Two-port Vector Network Analyzer Research Articles

A calibration method for vector network analyzers using a line and three or more offset-reflect standards

Abstract This article proposes a method for two-port vector network analyzer (VNA) calibration that uses a line standard and three or more offset-reflect standards (LnΓ, where n is the number of offset-reflect standards and n ≥ 3). The reflection coefficient of the highly reflecting offset-reflect standards and the propagation constant of the transmission line are unknown for this calibration method, but they can be obtained in the process of calculating the VNA calibration error coefficients. In the LnΓ calibration method, the calibration fixtures are all equal in mechanical length. Therefore, compared with the thru-reflect-line (TRL) method, this calibration method is convenient when the test fixtures of both sides of the VNA are fixed or difficult to move during the measurement. To further simplify the experimental operation, a corrugated reflector is also designed. In this calibration method, only one corrugated reflector used as the offset-reflect standard is needed. Different offset-reflect standards are realized easily by changing the position on the microstrip line. The proposed LnΓ calibration method is accurate, which is proven by theoretical analysis, simulations, and measurements.

Robustness Improvement for Chipless RFID Reading Using Polarization Separation

In this article, a novel method to improve the readability of chipless tags is presented. When the EM wave falls upon the tag’s surface at normal incidence or with a known incidence, the proposed approach permits to obtain a higher signal-to-noise ratio (SNR) of resonators whose orientation with respect to the normal to the plane in which the tag is positioned is unknown and, thus, to increase the reading distance of the tag. It introduces a technique based on the detection of this tag orientation in relation to the antennas; a projection of the signals will allow correcting this misalignment in order to always be able to decode the tag identifier on a signal that corresponds to an ideal alignment between the tag and the antennas. With this principle, it is also possible to separate the resonance mode of the resonator that is connected to the identifier from other parasitic resonance modes that may appear due to a possible misalignment between the tag and the antenna. This principle is applied to loop resonators commonly used in chipless radio frequency identification (RFID), which can present strong parasitic modes of resonance. Also, this method makes the reading orientation invariant, allowing to read the resonator whatever its orientation with respect to the normal to the plane in which the tag is positioned. Likewise, the proposed method can be also used to sense this resonator orientation. This method requires a dual-polarization antenna and a two-port vector network analyzer (VNA). Results are validated in simulation and with real-environment measurements for a single resonator and multiple resonators, as it is classically used in chipless RFID technology. Measurements in a harsh environment, as well as comparisons with classical techniques, such as time gating (TG), are presented. A study on the improvement of the read range is achieved to highlight the potential of the proposed technique. A reading distance of more than 80 cm was, thus, obtained, which represents 20 cm more than with the classic use of a postprocessor based on time gating. Finally, the principle of the measurement is generalized by applying it to a tag with a ground plane and considering that the antenna is not necessarily placed in normal incidence with respect to the plane of the tag.

Broadband Measurements of Soil Complex Permittivity.

Agriculture is a major consumer of freshwater and is often associated with low water productivity. To prevent drought, farmers tend to over-irrigate, putting a strain on the ever-depleting groundwater resources. To improve modern agricultural techniques and conserve water, quick and accurate estimates of soil water content (SWC) should be made, and irrigation timed correctly in order to optimize crop yield and water use. In this study, soil samples common to the Maltese Islands having different clay, sand, and silt contents were, primarily, investigated to: (a) deduce whether the dielectric constant can be considered as a viable indicator of the SWC for the soils of Malta; (b) determine how soil compaction affects the dielectric constant measurements; and (c) to create calibration curves to directly relate the dielectric constant and the SWC for two different soil types of low and high density. The measurements, which were carried out in the X-band, were facilitated by an experimental setup comprising a two-port Vector Network Analyzer (VNA) connected to a rectangular waveguide system. From data analysis, it was found that for each soil investigated, the dielectric constant increases notably with an increase in both density and SWC. Our findings are expected to aid in future numerical analysis and simulations aimed at developing low-cost, minimally invasive Microwave (MW) systems for localized SWC sensing, and hence, in agricultural water conservation. However, it should be noted that a statistically significant relationship between soil texture and the dielectric constant could not be determined at this stage.

