RF Absorber Research Articles

Over-the-air testing of LTE-A PHY layer in reverberation chamber

Reliable, repeatable, and flexible testing is crucial for assessing system performance and ensuring the quality of communication. In reverberation chambers (RC), real-life propagation environments can be emulated by loading absorbers, facilitating controlled testing of the system. This work presents over-the-air (OTA) testing of the LTE-A PHY layer in the RC. We assessed the performance of the LTE-A link using key performance indicators (KPIs) such as error vector magnitude (EVM), bit error rate (BER), and signal-to-noise ratio (SNR) for varying transmitter (Tx) and receiver (Rx) gains. Additionally, we have compared the results both in an empty RC and when the RC was loaded with RF absorbers. We used software-defined radio (SDR) for OTA transmission of LTE-A frames. The measurement results indicate that loading the RC with RF absorbers improves EVM, BER, and SNR. We also quantified the performance of the LTE-A link by changing the position of RF absorbers. Results showed that loading absorbers yielded up to 72.8% improvement in EVM, and placing absorbers closer to Rx helped reduce the amount of multipath, resulting in better transmission performance.

Calorimetric Measurements of a High-Power RF Signal

Development of a high-power RF signal meter based on the method of calorimetric measurements, which in some cases is the only possible one is presented. The device is as a system of elements exchanging either energy or information, the operation and interconnection of which ensures the stability of its functioning. The main elements of the device are: a coordinated load and a two-circuit water-air cooling system that convert RF energy first into thermal electrical energy, and then into internal energy of the coolant (working fluid). The measurement and control system provides information display on the monitor and generation of internal control signals. The work focuses on the process of converting electromagnetic energy into internal energy of the coolant, since the matched RF load is integrated into the cooling system and requires special design and technological solutions to ensure efficient operation. A cooling and measurement system has been designed to test the developed RF absorber with a power of up to 5 kW. The parameters of the cooling system were determined based on minimizing the time to achieve thermal equilibrium (time to establish readings) and the set parameters (input temperature no more than 35 °С, temperature difference no more than 40 °С). Based on calculations of the electrodynamic model constructed by the partial domain method, the geometric dimensions were determined at which the voltage standing wave ratio of the load in the operating frequency range (from DC to 1300 MHz) will be the smallest. The article presents the results of designing a high-frequency wattmeter cooled by a liquid (water), examines the thermophysical processes occurring in it and the influence of the presence of a coolant in the cooling circuit.

A review: Advancement in metamaterial based RF and microwave absorbers

Microwave absorbers (MAs) are specially designed structures which dissipate an incident electromagnetic (EM) wave by converting it into thermal energy and thereby prevents reflection and/ or transmission. The MAs are extensively in demand due to their property to reduce the electromagnetic wave interference (EMI) and to enhance electromagnetic shielding (EMS). MAs are also used in potential military applications as a stealth technology to design stealth aircrafts, warships and army clothing etc. due to the reduction of radar cross section (RCS) of the defense equipment by the absorber coating. In this review work, study of metamaterial-based MAs, their characteristics viz. left-handed material property, permittivity and permeability have been discussed in detail. Metamaterials (MMs) or artificial materials that play a significant role in reducing the size and thickness of MAs depending upon the frequency of operation. In this work, theoretical background of MM-based absorbers detailing the effect of permittivity and permeability on absorber performance has also been discussed. Different applications of absorbers are elaborately covered in this paper. Furthermore, frequency selective surface (FSS) which is a two-dimensional array has the capability to reflect or transmit EM wave completely or partially depending upon the nature of FSS. Therefore, distinct categories of FSS-based absorbers are discussed according to the frequency of operation, bandwidth and material specification where the effect of different design parameters is also critically analyzed. The objective of this review paper is to provide an exhaustive study of the unique attributes of MAs having a diverse range of applications for frequencies ranging from microwave, terahertz upto optical frequency domain. This review paper is certainly a vital tool for researchers to accurately understand, explore and work on MA design and selection as per user defined specific frequency and applications.

Design of an Anechoic Chamber for W-Band and mmWave

In this paper, we describe the design of an electrically large anechoic chamber for usage on millimetre-wave bands. Ansys Savant sotware was used to perform a simulation of the chamber, using physical optics coupled with uniform theory of diffraction (PO/UTD). Moreover, a method based on an open waveguide probe is described in this paper to obtain the electrical properties of the RF absorbers at millimetre-wave frequencies. Two different source antennas were simulated in this work and the corresponding quiet zones predicted. The largest quiet zone was 30 m m × 30 m m × 50 m m , for a chamber size of 1.2 m m × 0.6 m m × 0.6 m .

