Open-ended Waveguide Research Articles

High-Resolution UHF Near-Field Imaging Probe

Imaging probes which work at relatively low microwave frequencies in the ultrahigh frequency band (UHF) facilitate imaging through high loss dielectric materials and yield manageable imaging system complexity. However, acquiring images with resolutions on the order of few millimeters at frequencies within the UHF band requires super-resolution imaging methods and probes. In this paper, a compact near-field imaging probe working in the UHF band is proposed for high-resolution noninvasive imaging applications. The proposed probe is based on an electrically small loop loaded with a miniaturized spiral resonator. The probe is analyzed in this paper using numerical electromagnetic simulations and its equivalent circuit model is consequently developed. A prototype of the proposed probe operating at 426 MHz is fabricated and its spatial resolution as well as sensitivity are experimentally verified. A spatial resolution of around $\lambda $ /140 is demonstrated in simulations and verified experimentally. In order to highlight the utility of the probe for practical applications, images of many test samples including multilayer dielectric composite structures and corroded metallic substrate are acquired using the fabricated prototype. It will be shown that the images obtained using the proposed probe is comparable with those produced using an open-ended rectangular waveguide operating at 24 GHz. The salient features of the proposed probe include its high sensitivity and resolution while operating at relatively low frequency.

A Quantum-Based Microwave Magnetic Field Sensor.

In this paper, a quantum-based method for measuring the microwave magnetic field in free space is presented by exploring atomic Rabi resonance in the clock transition of 133Cs. A compact cesium glass cell serving as the microwave magnetic field sensing head was used to measure the spatial distribution of microwave radiation from an open-ended waveguide antenna. The measured microwave magnetic field was not restricted by other microwave devices. The longitudinal distribution of the magnetic field was measured. The experimental results measured by the sensor were in agreement with the simulation. In addition, a slightly electromagnetic perturbation caused by the glass cell was investigated through simulation calculations.

An electromagnetic sensor to locate service laterals during the trenchless lateral reinstatement

Lateral reinstatement is the last step in the rehabilitation of buried pipes using cured-in-place pipe (CIPP) liners, where the service laterals sealed by the freshly installed liner are cut open. In this paper, a low cost sensor based on the open-ended rectangular waveguide based non-destructive testing (NDT) technique that was developed in the past is extended for lateral detection. Its performance is tested inside metallic and non-metallic host pipes. Numerical modeling of the sensor and experimental validations are carried out. A dedicated electronic circuit and a servo controlled scanning mechanism are developed so that the sensor can be retrofitted on the robotic cutters used for lateral restoration in the trenchless industry.

Source Current Polarization Impact on the Cross-Polarization Definition of Practical Antenna Elements: Theory and Applications

With the growing interest in polarization diversity in communications and radar systems, the use of Ludwig's second and third definitions has become controversial among scientists and antenna engineers. Therefore, this paper is an attempt to clarify some of the ambiguity and confusion caused by these definitions. A detailed comparison of Ludwig's second and thirrd definitions of cross polarization, as applied to linearly polarized antennas, was performed. The results show that, in the diagonal plane, Ludwig's second definition leads to a lower crosspolarization level than the third definition for x- or y-polarized current sources. For a Huygens source, by Ludwig's third definition, the radiation pattern has a lower cross-polarization level than that obtained by Ludwig's second definition. For some applications, the antenna is usually placed in the yz plane. Therefore, new polarization bases are proposed according to which the source is used as a reference, and also on how this source is oriented in the yz plane. To complement the theoretical framework demonstrated in this contribution and to provide readers with a better and simpler understanding of the crosspolarization definition, the analysis of several practical antennas for diverse applications was presented. Numerical and measured radiation patterns of wire and printed dipoles, rectangular patch, pyramidal horn, and open-ended rectangular waveguide (OEWG) antennas were investigated according to the polarization formulations presented in this paper. In addition, a dual-polarized element and a dual-polarized active phased array at broadside were utilized to generalize the application.

The Dirichlet Casimir energy for ϕ4 theory in a rectangular waveguide

In this paper, we presented the zero- and first-order radiative corrections to the Casimir energy for a massive scalar field confined with Dirichlet boundary condition in an open-ended rectangular waveguide. In the calculation procedure, we applied a systematic renormalization program that allows all influences imposed by dominant boundary conditions in a problem be automatically reflected in the counterterms, leading the counterterms to be obtained in a position-dependent manner. To remove the appeared divergences in the computation task, the box subtraction scheme as a regularization technique was used. In this regularization technique, usually, two similar configurations were introduced. Then, to find the Casimir energy, the zero-point energies of these two configurations were subtracted from each other via defining appropriate limits. In the present work, first, the leading order Casimir energy for the massive scalar field in a waveguide is briefly presented. Next, by applying the renormalization and regularization procedures, the first-order radiative correction to the Casimir energy in the waveguide is calculated. Finally, all the necessary limits of the obtained answers for massive and massless cases are computed and the consistency of the obtained results are discussed.

