A new portable ELF Schumann resonance receiver: design and detailed analysis of the antenna and the analog front-end

The Schumann resonances (SR) are electromagnetic oscillations of the Earth-ionosphere cavity at frequencies of 7.8, 14, 20, 26, 33, 39, and 45 Hz. The long-term monitoring of the Schumann phenomenon has recently drawn attention, not least from the space-geophysics community [1‐3]. SR measurements and analysis provide information on the planetary thunderstorm activities, the properties of lower ionosphere layers, the Earth surface and atmosphere temperature variations, and the properties of earthquakes as well as on the studies of other celestial bodies [4‐9]. Consequently, increased interest has been shown by the scientific community on the methods and techniques employed in SR experimental detection. The detection of SR is a complex procedure that employs the limited energy generated and dissipated by the global lighting activity. This total energy is then smeared inside the huge volume of the Earth-ionosphere cavity, providing electric and magnetic field components. The prevailed electric component is vertically oriented, and the corresponding amplitude is close to 10⁻⁷ V/m. The magnetic field exhibits two horizontally potential components at N-S and E-W orientation with amplitudes of few tenths of picotesla. The detection of such weak electromagnetic fields in noisy environments is too difficult. Additionally, hardware imperfections can significantly reduce the performance of the system [10, 11]. To improve the signal-to-noise ratio (SNR), it is necessary to use specialized sensors and electronic equipment [12]. Especially, in the ELF band, where Schumann resonances lie, very few works give details about measurement equipment used regarding electrical and magnetic antennas, the analog front-end, and the data acquisition module [13]. In the observation system of ULF/ELF emissions at Nakatsugawa, the signal observed by the N-S sensor (each coil consists of perm alloy of 1.2 m long with 100,000 turns of the copper wire) is fed to a preamplifier, then to a low-pass filter of 10 and 30 Hz and main-amplifier, stored on DL-708 data recorder and saved on a hard disk. Summarized values of the induction coils, the amplification, and filtering are given for the observation system of ULF/ELF emissions at Nakatsugawa [14]. Two research teams from Mexico have developed a Schumann resonance station with two inductive antennas. Details about the structure and development of the magnetic antenna are given. This station measures the first three harmonics [15]. In SR observatories in southwestern China, the frequency band of the instruments is about 3–29 Hz (in the range of 3 dB), and the sampling frequency is 100 Hz. There is a notch filter at 50 Hz to suppress industrial interference in the electronics part of [16]. Details and a block diagram, concerning associated electronics, of the TNB Antarctica Schumann measurement platform are presented by a research team from Italy [17]. In the Schumann station located at Calar Alto (Spain), modes are captured through the 2-m magnetic antenna with an acquisition time of 30 min. Characterization of the sensor and detailed description of the amplification system are also presented [13]. The magnetometers, noise of the system, and the ELF measurement station that has been deployed in Sierra Nevada (Spain) are described and discussed by five research teams from Spain and Sweden [18]. The technical setup of the Hylaty geophysical station with a frequency range up to 300 Hz, as well as the design of ELF equipment, including antennas and receivers, is discussed by three research teams from Poland [19]. In this paper, a versatile receiver for SR detection and monitoring is presented. The SR detection and monitoring system is portable, low-cost, battery-powered, autonomous for nearly 45 days’ time, and able to measure up to six SR harmonics. The system, which consists of the induction coil antenna and the amplifying and filtering chain as well as the data acquisition and processing system, was designed and implemented entirely in the Electronics-Telecommunications and Applications Lab (ETA Lab) of the Physics Department of the University of Ioannina. The constructed magnetic field antenna is much smaller than the antennas described in the literature with cores 0.8–2 m long and a diameter of a few centimeters. The magnetic field antenna is implemented by two back-to-back identical induction coils [20] which are presented in detail and discussed in the following section. Moreover, the filtering and amplification chain exhibits an experimentally measured total passband gain equal to 410,000 or 112 dB (at 10 Hz). The preliminary induction coil sensor antenna design has already been presented and described [21]. The design and implementation stages of the induction coil sensor were based on the fact that the induction core should avoid saturation due to external electromagnetic fields which were mostly originated from 50-Hz power lines. Therefore, the SR measuring equipment was installed in relatively low “EM pollution” areas. In order to evaluate the portable SR system, measurements were acquired at various spots which were located at 1-km far distances from man-made electromagnetic pollution in the area of Northwest Greece. From these measurements, it is obvious that up to six harmonics can be detected within 10-min acquisition time. Schumann resonance is a global phenomenon with numerous applications and many open questions [22]. As already mentioned, there is a very limited number of ELF measurement stations around the globe, based on synchronous electronic methodologies and techniques of signal reception, conditioning, and processing. The contribution of this new ELF Schumann resonance receiver includes (a) signal conditioning stages with an equivalent input noise as low as 2.88 nV/√Hz and a total passband gain from 86 to 112 dB at 10 Hz, (b) monitoring and recording of six SR harmonics through a two back-to-back magnetic field antenna with total weight of 2.2 kg and 60 cm length, and (c) portability and over one and half month autonomy. Furthermore, discussion and detailed analysis of the efficient induction coil magnetic antenna and the low-noise amplifying-filtering chain through this work could motivate other researchers to create new and improved ELF measurement stations. The implemented induction coil antenna is presented and discussed in Section 2. The filtering and amplification chain we developed is extensively described in Section 3. Experimental results are present in Section 4, and finally, the article concludes with Section 5.