Propagation Constant Measurement Based on a Single Transmission Line Standard Using a Two-Port VNA.

This study presents a new method for measuring the propagation constant of transmission lines using a single line standard and without prior calibration of a two-port vector network analyzer (VNA). The method provides accurate results by emulating multiple line standards of the multiline calibration method. Each line standard was realized by sweeping an unknown network along a transmission line. The network need not be symmetric or reciprocal, but must exhibit both transmission and reflection. We performed measurements using a slab coaxial airline and repeated the measurements on three different VNAs. The measured propagation constant of the slab coaxial airline from all VNAs was nearly identical. By avoiding disconnecting or moving the cables, the proposed method eliminates errors related to the repeatability of connectors, resulting in improved broadband traceability to SI units.

Shielding effectiveness performance of polyaniline-NiFe2O4:Cu composites for sub-8 GHz applications

Herein, NiFe2O4 doped Cu was synthesized using a mixed-oxide method to investigate its potential for creating composites with high microwave shielding effectiveness. The compound NiFe2−xCuxO4 was synthesized with x values of 0.1, 0.3 and 0.5, respectively. After sintering at 1250 °C for 4 h, single-phase Ni ferrite was formed. To analyze the phase composition and the structure of the synthesized compound, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were employed. The study's findings showed that NiFe2−xCuxO4 did not exhibit a second phase. To create composites with high microwave shielding effectiveness, polyaniline-NiFe2O4:Cu composites were fabricated using a hot-pressing technique, with compositions of NiFe1.9Cu0.1O3.95, NiFe1.7Cu0.3O3.85 and NiFe1.5Cu0.5O3.75 with the aniline, The weight ratios of Cu-added nickel ferrite and aniline were changed from 1:1 to 1:3, and epoxy resin was used. Using a two-port vector network analyzer, the polyaniline-NiFe2O4:Cu composites’ microwave shielding effectiveness performance was examined in the range between 0 and 8 GHz. The study found that the shielding effect of the composites could be easily modified by changing the amount of polyaniline present in the specimens for the appropriate frequency bands. At 6.82 GHz, using a sample with a thickness of 2.0 mm, a minimum shielding effect performance of − 29.74 dB was achieved. Overall, the results of this study demonstrate the potential of polyaniline-NiFe2O4:Cu composites as effective microwave shielding materials.

Методика измерения матрицы рассеяния многопортового устройства двухпортовым векторным анализатором цепей

When designing transceiver paths, the problem of modeling the characteristics of individual components arises. The most universal and convenient for describing the microwave device is the scattering matrix. The use of two-port and four-port vector network analyzers allows to adjust the parameters of most of the developed microwave devices (dividers, filters, amplifiers,etc.).Often, the task is to use a two-port vector network analyzer to measure the parameters of a multi-port device (ports more than 4) to solve the problem of calculating the scattering matrix of the bone connection of adjacent industrial units. The purpose of this article is to compile a methodology for measuring the scattering of multiport devices with a two-port vector network analyzer with a sequence to obtain a measurement of a multi-channel final device (multi-channel radio path). Achieving this goal allows us to provide the necessary basis for the synthesis of domestic software used to develop microwave paths from the point of view of an overview of system analysis. Keywords: S-parameter measurement technique, S-parameter measurement, scattering matrix, vector network analyzer, microwave device cascading.

Pulsed free space two-port photonic vector network analyzer with up to 2 THz bandwidth.

We demonstrate a free space two-port photonic vector network analyzer capable of measuring the scattering parameters of devices and materials in the terahertz range with a frequency coverage of 0.2 - 2 THz in a single system. It is based on photoconductive terahertz sources and detectors driven by a telecom-wavelength femtosecond laser. Being able to cover a bandwidth of one order of magnitude, the system is capable of performing S-parameter measurements deep into the terahertz range, beyond frequencies reachable by their electronic counterparts. We demonstrate high performance at three application examples, namely S-parameter measurements of a split ring resonator array and a distributed Bragg reflector, as well as material parameter extraction of several materials.

Monitoring Wound Health through Bandages with Passive LC Resonant Sensors.

