Low Microwave Band Research Articles

Carbon based Composite Materials for Microwave Absorption in Low Frequency S and C Band: A Review

AbstractWith the rapid advancement of 5G technologies and electronic devices, there is a growing demand for microwave absorbing materials, especially those effective in low frequency microwave bands (2–8 GHz), making it an essential requirement. In recent years, many innovative microwave absorbers have been developed for low frequency absorption, with carbon/magnetic composite materials becoming particularly promising due to their various loss mechanisms and optimized impedance matching characteristics over a wide frequency range. However, there is currently a lack of a comprehensive review that summarizes the findings on low frequency absorbers. This review article thoroughly examines the current research on carbon/magnetic composite materials with efficient absorption performance in the low‐frequency S and C bands. It provides a detailed discussion of the microwave absorption performance of these composite materials. Furthermore, the composite design strategies, synthesis techniques, microstructure performance relationship, and microwave attenuation mechanisms are summarized. Lastly, the challenges and future outlook for low frequency microwave absorbing materials are addressed. This review article aspires to provide new insights into designing and synthesizing composite materials to accomplish effective low‐frequency microwave absorption, thereby promoting practical applications.

Improving signal isolation in hybrid RF duplexer utilizing a band-pass filter

This study deals with a passive RF duplexer integrated with a two-notch band. To design the model, a band-pass filter is considered. Using micro-strip technology, the RF duplexer substation is simulated. It is a rectangular in parallel coupling with frequency bands of 1 and 5 GHz while existing three ports. Moreover, to enhance the impedance coefficient and decrease the admittance, the method of complementary paired resonators is taken into account. Furthermore, scattering parameters were used by the step impedance method to make an integrated monolayer substrate from signal branching in duplex mode. Thus, the band-pass filter making the frequency cut-off bands allows designing GSM-4G radars. The low cut-off microwave band is included in these bands at the 77 MHz central frequency and the second cut-off band for GSM-4G radars at the 437 MHz central frequency. The duplexer has the total dimensions of 14 mm × 99 mm and the presented RF duplexer is simulated in CST.

Defect-related broadband dielectric loss mechanisms of Na1/2Sm1/2Ti1–z(Al1/2Nb1/2)zO3 ceramics

Dielectric ceramics with low dielectric loss in the microwave bands are key materials used in fifth/sixth-generation (5G/6G) telecommunication. Exploring the defect behavior and the broadband dielectric response of ceramics is helpful to understand the loss mechanism and develop novel low-loss microwave dielectric ceramics. In this work, Al/Nb was co-substituted for Ti4+ in Na1/2Sm1/2TiO3 (NST) ceramics with a perovskite structure, and the low frequency, microwave and terahertz (THz) dielectric responses were investigated. In different frequency bands, the variations of dielectric loss with substitution amount are inconsistent. The Al/Nb-substitution increases the dielectric loss in the low frequency and microwave bands, while the dielectric loss in the THz band decreases. The changes in dielectric loss are closely related to the lattice defects in Na1/2Sm1/2Ti1–z(Al1/2Nb1/2)zO3 (NST-zAN, 0 ≤ z ≤ 0.1) ceramics, which are studied by thermally stimulated depolarization current (TSDC) technique. The dielectric relaxation of the defects including oxygen vacancies (VO··) and defect dipoles leads to the dielectric loss in the low-frequency and microwave bands. The dielectric loss in the THz band is mainly affected by the damping of lattice vibration, which is associate with the defect concentration. This work might provide a considerable reference for the development of low-loss dielectric materials.

Exposure systems used in the assessment of EMF impact on living organisms

The subject of the study in the article is the processes of analysis and exposure assessment of electromagnetic field on humans. The main goal is to optimize size of the exposure antenna systems, its electrical parameters and generated electromagnetic field (EMF) for different frequency range. Objectives: Electromagnetic field (EMF – Electromagnetic Fields) studies on living organisms are one of the important branches of biomedical research. Most of this type of research is conducted using dedicated exposures system with fixed and controlled EMF exposure conditions. The purpose of biomedical research is to determine the influence of electromagnetic fields on living organisms. For this purpose the exposure systems are designed. The main task of the exposure antenna system, is to produce the EMF with known and controlled parameters in a specific EMF area. Depending on the frequency and the component (E or H) of the electromagnetic field used, different types of systems are used. For low frequencies (for instance 50 Hz common frequency) and magnetic fields, Helmholtz solenoids or coils are used, and for electric fields - flat capacitors (E Field). For higher frequencies field E is used for systems with linear antennas or TEM lines. There are also dedicated probes for invasive tissue heating. Each solution has its advantages and limitations and usually there are no universal solutions for all cases. Results: For the generation of a low frequency magnetic field up to several hundred Hz, the Helmholtz solenoid or coil is generally used because of its relatively simple construction, the ability to obtain high intensity and good homogeneity of the field in a relatively large area relative to the dimensions of the whole system. The homogeneous field area can be determined analytically based on system geometry or measurement. Otherwise it looks for exposure system above few MHZ. To create a homogeneous EMF in the field of radio frequencies, the exposure systems - usually the TEM lines (Crawford compartment) or sometimes the GTEM are very likely to be used. In both cases there are frequency and spatial limitations - the homogeneous field is assumed to be maximally present. At 1/3 the height between the plates of the chamber and at the same time the maximum frequency of operation of the TEM chamber can be described by the approximate dependence of fmax [MHz] = 50 / d [m], resulting in a chamber operating at 1 GHz. The work piece does not exceed a height of about1.5 cmwith a diameter of about4 cm. Larger working areas, but at the same time, a lower homogeneity of the field can be obtained in the GTEM chamber - but at the same time the GTEM chamber has much larger geometric dimensions. Another way is to produce PEM with the parameters in open space - in the vicinity of the antenna. In this case, it is very important to select an antenna so that the expected homogeneous area can be obtained at an acceptable distance from the antenna - in order to maintain high EMF intensity and to reduce the radiation of the antenna beyond the desired area. This can be achieved by limiting the emission area by, for example, shielding or using EMF absorbers. In addition authors simulate novel ablation antenna for ablation treatment. Conclusions: Assessing the impact of EMF on biological objects is an interdisciplinary studies that requires the involvement of biological or medical specialists, as well as the EMF standard field and metrology specialist. In this work we have taken the subject of correct selection and description used in the experience of exposure systems. The authors presented the most commonly used EMF emitters in the low frequencies and microwave bands, as well as new model of the ablation treatment antenna.

