In light of our current knowledge our, the first radio transmitters of history, constituted by an induction coil connected between an antenna cable and an earthed socket, manufactured by Marconi between the 1895 and the 1900, They were extremely rough.
Among the antenna cable and earth plug was also installed a spark gap, It consists of two metal tips, They are including scoccavano of sparks due to the high voltage, that could reach a potential of thousands of volts, produced by the induction coil.
The spark gap functioned as a switch, periodically loaded by connecting the antenna to the earth, and thereby producing a short pulse of electromagnetic radiation.
The energy produced by spark-gap transmitters were distributed over a wide bandwidth, and in this way the interfering transmitters to each other.
Furthermore, initially it was not possible to modulate the transmission with an audio signal, therefore emissions were exclusively in morse.
Siemens, in Germany, He managed to build a spark-gap transmitter that could transmit voice.
They were made many efforts to create a transmitter that could produce an emission in continuous wave, or a sine wave of a single frequency.
You must Frederick Thomas Trouton, who spoke at a conference of 1891, the intuition that if an electric alternator, constructed with a sufficient number of poles on its armor, It was made to rotate at a sufficiently high speed, He could have generated continuous waves.
This idea was thorough in practice by various researchers, among which Nikola Tesla, who tried to build machines of this type.
In the period between 1920 and the 1950, They were built some transmitting stations that used this system, America and Europe.
These were installations with capacities of the order of 500 kW or more, requiring much maintenance.
All these stations were then abandoned in favor of more efficient transmission systems, and in some cases destroyed during World War II.
Currently, there is only one transmitter still functioning alternator, and is Grimeton, in Sweden. The alternator used is named after its inventor, Ernst Alexandersson. In view of its great historical, is’ It was declared World Heritage by UNESCO.
This installation, lovingly preserved and maintained, It is activated in transmission usually twice a year. A commemorative broadcast on Sundays is done, previous or next, closer to 2 July, and the other on the day of Christmas Eve. Furthermore, It is possible in other special events broadcasts, or for technical tests. The transmission frequency is 17,2 kHz.
By sending a communication to the association Alexandersson pounced reception, with data about your location, the apparatus used and the intensity and quality of the signal, you will receive a postcard.
In the world of amateur radio, This card is called QSL, and it is the tangible proof of signal reception. Radio amateurs jealously guard their QSL, usually hung and framed proud to show to friends. I know one who has received some from the ISS space station.
Below a Radio Moscow QSL. The rear face carries the date, time and name of the amateur radio operator who received the transmission.
The alternator Alexandersson broadcasts can not be received with the radios we have at home. The frequency used is in the range of VLF (Very Low Frequencies, ie extremely low frequencies, but better translatable with Waves Very long). In this range you can listen to some so-called programs 'utility', ie fixed transmissions with commercial or institutional purposes, but also communications between submarines. But this band is best known for the opportunity to hear the sounds related to the terrestrial atmosphere.
Start listening to the VLF is actually very simple. To do this simply use a personal computer with a sound card. The frequencies involved are so low that it is possible to use the microphone input connected to a long electric wire. A free program to use the soundcard as receiver complete our VLF station. Unfortunately, as often it happens the cost of the system goes hand in hand with some limitations. In the first place, the antenna made with a piece of wire is not very sensitive to the signals that we want to listen, then we can only capture the stronger ones. Furthermore, if we live in a place with many electrical appliances (for example in a condominium), the noise generated by these appliances will give a lot of discomfort.
We could go out in the car with laptop, climb a mountain, as far away from electrical noise, and spread a long electric wire, but this solution, albeit fascinating and achievable, it is certainly not the most comfortable.
The technology and research enthusiasts there is help in the form of active mini whip antenna.
This antenna is small motorcycle, very sensitive to signals useful and little sensitive to disturbances, and you can also be used in a condominium, provided you install it out of the house, on a terrace or out the window. obviously, we can also take it with us on top of the mountain, and in this case the reception will be even better.
Despite being very inexpensive and practically available to all, The receiver consists of the computer's sound card has certain birth defects. If we use as a long wire antenna, we must be careful not to inadvertently introduce static voltage spikes, that would burn the card input stage, making it unusable. There are many tips network to protect the entrance.
Another problem of the sound card is that it could happen to us to hear sounds that do not actually come from outside, but from within the computer. E’ easy to distinguish, because if we detach the antenna internal signals remain listenable with the same intensity.
Furthermore, the bandwidth of the sound card is limited to about 30 kHz, so our VLF receiver can tune into only a limited number of emissions.
If we are passionate about listening to the VLF, and we consequently improve our receiving system, the second step after the active antenna is adopt an upconverter. This apparatus makes a frequency conversion of the VLF signals, moving them over a range of frequencies available on normal (albeit not very widespread) commercial receivers. If you already have a receiver for shortwave, You can use this method. But even in this case, the range of frequanze although much larger, It is however limited by the maximum frequency of the shortwave receiver, or 30 MHz.
Below a picture of a famous receiver capable of receiving shortwave.
There is a modern solution, that while remaining within a very affordable price range, It allows us to listen to a wide range of frequencies: The SDR system, ie Software Defined Radio. This topic deserves an article on its behalf, so I do not pursue it in this venue. For the moment I will only say that the hobby version uses a USB stick for receiving digital terrestrial TV.
We can listen to broadcasts of a large slice of frequencies.
Stumble this regard, the system described on the site Theremino, offering both devices schemes
that of the printed circuit projects (below an upconverter)
Example of an active antenna
and some free programs
all open source and freely available.
Furthermore, Research is also encouraged by the users.