A "BAD DEVICE", DIFFICULT TO MANAGE
The induction coil is nothing more than a multiturn loop and works with the same principle: there are many turns, a variable magnetic field passes through them, this creates a voltage across its terminals. In some loops we have no magnetic core and only mechanical support. Here we have a core material with high permeability, which virtually increases the efficiency and response of the loop/coils.
The reasoning appears simple, however, if we attempt to calculate this type of sensor we realize that it is almost impossible. Even the texts of physics and research institutes indicate that an induction coil is almost impossible to model. Fortunately we can resort to experimental tests to arrive at excellent results. Why? The reason is: there are too many unknown, unsuspected, and interactive parameters.
The first parameter that is difficult to calculate is the coil inductance. Even without considering the presence of a high permeability core, multilayer coils with thousand of turns are difficult to calculate. The final value of the inductor always has a different value from those calculated.
The second unknown is the parasitic capacitance: it changes its value depending on how the coils are wound, with what order, with how many layers. Two induction coils with the same look, same number of turns may have very different parasitic capacitance values: it involves two different values of self resonance, for example: 500 Hz and 4500 Hz... for two quasi-identical coils!
The third unknown factor is the
an equivalent circuit. With an air core loop, we
can, with good approximation,
reducing everything to some equivalent components:
generator, resistance and capacity. In an induction coil
we have the capacity
between the windings, capacitance between the layers,
the capacity between
the sections, distributed resistance and mutual
induction is not constant
between all of these. The model presented in the picture
does not accurately
simulate the approximate behavior of the induction coil
as it should.
The fourth and most serious unknown
factor is the
magnetic permeability of the system: we have to use a
core with permeability
much higher that the air to increase the effective
However, full effect of the high value of the
permiability is only
realised with a closed magnetic circuit, as in a
toroid. In an induction
coil the core is open, which means that the effect is
very small if compared
to original permeability and there are considering
Now you understand now why an induction coil is a difficult product to design? And perhaps this is the reason why they are so expensive.
THE CHOICE OF THE CORE: TYPE OF MATERIAL AND SIZE
The best results are obtained with
cores. The best choice would be mu-metal or permalloy.
But they are very
expensive and difficult to find. We therefore opted for
are many types and sizes, so there are many choices.
Also the choice of
the length/diameter ratio is very important: it must be
as high as possible
because the rod-µ and µ-coils are heavily dependent by
value. For ferrites like the one we chose, it must be at
least 25 for a
good coil efficiency. The numbers and the choices that
follow are the result
of calculations and measurements made on different
The final results, following the instructions that will be provided for the coil, are as follows: µ-Rod (ferrite reception gain) 158 times, and µ-Coil (ferrite inductance increasing) 71 times greater. These measured values indicate how large a change in the coil performance is achieved by adding the ferrite core. Both parameters are important: the µ-Rod will be useful to estimate the number of turns and µ-Coil to know the impedance and therefore properly design the preamplifier.
With these data we are now able to determine the number of turns, referring to the formula to calculate the voltage at the terminals of a loop, immersed in a known field:
E[v] = (4 x 3,14 x N x A x 6,28 x F x H) / 10.000.000
The coil must be sensitive enough to receive signals such as magnetic pulsations and the Schumann resonances with an intensity much greater than its intrinsic thermal noise, and the noise that will enter the preamplifier front-end. We refer in this case the natural background signal levels indicated by Maxell and Stone in their treatise of 1963, which were as follows:
As a reference for the intrinsic thermal noise we have used the simplified formula:
Noise floor [nV/ sqrt Hz] = 4 x sqrt R [kohm] (sqrt stands for "square root")
We here omit the calculations and tests that led us to the final result: it was a long push and pull between calculations and experimental tests. Let's go straight to the final results. The coil will be built as follows:
The winding will occupy 8 of the 10
leaving the two ends free. This is useful since the
value of µ-coil
decreases as you move away from the center of the bar.
The two core elements
at the ends add to the total µ-coil but carry no
windings. In addition,
the winding must not be done in a single layer. It must
be broken into
sections: this helps to decrease the parasitic
capacitance by raising the
self resonance frequency.
