IEEE microwave and wireless components letters : a publication of the IEEE Microwave Theory and Techniques Society

The knowledge of the electromagnetic constitutive properties of materials is crucial in many applications. Free-space methods are widely used for this purpose, despite their inherent practical difficulties. This paper describes an affordable free-space experimental setup for the characterization of flat samples in 1–6 GHz in a non-anechoic environment. The extracted properties are obtained from the calibrated Scattering Parameters, using a frequency-by-frequency solution or a multi-frequency reconstruction. For the first, we describe how the Time-Domain Gating can be implemented and used for filtering the signals. For the latter, a weighting factor is introduced to balance the reflection and transmission data, allowing one to have a more favorable configuration. The different role of transmission and reflection measurements on the achievable results is analyzed with regard to experimental uncertainties and different noise scenarios. Results from the two strategies are analyzed and compared. Good agreement between simulation, measurement and literature is obtained. According to the reported results for dielectric materials, there is no need of filtering the data by a Time-Domain Gating in case of the multi-frequency approach. Experimental results for Polymethylmethacrylate (PMMA) and Polytetrafluorethylene (PTFE) samples validate both the setup and the processing.

https://www.mdpi.com/2079-9292/7/10/260/pdf

Free-Space Materials Characterization by Reflection and Transmission Measurements using Frequency-by-Frequency and Multi-Frequency Algorithms

Abstract Low-loss tungsten-bronze microwave dielectric ceramics are dielectric materials with potential application value for miniaturized dielectric filters and antennas in the fifth-generation (5G) communication technology. In this work, a novel Al/Nd co-doping method of Ba 4 Nd 9.33 Ti 18 O 54 (BNT) ceramics with a chemical formula of Ba 4 Nd 9.33+ z /3 Ti 18− z Al z O 54 (BNT-AN, 0 ⩽ z ⩽ 2) was proposed to improve the dielectric properties through structural and defect modulation. Together with Al-doped ceramics (Ba 4 Nd 9.33 Ti 18− z Al 4 z /3 O 54 , BNT-A, 0 ⩽ z ⩽ 2) for comparison, the ceramics were prepared by a solid state method. It is found that Al/Nd co-doping method has a significant effect on improving the dielectric properties compared with Al doping. As the doping amount z increased, the relative dielectric constant ( ε r ) and the temperature coefficient of resonant frequency ( τ f ) of the ceramics decreased, and the Q×f values of the ceramics obviously increased when z ⩽ 1.5. Excellent microwave dielectric properties of ε r = 72.2, Q×f = 16,480 GHz, and τ f = +14.3 ppm/°C were achieved in BNT-AN ceramics with z = 1.25. Raman spectroscopy and thermally stimulated depolarization current (TSDC) technique were firstly combined to analyze the structures and defects in microwave dielectric ceramics. It is shown that the improvement on Q×f values was originated from the decrease in the strength of the A-site cation vibration and the concentration of oxygen vacancies ( $${\rm{V}}_{\rm{O}}^{ \cdot \cdot }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mtext>V</mml:mtext> <mml:mtext>O</mml:mtext> <mml:mrow> <mml:mo>⋅</mml:mo> <mml:mo>⋅</mml:mo> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ), demonstrating the effect and mechanism underlying for structural and defect modulation on the performance improvement of microwave dielectric ceramics. 

Structure, defects, and microwave dielectric properties of Al-doped and Al/Nd co-doped Ba4Nd9.33Ti18O54 ceramics

Permittivity of biological tissue is a critical issue for studying the biological effects of electromagnetic fields. Many theories and experiments were performed to measure or explain the permittivity characteristics in biological tissue. In this paper, we investigate the permittivity parameter in biological tissues via theoretical and experimental analysis. Firstly, we analyze the permittivity characteristic in tissue by using theories on composite material. Secondly, typical biological tissues, such as blood, fat, liver, and brain, are measured by HP4275A Multi-Frequency LCR Meter within 10 kHz to 10 MHz. Thirdly, experimental results are compared with the Bottcher-Bordewijk model, the Skipetrov equation, and the Maxwell-Gannett theory. From the theoretical perspective, blood and fat are regarded as the composition of liver and brain because of the high permittivity in blood and the opposite in fat. Volume fraction of blood in liver and brain is analyzed theoretically, and the applicability and the limitation of the models are also discussed. These results benefit further study on local biological effects of electromagnetic fields.

/pdf/effective-permittivity-of-biological-tissue-comparison-of-3mtvbj4i5o.pdf

Effective Permittivity of Biological Tissue: Comparison of Theoretical Model and Experiment

A fast calibration-independent method for complex permittivity determination at microwave frequencies

Effective permittivity of periodic composite materials: Numerical modeling by the finite element method

An improved microwave method to extract the complex permittivity of solid and liquid materials filled in a short-circuited waveguide is developed. The method determines accurately the dielectric constant of thin and moderate thick samples. It eliminates the problems arising from any position offset of the dielectric slab in transmission / reflection methods. The proposed method is iterative and the initial value is calculated by using the 7th approximation order of trigonometric terms in the exact reflection coefficient equation. This approach is applied to the simulated data of low loss and dissipative materials in limited frequency band.

https://www.scielo.br/pdf/jmoea/v11n2/a06v11n2.pdf

A Microwave Method for Complex Permittivity Extraction of Thin Materials

Complex permittivity determination with the transmission/reflection method

This article presents new structures and methods of design of miniature third-order substrate integrated waveguide (SIW) bandpass filters on a high-permittivity ceramic substrate for the C-band. The aim was to appraise the feasibility of such filters by using a 3D electromagnetic (EM) simulator. The substrate integrated waveguide (SIW) offers good quality factors and electrical performances compared with other planar techniques. Its integration capabilities and fabrication cost are other benefits that make it attractive. Ceramic material offer electrical properties suitable in designing of passive devices. High relative permittivity with low dielectric losses makes it possible to miniaturize passive components while exhibiting high temperature stability, which is an important selection criterion for a filter designed to equip the payload of a satellite. Three SIW filters were designed on a Trans-Tech ceramic substrate (thickness = 254 mm, er = 90, and tand = 0.0009) with drastic specifications for space application. The first filter is composed of three SIW resonators with direct coupling, the second is composed of three SIW resonators with a cross-coupling to create a transmission zero, and the third is composed of three SIW resonators with circular holes etched on the top of the metal layer to achieve a super-wide band. The obtained results for the proposed filters are presented, discussed, and compared with relevant published literature. The proposed filter can be used to enhance the performance of microwave devices used for C-band, especially Satellite communications.

/pdf/new-design-of-miniature-c-band-substrate-integrated-2yrp5ifyd6.pdf

Nawfal Jebbor

Papers

Effective permittivity of periodic composite materials: Numerical modeling by the finite element method

A fast calibration-independent method for complex permittivity determination at microwave frequencies

A Microwave Method for Complex Permittivity Extraction of Thin Materials

Complex permittivity determination with the transmission/reflection method

New design of miniature C-band substrate integrated waveguide bandpass filters using ceramic material