Welcome to the page dedicated to radiofrequency and radiotechnics

In this page I will try to transfer the basics of radio technology to anyone who takes a few minutes to read this web page, I will try to use a simple language as much as possible, so as to help those who approach this complex topic to understand its meaning. As always, I repeat that it is impossible to condense such a vast topic into a few lines, but if you need to know more you can contact me by visiting the "Contacts" page. A thought and thanks go to the person who made everything we are going to talk about possible, 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, physician and biologist Dott. Giuseppe Tallarico.

                                                                                                          Introduction

Radio technology is the science that uses electricity, electromagnetism and therefore electronics to send and receive radio waves for communication (radiocommunications) or control purposes. It provides for the study and design of electronic devices for the purpose, ie radio transmitters, radio receivers and transmission and reception antennas. Trying to simplify the concept, it is a question of sending electromagnetic waves (carrier) having an appropriate frequency and wavelength onto which the information to be transported is appropriately superimposed into the surrounding space (modulation).

This work is carried out by the transmitter, which has the transmitting antenna connected to its output through a tuned transmission line (coaxial cable or wave guide), which is responsible for transducing the electrical signal in an electromagnetic field of intensity. proportional to the power present at its input. Further information on electromagnetic fields can be found on this page. Everything is based on oscillatory type circuits, which can be of the series type or more often of the parallel type, they are electrical circuits that exploit the physical property of the inductors and capacitor and their skill to tune (resonance) on a single frequency depending on the value of the components used, it can be deduced from this that to modify the resonant frequency of an oscillatory circuit and then select (receiver) or produce (transmitter) a high frequency signal it is sufficient to vary the value of one of the components of the oscillatory circuit.

The electromagnetic waves will then be received and amplified by the receiving antennas, which are also oscillatory circuits tuned to a certain frequency, and operate the reverse procedure, that is, they transduce an electromagnetic field into an electric signal. The signal obtained is then sent through another line tuned to the reception apparatus (radio receiver), which through 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. radio frequency (demodulation). We will address the chapter on transmitters, receivers and antennas later, in separate treatments to better understand them. Sometimes the radio transmitter and receiver can coexist in a single electronic device, in this case we speak of a transceiver. The frequencies and therefore wavelengths on which it is possible to transmit are shown in the electromagnetic spectrum and indexed in various bands visible in the zoomable tables found in the line below.

The choice of the band and frequency in which to transmit depends on the distance to be covered and on the orography of the terrain, this is 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 over the electric one), but must be supported by high powers with relative proportionate pollution by electromagnetic pollution

Furthermore, up to about 30 MHz the reception of these emissions is affected by the so-called fading, i.e. the continuous variation of the intensity of the received signal, this is due to the continuous variation in thickness of the layers of the earth's atmosphere that reflect this type of signal and they allow propagation over long distances. The propagation by direct wave, on the other hand, remains stable.

Unfortunately, the use of low bands, due to the limits set out above, is gradually falling into disuse, they are replaced by internet streaming and digital satellite communications which allow good quality without the intrinsic limits to this type of emissions. They are used anyway, still mainly when it is necessary to cover large spaces with a single transmitter.

As the frequency rises, the electromagnetic waves tend to be absorbed and shielded by obstacles that stand in the way, until they have an optical range after 30 MHz, i.e. the transmitter must see the receiver, otherwise the reception of the radiated signal becomes poor or impossible, but there is the advantage of being able to use relatively low transmission powers, given the need to install more than one transmission system to cover a given area. 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 must also be borne in mind that the choice of the transmission band directly conditions the quality of the information transmitted, this is because in the low bands one is obliged, given the limited space available, to use narrow transmission channels, all this implies that regardless of the type of method used to superimpose information on the carrier (modulation) the maximum modulable frequency is 4.5 KHz if the transmission channel is 9 or 10 KHz, and 3KHz if the channel is 5 KHz wide.

It goes without saying that using the narrow band FM frequency modulation, which is insensitive to disturbances of any kind, the quality improves a lot, but the limitation remains.

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 the quality depends on the B.E.R. (bit error rate) and the type of data compression used, there is however the advantage of the very high immunity to disturbances and the possibility of transmitting large quantities of information in a narrow channel (compression).