Millimeter-Wave Wideband Circularly Polarized Planar Complementary Source Antenna With Endfire Radiation

A novel circularly polarized (CP) complementary source antenna with endfire radiation is proposed in Ka-band. With the existence of two antipodal notches etched at the edges of the two broad walls of an open-ended substrate-integrated waveguide, two orthogonal electric field components radiated from the equivalent magnetic current, and the electric currents separately can be excited simultaneously. The magnitude and phase differences between the two filed components can be controlled effectively by properly tuning the dimensions of the notches. The operating mechanism and the design procedure of the antenna are analyzed in detail. Wide −10 dB impedance and 3 dB axial ratio bandwidths of 64% and 51%, a gain varying from 3.1 to 6.4 dBic, and the symmetrical radiation pattern are obtained. In order to further increase the gain and front-to-back ratio (FTBR) of the antenna, a dielectric rod structure is then integrated with the antenna. An overlapped operating bandwidth of 41%, an improved gain up to 12 dBic and the stable radiation pattern with an FTBR close to 20 dB are verified by a fabricated prototype. The antenna presented in this paper provides a new mean to design the wideband endfire CP antenna with a simple configuration, which would be attractive for future millimeter-wave wireless systems.

Modeling of the multimodal radiation from an open-ended waveguide.

The multimodal radiation from the open end of a cylindrical waveguide with arbitrary wall thickness is solved by deriving algebraic solutions of the radiation impedance matrix, without restrictive hypothesis on the frequency range. The basic idea of the method is to turn the original unbounded problem into the problem of a cylindrical waveguide embedded in an infinite waveguide with an annular perfectly matched layer (PML) on its wall. Then, using a multimodal formalism of the guided wave propagation and a complex coordinate stretching PML, algebraic expressions are derived for the continuity and radiation conditions in this coupled system.

A Simulation-Assisted Non-destructive Approach for Permittivity Measurement Using an Open-Ended Microwave Waveguide

A new convenient and non-destructive permittivity measurement method is presented. No physical cut of specimens is needed here for material characterisation. In the setup, the material under test is placed in the near-field region of a microwave open-ended waveguide. An electromagnetic model of the setup is built in the Computer Simulation Technology simulation software. Employing optimisation, the permittivity is obtained from the measured reflection coefficients hbox {S}_{11}. Using the same technique, the effect of the model size is investigated that could reduce the modelling effort for large structures. The efficiency of a traditional method (i.e., Newton) and an intelligent algorithm (i.e. particle swarm optimisation) for permittivity calculation is thoroughly studied and compared. The proposed methodology is validated by experimental data. It is demonstrated that the proposed method can provide more accurate permittivity results than the intrusive in-waveguide measurement. The proposed methodology can contribute to electromagnetic analysis, thickness measurement and non-destructive evaluation.

A microfluidic sensor in coplanar waveguide configuration for localized micrometric liquid spectroscopy in microwaves regime

In this work, we present a sensor suitable for performing the spectroscopy on a localized micrometric volume of a generic liquid, in the spectral range comprised between 1 and 20 GHz. The sensor is based on two ungrounded, open-ended coplanar waveguides, acting as Input/Output ports, with a microfluidic channel passing through them. It works by detecting the change of the reflection and transmission of microwaves due to the variation of the dielectric properties characteristic of the liquid. The devices have been designed and numerically simulated to obtain the electromagnetic response in different conditions: an empty channel for having a zero-reference response and for observing variations as a function of liquids filling the channel. We considered standard (i.e., common in literature) liquids (deionized water, ethanol, and glycerol); by means of that set of simulations we were able to start the creation of the calibration curves and defining the sensitivity of the sensor with respect to dielectric properties of the liquid under test. Then, we realized a batch of prototypes utilizing a standard photolithographic process. Later we proceeded to check the operation of the sensors. We recorded experimental data by a Vector Network Analyser connected to the devices by means of microwave probes. Sensors are designed to perform both reflection and transmission measurements on the liquid sample, and local microscopy on a micrometric volume of the solution under test. Actually, the device works avoiding any contact between the metallic parts and the liquid that can flow uncontaminated through the microfluidic channel.

