Low-frequency Antenna Research Articles

The Ultra-Compact ELF Magneto-Mechanical Transmission Antenna With the Speed Modulated EM Signal Based on Three-Phase Induction Motor

An extremely low-frequency (ELF) magnetomechanical antenna with the size of one-millionth wavelength based on a three-phase induction motor is reported, whose working frequency is no longer limited by the resonant wavelength. The rotating three-phase induction motor with nonmagnetic material shell is just equivalent to a rotating magnetic dipole with 1.5 times magnetic dipole moment. It can transmit the modulated electromagnetic (EM) signal by modulating the rotating speed of the motor. A near-field ELF EM signal transmission experiment system is constructed based on the magnetomechanical antenna with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi 10$ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times50$ </tex-math></inline-formula> cm size, and the transmitted EM signals were received by a 200-turn coil, which is 10–200 m far away from the transmitter. The EM signal frequency is almost the same as the rotating speed of the motor, and it varies with the rotating speed too. With arbitrary waveform generator and variable-frequency drive (VFD) modulating the rotating speed of the motor, a near-field ELF transmitter with modulating EM signal is built, and the sawtooth wave (STW)-modulated EM signal was transmitted and received clearly during 10–200 m. The square wave (SW)-modulated EM signal was communicated too. The proposed magnetomechanical antenna has a compact size and is very suitable for ELF remote wireless communication.

Numerical Simulation of Subsurface Penetrating Radar in Isidis Planitia on Mars for China’s First Mission to Mars

The water ice exploration mission is an important scientific target of China’s first Mission to Mars. The high frequency full polarimetric subsurface penetrating radar (SPR) system onboard the landing rover can carry out high-resolution imaging of the near-surface area of Mars, which is an important tool for detecting the presence of shallow water ice and soil structure. At present, most of the radar detection simulations are based on low-frequency antennas to analyze the geological structure of Mars down to several kilometers, with low resolution, which is difficult to detect the subsurface structure in the near-surface accurately. Considering topographic relief, interface roughness, subsurface rock, water ice, and permittivity changes at different layers, a three-dimensional near-surface model in the Isidis Planitia on Mars is established. The interpretation of the forward simulation results is of reference value for future Mars exploration missions.

Application of Ground Penetrating Radar in Detecting Deeply Embedded Reinforcing Bars in Pile Foundation

Ground penetrating radar (GPR) has been widely used for nondestructive testings in civil engineering. However, the GPR has not been adequately applied in detecting deeply embedded reinforcing bars, which is usually difficult to be revealed in radar image due to the wave interference and attenuation in large depth penetration. This study presents a new approach for the GPR detection of deeply embedded reinforcing bars in the reinforced concrete pile foundation. The aim of the GPR survey is to determine the existence and the depth of internal reinforcing bars in the pile foundation for solving engineering dispute. Low centre frequency antenna was used in GPR field testing to obtain the reflected raw data. Optimized procedures of digital filtering techniques were applied to process the GPR raw data. The deeply embedded reinforcing bars are revealed in the radar image after the field testing and postprocessing procedures. The depth of the reinforcing bars was estimated based on the hyperbola match method. The GPR test results were validated by the excavation of the pile foundation. The low centre frequency antenna has been found to be essential to obtain the reflected wave signals of deeply embedded reinforcing bars. The optimized processing procedures is useful to identify and display the reinforcing bars in radar image. The combination of low centre frequency antenna and the postprocessing procedures make the detection of deeply embedded reinforcing bars feasible. The proposed GPR testing method has been found to be effective to estimate the depth of deeply embedded reinforcing bars, which provides the key information for solving engineering dispute.

VHF/UHF Ultrawideband Tightly Coupled Dipole Array for CubeSats

To date, CubeSat radars and imagers have been limited to operations beyond S-band due to the challenges associated with the design of wideband, compact, low-frequency antennas. Concurrently, the frequencies at VHF/UHF bands can image through clouds and foliage, and are very attractive for ice, water and biomass sensing. There is thus a need to develop wideband antennas that can operate at VHF/UHF bands which are low cost, light-weight, and packable. In this paper, we present a CubeSat deployable Tightly Coupled Dipole Array (TCDA) that achieves VSWR<; 3 from 80 MHz - 600 MHz, and VSWR<; 4 from 65 MHz - 600 MHz. While the array itself is 1.2 m in length, 10 cm in width and 1.5 mm in thickness, it can be folded and stowed in a compact volume of 0.40U. We detail the design of the array, demonstrate the folding mechanism, and validate its performance through measurements.