SOME TECHNICAL DETAILS AND FINAL VALUES
Ferrite blocks are readily available
large distributors of electronic components such as
cost of each ferrite block is approximately € 20.00. The
assembled together using a bicomponent glue. The joints
with high strength adhesive tape, in a longitudinal
direction at each joint
and along the entire length of the bar. It is necessary
to be careful at
this stage. The built bar is very delicate and can
easily break. The ferrite
material is very brittle and if it breaks, or falls, it
splits into a thousand
crumbs, as if it were made of glass.
To build the winding we have approached manufacturers of transformers: and they have wound 8 spools with 12.000 turns each. They fit perfectly with the ferrite cores and give to the construction a very professional end result. Each winding is wrapped approximately with 0,33 kg of copper wire (2,5 kg for the entire induction coil) and the cost realized in this way is Euro 55,00 each section for a total of Euro 650,00 (10 ferrite blocks + 8 wound sections).
And these are the final values of the coil, measured with a HP 4274A RCL bridge, an oscilloscope, a function generator, a decade resistor box and a transmitting loop:
With such high values of impedance
of a shield is not just a possibility but is a
necessity. If the induction
coil is not shielded it will be sensitive to the
electric field and the
quality of the reception will be compromised. We can use
a grid of thin
metal to shield the sensor, like the one used to make
for windows. This material is readily available at
Before being installed the grid must be isolated: I got the insulation by mounting the moschito grid inside of two larger sheets of nylon bubble. The isolation is necessary because it is essential to make sure that the ends of the metallic net are not touching. Otherwise they form a single loop, a net metallic tube, that would work as short-circuit loop, lowering the sensitivity of the coil. The shield must cover the entire object, windings and ferrite bar: with high impedances ferrite rod is coupled capacitively to the windings becoming itself an electric field sensor. If you do not believe it try to to touch the ferrite with a finger: you will see the hum noise output from the coil to increase immediately, as if you were touching an oscilloscope probe.
Choosing the circuit configuration and components
Here is the
schematic of the
preamplifier used with our coil:
The gain of the first stage has been
chosen to be
about 55 dB below 1 Hz, 12 dB at 100 Hz and reaching the
unity gain at
400 Hz : it is determined by the ratio R1 + R2 resistors
coil impedance. This ensures that the entire operating
range from 0,1 to
100 Hz output overrides the input noise of the
operational noise input
of the next stage. At the same time it is not so high as
to require offset
compensation and in any case is tolerated in terms
of product gain
for bandwidth by the operational amplifier.
ICS101 Frequency response with LPF turned OFF: the device works in flat from 0.6 to 200 Hz (+/- 3 dB)
The choice of the operational amplifiers has been very laborious. The first selection was made based on values of voltage noise and current noise indicated in data-sheet, considering their effect on the coil impedance. The following amplifiers were taken into consideration: OP27, OP07, TL071, AD820, AD743, AD797, OP97 and LT1113. Unfortunately, many documents do not report data below 10 Hz. For a low noise performances in the available data sheet were the best the AD743 and OP07. Testing them we found that the first is better at 10 Hz, while the latter works better under 1 Hz. For this reason we decided to install in a first stage an OP07: choosing to give advantage in the coil performances to frequencies below 10 Hz.
Finally the capacity of 220 pF C8
introduces a low
pass filter at about 220 Hz, to avoid signals at higher
the stage generating artifacts of measurement below 10
Hz. The low frequency
corner is instead determined by the inductance of the
coil and its intrinsic
resistance: it is a low pass filter at 1,3 Hz. A second
pole is given by
the output capacitor of the amplifier circuit. However,
they are not a
problem because under 1 Hz natural radio signals grow in
compensates for loss of sensitivity, giving a quasi-flat
low pass filter
We rejected the idea of the notch
to produce and especially difficult to maintain a stable
changes in temperature (do not forget that the coil
operates usually outdoors,
with temperatures that can range from +40 ° C to - 30 °
frequency of notch filter, especially if very narrow and
sharp, would be
influenced by temperature.