Bunch Imaging at the Open End of an Embedded Circular Waveguide

We consider a charged particle bunch passing along the axis of a semi-infinite open-ended circular vacuum waveguide placed into a collinear infinite waveguide with larger radius. The rigorous solution of this problem is constructed using residue-calculus technique. Analytical expressions for the Fourier harmonics of the field components are obtained in each domain of the structure. Numerical integration over frequency is also performed and time-domain field dependencies are obtained. The analytical results are compared with the results of the numerical simulation and an excellent agreement is observed. In particular, we report on the effect of a “bunch image,” a radiation pulse is generated in the coaxial area and has the form of the transverse electric field correlating with the longitudinal charge distribution within the bunch.

Open-Ended Half-Mode Substrate-Integrated Waveguide Sensor for Complex Permittivity Measurement

A novel open-ended half-mode substrate-integrated waveguide (HMSIW) sensor with dielectric flange for measuring complex permittivity of liquids, semisolids, and granular and particulate materials is presented. The open-ended HMSIW is designed and fabricated on FR4 substrate. The sensor model is described by using the aperture admittance model of open-ended waveguide. The complex permittivity of a given material is determined from measurement of the one-port reflection coefficient. For accurate measurement of the complex permittivity, the well-known three-material (air, distilled water, and 25% ethanol aqueous solution) calibration method was used. Complex permittivity of ethanol and 50% and 76% ethanol aqueous solutions measured with the HMSIW were in good agreement with those obtained by using an open-ended coaxial-line probe. In addition, results of measurements on saline solutions, gelatin samples, and canola seed samples of different moisture contents are shown.

Feasibility of using thermal response to Ka band millimeter wave heating to assess skin blood flow

Objective: Implementation of clinical guidelines for diagnosing peripheral artery disease will demand screening many millions of patients who are considered at-risk. This will require faster, easier screening technologies to identify patients with compromised blood flow to the extremities. Approach: The feasibility of using surface temperature response to Ka band (26.5–40 GHz) near-field irradiation to assess skin blood flow was explored using an animal model. Ears of domestic rabbits were subjected to low-power continuous wave radio frequency heating from an open-ended waveguide (WR-28) at f = 35 GHz. Three flow conditions were evaluated: (1) a baseline flow condition, (2) occluded flow and (3) reactive hyperemia. Surface temperatures were monitored continuously by means of an infrared thermography camera during each 2 min exposure. Main results: Ensemble average results showed significant differences (p < .05) at exposure times 30, 60, 90 and 120 s between baseline and occluded conditions, and between baseline and reactive hyperemia conditions. The occluded condition (N = 12) resulted in an average temperature increase of 21.4 °C ± 3.9 after 2 min, compared with an average increase of 12.1 °C ± 1.6 for baseline conditions (N = 9) and 4.7 °C ± 3.6 for post-occlusion/hyperemic conditions (N = 8). Significance: Results are compared with the results of a simple two parameter mathematical model. These results suggest a method for non-invasive skin blood flow assessment to screen for peripheral artery disease and associated risk of cardiovascular events.

Selection of Parameters of the Primary Feed of the Luneberg Lens for the Sum–Difference Operating Mode

Excitation of the Luneberg lens by an arc antenna array of open-ended rectangular waveguides forming the H10 wave is studied. Technical limitations imposed on the array of primary feeds in order to form the sum and difference operating modes of the antenna system are discussed.

Steel Corrosion Stages Characterization Using Open-Ended Rectangular Waveguide Probe

Safety evaluation of steel structures requires knowledge of corrosion progression stages. The deteriorative stage of corrosion involves multiple parameters, and thus it is difficult to be characterized by the model-based approaches. In this paper, we propose a steel corrosion stages characterization method using microwave open-ended rectangular waveguide (ORWG) probes and a statistical-based principal component analysis (PCA) method. Two ORWG probes operating in successive bands, ranging between 9.5 and 26.5 GHz, are utilized to obtain reflection coefficient spectra from specific sets of corrosion samples; i.e., uncoated corrosion progression, coated corrosion progression, and surface preparation. PCA is applied to extract corrosion progression feature from spectral responses of training samples. The robustness of the PC-based features is analyzed with influences of operating frequency, coating layer, and surface condition. It is found that the corrosion features extracted by the first principal component (PC1) from coated and uncoated corrosion samples are highly correlated to the corrosion progress regardless of probe parameter and coating layer.

Non-invasive Continuous Monitoring of Cerebral Edema Using Portable Microwave Based System

A portable non-invasive head detecting system based on microwave technology was developed for evaluation of cerebral edema change inside human brain. Real-time monitoring of cerebral edema in the brain helps the clinician to assess medical condition and treatment. In this work, a microwave signal was transmitted and coupled into an open-end circular waveguide sensor, incident on a 3D printed head phantom, and reflected back to receiver. Theoretically, the operation of this instrument depends on the conductivity contrast between cerebral edema and healthy brain tissues. The efficacy of the proposed detecting system is verified using 3D printed anatomically and dielectrically realistic human head phantoms with simulated cerebral edema targets with different size. Changes in the amplitude of time domain result were shown to be induced by the expansion or decrease of the edema volume. The eventual goal of this proposed head evaluating system is use in the hospital as an effective real-time monitoring tool.