Improved radiation characteristics of compact antipodal Vivaldi antenna with the hybrid technique for UWB applications

ABSTRACT A compact antipodal Vivaldi antenna with stable end-fire radiation properties over the bandwidth of more than 15:1 is proposed for UWB applications. The gradient titled corrugated edges used to redistribute the antenna surface electric fields are employed to extend the low-end cutoff frequency and increase the low-frequency antenna boresight gain. The asymmetric parasitic patch composed of two triangular patches with different height is introduced to couple more energy from radiating arms of the antenna and thus enhancing mid- and high-frequency gain. The proposed antenna is designed and optimized in detail. To validate the proposed design, a prototype is fabricated and measured. The simulation results are in good agreement with the experimental data. Measurement results show that, the gain variation of the proposed antenna is in the range of 5.4–13.7 dBi over 2.88–40 GHz with the a compact dimension of 40 mm×73 mm×0.508 mm. The compact dimension of the antenna makes it a good candidate for UWB applications.

German Aerospace Center's advanced robotic technology for future lunar scientific missions.

The Earth's moon is currently an object of interest of many space agencies for unmanned robotic missions within this decade. Besides future prospects for building lunar gateways as support to human space flight, the Moon is an attractive location for scientific purposes. Not only will its study give insight on the foundations of the Solar System but also its location, uncontaminated by the Earth's ionosphere, represents a vantage point for the observation of the Sun and planetary bodies outside the Solar System. Lunar exploration has been traditionally conducted by means of single-agent robotic assets, which is a limiting factor for the return of scientific missions. The German Aerospace Center (DLR) is developing fundamental technologies towards increased autonomy of robotic explorers to fulfil more complex mission tasks through cooperation. This paper presents an overview of past, present and future activities of DLR towards highly autonomous systems for scientific missions targeting the Moon and other planetary bodies. The heritage from the Mobile Asteroid Scout (MASCOT), developed jointly by DLR and CNES and deployed on asteroid Ryugu on 3 October 2018 from JAXA's Hayabusa2 spacecraft, inspired the development of novel core technologies towards higher efficiency in planetary exploration. Together with the lessons learnt from the ROBEX project (2012–2017), where a mobile robot autonomously deployed seismic sensors at a Moon analogue site, this experience is shaping the future steps towards more complex space missions. They include the development of a mobile rover for JAXA's Martian Moons eXploration (MMX) in 2024 as well as demonstrations of novel multi-robot technologies at a Moon analogue site on the volcano Mt Etna in the ARCHES project. Within ARCHES, a demonstration mission is planned from the 14 June to 10 July 2021,1 during which heterogeneous teams of robots will autonomously conduct geological and mineralogical analysis experiments and deploy an array of low-frequency antennas to measure Jovian and solar bursts.This article is part of a discussion meeting issue ‘Astronomy from the Moon: the next decades'.

Experimentally validated smart card ultra‐high frequency tag antenna for free space and near body scenarios

This investigation primarily promotes a ultra-high frequency radiofrequency identification (RFID) tag antenna for complex environment applications of smart card in free space and near body scenarios. It also considers other high dielectric materials such as water and metallic objects. A dual dipole antenna with T-match structure is chosen due to the advantage of providing high resilience towards change in impedance of high dielectrics, and to ensure the reception of maximum signals at the blind spots of the antenna. The proposed geometry facilitates placement of other off-the-shelf high and low-frequency antennas on the card, enabling more features, higher performance, and minimum field interaction between different frequencies. A presented RLC equivalent circuit model estimates the impact of body layers on antenna impedance. A high read range of 7.7 m in free space, 5.2 m near body and 2.8 m at the blind spots is obtained. The novelty is shown by testing the antenna near metals and water, achieving high read range without any design modifications. The proposed tag aims to provide new RFID applications that demand small size, less volume, low cost and multifunctionality over various constraints while showing good agreement with sensitivity, impedance matching and the read range.

Subsurface object detection and characterization using Ground Penetrating Radar

Subsurface characterization and information about buried utility infrastructure is an important issue affecting the public safety and progress of development projects. A heterogeneous subsurface environment is often insufficiently characterized by the data collected through various direct and indirect means, particularly in dense urban areas. The present study aims to detect the subsurface objects and map the stratigraphic environment in a city region using a non-invasive geophysical technique called Ground Penetrating Radar (GPR). In this study, antennas of central frequency 200 and 80 MHz have been used to identify the underground utilities and subsurface layer information, respectively. A methodology based on a geometrical approach using Support Vector Machines (SVM) is developed for computing the depth and radius of buried pipes. Also, the electrical discontinuities in the GPR profiles are identified through various processing techniques to extract the subsurface layer information. The results indicate that the 200-MHz antenna and SVM-based methodology estimate the buried pipe parameters with reasonable accuracy at various site combinations. It is found that the bistatic low-frequency 80-MHz antenna suitably characterizes the subsurface layers, which are in close agreement with the borehole data. The processed data illustrate a strong correlation between the radar signals and the characteristics of the strata resolving the uncertainty. The study highlights the capability of GPR in extracting the subsurface data and recommends a multi-frequency approach to map and interpret the complete subsurface environment at a specific site.

