Basic principles of antennas
In Telecommunications, one of the elements fundamental in the process to send and to receive data is the antenna, and for this reason, we are going to dedicate a little to study the fundamental elements and their more important parameters.
The antenna as a device (of one or several metallic drivers) can be defined designed to emit and/or to receive electromagnetic waves towards the free space.
The transmitting antenna as transforms electrical energy into electromagnetic waves to the space
The receiving antenna as transforms the electric field into electrical energy
In the image superior (obtained of wikipedia), we have a doublet antenna of average wave that receives a radio signal. This antenna is composed by two metallic barillas (dipoles) of equal length to the quarter of the wavelength. Joining dipoles, sum both half of the wavelength of the signal. The dipoles are connected to the receiver whose impedance imagines by letter R. That R must be equal to the impedance characteristic of the antenna so that the received and given power is optimal.
In the representation we are only going to show to the electric field (in green) but not the magnetic one, to simplify the model. Also it is necessary to emphasize that the represented thing is in the plane where we have the dipole. If we moved a centimeter above but, in a flat parallel to the previous one, we would have the same wave, just like to any distance by above or below the drawn plane. The oscillation of the electric field (green) causes that the electrons that are in the metallic bar (dipole) move to right and left (according to the field), bringing about greater or smaller concentration of them throughout the bar (to see how it varies the red zone). This brings about a change of potential in opposite dipoles and, therefore, a current by the receiver with the same frequency settles down with which it varies field E.
2. More important parameters of the antenna
With the old analogical TV, at the time of installing his antenna and in the process of direction to the repeater, we could observe how when being rotating the same on the axis of the mast, the signal was appearing in the screen, from an image with “*nieve” to seeing it well and then, again, the “snow”.
*Nieve = Denomination that the jargon of the installers was denominated to the lack of signal, showing in screen black and white grains (similarities to the snow).
Normally of it defines the Radiation pattern as the behavior of the density of power broadcast by an antenna when varying the coordinates of azimuth and elevation.
Also it is possible to be defined by other parameters, but the density of power is most habitual.
In the image we have a representation of this concept in two planes (lack the elevation). We see that to 0º we have the maximum power broadcast with the formation of a lobe that is denominated main.
By the back part another one of a considerable magnitude and some is created third parties of smaller importance (lateral lobes). We enclose a real lobe 3D of how it would be the density of density of power of the antenna. Another example that we can put as radiation pattern is the one that emits a dipole. In this case, a signal with form of doughnut, where the maximum has it in the horizontal axis, when installing is generated the vertical antenna.In the figure that is enclosed, is the dipole and how it varies the signal. As the energy in that zone is concentrated but (horizontal axis), the gain is 2.14 Db greater than the isotropic one. Other important parameters are:
2.1 Ancho of beam.
Angle where the broadcast power falls to half (3 dB). While smaller it is, but selective it is the antenna. In the case of the figure, a width of beam of 44 degrees is had.
2,2 Relation of main and secondary lobe, that is obtained when dividing between maximum value of the main and secondary lobe
2,3 Relation main and back lobe, would be just like the previous one, but taking the back lobe.
2.4 the antennas do not work for all the frequencies. A margin of them is defined to them for who the antenna has certain yield. Evidently, for one of them, it will work in the best conditions.
Ancho de Banda as the rank of frequencies can be defined where the broadcast energy is not inferior to the broadcast maximum power (3 dB)
2.5 At the time of to make antenna, if we want that the power is transmitted in a determined direction, interests that in the rest of directions, the power is 0. This aspect the Directivity enters game, that is defined as the maximum power broadcast in that direction and the power broadcast by an isotropic antenna (it broadcasts the same energy in all the directions), being equal the total power broadcast by each one of the antennas.
The Directivity usually is expressed in (dBi) as:
2.6 Now we have a parameter that perhaps, by its name, from place to confusion. We speak of the Gain.
As term, it gives the idea us than we won (what we multiplied some parameter, as the gain in tension, etc).
In the antennas, the gain is what radiates with respect to another antenna.
The Gain as the relation between the density of power that broadcasts an antenna in a direction (given by its directivity) with respect to an isotropic antenna or dipole can be defined that broadcasts in that same direction and distance, giving to both the same power of entrance.
One is moderate in decibels and according to the referring one it is the dipole or the isotropic antenna, we have dBi or the Dbd.
