Free-space Microwave Measurement Research Articles

Hybrid ECM Blended Whale Optimization Derived Frequency-Selective Conformal EMI Shielding Structure Using Ferrite Substrate

In this work, both planar and conformal novel electromagnetic interference (EMI) shielding structures with customized properties are developed using a unique coupling of frequency-selective surface (FSS) and magnetic substrate. A hybrid equivalent circuit model (ECM) blended whale optimization (WO) technique is offered in the quest of customized shielding of L-, S-, C-, X-, and Ku-bands using a simple FSS geometry. The proposed structure is designed to shield L-, S-, C-, X-, and Ku-bands with a fractional bandwidth of 66.67%, 66.67%, 66.67%, 50.2%, and 40.50%, respectively. The frequency-dependent complex dielectric permittivity ( ε'- jε") and complex magnetic permeability ( μ'- jμ") of ferrite are measured using a waveguide measurement setup at X-band. The measured electromagnetic parameters are utilized in the modeling of planar and conformal ferrite-based frequency-selective shielding structures. An optimal ferrite coupled FSS shielding structure is fabricated, and its performance is evaluated using a non-destructive free-space microwave measurement setup. The measured results are found in congruence with the full-wave simulation and WO results. Besides, the conformal configuration of the proposed shielding structure is peculiarly investigated for inward and outward curved shielding structures for different values of radius of curvature. The proposed technique is intended to achieve a customized, simple, wideband, polarization-insensitive, and angular stable structure in the desired frequency regime, thus verifying the potential of the aforementioned technique for shielding applications.

Design, Fabrication and Non-destructive Microwave Measurement of a Quad Resonance, Mono-band Metamaterial Polarization Converter Realized by a Fractal Inspired Split-Ring Resonator

The development and measurement of a single layer, wideband, and angularly stable microwave polarization converter (PC), specifically at a low-frequency regime, is still a very challenging task. In this article, a quad resonance, mono-band metamaterial polarization converter is fabricated and measured using a non-destructive free-space microwave measurement setup. A comprehensive study of a single-layer metamaterial (MTM) structure using a miniaturized fractal element array reveals that four resonances can be achieved within its operating frequency band to realize a wide operating band. The proposed MTM-PC is inspired by the split-ring resonator (SRR) based fractal geometry to control the linearly polarized electromagnetic (EM) wave in the C-band. The result shows the average polarization conversion ratio over 87% in a single broad frequency band (i.e., 5.5 GHz to 8.0 GHz). A careful study reveals that wideband response at lower frequency is achieved by introducing four resonances due to fractal inspired SRR geometry. Moreover, the proposed structure is the oblique angle insensitive up to 45°. The results reflect the enormous potential of the proposed approach for various practical EM applications.

Tailoring Antenna Focal Plane Characteristics for a Compact Free-Space Microwave Complex Dielectric Permittivity Measurement Setup

This article presents a compact precision free-space microwave measurement setup with a choice of three dielectric lenses to tailor the antenna focal plane characteristics for extracting complex dielectric permittivity of small samples. Custom designed spot-focusing horn antenna pairs were used to achieve a compact setup with antenna separation distance, 2 f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</sub> :4λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> -8λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and focal spot size, f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> :1λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> -1.5λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , where λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is the wavelength at center frequency. Using the compact free-space setup, relative complex permittivity (e' - je'') was extracted over 8-12 GHz for lowand high-loss dielectrics with lateral dimensions, 3.3λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> × 3.3λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and 10λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> × 10λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . For large materials under test (MUTs), i.e., 10λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> × 10λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , measurement accuracy in dielectric constant, Δe'% was <; 0.65% and <; 1.14% for low- and high-loss dielectrics, respectively. For smaller MUTs (3.3λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> × 3.3λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ), Ae'% was <; 0.89% and <; 2.29% for low- and high-loss MUTs, respectively. The error in loss tangent (Δtanδ) varied over 0.002-0.016 and 0.015-0.056 for large (10λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> × 10λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and small MUTs (3.3λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> × 3.3λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ), respectively. For large MUTs, biconvex lens pair with the smallest f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> (1λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</sub> (4λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) among the three lenses yielded the best accuracy in dielectric constant (e') due to tight field focusing at the focal plane. The plano-convex lens pair yielded the best accuracy in loss tangent (tanδ = e''/e') for large MUTs due to slow variation in the phase of the local plane wave. By tailoring antenna focal plane characteristics, a compact free-space setup that is 6×-10× smaller than the classical setup for handling MUTs that are 1/5th of the size used in classical setup is demonstrated without compromising the measurement accuracy.