Experimental measurements on Schumann resonance magnetic field components involve antenna implementations that are usually induction coils with efficient magnetic properties and enhanced detection sensitivity. Ferromagnetic core exhibits a relative magnetic permeability of the order of 10⁵, providing high induced voltage amplitude. Mumetal is a soft ferromagnetic material that exhibits a maximum magnetic permeability of the order of 2 × 10⁵. Our antenna implementation is based on induction coil architecture with mumetal core material. That mumetal material (ASTM A753 Alloy 4) exhibits very small coercive force, very low core loss, and remanence. Due to the fact that our core is a rod that is open-ended, the resultant equivalent magnetic permeability is much smaller than the mumetal relative magnetic permeability and is given by Eqs. (1) and (2). Also, it strongly depends on the relative permeability of the core material as well as on the induction coil geometry through demagnetization factor N. Equations (1) and (2) give demagnetization factor N and the resultant magnetic permeability (μ) of an induction coil.where m is the length to diameter core ratio (m = 300/25 = 12 in this research) and μ _r is the relative magnetic permeability of the core material. In this work N = 0.0151.

In practice, choosing a quite large induction coil length is a benefit due to the resultant high magnetic permeability [23]. But taking into account that as the length of the induction coil increases, the ratio m also increases eliminating the demagnetization factor value N, as observed from Eq. (1). That yields to the fact that the resultant magnetic permeability (μ) exhibits quite strong dependence on relative magnetic permeability (μ _r ) of the core material due to Eq. (2). Moreover, the value of μ _r presents crucial instability resulting from temperature or applied field frequency variations. Therefore, the induction coil sensor performance may be crucially affected and degraded through variations of magnetic permeability μ. As that material presents high relative magnetic permeability, Eq. 2 is modified to Eq. 3 and the induction coil magnetic permeability depends on the value of demagnetization factor N.

Figure 1 depicts a block diagram of the single induction coil, and Table 1 summarizes the corresponding geometry values. The windings should be laid on the core within 70 to 90% of its total surface in order to take advantage of the maximum possible flux.

The self-inductance, resistance, and capacitance of the single induction coil were firstly computed through the calculated value of the demagnetization factor as well as the value of other geometry parameters and core material characteristics. Using Nagaoka’s formula in the case of an air-coil inductor and a correction factor λ which was proposed by Lukoschus [24] for core material inductors, the magnetic antenna self-inductance was calculated [25, 26]. The induction coil resistance was then calculated through [27]. The magnetic antenna self-capacitance exhibited a strong dependence on the geometry of the coil as well as on the wire insulator electric permittivity and the shielding that may be used between the coil layers. Also, for the purpose of computing such a parameter, several extended computations [25] were needed. In practice, the induction coil self-capacitance was measured experimentally. Both calculated and measured values of the magnetic antenna self-inductance, resistance, and capacitance are summarized in Table 2.

