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Welcome at the page dedicated to the radiotechnics
In this page I will try to transfer, to anyone who takes a few minutes to read this web page, the basics of radiotechnics, I will try to use as simple a language as possible, so it helps those who approach this complex topic to understand its meaning.
I repeat as always that it is impossible to condense such a vast topic in a few lines, but if you need to deepen you can contact me by visiting the "Contacts" page
A thought and a thank you go to the person who made possible everything we will talk about, that is Guglielmo Marconi, without him and his discoveries everything we will talk about would not exist.
Guglielmo Marconi who, among other things, once came to my country, to the residence of his personal doctor, that is the illustrious scientist, doctor and biologist Dott. Giuseppe Tallarico.
Introduction
Radiotechnics is the science that uses electricity, electromagnetism and therefore electronics to send and receive radio waves for communication (radiocommunication) or control purposes. It envisages the study and design of the electronic apparatus for the purpose, namely radiotransmitters, radio receivers and transmitting and receiving antennas.
By trying to simplify the concept, it is to send,in the surrounding space, the electromagnetic waves (carrier) having appropriate frequency and wavelength to which the information to be transmitted (modulation) is appropriately superimposed, this work is performed by the transmitter, Which, connected to its output via a tuned transmission line (coaxial cable or waveguide), the transmitting antenna, which is responsible for transducing the electrical signal into an electromagnetic field of intensity proportional to the power present at its input.
Everything is based on oscillatory circuits, which can be of a serial type or more often of a parallel type, they are electrical circuits that exploit the physical properties of the inductors and capacitors and their skill to agree on a single frequency depending on the value of the components used , It is deduced from what to change the resonance frequency and then select (Receiver) or produce (transmitter) is enough to vary the value of one of the components of the oscillatory circuit.
Electromagnetic waves are then received and amplified by the receiving antennas, which are also oscillating circuits tuned at a given frequency, and operate the reverse process, i.e., transduce an electromagnetic field in an electrical signal.
The received signal is then transmitted via another line tuned to the receiving apparatus (radio receiver) which, by means of the usual oscillatory circuit, will select among the many, the frequency we are interested in receiving and will separate the transmitted information from the carrier Radiofrequency (demodulation). We will address the chapter transmitters, receivers and antennas later, in separate treatises to better understand them.
Sometimes the transmitter and the radio receiver can coexist in one electronic device, in this case it is referred to as a transceiver.
The frequencies and therefore the wavelengths on which it is possible to transmit are shown in the electromagnetic spectrum and indexed in various bands visible in the zoomable tables that are on the line below.
Magnifying electromagnetic spectrum Bands of the electromagnetic spectrum
The choice of the transmission band depends on the distance to be covered and the terrain, this because the signals with frequency in the lower part of the spectrum have the ability to overcome the obstacles that stand between the transmitter and the receiver (prevalence of the magnetic component on the electric one), but they must be supported by high powers with relative proportionate electrosmog pollution.
Moreover up to about 30 MHz the reception of said emissions is affected by the so-called fading, that is the continuous variation of the intensity of the received signal, this is due to the continuous variation of the thickness of the layers of the terrestrial atmosphere that reflect this type of signal and allow propagation over long distances. The propagation by direct wave remains stable instead.
Unfortunately the use of low bands, due to the limitations set out above is gradually falling into disuse, they are replaced by internet streaming and numerical satellite communications which allow good quality without the inherent limitations of this type of emissions.
However, they are still used predominantly when large spaces need to be covered with just one transmitter.
As the frequency increases, the electromagnetic waves tend to be absorbed and shielded from the obstacles that stand in the way, until an optical range is reached after 30 MHz, ie the transmitter must see the receiver, otherwise the reception of the radiated signal becomes impossible, but there is the advantage of being able to use relatively low transmission powers, given the need to cover a specific area to install more than one transmission system.
However, we always try to place them in a position as high as possible to extend the optical horizon and therefore increase the area served by the transmission system.
It should also be borne in mind that the choice of the transmission band directly affects the quality of the transmitted information, this because in the low bands it is obligatory considering the little space to use the amplitude modulation AM, all this implies that the maximum frequency can be modulated by 4.5 KHz if the transmission channel is 9 KHz, and 3 KHz if the channel is 5 KHz wide.
Instead using FM frequency modulation, which is insensitive to disturbances of any kind, the quality improves a lot.