Realization and high power test of damped C -band accelerating structures

The linac of the European project Extreme Light Infrastructure-Nuclear Physics (ELI-NP) foresees the use of 12 traveling wave $C$-band accelerating structures. The cavities are 1.8 m long, quasiconstant gradient, and have a field phase advance per cell of $2\ensuremath{\pi}/3$. They operate at 100 Hz repetition rate, and, because of the multibunch operation, they have been designed with a dipole higher-order mode (HOM) damping system to avoid beam breakup. The structures have symmetric input and output couplers and integrate, in each cell, a damping system based on silicon carbide (SiC) rf absorbers coupled to each cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to simplify the fabrication and to reduce the costs. The cavities have been fabricated, and the first full-scale prototype has been also successfully tested at the nominal gradient of $33\text{ }\text{ }\mathrm{MV}\mathrm{/m}$, repetition rate of 100 Hz, and pulse length of 820 ns. It represents, to our knowledge, the first full-scale linac structure with HOM damping waveguides and SiC absorbers tested at this high gradient. In the paper, we illustrate the realization process of such a complicated device together with the low and high power test results.

Compositions for transplantation by RF absorber with photosensitizer for brain tumors and cancer treatment

Compositions for transplantation by RF absorber with photosensitizer for brain tumors and cancer treatment

A low profile miniature RFID tag antenna dedicated to IoT applications

ABSTRACTRFID tag antennas with stable performance on the diverse electromagnetic mounting platforms are an integral part of the ubiquitous RFID systems. This research article presents a novel tag antenna design that facilitates the said objective. The proposed antenna consists of a modified H-shaped slot structure that ensures considerable robustness from the application environment through confining the surface current density within the antenna structure. The antenna offers a tunable bandwidth of 40 MHz within the microwave band of (2.4–2.5) GHz. The proposed tag antenna exhibits excellent response on metallic platforms of different sizes and thicknesses with an effective gain of almost four times of that in free space. Furthermore, the designed tag antenna performs adequately well on low–medium permittivity dielectrics (glass, paper, and plastic) and RF absorbers (water). The free space and on-metal performance of the proposed tag antenna are verified by testing a prototype realized on the FR4 substrate.

Performance Evaluation of Wide-Angle Ultrathin Microwave Metamaterial Absorber with Polarization Independence

A novel dual-band wide-angle ultra-thin absorber composed of dual closed-concentric ten-point stars with polarization independence is presented in this article. The ten-point stars formed resonators are engraved on low cost FR4 substrate. The EM absorber performance is studied in terms of electrical and physical parameters. The polarization independence is practically demonstrated using waveguide measurement method. The measured results are in good agreement with simulated results. The absorbance values of 99.37% and 97.18% are achieved at 2.13 GHz and 2.52 GHz, respectively. The presented absorber design is wide-angle RF absorber with high absorbance values of about 80% up to 60° under TE mode and 87% up to 75° of oblique incidence angle for TM mode excitations. The experimental results demonstrate the absorption effectiveness of the proposed design to be utilized in RF energy harvester modules for capturing ambient UMTS band signals and in RF absorber applications for S band radar systems. The presented absorber unit cell is ultrathin and compact with the thickness and size less than λ/56 mm and λ/5 mm, respectively, at lowest absorption frequency.

Design of high gradient, high repetition rate dampedC-band rf structures

The gamma beam system of the European Extreme Light Infrastructure--Nuclear Physics project foresees the use of a multibunch train colliding with a high intensity recirculated laser pulse. The linac energy booster is composed of 12 traveling wave $C$-band structures, 1.8 m long with a field phase advance per cell of $2\ensuremath{\pi}/3$ and a repetition rate of 100 Hz. Because of the multibunch operation, the structures have been designed with a dipole higher order mode (HOM) damping system to avoid beam breakup (BBU). They are quasiconstant gradient structures with symmetric input couplers and a very effective damping of the HOMs in each cell based on silicon carbide (SiC) rf absorbers coupled to each cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to simplify the fabrication and to reduce the cost of the structures. In the paper, after a review of the beam dynamics issues related to the BBU effects, we discuss the electromagnetic and thermomechanic design criteria of the structures. We also illustrate the criteria to compensate the beam loading and the rf measurements that show the effectiveness of the HOM damping.