ICS101 Frequency response with LPF turned ON. At 50 Hz the attenuation is 46 dB
The 22 and 6,8 µF capacitors used in
sections of the filter must not be polarized. This means
that you need
to use polyester capacitors connected in parallel to
reach the value shown.
Do not make compositions with electrolytic connected in
avoid these tricks. They do very strange things
deforming low frequency
There are three selectable gains: the
gain of 20
dB is usually the best and adapts to mostlocations and
sound cards. If
once turned on the coil the output signal exceeds 1
Vrms, you must
insert the low-pass filter or reduce the gain. The
maximum gain of 40 dB
is recommended only in combination with the low pass
filter or with sound
cards of low gain.
We now have all the data to estimate
of our coil thus constructed.
We know that:
E[v] = (4 x 3.14 x N x A x 6,28 x F x H) / 10.000.000 (http://www.vlf.it/minimal/minimal.htm)
we are now able to calculate the coil
10 Hz = 0,04 pT
By the same calculations at other
MEASURED SENSITIVITY: THE CHARACTERIZATION WITH A SOUND CARD
To make this measurement with the sound card we have to set the value of "equivalent noise bandwidth" to 1 Hz: it is different by the width of one FFT-bin. An example would be setting a sampling rate of 6000 S/s, 8192 FFT points and a "hamming" window of integration type. This will provide a value of 0,996 Hz equivalent noise bandwidth, suitable for our measure. You can also choose other settings to get this final value: SpectrumLab shows this value in the section "Configuration and display control / FFT proprieties".
Now we have to measure the background noise of our acquisition device (soundcard). To do this, once the coil is positioned far from house and connected to a PC by a coaxial line we turn (ON) off the power: the spectrum we detect is related to the input noise floor of our soundcard and all the noise coming from transmission lines we use to carry the signal from coil in the gardento the PC in the house.
Now we need to measure the noise of the preamplifier, but it varies with frequency because the inductor is not linear and varies its impedance: it changes so the value of noise generated by the current noise of the IC and changes the value of the operational amplifier gain. We need to realize therefore three resistance of 13,1 kOhm, 138 kOhm and 1,43 MOhm: these simulate the impedance of the coil at frequencies of 1, 10 and 100 Hz. We connect these resistors, instead of the coil, one by one, switching on the preamplifier with gain set to 20 dB and each time we read the values of noise in FFT curve at 1, 10 and 100 Hz. These three points will give the noise floor of our amplifier when it is fed by the coil. We can also do this for others but these three points are sufficient for an overview. With these three points we draw a curve that is the noise floor of our preamplifier.
Now we connect the preamplifier to
the coil and
test the strength of received signals. They must be at
least 15 dB above
the noise curve traced before. If we achieve this result
our device is
sensitive enough. But to know "how much" it is we have
yet to make a test:
transmitting a known signal strength. To do this I built
loop with 40 turns of diameter 45 cm.
The loop was placed 4.5 m from the coil, at 1.9 m height. In series is with the loop is a 1000 Ohm resistance connected to a function generator. After that signals are transmitted at various frequencies a floating oscilloscopeis used to measure the voltage across the resistance to then obtaining the current flowing in the loop. With this information, applying the law of Biot Savart we can calculate the field generated by our loop at a certain distance.
If you are unfamiliar with the
formula you can use
the service of a websites like this: http://www.netdenizen.com/emagnet/offaxis/iloopcalculator.htm
Applying the same signal strength for
and measuring a voltage of 3,66 V, then 3,66 mA of
current, we have generated
a field of 31 pT on the coil. This signal is much
stronger than natural
ones present and then we see them rise during normal
reception. The graph
below sums this up:
As we can see the sensitivity
obtained is very similar
to that calculated and at some frequencies
even better: the
gap between the noise floor (yellow line) and the
received signal (blue
line) is always maintained above 15 dB. A second graph
was made focusing
on the lower part of the spectrum: the most critical
part being below 3
Here too the results are very good: our device is quite capable of receiving the geomagnetic pulsations. With these measures we are now able to evaluate the quality of our construction by comparing it to the specifications of a professional product used for research. We chose the model of MGC-3 Meda: it is a tool used by many research centers and reported on many academic papers.
