IN THE ENGLISH
TRANSLATION SOME PARTS WERE PUT APART BECAUSE AT THE PRESENT THEY ARE TECHNICALLY
OVERTAKEN. THE PRESENT TREATMENT DOESN'T WANT TO BE A COMPLETE ONE:
SOMEWHERE ELSE THIS TREATMENT IS MUCH MORE DETAILED: LIKE IN A WEB SITES
OF THE LONG WAVE CLUB OF AMERICA, THE
INSPIRE PROJECT OR STEVE
MC. GREEVE SITE. THIS ONLY WANTS TO BE
A GENERAL INTRODUCTION ADDRESSED TO THE BEGINNERS.
RADIO SIGNALS
OF NATURAL ORIGIN, by IK1QFK
THE DISCOVER
We don't know who
was the first one to discover the presence of these signals; but, given
their maximal intensity around 5 kHz, the discover was connected to the
use of these frequencies. Telephonic societies noticed that first, but
they couldn't explain the origin of these interference (the long air wires
which unawares were used as antennas). First telegraphists noticed by experimenting
with low frequencies (thinking that to cover long distances were necessary
long waves). First military spy electronic department noticed that
by using triodes done to amplificate currents between two earth connections
(this in order to intercept communications inserted in the ground). Maybe
even us, we noticed that when unawares we have detached the ground
in a microphone connected to an audio amplifier and we thought at that
moment to hear our neighbor doing works at his home with his electric drill.
Despite the many
occasional meetings with those signals their origin was finally discovered
in 1957 after works done during the international geophysical year: during
that period, when scientists of different countries did together a scientific
research program just in order to study this phenomenon, 66 nations participated
to the study of geomagnetic phenomenon, of meteorology of atmosphere, of
cosmic rays and of many other phenomenas.
Begun on the 1st of July
1957 in correspondence with the maximum sun activity period, ended
up december the 31st 1958 a lot of money was spent and many devices as
observatory astronomic, probe balloons , meteorological missiles and
satellites for the study of high layer of the atmosphere. After this study
an answer has been given to the many questions up to that moment unanswered
about the provenance of natural origin signals.
ORIGIN OF THE
SIGNALS
The explanation of
this phenomenon concerns few physical fields as the terrestrial magnetism
, electrostatic energy in the atmosphere, and many others: we will
examine them one by one in the part dedicated to the genesis of these signals
in order to understand which device connects between then different fields.
A) The terrestrial
magnetism
The planet on which
we live is as a big magnet: with the two poles north and south different
and opposite, they are not the same as the geographic poles and their positions
changes from time to time. This makes the north measured with the
compass is different from the geographic one called magnetic declination.
Lines that connect points with the same magnetic declination, are called
isomagnetics and they are representative of the not regular distribution
of the magnetic field.
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B) The magnetosphere
The strength line of the earth magnetic field extend around our planet in the space so creating the magnetosphere: the place where generate part of the radio listened signals of natural origin. This region is 200 and further Km of altitude where the movement of charged electrical particle is controlled basically by the earth magnetic field and is not conditioned by the collision with themselves. The magnetosphere in its part in front of the sun, extends over 57000 Km about ten times the earth radius; in the other side of the sun it must probably be of hundreds of earth radius. The long shaped look is originated by the solar wind influence, or solar plasma which is formed by electrons and proton swarms, emitted by the sun, that strongly compress the magnetosphere in its sun oriented part.
During the international
geophysical year was also discovered the presence of huge radiation zones
inside the magnetosphere: these regions called "Van Allen's band",
contain protons and electrons full of energy cough by the earth magnetic
field. Even the sun is object of research for the magnetic field: it has
two poles which sign invert during the solar activity during the 11 years
cycle. Some of the space missions as Ulisse one, did fly over starting
from 1996 the two magnetic poles of the sun; the measuring from a
point of view never tested before and studying the intensity and
the orientation of the interplanetary magnetic field, allowed to have data
confirmed and sure and not simply supposed by theories. Now, done this
long but useful introduction, we can now start by describe
the phenomenon which is at the head of part of these signals, that is the
atmospheric electric discharge.
C) The atmospheric electric
discharge.
The earth crust is composed
by minerals and salty liquid (the oceans) and can so be considered very
good conductor. Measuring done around it show in a clean sky day the atmosphere
is home of a electric field with intensity of 100 V/m, produced of
a superficial negative discharge.