This paper details a passive, inductor-capacitor (LC) resonant sensor embedded in a commercial dressing for low-cost, contact-free monitoring of a wound; this would enable tracking of the healing process while keeping the site closed and sterile. Spiral LC resonators were fabricated from flexible, copper-coated polyimide and interrogated using external reader antennas connected to a two-port vector network analyzer; the forward transmission scattering parameter (S21) magnitude was collected, and the resonant frequency (MHz) and the peak-to-peak amplitude of the resonant feature were identified. These increase during the healing process as the permittivity and conductivity of the tissue change. The sensor was first tested on gelatin-based tissue-mimicking phantoms that simulate layers of muscle, blood, fat, and skin at varying phases of wound healing. Finite element modeling was also used to verify the empirical results based on the expected variations in dielectric properties of the tissue. The performance of the resonant sensors for in vivo applications was investigated by conducting animal studies using canine patients that presented with a natural wound as well as a controlled cohort of rat models with surgically administered wounds. Finally, transfer functions are presented that relate the resonant frequency to wound size using an exponential model (R2 = 0.58-0.96). The next steps in sensor design and fabrication as well as the reading platform to achieve the goal of a universal calibration curve are then discussed.

Influence of Noise on Scattering-Parameter Measurements.

We present a general model of noisy scattering-parameter (S-parameter) measurements performed by a vector network analyzer (VNA). The residual error of the S-parameter due to the noise is examined to appear like a complex Gaussian quotient. The statistical analysis of the residual error is given, and relevant statistical quantities are derived and discussed. Experiments were conducted on a two-port VNA to validate the noise-influenced S-parameter model. We show that the uncertainty due to the noise is often critical in S-parameter measurements, in particular for S-parameters of a small magnitude.

A Packaged 0.01–26-GHz Single-Chip SiGe Reflectometer for Two-Port Vector Network Analyzers

This article presents a packaged SiGe BiCMOS reflectometer for 0.01–26-GHz two-port vector network analyzers (VNAs). The reflectometer chip is composed of a resistive bridge coupler and two wideband heterodyne receivers for coherent magnitude and phase detection. In addition, a high-linearity receiver channel is designed to accommodate ~20 dBm of RF input power to the reflectometer. External sources are used to provide stimulus over the entire bandwidth, and a high-linearity shunt 50- $\Omega $ switch is placed at the input of each reflectometer chip to provide a matched condition for better measurement accuracy. The measured dynamic range (DR) of the heterodyne receiver is 129±3 dB at 0.01–26 GHz at an IF resolution bandwidth (RBW) of 10 Hz. The reflectometer chip is packaged on a low-cost printed circuit board (PCB) and connectorized for repeatable measurements. Several passive and active device-under-test (DUTs), such as filters and amplifiers, are measured in single- and dual-port complex $S$ -parameter setup at 0.01–26 GHz. The single-chip reflectometer results in excellent agreement with commercially available test equipment and with minimal magnitude and phase difference. Several definitions (mean deviation and error vector) are used to quantify the measurement accuracy. A measured $S_{21}$ of −80 dB is obtained using high-rejection filters. An in-depth DR analysis of possible limiting factors in the measurement setup is also presented. This article shows that silicon-based highly integrated reflectometers can be used in low-cost VNAs with excellent DR and accuracy.

Microwave Instrument for Simultaneous Wet Antenna Attenuation and Precipitation Measurement

Attenuation data from microwave links can be used to derive important near-ground precipitation estimates which complements observations from rain gauges and weather radars. Wetness on the cover of the microwave link antennas can, however, lead to additional attenuation, resulting in an overestimation of rainfall rates. To investigate the impact of this wet antenna attenuation (WAA) effect, a versatile microwave field experiment was built. It is based on a two-port vector network analyzer and equipped with six dual-polarized commercial microwave link antennas at different frequencies between 18 and 40 GHz. To a measurement frequency range of 150 MHz, range gating enables simultaneous measurement of antenna reflection coefficient and total path attenuation. An optical camera is integrated for visual inspection of the antenna covers. Appropriate housing and active temperature control makes the device weather-proof and enables all-season operation. Several supplementary meteorological instruments at the installation site provide information about the actual weather conditions. They can be used as the ground truth for evaluation. The presented instrument is a unique setup to study the WAA effect, by simultaneously providing measurements of the total path attenuation together with reliable WAA estimations, directly derived from measurements of the antenna reflection coefficient. Its data will give insights into WAA magnitude and temporal evolution in relation to meteorological variables. This will help developing improved WAA estimation algorithms, which will result in improved rainfall estimation from microwave links.