Generally, the optical propagation of electromagnetic waves with very high frequency is exploited for point-to-point communications, i.e. radio links 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 links can be represented as in the figure below.

In radio links very directional antennas are used, usually parabolic. Here are some zoomable images that will allow you to better understand what is exposed. 

                        Transmission system with radio links and RADAR                                                                                                                        Parallel oscillatory circuit

             Parabolic antenna for very long distance space communications.                                                                                  Oscillating circuit of series type

                                                                                      Transmitters

To transmit the radio waves through the antennas we make use of devices designed and built for the purpose, we are talking about radio frequency transmitters, they are very complex devices, formed by various stages, different depending on the type of modulation used, but which in any case provides always as the first stage an electronic oscillator which generates the carrier having the desired frequency and as the last stage a power amplifier which raises the voltage and current values to the appropriate values needed to generate the power suitable to cover the desired distance.

The oscillator is an electronic circuit using active and passive electronic components, which generates through a tuned oscillatory circuit and an active component (transistor or valve) an electronic signal of frequency established in the design phase, which must coincide with the transmission frequency that we are interested, it is very important that the oscillator supplies a signal with a frequency that is stable over time, to obtain this a particular component with a piezoelectric effect called quartz is introduced into the oscillatory circuit, which stabilizes the oscillation and makes it independent of the voltage power supply and within certain limits also the temperature. In multichannel circuits, to avoid using many quartz, a particular circuit called P.L.L. is used. (phase locked loop), which uses a single crystal to generate all frequencies in a stable way.

Lately a new concept digital circuit called D.D.S. (direct digital synthesis) with numerical control which has many advantages over analogue ones.

The output signal from the oscillator then reaches the modulator stage, that is the circuit which superimposes the information to be transmitted on the carrier to be radiated. It differs according to whether the amplitude modulation (AM) is used in which the carrier frequency remains stable and its amplitude varies, or the frequency modulation (FM) in which the amplitude remains stable and its amplitude varies. varies the frequency, or phase modulations such as FSK or AFSK.

There are also digital modulations used for wi-fi, for digital TV and for D.A.B. +, this requires a different modulator for each type.

Finally, the modulated signal arrives at the selective power amplifier stage, which raises the voltage and current values up to the power that the project needs in order to cover the predetermined area. The transmission line that will carry the signal to the antenna is connected to the transmitter output via a specific connector. 

It must be borne in mind that all coaxial cable or waveguide transmission lines have a characteristic impedance that must be respected throughout the entire path, to avoid mismatches and signal losses, both in transmission and reception. The characteristic impedance in transmission is usually established at 52 Ω in the transmitting devices and 75 Ω in the receiving ones. Below is the expandable block diagram of a generic transmitter and other explanatory images.

                           Zoomable view of a 27 MHz HF band transceiver                                                                                               TV radio transmission systems with relative network bridges

             RAI-WAY San Nicola dell' alto transmitting center                                                                                                             Waterproof connector with 50 Ω impedance of type N

                                                                                        Radioreceivers

The radio receiver is an electronic device that is responsible for restoring (demodulating) a signal previously transmitted by the radio transmitter.

The electromagnetic signal, collected by the receiving antenna, is transformed from an electromagnetic wave to a potential difference by the antenna itself, and is then sent via a transmission line with constant impedance to the input of the receiver, which has an amplifier-filter as its first stage. of band tuned to the frequency to be received, it is an amplifier for small and low noise signals, so as to be able to amplify even low intensity signals and therefore increase the sensitivity of the receiver.

The following stages in case of superheterodyne receiver are a variable frequency oscillator (local oscillator) tuned to the frequency to be received minus the value of the intermediate frequency (455 KHz in case of AM receiver and 10.7 MHz in case of FM) and a 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 of the converter we find a fixed frequency signal regardless of where we will tune the oscillator circuits of the input filter and of the local oscillator, this signal is the result of the beating 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 other unnecessary signals.

The stages of the receiver as described up to now are the same regardless of the type of modulation, instead the following stage, i.e. the demodulator is different depending on the signal to be demodulated, it is simple (a Germanium diode and a low value capacitor ) in the case of AM and much more complex in the case of F.M. (ratio discriminator) and of increasing complexity with phase or digital modulations.