Switched-Antenna Low-Frequency (LF) Radio-Frequency Identification (RFID) for Ornithology

Although low-frequency (LF) radio-frequency identification (RFID) is the de facto standard for many ornithological tracking studies, it is hampered by low range (5-10 cm) and the multiple tag collision problem. The simple concept of augmenting existing RFID transceivers with a switched antenna array is explored to overcome these difficulties. Analysis shows that non-ideal behavior of the switch elements can lead to detuning and cross-talk effects that should be considered when designing the LF antennas. An open-source prototype RFID reader with an integrated switch array supporting up to 12 antennas is developed and presented. Controlled laboratory tests indicate good read reliability of an 8-element antenna matrix covering a 37 cm × 37 cm ground area and a 4-element array placed on a bird feeder, but relative tag-antenna orientation must be carefully considered. A four-hour field test with tagged house sparrows indicates detection reliability of 94% and 78% for the 4-element feeder and 8-element mat arrays, respectively.

Multi-frequency GPR data fusion and its application in NDT

Ground-penetrating radar (GPR) is a non-destructive technique that utilizes high-frequency electromagnetic waves to detect and locate subsurface objects and interfaces. Resolution can be improved with an increase of the source frequency; however, this will lead to decreased investigation depth due to the stronger attenuation of the high frequency signal. Thus, there is a trade-off between the resolution and the investigation depth in the single central frequency-based GPR system. To obtain both high resolution and deep penetrating ability simultaneously, we applied multi-frequency GPR data fusion with three algorithms: time-domain fusion with weights/without, frequency-domain fusion with weights. The fusion effect is qualitatively and quantitatively evaluated and compared by the fused radar profile and Laplacian operator, which is a second derivative gradient operator and commonly used in image edge detection by detecting the zero-crossings of image intensity. The results of the numerical simulation and field data showed that the fused profile was able not only to retain the high resolution in the shallow area from the high-frequency antenna but also take advantage of the significant investigation depth of the low-frequency antenna by merging the multi-frequency data into a single profile. Thus, the fused GPR data has the ability to create a single profile with more information and detailed characteristics.

Investigation into CME Shock Speed Resulting from Type II Solar Radio Bursts

An investigation into Type II solar radio bursts was carried out to understand the frequency gap between fundamental and harmonic emissions of the radio burst. This investigation focused on Type II solar radio bursts with flares and coronal mass ejections by relating the separation between fundamental and harmonic emissions. We used the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Spectrometers (CALLISTO) and a newly designed low-frequency antenna array. This article describes the proposed new instrument in terms of its antenna design, the bandpass testing of the antenna, the new system significance in studying Type II solar radio bursts, and its comparison with other leading radio solar monitoring instruments. Upon setting up the new technology, the radio-frequency interference of the observation site at the University of Malaya was shown to emphasize the suitability of the selected site. This article also shows the preliminary results of the proposed new instrument by reporting the detection of a Type III solar radio burst that was confirmed by CALLISTO. Moreover, it also includes the optimal observation design and strategies for future detections.

Flexible Magnetic Polymer Composite Substrate with Ba1.5Sr1.5Z Hexaferrite Particles of VHF/Low UHF Patch Antennas for UAVs and Medical Implant Devices.

Our goal is to fabricate flexible magnetic polymer composites as antenna substrates for very high frequency (VHF)/low ultra high frequency (UHF) antennas for unmanned aerial vehicles (UAVs) and medical devices. Magnetodielectric materials, which have permeability (μ) similar to permittivity (ε), have attracted great attention, because they facilitate miniaturization of microwave devices while keeping or enhancing electromagnetic characteristics. Mechanically millled Ba1.5Sr1.5Co2Fe24O41 (Ba1.5Sr1.5Z) hexaferrite particles were used to increase permeability in the interesting frequency band. The microwave properties of Ba1.5Sr1.5Z composites were predicted and measured. Hansen’s zero-order analysis of antenna bandwidth and electromagnetic field simulation showed that the hexaferrite-based flexible composite could enhance a bandwidth and achieve the miniaturization of antennas as well. The magnetic antenna substrates can be a good solution to integrate antennas into the UAVs whose dimensions are comparable to or larger than communication wavelength.