Broadband Determination of the Even- and Odd-Mode Propagation Constants of Coupled Lines Based on Two-Port Measurements

We describe a method for the broadband measurement of the even- and odd-mode propagation constants of symmetrical coupled transmission lines. The main advantage of the method, its simplicity, lies in the lack of need for four-port measurements. Only two-port measurements are required, and thus just a two-port vector network analyzer (VNA). The accuracy of the results can be increased at the cost of a higher number of measured coupled-line lengths. The new approach has been successfully applied for the determination of coupled microstrip lines in the 2- to 20-GHz frequency band in both simulated and experimental cases. A brief discussion about the repeatability of the microstrip launcher and the adverse effects of radiation is included.

Magnetic Near-Field Measurement With a Differential Probe to Suppress Common Mode Noise

To measure the magnetic near field with high common-mode noise rejection, a probe with the differential and symmetrical structure forming a detection loop is developed. Such measurement is performed by using a two-port vector network analyzer (VNA) and validated by using a microstrip line as a device under test (DUT). In comparison to the measurement with traditional single-output magnetic near field probe, the electric field suppression of the proposed measurement can be improved by at least 10 dB in the frequency range of $3\sim 11$ GHz, while the other characteristics remain unchanged.

Complex Permittivity and Microwave Absorption Properties of OPEFB Fiber-Polycaprolactone Composites Filled with Recycled Hematite (α-Fe2O3) Nanoparticles.

Recycled hematite (α-Fe2O3) nanoparticles with enhanced complex permittivity properties have been incorporated as a filler in a polycaprolactone (PCL) matrix reinforced with oil palm empty fruit bunch (OPEFB) fiber for microwave absorption applications. The complex permittivity values were improved by reducing the particle sizes to the nano scale via high-energy ball milling for 12 h. A total of 5–20 wt.% recycled α-Fe2O3/OPEFB/PCL nanocomposites were examined for their complex permittivity and microwave absorption properties via the open ended coaxial (OEC) technique and the transmission/reflection line measurement using a microstrip connected to a two-port vector network analyzer. The microstructural analysis of the samples included X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR). At 1 GHz, the real (ε′) and imaginary (ε″) parts of complex permittivity of recycled α-Fe2O3 particles, respectively, increased from 7.88 to 12.75 and 0.14 to 0.40 when the particle size was reduced from 1.73 μm to 16.2 nm. A minimum reflection loss of −24.2 dB was achieved by the 20 wt.% nanocomposite at 2.4 GHz. Recycled α-Fe2O3 nanoparticles are effective fillers for microwave absorbing polymer-based composites in 1–4 GHz range applications.

Design and Experimental Assessment of a 2D Microwave Imaging System for Brain Stroke Monitoring

The aim of this paper is to present and experimentally verify the first prototype of a microwave imaging system specifically designed and realized for the continuous monitoring of patients affected by brain stroke, immediately after its onset and diagnosis. The device is a 2D version of the 3D system, currently under construction, and consists of an array of 12 printed monopole antennas connected to a two-port vector network analyzer through a switching matrix so that each antenna can act as a transmitter or receiver, thereby allowing the acquisition of the entire multistatic multiview scattering matrix required for the imaging. The system has been experimentally tested on 2D phantoms with electric properties mimicking the brain. The presence and the evolution of the stroke have been reproduced by filling a proper cavity in the phantom with a liquid having the electric properties of blood. A differential approach has been adopted by acquiring the scattering matrix before and after the filling of the blood cavity. The so achieved differential dataset has been processed by means of a linear imaging algorithm in order to reconstruct the stroke location and dimension. Moreover, the effect of pre- and postprocessing operations on the measured data is investigated. A good agreement has been obtained between the reconstructions and the actual scenario. As a final remark, it is worth noting that the entire data acquisition and processing are sufficiently fast to allow a real-time monitoring.