Analysis and Design of a Metamaterial Lens Antenna Using the Theory of Characteristic Modes

A new single-layer metamaterial lens antenna aimed to operate at 10 GHz is proposed in this paper. The lens antenna consists of twelve capacitively coupled unit cells distributed along a ring and illuminated by an open-ended circular waveguide with a metallic resonant ring. The theory of characteristic modes is used to analyze the metamaterial lens, in order to provide an insight into the radiation characteristics of the antenna. The proposed antenna has been optimized, obtaining a large bandwidth and a maximum directivity of 12.88 dBi at 10 GHz.

Accurate identification of whispering gallery mode patterns of gyrotron with stabilized electro-optic imaging system

The precise field pattern measurement and analysis of a typical whispering gallery mode excited in a gyrotron are important to understand the interaction physics of the gyrotron. We precisely analyzed the characteristic of a whispering gallery mode, rotating TE6,2 mode, by a photonic-assisted W-band (75–110 GHz) electro-optic imaging measurement system. The whispering gallery mode in the W-band region diverges fast in free space as it propagates from the radiation port. Therefore, scanning the field patterns of a device-under-test should be performed as close as possible to identify the device's characteristics. We successfully accomplished visualizing highly accurate field patterns of a rotating and mixed whispering gallery mode based on the measured electric field magnitude and phase by using dual optical fiber-scale electro-optic (EO) probes. We observed the distorted fields when the typical open-ended waveguide and a general EO probe were used in the extremely near-field zone, whereas a very precise f...

Smart Compressed Sensing for Online Evaluation of CFRP Structure Integrity

This paper investigates a smart sensing technique for online and automatic evaluation for carbon fiber reinforced polymer (CFRP) structures’ integrity. A compressed sensing (CS)-based algorithm framework integrating sampling and defect evaluation together is proposed enabling intelligence of an open-ended waveguide imaging system for the first time. Compared to traditional raster scan designs which require complete sampling, this smart CS technique can generate whole damage pattern while the scanning is conducting, the sensor intelligence is achieved by the proposed algorithm framework rather than seeking a hardware update. The CS enables accumulated downsampling and sparse recovery based on the frequency sparsity of impact damages on CFRP structures. Damage pattern in the reconstructed image can be detected using the proposed histogram threshold edge detection (HTED) algorithm when the image is stabilized enough. Edge-preserving smoothing is used to improve the stability of the reconstructed image. The experimental results illustrate time efficiency of the framework and more accurate damage localization using HTED. The proposed smart CS technique is attractive in quality control of CFRPs production. This technique can also be applied to situations where the sampled data is partly lost. Dispensing with hardware updates incurs minimum disruption and also benefits cost control and improves the productivity.

Polarization-dependent bi-functional metasurface for directive radiation and diffusion-like scattering

In this paper, we design a bi-functional metasurface with different spatial distribution of reflection phase responses depending on the incident polarization. The metasurface with a thickness of only 0.067 λ0 (λ0 is the working wavelength) is constructed by unit cells composing two orthogonal I-shaped metallic structures, and the reflection phase for x- and y-linearly polarized incidence can be independently controlled by the geometric parameters. The metasurface can work as a flat parabolic reflector antenna with a maximum gain reaching about 22 dBi around 9.5 GHz, when it is illuminated by the x-polarized feed source of an offset open-ended waveguide antenna. Meanwhile, designed with randomly distributed reflection phases, the proposed metasurface can behave as an electromagnetic (EM) diffusion-like surface, which is capable of suppressing the backward scattering in a broadband from 8.5 GHz to 14 GHz for y-polarized incidence. By this strategy of EM functionality integration, a metasurface reflector antenna equipped with stealth technique to achieve simultaneously high gain and low backward scattering is obtained. Finally, experiments have been carried out to demonstrate this design principle, which agree with the simulation results. The proposed metasurface could offer a promising route for designing EM devices with polarization-dependent multi-functionalities.

Improved finite‐range gain formula for open‐ended rectangular waveguides and pyramidal horns

A finite-range gain expression given in the literature for open-ended rectangular waveguide is improved by employing the gain correction factor approach of Polk by returning to the original aperture field integrals. In these, the phase factor is approximated to third order and the amplitude factor to second order with distance from the antenna from which series approximations have been obtained to simplify computation of gain. Comparison with measured and/or simulated results shows that the closed-form solution is as accurate as a previous expression beyond the far-field boundary, and is more accurate at shorter distances. The formulation is extended to pyramidal horns where results obtained compare well with experiment.