Systems of radio communications
In order to review the concepts, we are going to review a pair of times this work on signals and waves that we can see in
Equation of transmission of Friis.
This equation will be seen ahead but given the importance that it has in the telecommunications (radio I connect). There are many forms to indicate it and, for this reason, the best thing is to use it of the direct way, deducing the Db received, from the formula: , where Pt is the transmitted power, Gt the gain of the antenna that it transmits, gr. the gain of the receiving antenna, moor is the wavelength of the signal and R the distance that separates receiver emitter.
In the previous presentation, was a relation as it is in the image
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Lfs as log of the relation between transmitted and received Power is defined. We apply the equation of friis and it does not give:
Also we know that to the relation between the speed of the light and wavelength, comes dice by c = λ/T (with λ T and the wavelength the period, that is the inverse one of the frequency. Therefore, λ as c/f can be replaced. The previous expression we have left:
Now, we continued moving terms, but we are going to pass the meters of r (distance) to km and Hertz to GHz. In the case of the distance, we have left r= 103 km.
When high being to the squared one, that expression stays to us as r2 (in km) * 106
In the same way, the frequency stays as F2 (in GHz) * 1018. It leaves a so great number because we have elevated the 9 of the Ghz to the squared one. The previous equation we have left:
The previous expression, developing the quotient, is
The first term gives 92.44 us, which gives rise to the initial expression
Zone of fresnel
In order to avoid interferences being two points where one interchanges signals of high frequency, not only it is necessary to have cleared the reporting line between the emitters, but it is necessary to consider the zone of Fresnel.
It is possible to be shelp that the zone of Fresnel is the volume between emitter and receiver, where it does not have to exist obstacles that can disturb the transmission of the signal.
In the Image superior one is the Ruptures of the zone of fresnel by passage of vehicles, low height of emitting-receiving or growth of plants.
A formula exists that allows us to calculate if between two meeting points (although visibility exists), we have possibility of establishing communication. The calculation of the value of r (Distance from the horizontal of connection to the obstacle) comes given by:
Where d1 and d2 are the distances that there are between the obstacle and emitter and obstacle and receiver As we can see, the form of the zone of fresnel is oval. This takes to us to think that the maximum we have it in the central zone, where d1 = d2. Of here, that stops the maximum distance we have the formula:
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We have them distances are equal, and for that reason we are going to replace d1 = d2 = D/2, being distance D the value that we have between emitter and receiver. In addition we are going to replace the wavelength by the frequency so that λ = c/f Replacing in the formula we have:
If now we replaced c, we have:
The 106 are used next to another 103 uses that it comes from km to have 109 that. That is the reason for which the frequency that there is to put is GHz
Therefore, D comes in km and f in GHz.
2,2 Systems of radio and television
Digital radio by satellite
DAB (Digital Audio Broadcasting). Of little implantation due to the price of the receivers, it offers characteristics as high quality of sound (CD) and unique frequency.
Systems of movable telephony
The movable telephony has evolved as much, that many of the systems have been obsolete already. In this section we will take care but thorough of the recent systems but. We will do one first
Systems movable cellular deprived public and
Within the Private ones:
The necessity to contact with characteristics different from the public ones, produces the development of other systems that are used by security bodies, private companies, services of help, etc
The advantages that offer these systems are:
1º Communications of user to several users. Of this form, a worker speaks and several listen. This does not happen with the public one.
2º Can define a closed group of users.
3º Technology PTT (push to talk). If you beat you speak. If you stop beating, listening
We have two systems (PAMR, PMR and TETRA), each with specific features. We will emphasize the TETRA, that it offers some advantages as they are
If necessity exists to take a complete system TETRA to a distant point to the base, by means of movable systems, required time to do it operative small is compared with other systems
1º an electromagnetic wave loses energy due to ......
2º a coaxial cable with respect to a pair of copper has more or less lost. The answer on the basis of the diagram of the presentation reasons
3º What is the Skin effect. Help with three drawings
4º Tenemos in store two coaxial cable roll types. One of low other losses and normal. To that this is due? What element changes in the cable?
5º a fiber cable works with light of 850 nm.
a) To calculate that loss we have to 10 km if the length overall is of 60 km, the exit of the transmitter are 30 mW, the loss of the 4 connectors is of dB, the loss by joint is 0,15 dB, the joints are placed each 2 km and the loss of the fiber is of 0,35 dB/km.
b) To calculate the power in the receiver given in dBm
We elevate to the power of 10 to clear lg =>
7º In a tower we have a generator of 20W to feed an antenna that is to 40 meters. If the cable has lost of 2dB/100 meters, that power arrives to him at the antenna.
8º To calculate the power that we have when coming out of the receiving antenna whose directive gain has value of a 5 and one efficiency of 80%, located to a distance of 20Km of the transmitting antenna of directive gain 4 with efficiency of 70% that receives a power of entrance of 40W. The work frequency is of 150MHz.
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9º Busca in Internet what models of cars have the DAB
10º the compass is a simple and very useful instrument for our work. In addition, smartphone already has one including. a) Opens the compass and looks for the direction of a point anyone of your surroundings. It compares the result with your companion b) Looks for facilitated the North and South coordinates. It compares these values with facilitated by google maps. It compares results both. Then, Raisin the values of the compass to the data of google maps to see that result offers to you. You in account that one comes in the decimal system and the other in minutes and seconds. PS. In the map of google, we have a positive value in the coordinate that indicates the degrees that are on the Equator and another negative, because we are before the meridian of Greenwich
Projects of radio link:
Part 1º. (I have left to a pdf in DRIVE with the title Calculation De Radioenlace where radio link examples are exposed)
The idea of this first practical one is to realise a radio link from two points to choose of your surroundings, with real components. For the characteristic of the station it bases, that feeds the antenna, I have left a document pdf in the shared folder of DRIVE.
Rocket can be looked for in Internet as ubiquiti m5 datasheet.
The listening margin obtains when remaining the total gain and total losses (156,52 - 94,45) = 62,07 dB
To show/To hide to Practice Installation Mobile Radio
Installation of the complete program in the PC. First of all, we have these videos of a colleague of Venezuela:
1º Ir to the Web http://www.cplus.org/rmw/english1.html to make a registry of user. To puncture in Radio mobile online to make a registry online and to work on the program already lodged in a servant
2º If we want to install it, (we will do it) we must follow the guide indicated in the section download
3º Tenemos to choose two points now anyone. That they are not much to avoid the terrestrial curvature.
4º Hacemos 1º measurement and in the results we have a sample of the zones of fresnell, reporting line (in red) and the data of the transmission and reception
The significant values but are those that we have put) (These data are the characteristics of equi In the transmitter: tx is the power transmitted by the system in dbm. The power of dbi is obtained developing this process:
Where x is the power in dBm (decibels in. To see https://es.wikipedia.org/wiki/DBm page and to verify as it evolves dbm from his maximum value to the minimum. Rx occurs in dbi, that is gain with respect to the isotropic antenna.
Rx sensitivity makes reference to the minimum level of signal that is needed for a correct operation of the equipment. In the right part we have the data regarding the lost one by the propagation In performace indicates the results given the orographic and technical characteristics to us