Nonlinear, Active, and Tunable Metasurfaces for Advanced Electromagnetics Applications

We demonstrate a series of nonlinear, active, and tunable metasurfaces for a variety of electromagnetic applications. The metasurfaces have achieved a range of exotic properties by populating nonlinear or active circuit components on a periodically patterned metallic surface. The circuit components such as diodes, varactors, transistors, and other devices can be controlled manually, actively, or self-adaptively. This allows nonlinear metasurfaces to have active tuning, power-dependent behavior, self-focusing, reconfigurable surface topology, or frequency self-tuning capabilities. The power-dependent metasurfaces can be applied to active RF absorbers that only absorb high-power surface waves to prevent malfunction or damages to sensitive devices. The rectifier-based waveform-dependent metasurface absorber can be specifically designed to absorb either high power pulsed waves or continuous waves. The transistor-based surface wave metasurface absorber provides another degree of freedom in that it can be manually switched to tune the absorber, or it can be tuned using computer controlled feedback. The self-focusing effect has been demonstrated for the first time at RF frequencies to automatically collimate high-power surface waves. The reconfigurable and self-tuning metamaterial surfaces can be implemented to support a broadband reconfigurable antenna system or to adapt to a wide range of incoming frequencies. In this paper, the concepts of nonlinear and active tunable metasurfaces are discussed, including results of full-wave simulation analysis, EM/circuit co-simulation, and experimental results in waveguides, using a near-field scanner, as well as far-field measurements in an anechoic chamber.

Metamaterial based broadband RF absorber at X-band

Metamaterial based electromagnetic wave absorbers may provide perfect absorption over significant range of electromagnetic spectrum. With perfect and broadband absorption, metamaterial absorbers play an important role in stealth technology. In this paper, design, simulation and measurements of a novel X-band (8–12GHz) metamaterial broadband absorber are presented. An absorber operating in the middle of the microwave X-band is obtained by selecting differently sized unit cells and placing their resonant frequencies close to each other. The paper presents two absorbers which consist of different number of unit cells. The first absorber has 12 unit cells (KOH12) while the other has 16 unit cells (KOH16). It is observed that the simulation results are in good agreement with the measurements. Measurement bandwidth for an absorption level of 80% is 2.73GHz for KOH12 and 2.55GHz for KOH16. These are the highest absorption levels for planar metamaterial absorbers at X-band. The absorption mechanism of broadband metamaterial absorber is also investigated by using electric and magnetic field distribution. Results of this study are promising for the stealth applications as absorbing elements against X-band radars.

발사체의 RF 호환성 시험을 위한 소형 S-밴드 안테나 햇

Abstract In this paper, we propose a compact antenna hat to perform RF compatibility testing efficiently between the launch vehicle and ground stations. The proposed structure implements a small size and low loss using the conductive shield instead of the conventional RF absorber. The S-band antenna hat, which is fabricated for an inverted-F onboard antenna with the size of 74 mm×13 mm×16 mm, has the small enclosure of 88 mm×35 mm×44 mm, the return loss of 25.6 dB, the insertion loss of 0.26 dB, and the leakage loss of 49.4 dB at the center frequency of 2.25 GHz. The simulated and measured results show a good agreement. Key words: Antenna Hat, Antenna Test Coupler, Launch Vehicle, RF Compatibility Testing, Telemetry 'Ÿ ‘ Ri(Korea Aerospace Research Institute) *¢£¤¥¦6p¥ (Department of Radio Science & Engineering, Chungnam National University)uManuscript received October 20, 2014 ; Revised December 12, 2014 ; Accepted December 23, 2014. (ID No. 20141020-085)uCorresponding Author: Dong-Chul Park (e-mail: dcpark@cnu.ac.kr)