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The effect of
the earth magnetic field is to put on movement the atmospheric charges
originated by the ionization produced by the cosmic ray and
by the solar wind taking down the positive ones. The density of this current
is very low ( few amperes by square meter) but, given the big
earth surface the total current that goes on the ground
has an intensity of about 1800 A. this kind of “dark discharged”
would be able to neutralize the earth charge in few hours if there
were no about 300 storms per day on the earth (hundred lightning hit the
earth every second).
When a storm is approaching
the discharged are distributed as shown in picture: the clouds base is
strongly negative, so, given its closeness to the ground of 2-3 Km.,
under it is it formed a en electric field of about 1000 V/m with a potential
difference between clouds and ground of 2 – 3 millions volts.
This field versus is opposite to the existent one in a clean sky day and, given its intensity, does intense ionization processes of molecule in the zone under the could making it conductor: this allows the making of the lightning. |
KIND OF SIGNALS
A) The spherics
The discharge is very violent and can easily arrive to 10.000 Amper, taking to the earth something like ten coulomb. Sound and light effects during the lightning are not everything; there also is a big electromagnetic component which is radiated end which is named spheric or static. |
It's a electromagnetic
signal of wide spectrum, composed by dumped waves of relatively low
frequencies, rich of harmonic components: it spreads in a long distance
because of the earth wave; this has big influence in order to spread in
low frequencies. Even Shumann resonances are involved: they happen
because of the natural wave's guide formed by the earth and by the ionosphere
(freq. 7.8 Hz and harmonic frequencies 15.6, 23.4, 31.2 Hz ).
As in a wave's guide
the right frequency signals can travel inside this natural channel
with very low attenuation values. The spheric decametrics components
spread by ionosphere reflection the same way as the radioamatorial signals.
A long time ago it was supposed that the distance a spheric up to
15 kHz could run was about 2000 Km, but coordinated listening
in VLF denied this data: done on the occasion of the Shuttle fly (Sepac
mission) they allowed the verification the spherics presence
even at a distance of 7000 Km denying a theory which seemed a good one.
This episode confirms that VLF are a unexplored field yet, so with plenty
of satisfactions for who study it.
Tweeks are spherics
in the atmosphere below the ionosphere. They sounds like a bird's sing,
in a frequency range of 1-7 kHz. In a spectrogram they result like a descending
tone on the order of 25 to 150 mS. Tweeks are normally heard in the evening,
after the sunset.
B) The whistlers
One part of the spheric
spreads inside the ionosphere, and we as can notice (if we are listening
in medium waves) the approaching of a storm even if it's still very far.
The other part of the spheric escapes the ionospheric barrier, it overtakes
it and keeps running its path in the empty.
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If the attenuation is low
, also because of the many kilometers done, it's not possible to
say the same thing about the dispersion that is under that influence:
this dispersion is caused by the interaction between the spheric
end the magneto plasma which is in the ionosphere, and its intensity
distorts the original signal.
The result
is a whistler lasting few seconds with a frequency going down
from 5 - 6 Hz down to 1 Hz; they also exist whistlers with a growing
frequency but they are less frequent compared to the other ones. The effect
is very suggestive and whistlers modulated and places upon make think the
noise made by the dentist's drill, or the sounds of science fiction
movies as Star Trek..
This kind of propagation
that we can call "magnetospherics", happens in VLF only, and we haven't
noticed similar behaviors in higher frequencies: practically
it happen in a North South direction connecting between them geomagnetic
conjugate point. This doesn't mean that we can calculate a whistler
propagation path on a geographic map.
C) Discrete Emissions
We can also find in this weird world discreet
emissions which last less then whistlers, but linked to them for
a common origin: in the closest zones to equator they interact with themselves
whistler waves and electron plasma feeds energetic shaped:
like in a particle accelerators (cyclotron), the electrons which are present
under the effect of the terrestrial magnetic field and for the action of
the variable electromagnetic field coming from the whistler wave
, are under the effect of an acceleration, which generates falling
or rising tones; when these two signals follow at the same time, they origin
a hook which lasts not more then 2-3 seconds.
When the natural radioactivity is strong
two or more of these signals can lay on, in this case they are called chorus.
It's known that these discreet phenomena happen not just with spherics
but also with telegraphic signals under 20 kHz; these artificial emissions,
strong as intensity, are obviously of military origin. The terminology
of these signals takes its origin from their spectrum analysis, done by
calculator help with special FFT programs.