Here is the chart that compares the
in the region of Schumann resonances. Two references
were taken: the minimum
signals detected by Maxell / Stone in their famous paper
and signals detected
in the daytime in our monitoring station, which
obviously are stronger.
The graph shows the signals expressed in dBpT: 0 dBpT
correspond to 1 pT,
20 dBpT to 10 pT, -20 dBpT to 0,1 pT and so on.
In this second picture we see how our
in the most difficult region of geomagnetic
under 6 Hz.
The measurements can be affected by some error: do not forget what we have done with the help of a single sound card, but, it appears clear that the device we constructed has characteristics very similar and comparable to a professional product. We can be very satisfied with this. Even the values of sensitivity that we calculated in the previous section are quite satisfied:
Final results: what can we really get with this coil?
The initial goal was to build an
able to receive the Schumann Resonances and geomagnetic
a soundcard: let us see how it has been observed.
Here we can se a good PC1 geomagnetic
It extends from 0,8 to 1,2 Hz and it emerges from the
some times by more than 15 dB. Schumann resonances are
also clearly visible.
Our coil is doing a good job!
Here too we have received a
of type 1, even stronger than before. Note that the
pulse has the typical
vertical stripes that identify in an unequivocal way
this type of signal.
But here we have another type of signal, a guest more
difficult to receive:
a spectral resonaces structure (SRS). It looks similar
to the Schumann
resonance but with frequencies less than 3 Hz and with a
spacing of about
The signal SRS is much stronger than
in the previous
example. According to a lot of documentation that is
available on the web
they would be due to the fluctuation of the
magnetospheric tail under the
influence of the solar wind. They would be a direct
consequence of the
Alfven resonance. Also are visible in the spectrogram
and a weak PC1 pulsation. Our coil works just fine!
Beware of false signals
The position of the coil is very important: it should be situated away from power lines and devices that work on alternating current. A house in the countryside can be a good site but this is not always true: the power lines can disturb the reception even several miles away. A place in town is certainly a very critical position with little chance of success for good reception. Many signals we receive may not be natural but have the following origins:
a) Microphone effect. The coil also functions as a microphone: it hears the footsteps, the passage of vehicles and thunder. A geophone positioned near the coil can help to distinguish true signals from false signals.
b) The disturbance of the earth's magnetic field. It can be disrupted by any ferromagnetic metal object that moves close to the coil: just a bunch of keys to rotate a few feet away, and their track appears on the In the spectrogram. This also means that the coil must not be near where the metal parts that can vibrate, like fences or iron gates. Even the passage of cars causes of changes in the magnetic field within ten meters away.
c) Local sources. The coil should be located away from local ELF sources, such as: electric motors, pumps, washing machines, generators, charge controllers for solar panels, tube lamps, dimmer, CRTs,TVs, computers and any type of switching power supply.
d) Main power interharmonics (or Main power AM modulation) : Many signals arrive from the power supply: they are caused by non-linear loads that modulate in amplitude the network current. Many of these signals appear just like an AM modulation: for example you can see signals similar to PC1 at 1,5 Hz but are also present to 98,5 to 101,5 Hz. They are certainly not geomagnetic pulsations. To discover they sometimes need to run several spectrograms of the same signal at different FFT resolutions: this allows you to highlight details and find which are false signals. See for example the page of the observatory of Cumiana (http://www.vlf.it/cumiana/livedata.html) where the region of frequencies below 30 Hz is examined simultaneously in different ways: at high resolution with slow scrolling and with faster scrolling in multistrip graph.
There we have done it: we have built an induction coil with professional specifications, using as acquisition device a simple sound card. Now we too can receive similar geomagnetic pulsations such as HAARP .
Many Thanks to: Claudio Re for support during testing, advices and project documentation, Marco Bruno for advices and equipment provided, Andrea Dell'Immagine for various suggestions and the idea to use rectangular ferrites, Dave Ewer for grammar correction.
A commercial version of this product with the pre-amp built in, and coil drowned in resin is distributed by SISTEL (www.comsistel.com)
Induction coil sensors—a review, by
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