A conductive shielded compact antenna hat for inverted‐F antennas

ABSTRACTWe propose a conductive shielded compact antenna hat, which can be utilized as a low loss passive RF repeater or coupler, to perform the RF compatibility testing of the aircraft, rocket, and launch vehicle using inverted‐F type antennas. Although the onboard antenna is very closely enclosed by the proposed antenna hat without RF absorbers, the resonance of the antenna itself is hardly affected compared to the free space radiation. The antenna hat which is fabricated for the S‐band inverted‐F antenna with the size of 74 × 13 × 16 mm3 has the small enclosure of 88 × 35 × 44 mm3, the return loss of 25.6 dB, the insertion loss of 0.3 dB, and the leakage loss of 52.5 dB at 2.25 GHz. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:220–223, 2015

Interaction of Active MMIC with Package/Housing -Theory, Analysis and Measurement

This paper presents enclosure effect comprehensive by 3D Electromagnetic modeling, simulation and measurement of active MMIC with Package/housing. When put inside a test box or package with a cavity, MMIC shows significant deviation from it RFOW measurement data. Cavity resonance’s adverse effects are analyzed by Eigen mode solver using CST Microwave Studio and RFOW measurement and in test box data is matched by simulating package/house with the best suited RF absorbers.

Gap Waveguide PMC Packaging for Improved Isolation of Circuit Components in High-Frequency Microwave Modules

In this paper, perfect magnetic conductor (PMC)-based packaging technique was used to improve the isolation performance among various microwave circuit components such as high-gain amplifier chains. In this approach, a periodic structure (such as metal pin rows) together with the ground plane of the substrate created a stopband for unwanted parallel plate or cavity modes as well as substrate modes, and thereby suppressed the problems of circuit resonances and related package phenomena. This paper describes two Ka-band amplifier chains that were tested with this new packaging technique. Firstly, a single amplifier chain was tested for maximum stable gain operation, and it was found that the stable gain of was achieved, whereas traditional metal wall package with RF absorber offered stable gain of 40 dB, thus showing significant isolation improvement. Secondly, two high-gain amplifier chains were placed side by side and their mutual isolation was tested. With the proposed gap waveguide packaging, a minimum isolation of 78 dB was achieved, whereas a complete metal shield provided a minimum isolation of only 64 dB over the band of interest.

Tapered channel for six-dimensional muon cooling towards micron-scale emittances

A high-luminosity muon collider requires a significant reduction of the six-dimensional emittance prior to acceleration. Obtaining the desired final emittances requires transporting the muon beam through long sections of a beam channel containing rf cavities, absorbers, and focusing solenoids. Here we propose a new scheme to improve the performance of the channel, consequently increasing the number of transmitted muons and the lattice cooling efficiency. The key idea of our scheme is to tune progressively the main lattice parameters, such as the cell dimensions, rf frequency, and coil strengths, while always keeping the beam emittance significantly above the equilibrium value. We adopt this approach for a new cooling lattice design for a muon collider, and examine its performance numerically. We show that with tapering the cooling rate is not only higher than conventional designs, but also maintains its performance through the channel, resulting in a notable 6D emittance decrease by 3 orders of magnitude. We also review important lattice parameters, such as the required focusing fields, absorber length, cavity frequency, and voltage.

Demountable damped cavity for HOM-damping in ILC superconducting accelerating cavities

We have designed a new higher-order-mode (HOM) damper called a demountable damped cavity (DDC) as part of the R&D efforts for the superconducting cavity of the International Linear Collider (ILC). The DDC has two design concepts. The first is an axially symmetrical layout to obtain high damping efficiency. The DDC has a coaxial structure along the beam axis to realize strong coupling with HOMs. HOMs are damped by an RF absorber at the end of the coaxial waveguide and the accelerating mode is reflected by a choke filter mounted at the entrance of the coaxial waveguide. The second design concept is a demountable structure to facilitate cleaning, in order to suppress the Q-slope problem in a high field. A single-cell cavity with the DDC was fabricated to test four performance parameters. The first was frequency matching between the accelerating cavity and the choke filter. Since the bandwidth of the resonance frequency in a superconducting cavity is very narrow, there is a possibility that the accelerating field will leak to the RF absorber because of thermal shrinkage. The design bandwidth of the choke filter is 25kHz. It was demonstrated that frequency matching adjusted at room temperature could be successfully maintained at 2K. The second parameter was the performance of the demountable structure. At the joint, the magnetic field is 1/6 of the maximum field in the accelerating cavity. Ultimately, the accelerating field reached 19MV/m and Q0 was 1.5×1010 with a knife-edge shape. The third parameter was field emission and multipacting. Although the choke structure has numerous parallel surfaces that are susceptible to the multipacting problem, it was found that neither field emission nor multipacting presented problems in both an experiment and simulation. The final parameter was the Q values of the HOM. The RF absorber adopted in the system is a Ni–Zn ferrite type. The RF absorber shape was designed based on the measurement data of permittivity and permeability at 77K. The Q values of the HOM in the DDC are 10–100 times lower than those of a TESLA-type HOM coupler.