The Gain and directivity are analogous concepts
2,7 Efficiency defines as the quotient between the broadcast power and the power that is provided to the antenna. We can use the Gain and Directivity to obtain the efficiency.
On the one hand we have:
We do the same with the Directivity
and clearing again the power broadcast in a direction,
Broadcast P is the total power broadcast by the antenna
Broadcast D * P = entered G * P
As , we have:
It makes reference to the position of the electric field with respect to the Earth plane, of this form we have
3,1 Vertical Polarization: If the electric field generated by the antenna (denominated E) is vertical with respect to the Earth. One moves from top to bottom
3,2 Horizontal Polarization: In this case, and he is parallel to the Earth plane.
3,3 Circular Polarization: The electric field generated by the antenna (e) moves generating circles in its displacement, that can on the contrary be in the sense of the needles of the clock or.
4. POWER TRANSMITTED by an antenna
The antennas, aside from receiving the signal, can transmit, and they do it with different powers, according to his feeding and design. In order to measure this value dBm is used (decibels regarding a reference of 1 milliwatt). As it is logical, the maximum radiation emitted by the antenna is limited.
In the USA it is of a watt and in 250 the European Union of mW.
We will see that values in db are equivalent each of these powers.
a) For the Union, we have 250 mW. We passed it to the formula and we have:2º CompraWifi has an application for the Calculation of distance between antennas. In the following page (http://www.comprawifi.com/index.php?act=calculo #), maximum compute the range that is obtained with different elements. To compare results and of drawing referring conclusions to how it varies D when changing f, the losses, etc
Practices of Antenna
Before nothing, we are going to verify within the classroom, the signal received by an antenna, using our measurer of field promax
For it, we have to do the first section Practical with the field measurer
In order to make a fast view and to verify the operation, we verified the following steps
1º Seleccionamos 0 or 1 for analogical or digital. To make sure that it is in digitalis, verifying that in the image appears cofdm. On the other hand, when having in digital way and looking for phantom, we will see three verticallines that they pick up the bandwidth
2º Vamos to button 1, where we have the phantom. We move to channel 42. We return to press button 1. The search of services begins. It takes awhile.
3º Pulsamos in revolving button and we looked for list of services. It beats and we will see the channels that arrive us in channel 42. We see that we have antenna 3 HD and that before we did not see it. This is because the measurer cannot decode channels HD. We select another one, for example, Neox. Now if we can see the channel in screen.
4º Pulsamos the 3 and we see the channel power. In our case Dbµv gives 34, with channel width of 8 MHz.
5º Vamos again to the rotating button. We see that aside from the list of channels we have the carriers of the channel 8k, the period of guard of 1/4, bandwidth, not to touch the spectral investment, have a Code this of 2/3, modulation 64qam, etc
6º Finally, we have important data on the signal within the button between 3 and 4, the marked one by a folder and I sweep verticals and in the figure indicated as 30.
When pressing, we have the CIS (Channel Status Information) and Seeing after viterbi
To have well-taken care of with button located underneath the dull one. It serves to show us to way monitor or way data. It shows in Neox a measurement to us of CIS 17 and To less see 1 by 10 high 7
7º Pulsamos in 1 search another channel. In the 45 they are all HD, we cannot visualize
We go to the 39. We have another channel. We see in list of services that we have Disney Paramount, DMAX, goal etc
Practice of the radiation pattern
The radiation pattern of an antenna serves to know the answer in gain that has an antenna based on the direction with respect to a main axis.
This answer is worth so much in transmission as in reception.
We must fix us to the characteristics of the manufacturer to what value talks about (Electric field, magnetic or the density of power).
In the following image the gain in electric field and magnetic of an antenna imagines
to see folder
Also to emphasize that the gain of the antenna depends on the work frequency
Process of the practice
1º Instalar the base of mast on a cleared land. Within the patio, to choose the best place whereas clause than we are going to receive signals from other points as a result of the reflection of signals by near objects
2º Orientar the antenna to the repeater of Mijas.
3º Conectar our measurer of field. To fit pair to take the signal from a channel of the UHF band, for example 55
4º Medir the maximum gain that we have. To that value we will give the maximum gain him, with 0 db. For example, if with the measurer we received 80dBµV when we oriented to the repeater of Mijas, to that value we give to a gain max him of 0dB. S.A. to take another measurement we received 77dBµV, we have lowered 3 dB, and therefore, the new S-value -3dB.
5º Tomar each 5 degrees the value from electric field and we are it writing down in a table. For this task, to use some compass of the mobile