Design and characterization for a high-temperature dual-band radome wall structure for airborne applications

Modern aircraft have reached higher and higher flight speed, resulting in the urgent demand of design and characterization for high-temperature airborne radomes, especially for those with dual-band or multi-band properties. In this paper, an A-sandwich radome wall structure made of quartz is designed for both centimeter and millimeter wave applications at high temperatures. Its transmission performance in the 4–40GHz frequency range at ambient temperature to 800°C is characterized by a broadband free-space microwave measurement system, which is mainly composed of a vector network analyzer, spot-focusing lens horn antennas, and a furnace for heating sample. A calibration method is proposed to eliminate the temperature-dependent distortion of microwave signals by the microwave-transparent walls of the furnace, and the errors caused by the wall position variations as well. Transmission measurements show that the radome wall structure is feasible for dual-band applications in the 4–10GHz and 24–40GHz frequency spans with the transmission efficiency higher than 70% at temperatures up to 800°C, and demonstrate the effectiveness of the design method. It can be expected that the free-space microwave measurement system in conjunction with the proposed calibration method is capable of damage detection for aerospace structures under high-temperature conditions.

Ultra-lightweight hybrid woven fabric containing stainless steel/polyester composite yarn for total EMI shielding in frequency range 8–18 GHz

This article describes the preparation of composite conductive yarn from stainless steel and polyester staple fibre with different fibre composition adopting worsted spinning system. The conducting yarn was successfully woven into variety of hybrid woven structures in the Picanol Gammax Rapier Loom. The aim of this research was to develop conducting yarn of different conductivity level (linear resistance 6–9 kΩ/m to 150–200 Ω/m) and subsequent conversion of yarn into ultra-lightweight woven fabric (~100 gsm) for providing electro-magnetic interference/radio frequency interference shielding effectiveness to the electronic equipment/devices. The electromagnetic shielding effectiveness of these woven fabrics was evaluated in free space microwave measurement system in microwave frequency (8–18 GHz) for vertical and horizontal polarization of electromagnetic wave. EMSE of ~31 dB in single layer of fabric is achieved. The effect of various fabric parameters, i.e. yarn spacing, yarn density and weave structure, on behaviour of electromagnetic wave has also been studied.

Carbon black/ polyurethane nanocomposite-coated fabric for microwave attenuation in X &amp; Ku-band (8–18 GHz) frequency range

In the present study, coating formulations based on carbon nanoparticle (125–150 nm) and two component polyurethane resin [Polyester polyol-8 and hexamethylene diisocyanate [C8H12N2O2 (NCO-(CH2)6–NCO)] have been prepared in concentration of 5,6,7,8, 9 and 10 g of carbon black in 100 ml of polyurethane solution and was applied on cotton fabric adopting knife over roller coating technique. The coated fabrics were evaluated for surface resistivity and relevant microwave properties i.e. permittivity, scattering parameters, reflection, transmission, absorption, reflection loss and electromagnetic interference shielding in HVS free-space microwave measurement system. It is observed that increasing the carbon content increases the permittivity values and decreases the impedance. It is seen that with increase of carbon content up to 7 g, absorption increases due to increase of permittivity while with further increase of carbon content from 8 to 10 g, absorption decreases due to increase of conductivity and thereby increased reflection. The coated fabric at 6.55 wt% carbon and 93.45 wt% PU in thickness of 1.3 mm has shown about 46% absorption, 29.49% transmission and 24.5% reflectance in X (8.2–12.4 GHz) and Ku (12–18 GHz) frequency band. When electrical thickness of coated fabric equals λ/4, a peak of reflection loss is observed due to Salisbury absorption mechanism. Highest reflection loss of –31.39 dB at 17 GHz, more than 20 dB in bandwidth of 16–18 GHz and more than 10 dB in entire Ku- band is achieved. Coated fabric may be used as apparel for protecting human being from hazardous microwave and for radar absorbing structural materials for defence applications.

Microwave absorption in X and Ku band frequency of cotton fabric coated with Ni–Zn ferrite and carbon formulation in polyurethane matrix

The present study highlights various microwave properties, i.e. reflection, transmission, absorption and reflection loss, of the coated cotton fabric [formulation: Ni–Zn ferrite (Ni 0.5Zn0.5Fe2O4) and carbon black (acetylene black) at concentrations of 30, 40, 50, 60 and70g of ferrite and 5g carbon in each 100ml polyurethane] evaluated at 8–18GHz frequency. The uniform density of filling materials in coated fabrics (dotted marks in SEM micrograph) indicates homogeneous dispersion of conducting fillers in polyurethane and the density of filling material cluster increases with increase in ferrite concentration. SEM images also show uniform coating of conducting fillers/resin system over individual fibers and interweave spaces. The important parameters governing the microwave properties of coated fabrics i.e. permittivity and permeability, S-parameters, reflection loss, etc. were studied in a HVS free space microwave measurement system. The lossy character of coated fabric is found to increase with increase of ferrite content; the ferrite content decreases the impedance and increases the permittivity and permeability values. The 1.6–1.8mm thick coated fabric sample (40wt% ferrite, 3wt% carbon and 57wt% PU) has shown about 40% absorption, 20% transmission and 40% reflectance in X (8.2–12.4GHz) and Ku (12–18GHz) frequency bands. The reflection loss at 13.5GHz has shown the highest peak value (22.5dB) due to coated sample optical thickness equal to λ/4 and more than 7.5dB in entire Ku band. Owing to its thin and flexible nature, the coated fabric can be used as apparel in protecting human being from hazardous microwaves and also as radar camouflage covering screen in defense.