Smart receiver system of green hybrid antennas

A hybrid of multiformities receiver system is proposed in this article.The system is developed through a combination of reconfigurable beam-steering antenna, reconfigurable beam-shaping antenna, and data acquisition (DAQ) boards. The combination of beam-steering antenna with beam-shaping antenna formed a hybrid four-layer structure with fabricated element plank of rice husk placed between the beam-shaping antenna's feed line and beam-steering antenna's radiating element. This four-layer hybrid and element plank are held together by 5 mm PCB supports at every edge of the antennas forming a stack configuration. The PIN diode switches of both antennas are connected with DAQs to adaptively control the degree angle and beamwidth of the radiation patterns. The activation of either beam-steering antenna or beam-shaping antenna depends on the DAQs' Beam Manager memory and the weakest level of received signal strength. When the transmitted signal is coming from the angle of 0°/360°, 90°, 180°, and 270°, its range and angles' location will automatically be detected by the first DAQ that is connected to the beam-steering antenna. Meanwhile, the beam-shaping antennas are activated by the second DAQ, to receive the transmitted signal for a longer range at four different gains: 3.6 dB, 7.2 dB, 9.9 dB, and 14.64 dB. Hence, this hybrid receiver system collectively contributes to the well-developed eight-multiformity radiation patterns that can be inter-changed within only 0.01 ms. Furthermore, signal reliability is paramount since the system could self-elucidate the mutual coupling effects with the aid of rice husk element plank, which acts as a RF absorber. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:800–809, 2013; View this article online at wileyonlinelibrary.com. DOI: 10.1002/mop.27412

Photoinduced spin polarization and microwave technology

We report here on studies of optically pumped active microwave media based on various fullerene derivatives, with an emphasis on the use of these materials in microwave electronics. We have investigated a class of optically excited paramagnetic materials that demonstrate activity in the X-band as candidate materials. We found that a particular fullerene derivative, Phenyl-C61-butyric acid methyl ester (PCBM), produced the largest electron paramagnetic resonance (EPR) emission signal compared to other organic compounds that have been suggested for use as microwave active materials. We also studied the effects of concentration, temperature, solvent etc. on the activity of the material. In these experiments, EPR studies using a commercial spectrometer were followed up by measurements of an RF signal reflected from a resonator loaded with the PCBM-based material. The activity was directly demonstrated through the change in the quality factor and RF coupling between the resonator and waveguide feed.At the inception of these experiments the primary interest was the development of a microwave PASER. The PASER (particle acceleration by stimulated emission of radiation [1]) is a novel acceleration concept that is based on the direct energy transfer from an active medium to a charged particle beam. While the previous work on the PASER has emphasized operations at infrared or visible wavelengths, operating in the microwave regime has significant advantages in terms of the less stringent quality requirements placed on the electron beam provided an appropriate microwave active medium can be found. This paper is focused on our investigation of the possibility of a PASER operating in the microwave frequency regime [2] using active paramagnetic materials. While a high level of gain for PCBM was demonstrated compared to other candidate materials, dielectric losses and quenching effects were found to negatively impact its performance for PASER applications. We present results on development and bench testing for these new fullerene-based materials along with some conceptual designs for microwave PASERs. Other possible applications for active paramagnetic materials are suggested including low noise microwave amplifiers and tunable RF absorbers.

Evaluation of Uncertainties in Electromagnetic Disturbance Measurement above 1 GHz due to Site Imperfections

The data dispersion of the measurement of electromagnetic disturbance above 1GHz is mainly affected by site imperfections (expressed by the site voltage standing wave ratio (SVSWR)). To confirm the relationship between site imperfections and the measured field strength, we measured the SVSWR and the field strength radiated from the equipment under test (EUT) by changing the area covered by the RF absorber on the metal ground plane. From the results, we found that the data dispersion of measured field strength can be estimated from the measured SVSWR, and therefore, we can determine the measurement uncertainty of the measured field strength at the test site.