Simultaneous Measurement of Electric and Magnetic Fields With a Dual Probe for Efficient Near-Field Scanning

A near-field scanning is usually time-consuming to map the electromagnetic field along the surface of a device. In this communication, a dual probe is proposed with a U-shaped loop to sense radio frequency (RF) magnetic and electric fields simultaneously, so that the one-time scanning by the dual probe can replace the two-time scanning by single-detector probes. Different from the traditional near-field scanning by using a single-detector probe and spectrum analyzer, the proposed scanning can be achieved by the two-output probe and a two-port vector network analyzer connecting to the probe. The efficient near-field scanning measurements of the electric and magnetic field on a conductor-backed coplanar waveguide transmission line and an RF power divider by using the dual probe are in agreement with those by using two single-detector probes.

The Relationship Between Switch-Term-Corrected Scattering-Parameters and Wave-Parameters Measured With a Two-Port Vector Network Analyzer

We explore the relationship between scattering-parameters (S-parameters), corrected for switch terms, and S-parameters calculated from wave-parameters measured with a two-port vector network analyzer. By deriving the equations for both representations using consistent notation, we clearly demonstrate their equivalence. This result proves that measuring wave-parameters enables us to obtain uniquely defined S-parameters for any changes of impedance behind the signal-separation hardware.

A Multistandard Method of Network Analyzer Self-Calibration—Generalization of Multiline TRL

This paper presents a statistical self-calibration procedure for two-port vector network analyzers (VNAs). It is a generalization of the Multiline thru-reflect-line (TRL) method and allows to combine the information of multiple A-standards, which can be realized by any matched networks. Moreover, the reflect standard is replaced by a more general network standard as a symmetrical network that may have nonzero transmission. In consequence, this method offers more flexibility in the choice of calibration standards and an extended bandwidth especially toward lower frequencies. At the same time, the improved accuracy over conventional methods persists, as well as the simple implementation, avoiding iterative solutions. The presented error analysis yields some new insights on the choice of calibration standards. In addition, a method is provided to modify the performed weighting of the information of different standards.

Experimental setup for transfer function measurement to assess RF heating of medical leads in MRI: Validation in the case of a single wire.

The measurement of the transfer function is a good tool to evaluate the radiofrequency heating of complex conductive wires, such as pacemaker leads. The aim is to describe precisely the design of a transfer function bench and compare the measurements to simulations. The transfer function was measured by mean of an excitation probe and a receiving probe, both connected to a two-port vector network analyzer. The experimental results were compared with the simulated results, reproducing the excitation scheme. This procedure was applied to two different cables with different geometrical and insulation properties to test the robustness of the setup. It is possible to touch the cable electrode with the excitation probe without inducing an error in the measured transfer function, which solves the direct coupling problem. There is a good agreement between the measured and simulated transfer function for both tested cables. A valid transfer function measurement bench is described. Magn Reson Med 79:1766-1772, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Microwave Interferometry for High Sensitivity VNA-FMR Measurements

Two-port vector network analyzer ferromagnetic resonance (VNA-FMR) measurements using coplanar waveguides (CPWs) are a fast and powerful tool for the analysis of ferromagnetic film parameters, including damping and effective field contributions like anisotropies, exchange coupling, and demagnetization fields. Thereby, VNA-FMR appears to be a good choice for the analysis of patterned magnetic nanostructures showing a rich spectrum of different anisotropy terms. However, due to their small volume, the absorption of nanostructures in FMR is low and the measurement signal is superimposed by the excitation signal. To eliminate the background signal, we propose an interferometric approach with the following two key features: 1) the reference path has the same geometry as the measurement path, including an identical but empty CPW parallel to the measurement CPW to remove spurious field dependent contributions to the signal and 2) additionally, we extend the reference path by 0.5 m of an RF coaxial cable. This path length difference leads to a frequency-dependent phase difference of the two paths, assuming a value of 180 ° in ≈ 500 MHz intervals. Here, the excitation RF signals from both paths add to zero. In broadband measurements, this simple approach allows to obtain a sufficient number of grid points for dispersion analysis without the need for a phase shifter. Our experiments reveal an improvement of the sensitivity by up to two orders in magnitude under optimum conditions. The amplitude and the shape of the resonance peaks will be discussed.