Corrections to “Error Correction for Diffraction and Multiple Scattering in Free-Space Microwave Measurement of Materials”

Corrections to “Error Correction for Diffraction and Multiple Scattering in Free-Space Microwave Measurement of Materials”

Error correction for diffraction and multiple scattering in free-space microwave measurement of materials

Metamaterials often have sharp resonances in permittivity or permeability at microwave frequencies. The sizes of the inclusions are of the order of millimeters, and this means that it is more convenient to carry out the measurement in free space. Time gating is often used in the free-space method to remove multiple scattering from the antennas and the surrounding objects. However, this lowers the resolution in the frequency domain, making it difficult to resolve the resonances reliably. Diffraction around the sample could also reduce measurement accuracy. A calibration procedure, based on the 16-term error model, which removes the need for time gating by correcting for both multiple scattering and diffraction, is developed. This procedure is tested on carbonyl iron composite and split-ring resonators, and the results are presented.

Microwave Non-Destructive Testing of Coatings and Paints Using Free Space Microwave Measurement

Microwave nondestructive testing (MNDT) techniques are applied to evaluate quality of anti-corrosive protective coatings and paints on metal surfaces. A tree-space microwave measurement (FSMM) system is used for MNDT of protective coatings. The FSMM system consists of transmit and receive spot-focusing horn lens antennas, a vector network analyzer, mode transitions and a computer. Diffraction effects at the edges of the sample are minimized by using spot-focusing horn lens antennas. Errors due to multiple reflections between antennas are corrected by using free-space LRL (line, reflect, line) calibration technique. We have measured complex reflection coefficient of polyurethane based paint which is coated on brass plates.

Left- and right-handed transmission peaks near the magnetic resonance frequency in composite metamaterials

We present free-space microwave measurements on composite metamaterials (CMMs) consisting of split ring resonators (SRRs) and wires either on the same dielectric board or on alternating boards. Our experimental results disprove the widely held belief that the occurrence of a CMM transmission peak within the stop bands of the SRRs alone and wires alone constitutes a clear demonstration of left-handed (LH) behavior. This belief is based on the assumption that the stop bands of SRRs alone and wires alone are not affected by the simultaneous presence of both. We show here that this assumption is wrong: The effective plasma frequency, ${\ensuremath{\omega}}_{\mathrm{p}}^{\ensuremath{'}}$, of the CMM is actually substantially lower than the wires-only plasma frequency, ${\ensuremath{\omega}}_{\mathrm{p}}$; furthermore, the in-plane wires, as opposed to the off-plane case, push the magnetic resonance frequency of the SRRs, ${\ensuremath{\omega}}_{\mathrm{m}}$, to a higher value, ${\ensuremath{\omega}}_{\mathrm{m}}^{\ensuremath{'}}$, for the CMM. We conclude that the criterion for deciding whether a peak in the transmission spectrum through a CMM is really left-handed is for the peak to be located above ${\ensuremath{\omega}}_{\mathrm{m}}^{\ensuremath{'}}$ and below ${\ensuremath{\omega}}_{\mathrm{p}}^{\ensuremath{'}}$. Our results provide a definite way for experimentally identifying ${\ensuremath{\omega}}_{\mathrm{p}}^{\ensuremath{'}}$.

Free-space, broadband measurements of high-temperature, complex dielectric properties at microwave frequencies

A free-space microwave measurement system that is used for the high-temperature measurement of dielectric constants and loss tangents of homogeneous materials and that is applicable to composite materials as well is discussed. The system is capable of operating in the 5.85-40-GHz frequency range and ambient to 850 degrees C temperature range. A computer is used to control and coordinate furnace temperature, network analyzer functions, and data storage. Dielectric constants and loss tangents of the materials are calculated from the measured values of S/sub 21/. The measurement system, including the high-temperature furnace and the calibration technique, is described. Dielectric constants and loss tangents are presented for fused quartz and boron nitride grade HP in the frequency range 13.0-17.4 GHz and the temperature range from ambient to 850 degrees C. >

Determination of Plasma Properties by Free-Space Microwave Techniques

Time-resolved free-space microwave measurements are used to study the loss mechanisms in the afterglow of an intense He discharge between 100 and 1000 μ. The method of evaluation is developed in detail and the results of the measurements and their accuracy are given. Some of the problems and limitations of the microwave method are discussed; in particular, plots of the field pattern between the horns are presented indicating that a reasonable interpretation in terms of an integrated path length is possible.