Then using the broadband FM, which unfortunately is necessarily confined to the high bands given the 75 KHz channel, the maximum transmissible frequency becomes 15 KHz therefore of Hi-Fi quality.
With digital modulations then quality depends on B.E.R.
(bit error rate) and the type of data compression used, there is however the advantage of very high noise immunity and the possibility of transmitting large amounts of information in a narrow channel (compression).
Generally, the optical propagation of electromagnetic waves is exploited with very high frequency for point-to-point communications, ie radio bridges are created, which are used to create communication backbones (telephone, TV, radio or other), to which they will be then connected the local transmitters. The radio bridges can be represented as in the figure below.
In radio links, very direct antennas are used, usually parabolic antenna.
Here are some zoomable images that will allow you to better understand what is exposed.
Transmission
system with radio links and RADAR Series oscillatory
circuit
Parabolic antenna for spatial communications Parallel oscillatory circuit
Radio transmitter
To transmit radio waves through the antennas, we use the devices designed and built for the purpose, we are talking about radiofrequency transmitters, which are very complex apparatus, formed by different stages, depending on the type of modulation used but which Always provides as the first stage an electronic oscillator that generates the carrier having the desired frequency and as a last stage a power amplifier that elevates the voltage and current values to the appropriate values that they need to cover the desired distance.
The oscillator is an electronic circuit using active and passive electronic components, which generates by means of a tuned oscillation circuit an electronic frequency signal determined at the design stage, which must coincide with the transmission frequency we are interested in, is very important that the oscillator provides a frequency signal that is stable over time, to achieve this, a particular piezo-electric component called quartz crystal is introduced into the oscillatory circuit, which stabilizes the oscillation and makes it independent of the supply voltage and within certain limits even from the temperature. In multichannel circuits to avoid using many quartzs, a particular circuit is called a P.L.L. (Phase locked loop), which uses only one quartz to generate all frequencies in a stable manner.
Lately, a newly conceived digital circuit called D.D.S. (Dyrect digital synthesis) with numerical control which has many advantages over analog ones.
The output signal from the oscillator then passes to the modulator stage, this circuit overlapping the information to be transmitted to the carrier to be radiated, it is different depending on whether amplitude modulation (AM) is used where the carrier frequency It remains stable and varies its amplitude, or the frequency modulation (FM) in which the amplitude remains stable and varies its frequency, or phase modulations such as FSK or AFSK.
There are also digital modulation (used for wi-fi, TV D.V.B. and D.A.B) and this assumes a different modulator for each type.
Finally, the modulated signal arrives at the selective power amplifier stage, which elevates the voltage and current values up to the power required by the project to cover the predetermined zone.
At the out of transmitter is collected the transmission line, that will carry the signal to the antenna and is connected to the transmitter output via a specific connector.
It should be borne in mind that all coaxial or waveguide transmission lines have a characteristic impedance that is respected in the totality of the path, in order to avoid signal misalignment and loss, both in transmission and in reception.
The characteristic impedance in transmission is usually set at 52 Ω in the transmission apparatus and 75 Ω in the receiving ones.
Below is the zoomable diagram of a generic transmitter and other explanatory images.
Magnifying view of a HF band transceiver TV-radio transmission systems
RAI-WAY San Nicola dell' alto transmitting center Type N connector with 50 Ω impedance
Radio receivers
The radio receiver is an electronic apparatus which is responsible for restoring (demodulating) a signal previously transmitted by the radio transmitter. We will consider only the most common, that is the superheterodyne type.
The signal collected by the receiving antenna, transformed by electromagnetic wave, unlike potential from it, is then sent
At the input of the receiver, which as a first stage has a tuned amplifier-filter on the frequency to receive, it is a low-noise and low-power amplifier so as to also amplify low-intensity signals and thus increase The receiver sensitivity.
The following stages in the case of a superheterodyne receiver are a variable frequency oscillator (local oscillator) tuned to the frequency to receive less than the intermediate frequency value (455 KHz for AM receivers and 10.7 MHz for FM) and one stage Converter-mixer where the signal coming from the antenna-amplifier-filter (front-end) is mixed with that of the local oscillator.
At the output from the converter we find a fixed frequency signal, no matter where we will tune the oscillator circuits of the filter Input and local oscillator, this signal is the result of the beat between the two frequencies (intermediate frequency).
The intermediate frequency signal is then sent to two or three tuned and selective amplifier stages, which only amplify the intermediate frequency and eliminate any unwanted signal.
