of self shielded and low loss transmission line at millimetre frequencies. A SICL cavity is designed by shorting the middle conductor at the two edges and keeping the other two edges separated from the sidewall. The eigenmode analysis affirms the presence of a dominant TEM mode at 28 GHz along with the presence of SICL even and odd modes. A half wavelength diagonal slot is etched on the bottom plate of the cavity to disturb the TEM mode surface current at 28 GHz. Measurement results show the SICL cavity backed slot antenna exhibits a gain of 5.6 dBi at 28.6 GHz and high front-to-back ratio better than 17 dB. The SICL-based cavity exhibits millimetre wave resonance with much smaller size, which helps to improve the compactness of the design. The proposed antenna can be used for millimetre wave applications like handheld mobile devices at millimetre-wave frequencies.

Triode magnetron injection gun for the KIT 2 MW 170 GHz coaxial cavity gyrotron

Considering the recent understanding of the physics of electron trapping mechanisms taking place in the magnetron injection gun (MIG) region of gyrotrons and the sensitivity of the emitter ring manufacturing tolerances on the electron beam quality, a MIG has been designed and manufactured for the 2 MW, 170 GHz coaxial cavity gyrotron developed at Karlsruhe Institute of Technology. The new MIG has the following novelties: (i) the design satisfies the criteria for the suppression of the electron trapping mechanisms, (ii) a new type of emitter ring is used for the suppression of the influence of the manufacturing tolerances and misalignments on the quality of the generated electron beam, and (iii) the design was optimized to generate a good beam quality in a wide variety of magnetic field profiles to increase the flexibility. An additional important feature of the new triode MIG design is the possibility to operate with only two power supplies by using a special start-up scenario. The first experimental results of the coaxial cavity gyrotron with the new MIG are presented.

Generation of Plasma Flow Based on ECR Discharge in a Narrow Coaxial Cavity

The possibility of simultaneous extraction of the ion and electron components of plasma and the formation of a current-compensated plasma flow generated in a narrow coaxial resonator on ECR has been established. The characteristic dependences of the ion current on the mass flow rate of the gas (argon) and the microwave power introduced into the resonator are presented.

An Analysis of Metaheuristic Algorithms Used for the Recovery of a Failed Antenna Element in an Antenna Array

A ground-based synthetic aperture radar system (GB-SAR) comprising of 12 horn antennas in its transmitter section and 12 Vivaldi antennas in its receiver section is used for the monitoring of the Tessina landslide in Italy and the sinking of the Costa Concordia ship in Italy. Alternate 2×6 geometry of 12 coaxial cavity horn antennas have been proven to yield a higher half-power beamwidth and low peak sidelobe ratio resulting in a better quality of the image captured. This 2×6 planar array, when fed with power according to the Dolph Chebyshev weighting polynomial, removes side lobes entirely and results in an increase half-power beamwidth (HPBW) value. A failure of any single element in the 2×6 array could lead to the alteration of the radiation pattern thereby jeopardizing the monitoring activity. This paper explores the usage of metaheuristic algorithms in order to recover the weight of the principal antenna element of the 2×6 coaxial cavity horn antenna array. The algorithms explored are Gradient Descent, Simulated Annealing, Firefly algorithm, Bat algorithm, Flower Pollination algorithm, Cuckoo search algorithm, and Harmony Search algorithm. Among these ones, the Firefly algorithm performs the best in finding the weight of the failed antenna element of the 2×6 planar array.

Wideband Quasi-Monostatic Simultaneous Transmit and Receive Reflector Antenna

A dual polarized quasi-monostatic simultaneous transmit and receive (STAR) antenna system operating over 2:1 bandwidth with high isolation, co-polarized transmitting (TX)/receiving (RX) radiation, and high TX gain is proposed. The TX antenna is apex matched parabolic reflector illuminated by a coaxial cavity feed. The RX antenna is also coaxial cavity and mounted back-to-back with the TX feed. Thus, the configuration is termed quasi-monostatic. High front-to-back (F/B) ratio of the RX and TX feed antennas and the separation between them ensures high isolation. The fabricated system achieves 61 dB (average) isolation for CP and >38.5 dB for linearly copolarized TX and RX antennas. Moreover, the amplitude and phase imbalances in beam forming network (BFN) have lesser impact than the BFN-based STAR antenna systems. The antenna system operates from 4 to 8 GHz with VSWR 7 dBic, and >20 dBic for the RX and TX, respectively. Additional techniques to further improve isolation are outlined.

Tunable Coaxial Cavity Resonator-Based Filters Using Actuated Liquid Metal Posts

This letter presents a new tuning method for coaxial cavity resonator-based bandpass filters (BPFs). The proposed tuning mechanism lays its foundations on pneumatically-actuated Galinstan that functionalizes the inner conductor of a capacitively-loaded coaxial cavity resonator. By altering the volume of Galinstan (i.e., height of inner conductor), ultrawide (>octave) and low loss RF tuning can be obtained without the need for small capacitive gaps that typically limit the RF performance of coaxial BPFs. To validate the concept, a coaxial cavity resonator with frequency tuning between 2.9 and 9.9 GHz (3.4:1 tuning range), and unloaded quality factor between 120 and 625 was manufactured and measured. Furthermore, the RF performance of a two-pole BPF with: 1) tunable frequency between 3.4 and 7.5 GHz (2.2:1); 2) an intrinsically switched-off state; 3) insertion loss <1 dB; and 4) return loss <10.5 dB was experimentally validated.

Coaxial Cavity With Stepped Inner Conductor for a Sub-Terahertz Second-Harmonic Gyrotron With Broadband Continuous Frequency Tuning

It is found that a cavity design strategy proposed recently to enhance the continuous frequency tuning band of a sub-terahertz second-harmonic gyrotron aggravates the problem of competition between the operating mode and the first-cyclotron-harmonic (fundamental) modes. A stepped coaxial inner conductor is proposed in this article as a means for a selective suppression of the most dangerous fundamental competitors. It is shown that, with a proper choice of the operating mode, the effect of the inner conductor is only distinct for unwanted competing modes. In this case, the gyrotron performance is slightly sensitive to the longitudinal profile of the coaxial insert. Therefore, this profile can be selected in such a way as to reduce the effective cavity length for the competing modes, together with their diffractive quality factors. As a consequence, starting currents of these modes can be selectively increased. This is illustrated by a realistic design of a 0.4-THz second-harmonic gyrotron with 3-GHz continuous frequency tuning.

Permittivity Spectrum of Low-Loss Liquid and Powder Geomaterials Using Multipoint Reentrant Cavities

Permittivity is a useful tool to characterize the composition and quality of many geomaterials. In general, the non-resonant permittivity measurement methods exhibit a higher degree of uncertainty than their resonant counterparts. In resonant measurements, the reduction in uncertainty comes typically with a loss in broadband. This article describes the theory, design, and application of multipoint coaxial reentrant resonant cavities applied to low-loss geomaterials at different temperatures. Specifically, a full-wave method based on circuit analysis is developed and applied for a circular corrugated waveguide. Moreover, the mode-matching method is applied to calculate the generalized admittance matrix (GAM). Two multipoint cavities and software were built and validated. The first cavity has five resonant frequencies, between 170 MHz and 2.3 GHz, and the second has four resonant frequencies, between 1.3 and 8.6 GHz. Thus, this method allows for “broadband-resonant” measurements. The permittivity values of liquid hydrocarbons, powdered kerogen, and pyrite are shown.