The main self-resonance frequency (1/√LC) of the single induction coil is 480 Hz. However, as the bandwidth of the Schumann resonance frequency components lies in the ELF range, there is no risk of degradation on our induction coil’s performance due to these self-resonances which are also filtered, efficiently, through our filtering and amplification chain stages. The implemented detection system includes two back-to-back coils (Fig. 2) resulting in a total coil length of 60 cm. The housing of the induction coil antenna is provided by a Plexiglas container offering a versatile and portable setup for easy transportation and installation. The 60-cm induction coil antenna (inside the Plexiglas container) is depicted in Fig. 2.

Analog front-end, regardless of its architecture, carries out filtering and amplification of received signals to the level required for the data acquisition module [28]. Specifically in this work, the filtering and amplification setup consists of six cascading circuits which provide amplification and filtering of the induced very low-amplitude voltage signal (few tens of nV) coming out of the coil. Figure 3 shows a block diagram of the electronic chain. Our filtering and amplification chain consists of the following stages: preamplifier, low-pass filter and notch filter, double notch filter, double low-pass filter, high-pass filter, and variable gain amplifier. Since the maximum current of the signal condition setup does not exceed 120 mA, one 47-Ah battery with a 2.5-W solar battery charger extends autonomy for more than 45 days. Data acquisition and processing system has already been presented by Tatsis et.al [29] for the purpose of digitizing signals of the entire ELF band, with 16-bit resolution and an adjustable sampling rate.

For better noise performance and less signal degradation and distortion, the preamplifier was mounted directly on the coil inside the Plexiglas container as shown in Fig. 2.

In order to confirm and improve the system’s operation, we made short-term measurements (few hours) mainly in “almost” white zones in the region of Epirus (NW Greece). Those spots as well as geographical coordinates and distance from the EM pollution are shown on Table 6. Except the last spot (chapel at Doliana), all the other places are in the countryside not suitable for long-term housing of our SR detection and monitoring system. Finally, the measurement equipment have been installed at the fifth spot, a chapel located at a small gorge, 570 m above the sea level and around 1 km away from man-made electromagnetic pollution (Fig. 10). The SR system starts measuring on 19 January 2016 with concurrent recordings, with few time gaps due to electronic equipment maintenance and local interference. Since June 2016, ETA Lab’s SR measurement station has been added to the list of worldwide Schumann resonance stations [30].

To verify that the primary aim of this work was achieved, which was to design and implement a standalone, portable, and low-cost system able to measure as much SR harmonics as possible, we present power spectrums with 60 cm back-to-back magnetic antenna and N-S orientation with 10-min acquisition time (Fig. 11). Each figure additionally contains noise and fitting curves based on nonlinear least square regression, using Lorentzian-like function as described at [29]. For the first six modes, resonant frequencies, Q factors, and peak powers are summarized in Table 7. Although detailed analysis and validation of the record data is beyond the scope of this work, additional results are presented to document the system’s operability. Specifically, frequency mean value and standard deviation for the first six modes and for May 2017 are shown on Table 8. Frequency variation versus time, for the first week of May 2017, for mode 1 is shown in Fig. 12. Daily frequency variation, with maximum values around (8.00 ± 0.05) Hz for the first 4 days and (7.90 ± 0.05) Hz for the last 3 days, is noticed. Respectively, minimum frequency values lie around (7.75 ± 0.05) Hz and (7.55 ± 0.05) Hz. For all 7 days, maximum frequency values occur between 2:00 am and 6:00 am while minimum frequency values occur between 11:00 am and 3:00 pm, which yields similar frequency patterns for mode 1. These day-to-night and day-to-day variations are interpreted mainly by the inhomogeneities and the anisotropy in the Earth-ionosphere waveguide.

A new portable ELF receiver for Schumann resonance detection and monitoring is presented in detail in this paper. The system is comprised of a self-designed induction coil sensor and six filtering and amplification stages. In the presented system, two magnetic field antennas, with a weight of 2.2 kg and size of 300 × 25 mm each were developed. The main self-resonance frequency of the single induction coil is 480 Hz. The low-noise implemented signal conditioning stages have an equivalent input noise as low as 2.88 nV/√Hz and a total passband gain from 86 to 112 dB at 10 Hz. The experimental results have also demonstrated the system’s feasibility in measuring clearly up to the sixth SR harmonic. Since January 2016, the measurement system has been installed near North-West borders of Greece measuring and storing SR initially with one magnetic coil and since June 2016 with two magnetic coils back to back. The system’s remote monitoring, validation of 18 months of data, and installation of E-W magnetic coil antenna are included in a short-term future work.