On method of composing low frequency signals based on array structures

Generating low-frequency electromagnetic waves based on high-frequency antenna and illuminating targets with multi-band signals can be an effect way that can not only reduce the physical dimension of a low frequency antenna, but also improve the performance of radar detection. Combining the electromagnetic wave doppler effect principle and the array antenna architecture, a method of generating a low-frequency signal around the illuminated target is proposed based on the controlling of array antenna parameters, including array radiation element signal timing, phase and element spacing. The principles of array parameter design are described. Composite signals are simulated respectively under two typical geometric relationships between targets and array antenna, target located along the array direction and in the direction of 45° scanning angle. The peak sidelobe ratio (PSLR) and integral sidelobe ratio (ISLR) are used to evaluate the quality of the composite signals. Aiming at practical applications, the effects of array element spacing error, phase error and target location error on the composite signal are simulated and analyzed. Under the condition of sparse uniform array, the influence of the radiation element spacing on the composite signal is analyzed. The simulation results show that the harmonic components of the composite signal increase with the radiating element spacing error and phase error growing.

MIMO handset antenna for 5G/WLAN applications

We propose a dual-module multiple-input multiple-output (MIMO) antenna for portable terminals. The operating bands of the handheld terminal antenna are 5G (3.4–3.8 GHz) and WLAN (5.150–5.925 GHz). Antenna elements of 5G and WLAN are spaced to reduce coupling between antenna elements in the same module. The return loss of all antenna elements is larger than 6 dB. The isolation between all elements is larger than 14 dB. The radiation efficiency of the high-frequency antenna is greater than 50%, and the radiation efficiency of the low-frequency antenna is greater than 40%. The far-field gain of all elements is greater than 2.2 dBi.

Optimal Placement of Individual LF Antennas on Each Axis in a Small Mobile Device Through a Systematic Optimization Process

This letter proposes the optimal placement of individual low-frequency (LF) antennas on each axis integrated in a small mobile device platform to maximize the coverage range of the communication link. To achieve the maximum coverage, considering small platform effects, a systematic genetic algorithm optimization process was conducted using a readable volume metric as a cost function. A readable volume of 34.5 m3 at the optimum locations of the individual LF antennas was obtained compared with that of 21.6 m3 for the bundle of LF antennas. To verify the results, Poynting vector measurements were performed using LF antennas located at optimal positions inside the copper mobile device platform. The Poynting vector strengths at the range of 30 cm were −138.9, −140.7, and −138.7 dBW/m2 along the x -, y -, and z -axis, respectively.

Analysis of Bent Wire Antenna Resonant Frequency for Different Bent Angles

AbstractFor the bedrock survey in the temperate glacier regions, low frequency antenna system of the airborne ground penetrating RADAR plays a very important role. A small size antenna, working at 10 MHz, would make its use easier in various radar applications. Reducing the resonant frequency of the wire dipole antenna structure, without increasing the physical size, can be attained by introducing bents in the existing structure. This paper introduces a new bent wire dipole antenna and presents the effects of bent angle of 80°, 70°, 60°, 50°, 45° and 40°of the wire antenna on the antenna parameters such as resonant frequency, S11, VSWR, gain and radiation pattern. A broadband four element equivalent circuit model of a straight dipole is used with some modifications in the R, L, C equations for the new bent wire dipole antenna by utilizing the bent angle. The frequency response of the equivalent circuit model, calculated resonant frequency values using the equations and the simulated results of the bent wire dipole antenna is compared and analyzed. As the antenna placement area inside the anechoic chamber is limited, the size of the 10MHz antenna has been scaled down by a factor of 0.01. This modifies the resonant frequency of the new structure to 1GHz. The scaled down antenna system are simulated, analyzed and tested in a GTEM cell. For bandwidth improvement of the 10 MHz antenna, a lumped element matching circuit has been designed and simulated.

A roadmap towards a space-based radio telescope for ultra-low frequency radio astronomy