The next and last stage is an audio amplifier, which amplifies the demodulated signal in order to drive a loudspeaker, it can be of the type shown in the active components page of this site.

There are also receivers with direct amplification, ie 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 cannot have the characteristics of sensitivity and selectivity of superheterodyne, but to start they are fine, you can find a feasible example in the figure that you can find further down.

Recently they have entered the world market of new generation single chip receivers (DSP) in which the signal processing after tuning and conversion is carried out in a totally digital way after A/D conversion, the advantages are many, including filtering better, elimination of noises etc., but above all the greatest advantage is that the whole receiver is in a single integrated circuit of very small dimensions and of negligible cost, a typical receiver integrated circuit of this type is the SI4702 of which you can find the datasheet at this link, the disadvantages are sometimes a lower sensitivity, an adaptation to signal variations that is not really fast and an audio quality that is in some cases questionable or with a feeling of artificiality.

Here are some explanatory images, for a better view you need to enlarge them.

                                                                                                                                 Block diagram of a typical superheterodyne receiver

   Superheterodyne AM-FM commercial receiver seen inside with the various stages                                                                                          D.S.P all in one tuner stages SI4702

                                        Direct demodulation AM receiver                                                                                                                                      Tuned front-end without demodulator

                                                            Receiving and transmitting antennas

To conclude the topic, the vast topic of radio technology we will now talk about the component which is perhaps the least considered (at an amateur level) in this field but which is in fact the most important, that is the antenna.

It serves to radiate the signal produced by the transmitter and to receive the same signal at the receiver's input.

The receiving and transmitting antennas are based on the same physical laws and are reversible, i.e. 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, which supplies at its ends an electric signal proportional to the electromagnetic field in which it is immersed.

the transmitting antenna is a tuned resonant system which creates around itself an electromagnetic field proportional to the power of the signal applied to its ends.

The value of the frequency and wavelength in which it resonates, and therefore the one in which it has the maximum sensitivity or efficiency, depends on its physical dimensions, so the ideal antenna for a given frequency must be of the same length in meters of its wavelength, in this case we speak of a full wave antenna, which is the one with the highest performance. The antennas can also be made with submultiple measurements of the wavelength type 1/2, 3/4 and 1/4 accepting a small decrease in the conversion and propagation efficiency, but this allows for much smaller antennas in the case of transmissions in the lower part of the electromagnetic spectrum.

The simplest antenna that can be created and which forms the basis of all the others is the Hertzian dipole, it is composed of two elements of conductive material with a total length that can be calculated using the formula 300000 / frequency, in which 300000 is the speed of light in Km / s, and the frequency is that in which the dipole must work and is expressed in Hertz.

Starting from the dipole, which if placed in a vertical position radiates its own field omnidirectionally, we obtain by adding other passive dipoles directives or directional antennas, which have the characteristic of radiating in only one direction. The advantage of the directional antennas is to concentrate the radiated field or the received field in one direction only, so as to serve only a certain area, or to receive only in one direction and discard all the others. The most used directional antennas are the Yagi and the parabolic, used for its high directionality in radio links 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 of impedance (i.e. the ratio between resistance, inductive reactance and capacitive reactance) characteristic equal to that of the transmitter output or to the input of the receiver.

The coaxial cable is formed by a conductor immersed in an insulating material of cylindrical shape and in turn covered by a braiding of conductive material with the function of a screen towards the internal conductor. The whole is protected by a plastic or rubber sheath which isolates everything from rain and mechanical trauma. The section of the cylindrical insulator determines the characteristic impedance of the transmission line and as mentioned before, to avoid losses 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 made specifically by me with which it is possible to calculate a dipole starting from the frequency on which it must resonate. Here are some zoomable images.

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

                           Representation of a classic dipole with transmission line                                                                            Tower with radio link and directional panels for cellular telephony

                                  Three-element directional HF yagi antenna                                                                                                      Self-radiating cable-stayed pylons for OM medium wave

                                  Transmission line in "Cellflex"                                                                                                                                   Directional parabolic antenna for 2.4GHz WI-FI

To conclude, I want to reiterate that it is impossible to condense everything there is to say about radio technology in this small text, but if you intend to deepen or need professional advice, you can contact me via the "Contacts" page.

Thank you for visiting my site.

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