The Schumann's symphonies
by Pierluigi Poggi IW4BLG
Adapted and enached from the article
published on Radio Rivista March 2013 with curtesy
of its director Gabriele Villa, I2VGW
are a group of energy peaks in the portion of the
spectrum of extremely low frequencies (ELF), effect
of global electromagnetic resonances excited by
This phenomenon is named by physicist Winfried Otto Schumann who first predicted it mathematically in 1952.
cavity, whose boundaries are the Earth's surface and
the ionosphere, it is naturally excited by energy
from lightning discharges and its resonance is
revealed as separate peaks in the field of ELF
around 8, 14, 20, 26 and 32 Hertz.
These signals are collected and analyzed for various studies such as example:
not everibody is lucky enough to be able to have
room enought to its installation. The aim of this
work, is therefore, to propose some solutions and
practical suggestions to those willing to study the
matter but having limited means. The solution
presented is suitable for field day use, in order to
allow the enthusiast to take advantage of a "day out
of school" for experiment with success.
simplify at most the schematic of the necessary
elements to this study , we can assume the following
Figure 3: Block diagram of the acquisition chain
The system is composed of an element designed to capture the natural electric field, followed by a module that processes the signal to make it easy to be analyzed, then the last item, in chargheof the analysis.
Let us now, block by block, develop some considerations of design and use.
The newcomers less accustomed to these extremes frequency, could legitimately ask whether it is possible, as in the VLF and higher, use a kind of active antenna, made with a small stylus and an appropriate amplifier stage of high input impedance.
from a theoretical point of view the thought is
logical and motivated, unfortunately some practical
aspects make it very difficult if not impractical
Let see with some interesting observations
on this regard:
should worth to be considered how playing with an
antenna very short, every little disturbance of the
static field on its vicinity, becomes a signal or
better, a disturbance in measurement. Flying
insects, the grass moving in the wind, the movement
of some human in the vicinity of the antenna as well
as the atmospheric agents such as rain, fog, snow
are all elements that can create havoc in various
ways on the output signal of the system.
Marconi antenna, because of its more generous
dimensions, it solves many of these problems. Also
for the same reasons, it presents a high capacity to
the ground, which allows the design and
implementation of a front end much less critical and
solution proposed is a compromise between wishes and
real possibilities, with a goal to allow a certain
freedom of study and experimentation.
convince definitely the novice about the
difficulties, you can consider how 10 Hz are
associated to a wavelenght of 30.000km. A Marconi
20m long, therefore, it is equivalent to use for the
reception of the medium wave, a whip only 0.2 mm
From experiments made with the system described below, it appears that:
The front end
It is the stage that collects the signal received by the antenna and, using an appropriate treatment, makes it available to the device in charge of record and analysis. Its main tasks are:
The proposed scheme is quite classic, with some specific characteristics.
The antenna, as common practice in these
cases, is modeled as a voltage generator in series
with a capacity that identifies the coupling of
receiving structure with the electric field to be
received. The first stage follows the pattern
presented by Marco IK1ODO and published in various
literature (See bibliography), with some fine tuning
for the specific use herein defined. The various
elements have the following tasks:
The choice of the first integrated circuit
As often happens, the first stage of a receiver is what determines most part of the final result. The choice of the active device is therefore important and must be performed in this case following two guidelines:
To study the effects of the bias currents of the inputs, we can use the scheme shown here:
Figure 5: equivalent schematic for the effect of bias current
Considering the values of the resistance of the circuit, it appears that the key role is the difference of potential generated across of R3. Thus, it's important choose a device with low bias current. For the study of the noise we can use the diagram below:
Figure 6: Schematic equivalent for study on noise
The contributions to the total noise are due to:
With a little of calculations, we can put in evidence as with the values of resistance employed in the circuit, it is the predominant the contribution of the operational input noise current the to the total noise. By comparison of the performance of some of the devices mostly available, we obtain the following summary table calculated at 10Hz, 17 ° C and with the values of R1, 2,3 of the circuit before described.
Figure 7: Comparative table between different OpAmp
The first six devices are substantially equivalent and considering cost and availability perhaps it may be worth experiencing only the alternative between the TL071 and the AD820. The OP07 and OP27 and following ones, although excellent in general sense, for this specific application are not advised because of their high current noise at those frequencies. The breakdown of the various contributions to the total noise in the circuit, is summarized in the graphics below. Pay attention as the scale is a logarithmic type.
Figure 8: Graphical comparison of the noise of the circuit with different OpAmp
Even when you operate the system on a remote area, the 50 Hz of the mains network, will be always the louder signal, sometimes strong enought to defeat the acquisition system. Thus, it's introduced a notch filter centered at 50Hz in order to reduce the amplitude of that unwanted signal without affecting significantly the useful band of the system. For its maximum effectiveness should be made of components (Resistors and capacitors) with 1% accuracy.
The second stage has the following tasks
Putting "all together" you get the following response of the antenna system and the frontend:
where you can see:
The system is powered by a pair of rechargeble batteries of 8.4 V NiMh. With their 140mAh capacity, they are sufficient to guarantee a day of operation of the system. If you need a greater autonomy, you can assume the use of two small batteries Pbgel of 12V or maybe four cells LiPo if the weight factor is important.
The system PC + software
The simplest way today to analyze the signal, is the use of a computer with a sound card and an appropriate software. Unfortunately, not all the sound blaster work fine down to those frequencies and is therefore warmly advisable, to test them before using. As a programs for the acquisition, recording and analysis, among the most popular we can mention:
Please refer to their websites for each instruction on their use and configuration.
What do I receive?
As anticipated, the system presented offers good reception. In the figure below you can see a recording prepared with Spectrum Lab and acquired with the antenna T of 18m, supported by occasional means as visible in other images.
Figure 11: Screenshot of Spectrum lab with the Schumann resonances (source IK1QFK)
The result is very positive and achieved with little effort and investment.
The bands of Schumann resonances are clearly visible, as well as 50Hz albeit its considerable amount does not affect the cleanliness of the acquisition.
Figure 12: Setup capture: simple, functional and extemporaneous
To drive the testing activities quickly towards an efficient configuration, you can consider the following guidelines:
At the end of this paper, I would like to thank Marco and Andrea IZ5IOW, IZ5TLU for having me light up the curiosity on this matter and Renato IK1QFK and Marco IK1ODO for the competent and valuable help and support of experience.
wishing to explore some of the many topics covered in
this article, I may suggest to take a look at the
Renato Romero, Radionatura, Sandit Libri, 2006 Albino
Pierluigi Poggi, Active antennas, Sandit Libri, 2011 Albino
AA.VV, Low Level Measurements "Keithley
Daniel H.Scheingold, Transducer interfacing handbook "Analog Devices inc. 1980 - Norwood, Massachusetts USA
it.wikipedia.org / wiki / Risonanza_Schumann