The past two decades have witnessed a renewed interest in low frequency radio astronomy, with a particular focus on frequencies above 30 MHz e.g., LOFAR (LOw Frequency ARray) in the Netherlands and its European extension ILT, the International LOFAR Telescope. However, at frequencies below 30 MHz, Earth-based observations are limited due to a combination of severe ionospheric distortions, almost full reflection of radio waves below 10 MHz, solar eruptions and the radio frequency interference (RFI) of human-made signals. Moreover, there are interesting scientific processes which naturally occur at these low frequencies. A space or Lunar-based ultra-low-frequency (also referred to as ultra-long-wavelength, ULW) radio array would suffer significantly less from these limitations and hence would open up the last, virtually unexplored frequency domain in the electromagnetic spectrum.A roadmap has been initiated by astronomers and researchers in the Netherlands to explore the opportunity of building a swarm of satellites to observe at the frequency band below 30 MHz. This roadmap dubbed Orbiting Low Frequency Antennas for Radio Astronomy (OLFAR), a space-based ultra-low frequency radio telescope that will explore the Universe’s so-called dark ages, map the interstellar medium, and study planetary and solar bursts in the solar system and search them in other planetary systems. Such a radio astronomy system will comprise of a swarm of hundreds to thousands of satellites, working together as a single aperture synthesis instrument deployed sufficiently far away from Earth to avoid terrestrial RFI. The OLFAR telescope is a novel and complex system, requiring yet to be proven engineering solutions. Therefore, a number of key technologies are still required to be developed and proven. The first step in this roadmap is the NCLE (Netherlands China Low Frequency Explorer) experiment, which was launched in May 2018 on the Chinese Chang’e 4 mission. The NCLE payload consists of a three monopole antenna system for low frequency observations, from which the first data stream is expected in the second half of 2019, which will provide important feedback for future science and technology opportunities.In this paper, the roadmap towards OLFAR, a brief overview of the science opportunities, and the technological and programmatic challenges of the mission are presented.

A compensation method for the consistency of multi-channel mixing circuit for solar radio observation system

The observing frequency of Ground-based Solar Radio Observation Systems (SROS) ranges from tens of MHz (10-m-wave) to tens of GHz (millimeter-wave). Due to the limitation of the ADC input bandwidth and sampling rate, the broadband solar radio signals acquired by the SROS will be segmented by mixing circuit and then processed the analog-to-digital conversion. The analog device inconsistency leads to the inconsistencies of the amplitude and phase between different channels of mixing circuits, which affects the accuracy of the receiver. The compensation coefficient of each frequency point is first calculated by measuring the deviation of the amplitude and phase between different channels, and then stored in FPGA of the receiver. The data achieved by ADC are proceed by FFT and then compensated by the compensation coefficients. The validation experiments are carried out using 150–500 MHz SROS located in Chashan Solar Radio Observatory (CSO). The amplitude difference between different channels can be as small as 0.5 dBm. The phase consistency can also be significantly improved. The practical application shows that this method can improve the consistency of the wideband mixing circuit, and can be used to correct of multi-channel consistency of the digital beam forming (DBF) for the low-frequency log-periodic antenna array and interference of the multiple antennas.

Radio detection of cosmic rays in [1.7–3.7] MHz: The EXTASIS experiment

Since 2003, significant efforts have been devoted to the understanding of the radio emission of extensive air showers above 20 MHz. Despite some studies led until the early nineties, the band available below 20 MHz has remained unused for 20 years. However, it has been claimed by some pioneering experiments that extensive air showers emit a strong electric field in this band and that there is evidence of a large increase of the radio pulse amplitude with decreasing frequencies. The EXTASIS experiment, located within the Nançay Radioastronomy Observatory and supported by the scintillator array and the autonomous radio stations of the CODALEMA experiment, aims to re-investigate the low-frequency band, and especially to study the so-called ”sudden death” contribution, the expected electric field radiated by the shower front when hitting ground level. In this work, we present the instrumental setup of the EXTASIS experiment composed of 7 low-frequency antennas operating in [1.7–3.7] MHz and covering approximately 1 km2. We report the observation of 18 air showers detected in coincidence in the three instruments, and estimate a detection threshold of 23 ± 4 µV from comparisons with detailed SELFAS3 simulations. We also report a strong correlation of the low-frequency signal observation with the atmospheric electric field.

Performance of Electrically Small Conventional and Mechanical Antennas

Antennas that operate in the very low frequency (LF) band and below are useful for a number of applications, including long-distance and underwater communication. When constrained in size, the antennas are electrically small and very inefficient. This has motivated the need for novel approaches to LF antenna design. Here, we present concepts for antennas that generate electromagnetic signals from mechanical motion. We first review the generated fields and efficiency of conventional magnetic and electric dipole transmitters. This is then extended to their mechanical counterparts for comparison. Our results show that the motion of magnets or electrets (the electrical analog of a magnet) can efficiently radiate electromagnetic energy when coupled to a low-loss electromechanical suspension. Mechanical antennas, with spatial dimensions on the order of a meter, can theoretically exceed the performance of conventional short dipole and coil transmitters by more than eight orders of magnitude for frequencies of 1 kHz and below. This paper is intended to lay the foundation for future development involving the implementation of efficient, small form-factor, mechanically actuated antennas.