Armando Caligiuri, electrons in motion
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                       Welcome to the page dedicated to radiofrequency and radiotechnics


On this page I will try to transfer, to anyone who dedicates a few minutes to reading this web page, the basics of radiotechnology, I will try to use as much as possible a simple language as well as in the spirit of this site.
I repeat, as always, that it is impossible to condense such a broad argument in a few lines, but if you need to get in touch, you can contact me by visiting the "Contacts" page.
A thought and a recklessness goes to the person who has made it possible for everything we talk about, that is, Guglielmo Marconi, without him and his discoveries all this would not exist.
Guglielmo Marconi who, among other things, came once in my country, in the residence of his personal doctor, the illustrious scientist, medical and biologist of  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 radio transmitters, 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 the ability 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.


                         

  
       Table of electromagnetic spectrum enlargeable                                                                                   Subdivision of electromagnetic spectrum bandwidth


The choice of the transmission band depends on the distance to be covered and ground orography, because frequencies at the low end of the spectrum have the ability to overcome the obstacles between the transmitter and the receiver (the magnitude of the magnetic component On electric power), but must be supported by high powers with relative proportional electrosmog pollution.
In addition, up to about 30 MHz, the reception of such emissions is affected by so-called fading, that is, the continuous variation of the signal intensity received, this is due to the continuous variation in thickness of the Earth's atmosphere layers reflecting this type of signal and Allow propagation at long distances. Direct propagation remains stable.
Unfortunately, the use of low bandwidths, due to the limits set out before, is gradually falling into disuse, they are replaced by Internet streaming and digital satellite communications which allow good quality without the inherent limits to this type of emissions.
They are still used, even more so when large spaces are to be covered with only one transmitter.

As the frequencies rise, electromagnetic waves tend to be absorbed and screened by the obstacles they are facing, until after 30 MHz there is an optical range, 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 power transmitters, given the need to cover a given area to install more than one transmission system.
However, you always try to put them in a position as high as possible to extend the optical horizon and then increase the area served by the transmission system.

It should also be borne in mind that the choice of the transmission band directly influences the quality of the transmitted information, because in low bandwidth it has been obliged because of the little space to use amplitude AM modulation, all this implies that the maximum modulating frequency Is 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, quality improves very much.
Using the broadband FM, which unfortunately is necessarily confined to high bandwidths with the 75 kHz channel, the maximum transmitting frequency becomes 15 KHz so Hi-Fi quality.
With digital modulation then the quality depends on B.E.R. (Bit error rate) and the type of data compression used, there is however the advantage of the high immunity to the disturbances and the ability to transmit large amounts of information in a narrow channel (compression).

Generally, it uses the optical propagation of electromagnetic waves with very high frequencies for point-to-point communications, that is, they produce radio bridges, which serve to create communication backbones (telephone, TV, radio or other) to which they will be
Then connect the local transmitters. The radio bridges are depicted as shown below.

                                 
The radio bridges use very dirigible antennas, usually parabolic.
Here are some images that will allow you to better understand how exposed.


                           

 
   
Transmission system with radio bridges and RADAR                                                                              Series type oscillatory circuit
                           

    
Parabolic Antenna for Long Distance Space Communications                                                                 Parallel Type 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, i.e. the 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.

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.






                             

        
Enlargeable View of a 27 MHz HF Transceiver                                                                                     Television Broadcasting Equipment with Related Network Bridges



                                                                                          Radio receivers   

The radio receiver is an electronic apparatus which is responsible for restoring (demodulating) a signal previously transmitted by the radio transmitter.
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 tuner-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


                      

    
Commercial Receiver Supereterodine AM viewed internally with the various stages o                                 AM receiver with direct demodulation


                          
 
                      
                  
Tuned front-end without demodulator                                                                               Corded tuning scale of an old receiver
                                  


                                                                    
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 impedance cable equal to that 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.


                      

 
             Transmission lines to antennas                                                                                                  Short wave dipole antenna HF on board a ship


                      


 
          Graphic representation of a classic dipole with transmission line                                        Tower with radio bridge and directional panels for cellular phone signals


                      

      
 
Three-Directional HF-Yagi HF Antenna (Wikipedia)                                                              Self-evolving  and self irradiate tower  for OM waveform




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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Armando Caligiuri, Electronic senior expert, electronic and I.T. maintainer, I.T. consultant.      e-mail: info@armandocaligiuri.it
  Creato e mantenuto da Armando Caligiuri, (C) 2017, V.1.0.0