The stages of the receiver as well as shown up to now are the same regardless of the type of demodulation, but the next stage, that is, the demodulator is different depending on the signal to be demodulated, simple (a diode and a low-value capacitor) in case of AM
Much more complex in the case of F.M. And more and more complex complexity with phase or digital modulations.
The next and last stage is an audio amplifier, which amplifies the demodulated signal so as to pilot a loudspeaker, it may be of the type shown on the active components page of this site.
There are also direct demodulation receivers , without local oscillator and mixer, but only with an oscillatory circuit tuned to the antenna input, the demodulator and an audio amplifier. Of course, these types of receivers may not have the characteristics of sensitivity and selectivity of superetherodyne, but to get started well.
Recently, new generation receivers have come to the world market, where signal processing after tuning and conversion is performed in a totally digital format after converting A / D,
the benefits are many, better filtering eliminating noises, and so on, but most of all, the biggest advantage is that all the receiver is in only a small, unimaginative small chip, a typical receiver chip is the SI4702 whose datasheet is located at this link. The disadvantages are less sensitivity, lower adaptation to the very fast signal variation and audio quality in some cases questionable or with a sense of artificiality.
Below are some explanatory images, for better vision you need to enlarge them.
Block diagram of a typical superheterodyne receiver
AM-FM Superheterodyne Receiver seen inside Tuner D.S.P all in one SI4702
AM receiver with direct demodulation Tuned front-end without demodulator
Receiving and transmitting aerials
To conclude the topic of the radio technology we will now discuss the perhaps less considered (at amateur level) in this field but perhaps the most important, that is, the antenna.
It is used to radiate the signal produced by the transmitter and receive the same signal at the receiver input.
Receiving and transmitting antennas are based on the same physical laws and are reversible, that is, the same antenna can be used both in reception and in transmission, of course if it is sized to receive the power of the transmitter.
The receiving antenna is a tuned resonant system that provides its electrical signal proportional to the electromagnetic field in which it is immersed.
The transmitting antenna is a tuned resonant system that creates around an electromagnetic field proportional to the signal power applied to its heads.
The value of the frequency and wavelength in which it resonates, and therefore the one in which it is most sensitive, depends on its physical size, so the ideal antenna for a given frequency must be of the same length in meters Of its wavelength, in this case it is referred to as a full wave antenna, which then is the one of the highest performance. The antennas can also be made with underdimple measurements of the wavelengths 1/2, 3/4 and 1/4, accepting a small drop in conversion and propagation efficiency, but also much smaller antennae in the case of transmissions In the lower part of the electromagnetic spectrum.
The easiest and most easily accessible antenna, based on all the others, is the Hertzian dipole, it consists of two elements of conductive material with a total length that can be calculated using the 300000 / frequency formula, in which 300000 is the speed of the light in km / S, and the frequency is the one where the dipole has to work and is expressed in Hertz.
Starting from the dipole, which when positioned vertically radiates its own field omnidirectionally, is obtained by adding other passive dipole directional or directional antennas that have the characteristic to radiate in one direction.
The advantage of directing antennas is to concentrate the irradiated field or field received in one direction, so that only a given zone can be used, or receive only in one direction and discard all the others.
The most commonly used antennas are Yagi and Parabolic, used for its high directionality in radio bridges or for cosmic or satellite communications.
The antenna will then be connected to the transmitter or receiver via a transmission line formed by a coaxial cable or waveguide with a impedance equal of the output of the transmitter or receiver input.
The coaxial cable is formed by a conductor immersed in a cylindrical insulating material and in turn covered by a conductive material shoe with the screen function towards the inner conductor. All is protected by a plastic or gummy sheath
Which isolates it from rain and mechanical trauma.
The section of the cylindrical insulator determines the characteristic impedance of the transmission line and as mentioned above to avoid leakage, it must coincide with that of the apparatus to which it will be connected.
By clicking on this text you can download an XLS file that I created specially with which you can calculate a dipole from the frequency on which it should resonate.
below some enlargeable images.
Transmission lines to antennas Short wave dipole antenna HF on board a ship
Representation of dipole with transmission line Tower with directionals antennas LTE 4G
To conclude, I would like to emphasize that it is impossible to condense in this small text all that there is to say about radiotechnics, but if you are going to deepen or need professional advice, you can contact me via the "Contacts" page.
Thank you for visiting my site.
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