D) Signals linked to seismic phenomena
The very long waves
are studied by seismic observatories. The score of the study
of earth currents and of VLF and ULF emissions occurring together within
seismic phenomena, is establishing the relationship between those
signals and and the earthquake: the score is to be able to forecast,
one day, those terrible events.
Research done up to the present
days are divided in three frequencies bands:
- Between 0,01 and 12 Hz:
particularly between 3.1 and 12 Hz; the presence of weird noises would
be connected to those phenomena.
- Between 30 and 150 Hz,
where are controlled the level of background noise and the level of the
carrier of LW Broadcast station .
The origin of these
signals is in the compression of the rock layer that contain quartz and
the following electromagnetic emission before and during the earthquake.
These episodes, which are quite rare, are difiucultly observed; since
a real monitoring is deficient, the probability of meeting these signals
is random and not frequent.
THE RECEPTION
There are two systems to receive
natural radio emissions in VLF: by frequency conversion or direct
reception.
A) Frequency conversion
Usually done with
a IC mixer for HF (SO42P), consists of amplitude modulation of a carrier
(generated by quartz) by VLF signal deriving from the antenna. VLF converters
on the market the frequency carrier is chosen on not crowded HAM band (28
MHz) or lower frequencies (2-4 MHz). To the carrier (at example of 28 MHz)
are hooked, up and down, all frequencies from few Hz to a certain value
which arrives in commercial converters to around 1 MHz (changing from model
to model), quite as side bands. Converted as said, the signal can be normally
listened with a common HAM receiver, using all tool characteristics and
performances.
Carefully: the coupled
condenser at the SO42P input, that allows the passage of the signal to
the converter, decides also the lowest cut frequency.
It's better to get sure that the
frequency is not to high because in that case are not included frequencies
lower than 10 kHz. A dynamic microphone connected instead of the antenna
to the converter, could be easily used by tuning the signal at 28
MHz in AM and talking in the microphone you can hear your voice: if this
doesn't happen it's impossible to hear the natural origin radio signals
of maximum intensity of 5 kHz which is just a little bit upper the
human voice band (1 kHz): in this case the coupled condenser must be
replaced by another one more powerful.
Two are the
benefits of this systems: the first one is the possibility of
using the side bands and even the CW tight filter to select portions of
signal otherwise lost confuses between many other signals in a wide band
listening. The second benefit consists of approaching even the frequencies
near to few Hz in CW mode: the DC becomes in fact tone at 28 MHz and eventual
signals, even a few hundreds Hz, are around this carrier and they
are so perfectly receivable.
B) Direct reception
The reception without
frequency conversion is possible because of the these emission are inside
the band listenable from the human ear. The problem is to receive this
signal with a long wire antenna, with a whip or between two ground connections
and to amplifier it.
The simplicity of
this operation made possible the diffusion of electric field portable receivers
at a very low price.
They consists of a stilus antenna
and of a little audio amplifier which amplifies the stilus signal and sends
it to an auricular; the only existent control is the amplification
gain, and in more complicated models, the frequencies choice of cut between
changer. The signal listened is wide band type and frequency limits are
decided from our ear.
Since these receivers
are extremely easy to carry, it's possible to do listenings in open
country far from electrical noises, trying to use at the best dB
available. Even if the listening doesn't give computerized analysis,
we can have the beauty of the signals different from any others.
CONCLUSIONS
Since the radioamatorial
experience has begun, we keep discovering some new applications about the
big possibilities radio offers us every day in every way. VLF are just
a little part of all of this but they take us closer to human life of other
fields: the presence of magnetite in some migrant birds nostrils, and in
some parts of the human brain connects the existence of these signals to
our life.
It is possible to
suppose a link between these signals and the perception called "sixth sense"
like the nervousness in some animals just before the earthquake; in fact
electroencephalogram shows that our brain uses electric impulses in ULF
band. About this matter we don't have many contributions and this could
be a stimulus in order to make a careful study of the subject.
THANKS to:
ANDREA BERTOCCHI and MICHEL ANDRE' for translation. |
REFERENCES:
Scienze tecniche, De Agostini editions Fisica e laboratorio, Atlas editions Fisica moderna, Cremonese editions Dizionario delle tecniche e delle scienze, EP/SAIE editions Stelle e pianeti, Arnoldo Mondadori editions Top secret radio 1 and 2, CD editions Tuttoscienze, LA STAMPA daily paper insert n°573 Radiorama, Medicea editions CQ elettronica, CD editions |