The satellite or dish antenna is a device mainly responsible for receiving or transmitting high-frequency electromagnetic waves from the Earth to satellites that orbit in free space, and adapting these frequencies so that they can be distributed over coaxial cable networks.
Commercial satellites work in frequency bands classified as microwave, L, C, Ku and Ka band. However, these frequencies are so high that it is impossible to distribute them over coaxial cable and they have to be transformed to lower frequencies.
In the case of commercial satellite television signals, the band used for their distribution by coaxial cable is the IF Band (intermediate frequency). It is also common to use satellite dishes on terrestrial point-to-point microwave links.
How does it work? The shape of the dish satellite antennas does not allude to an esthetics issue or to an impulse of some manufacturer, but, on the contrary, responds to a purely mathematical question that uses a property of parabolas, known for a long time, almost 2,000 years in a very intelligent way.
But, what are parabolas? A parabola is a conic (the curve that arises when making a certain cut to a cone) that is defined as the set of points that are at the same distance from a specific point, called focus, and a certain straight line, called the directrix. In the following image, we can see a parabola (in red), its focus and its directrix, as well as the equality of distances from various points to those two objects:
From this definition, it is easy to construct a 3D object by rotating the parabola about a vertical axis through its focus. By doing this, we get a three-dimensional surface, called a paraboloid.
Exact! We have just created a parabolic satellite dish, which focus is the same as that of the parabola that we have rotated. A point inside the parabola that moves along a straight line parallel to the axis will “bounce” off the parabola and be sent toward the focus.
This means that, if we send signals towards the parabola, which are parallel to the axis, they will be reflected by it towards the focus, regardless of the line we use. This is very useful because, with a paraboloid that has a signal receiver placed at the focus, we can get all the signals that bounce off it to be sent to said receiver, without having to point directly at it. That is, with a small receiver we obtain a large signal reception, using the entire surface of the paraboloid in the manner described.
On the other hand, this property can also be used inversely. We place a signal broadcaster oriented towards the paraboloid at the focus of the paraboloid and broadcast the signal to as much of its surface as possible. All the signals will “bounce” off and will be reflected outwards parallel to its axis, thus achieving a greater signal broadcast than what we would obtain by broadcasting only from one point. For example, this can be used in vehicle headlights (by placing a bulb in the focus to emit more light) or parabolic microphones (with a mic in the focus to broadcast sound to a larger area).
The focus of the paraboloid is attached to a straight rod that rests on its surface (it has to be held it in some way). This generates the problem that some signals can bounce off this bar, both when sending and receiving, and therefore get lost. To avoid this, what is done on some occasions is to take for the satellite dish antenna a part of the paraboloid that is not “below” the focus, but “to one side”. This is how we avoid those bounces in the fastenings and we get a more efficient antenna.
What is a satellite dish made of? The main components of a satellite antenna are:
• A passive reflector, shaped like a paraboloid of revolution
The shape of the reflector gives the antenna different properties, such as its high directionality or its suitability to work with extremely high frequencies. However, its main characteristic is that all the electromagnetic waves that collide in it are reflected and concentrated in a single point called focus, generating a high gain in it.
The size of the reflector of the satellite antenna is a decisive factor for the correct functioning of communications since it is directly linked to the wavelength of the frequencies that we use, as well as the force (EIRP) with which said frequency signals are received. For this reason, to work with Band C frequencies, it is necessary to use reflectors with a larger diameter, since, when using a lower frequency range, the wavelength is greater. Thus, the minimum diameter of a satellite dish that works in Band C would be 120 cm, while for Band Ku it would be of 40 cm.
Likewise, the material in which the reflector is made also affects to the performance of the antennas, and not only in aspects such as their durability or resistance. Mesh or grid reflectors can work with C Band frequencies, but are not suitable for higher frequencies such as Ku Band.
• An active element (LNB or BUC)
The active element is in charge of adapting the frequencies so that they can be distributed, both by air and by cable. In addition, it is capable of polarizing electromagnetic waves in different axes (vertical, horizontal, circular, etc.), to optimize the spectrum and be able to use frequencies that are very close to each other, without interfering with each other.
The LNB (Low Noise Block) is responsible for capturing the high frequency signal from the satellite and converting it into a lower frequency signal, allowing its distribution through coaxial cabling. For its part, the BUC (Block Up-Converter) performs the inverse function, amplifies and converts the frequencies coming from the coaxial cable into higher ones, to send them to the satellite. Both the LNB and the BUC need a feed horn or feed, which is responsible for channeling the waves between the reflector and the active element.
Thus, depending on the active element to be installed, satellite dishes can be:
• An anchoring system that allows the orientation and subsequent fixing of the antenna.
Apart from these main components, they can also have a series of accessories to improve their performance, such as a motor (to automatically direct the satellite antenna to different satellites), heating blankets (to prevent the accumulation of ice or snow on the reflector), radomes (antenna coating that protects it from weather agents and fauna, without affecting its electromagnetic properties), etc.
What types of satellite dishes are there?
Over time, these types of antennas have been perfected, managing to reduce the size of the reflector without losing its performance. Thus, there are several types of parabolic or satellite antennas:
• Parabolic antenna with primary or centered focus: it was the first to be used, its focus is on the central axis of the reflector. It is absolutely symmetrical and has a parabolic shape. The active element is suspended in front of the reflector in its central axis, generating a shadow area. It is also known as a C Band parabolic antenna since it is the one used for this band, although it is also used for Ku Band frequencies and point-to-point terrestrial microwave links.
• Offset or displaced focus satellite dish: the reflector is not completely round; its shape is a section of an oval paraboloid reflector. As its name indicates, its focus is displaced from the center of the antenna, preventing the active element from casting a shadow on the reflecting dish. By having the focus displaced, this type of antenna does not point perpendicular to the satellite but is inclined about 25° downwards. They are not appropriate for Band C.
• Cassegrain parabolic antenna: it is an antenna with a main concave parabolic reflector and a secondary smaller convex one, suspended in front of the main reflector and near the focus, in such a way that the electromagnetic waves, when they hit a reflector, are reflected to the next one.
• Flat satellite antenna: also called planar array. Although it is not physically a satellite dish as such, it is a compact antenna for receiving satellite signals, which stands out for its small size and visual impact in the facilities. It is intended for domestic installations and the Ku Band, with a good level of satellite coverage footprint.