CPW‐fed low‐profile directional antenna operating in low microwave band for wideband medical diagnostic systems

Medical diagnostic systems demand wideband antennas with directive radiation patterns. In addition, portable systems require low profile and light weight antennas to be incorporated. These necessities of modern microwave-based diagnostic systems are addressed in this reported work. The antenna is built with two simple thoroughly printed structures connected to each other using two copper walls, forming a loop-like geometry. The top element is capacitively loaded with a pair of symmetrically placed slots and fed with a coplanar waveguide (CPW). The antenna works like a small loop in the lower resonating mode and as a folded-dipole in the higher modes. The antenna exhibits a 109% bandwidth with stable radiation patterns of an about 9 dB front-to-back ratio over most of the band with only a profile of 0.05λ (λ = wavelength at the lowest frequency). The measured and the simulated results show that the proposed antenna is suitable for wideband medical imaging systems.

Lumped-element compensated high/low-pass balun design for MMIC double-balanced mixer

We report a novel lumped element balun structure for both monolithic and hybrid circuit applications. The proposed structure utilizes two filters to compensate the amplitude and phase errors at the two balance outputs of a traditional out-of-phase power splitter. This circuit requires no multilayer or suspended substrates techniques; therefore, wide applications on many circuits operated especially, in the low microwave band, are expected. Two monolithic microwave integrated circuit (MMIC) mixers were built to demonstrate the electrical feasibility.

Microwave spectra and molecular conformation of methoxy difluorophosphinoxide

Two closely spaced a-type low resolution microwave band series were observed for the normal, 13C and d 3 isotopic forms of methoxy difluorophosphinoxide. The lower frequency series of the three isotopic species was found and assigned using a pulse beam Fabry—Perot cavity Fourier transform microwave spectrometer. The a- and b-type spectra of the three species were fitted to a Watson hamiltonian which gave A=4568.215(1) MHz, B=2500.471(9) MHz and C=2453.863(9) MHz for the normal species. Stark effect measurements of CH 3OP(O)F 2 led to the determination of the electric dipole component μ a =2.699(2) D and μ b =1.522(2) D. Moments of inertia and electric dipole data show that the lowest energy conformation has C s symmetry. The data are most consistent with an assignment of the observed conformer to the trans configuration. The second a-type low resolution microwave band series not observed in the pulsed beam cavity spectrometer arises from a methoxy torsional excited state fo the trans conformer or a second higher energy conformation. The absence of A–E torsional splittings in the a- and b-type spectra gives a lower limit of ∼2500 cal mol −1 for the V 3 methyl internal rotation barrier.

Improved agile notch adaptive equaliser

A major source of ‘outage’ in high-capacity digital radio-relay systems operating in the lower microwave bands is multipath propagation. Adaptive equalisation of the channel and other remedial measures such as the use of space diversity are often necessary for adequate system performance. A common type of adaptive frequency-domain equaliser employs a variable single-pole filter controlled by three fixed spectrum-sampling detectors. The effectiveness of this type is limited by extraneous causes of spectrum distortion such as tilts. The letter describes a new form of equaliser which searches for and equalises according to minima in the distorted spectrum and is insensitive to extraneous spectrum tilts.

Characteristics of low resolution Q-branch microwave bands: Applications to 2,2,2-trifluoroethyl vinyl ether and 7-methylbicyclo[2.2.1]-hept-2-ene-5-one

A rigid rotor analysis of μb- and μc-type Q-branch series indicates overlapping low J transitions giving rise to the low resolution microwave bands which follow a half-integer frequency rule. At higher J values certain Q-branch series form bandheads shifted to a lower frequency from the series origin. Under favorable circumstances low resolution bands are expected and the frequency of these bands may be used to estimate the rotational constant parameter (A−C). The microwave spectrum of 2,2,2-trifluoroethyl vinyl ether exhibits bands which are assigned to the bandhead phenomenon. This provides a value of (A−C) and in conjunction with previous data yields the three rotational constants A, B, and C uniquely from low resolution data. The results are consistent with the s-cis-planar as well as gauche conformations. Previously reported low resolution bands of 7-methylbicyclo[2.2.1]-hept-2-ene-5-one are interpreted as two half-integer band series arising from low-lying vibrational states with slightly different values of (2A−B−C).