A parabolic aerial , commonly called parabola by the general public, is a antenna having reflectors paraboloidal, based on the geometrical properties of the named curve Parabole and named surface paraboloid of revolution. This antenna qualified the universal one since it functions in theory on any Fréquence or Wavelength, is however only employed starting from the Bande L as of 1.1 [[GHz]] and when a high Gain of antenna is required. It is estimated that the interest of reflectors parabolic is felt only starting from one diameter higher than 4 times the wavelength of the signal to be transmitted. Except exception, the Radioamateur S especially use this type of antenna to the top of 430 [[MHz]], with parabolas which exceed consequently the 3 meters in diameter.

Reflectors

The reflectors parabolic is charged to concentrate the waves received or emitted (Radar, Télévision, ISM and WiFi, Radio-amateurism, radio-relay Systems, or waves emitted by the stars in Radioastronomie) towards the antenna-source, which is at the hearth of the parabola. The parabolic aerials of small diameter are manufactured out of buckled plate (Acier or Aluminum). For the antennas of large diameter, the reflectors are sometimes made out of netting, which causes to decrease the catch with the wind. The reflectors should not comprise hollows or bumps of an amplitude higher than 5% wavelength, which for memory, is of 2.5 cm in TV sat and 12.5 cm into 2.4 GHz.

The source

The electric cable of the antenna is connected to a antenna-source, commonly simply called " source" who is placed at the parabolic hearth of the reflectors. The goal of the source is d'" éclairer" entirely the surface of the reflectors.

In the antennas premium-x-ray ( primary education hearth ), the reflectors is circular and the source is placed in the axis of the lobe of the antenna. This provision makes that the source constitutes a screen for the waves and the output, therefore the total profit of the antenna is decreased. This type of assembly is used preferably for the reflectors of diameters higher than two meters, the shade of the source and the arms supports being relatively negligible.

To avoid the disadvantage of the assembly premium-x-ray , it is current to decentre the source, the reflectors is then a portion of paraboloid to elliptic contour: one speaks then about " parabola offset". The output is improved clearly, especially for the small antennas as those which are used by the general public for the reception of satellite television. Other favors: the reflectors can preserve a quasi vertical position even for the satellites placed rather high in the sky.

To make more compact one antenna of large Focal, one uses the assembly of the type common Cassegrain in the Télescope S. the reflectors secondary can be plane or hyperbolic.

The assembly called " parabola grégorienne" is an assembly " offset" using reflectors secondary like the assembly " Cassegrain". Its output is very good.

Owner of emission

Size of surface forming a " reflectors parabolique" is relatively small in general in front of the wavelength of the emitted signal and it is then not possible to neglect the phenomena of diffraction. Each point of the surface of the " réflecteur" will radiate like a point source, and the total field emitted in a point is the coherent sum of all the infinitesimal fields. All occurs as in the case from diffraction from a wave by an opening. Some is thus the parabolic type of aerial, the owner of emission comprises a principal lobe in the direction of emission and the secondary lobes all around the sphere centered on the antenna and which one tries to minimize.

Let us take a small portion of the parabolic aerial having more or less the shape of a rectangle of dimensions $l\; \backslash \; by\; \backslash \; L$. The theory of diffraction shows that the amplitude of the wave emitted in a direction located by the angles $\backslash ,\; \backslash \; theta$ (horizontal azimuth angle or layer) and $\backslash ,\; \backslash \; phi$ (Angle of elevation or site) at a distance $r$ of the sufficiently large antenna so that the approximation of Fraunhoffer is vérifée, is worth ($sinc$ is the function cardinal Sinus):

$E\; (R,\; \backslash \; theta,\; \backslash \; phi)\; =E\_0.\; \backslash \; frac\; \{L\}\; \{\backslash \; lambda.r\}\; .sinc\; \backslash \; left\; (\backslash \; pi\; \backslash \; frac\; \{L\}\; \{\backslash \; lambda\}\; .sin\; (\backslash \; phi)\; .cos\; (\backslash \; theta)\; \backslash \; right)\; sinc\; \backslash \; left\; (\backslash \; pi\; \backslash \; frac\; \{L\}\; \{\backslash \; lambda\}\; .sin\; (\backslash \; theta)\; \backslash \; right)$

The width of the beam in general is considered as the angle underlain between each side of the principal lobe where the value of the intensity of the signal does not go down under 0,5 the value from that from the peak, is -3 dB in logarithmic scale (in Décibel or dB). By solving with this value the equation, one obtains:

$R\_\; \backslash \; phi\; \backslash \; approx\; 0,886\; \backslash \; frac\; \{\backslash \; lambda\}\; \{L\}\; \backslash \; qquad\; and\; \backslash \; \backslash \; qquad\; R\_\; \backslash \; theta\; \backslash \; approx\; 0,886\; \backslash \; frac\; \{\backslash \; lambda\}\; \{L\}$

According to the type of antenna, one must add several rectangular portions to obtain the formula of emission for the final antenna. A very frequent particular case but is that of the antenna which forms a complete parabola of diameter D. In this case, the resolution R (or $\backslash ,\; \backslash \; Theta$) becomes:

$\backslash \; Theta\; \backslash \; approx\; 70\; \backslash \; frac\; \{\backslash \; lambda\}\; \{D\}$

The diagram of above gives the pace of the evolution of the power of the wave, standardized compared to the emitted maximum power, according to the site and of the layer of such a parabolic aerial. One sees appearing the principal lobe where the majority of emitted energy or receipt are. One it definite like the beam of emission or reception of the antenna.

It is seen that to reduce the angular opening of the antenna, there are two methods:

• either to increase the size of the antenna
• or to decrease the wavelength/to go up in frequency.

Applications

• terrestrial Telecommunications & professional transmissions (radio-relay systems)
• Connections between the Ground and the satellite S
• Radar S
• Broadcasting by satellite, MMDS, for the general public.
• Radio-amateurism, Wifi, ISM.
• Radioastronomy

Satellite television

In the " paraboles" used for the reception of satellite television, the source is directly connected to an element of receiver, the head ( LNB ), which is in fact a frequency converter to weak noise. The received signal, whose frequency is about 11 or 12 GHz, is transposed on a frequency band around 950 to 2150 MHz to lead to the television receiver. If only one satellite (or constellation of satellites) is collected, or in the case of a motorized antenna, only one head is used. If a second satellite (or a second orbital position) is wished, the system then uses the parasitic hearth, shifted of approximately 7 cm (but depends on the diameter length of the focal distance) compared to the axis of a parabola of 80 cm. This artifice validated makes it possible to collect two separate satellites of 6° (Hot-Bird 13°E and Astra 19°E, for example) with only one reflectors, the two hearths feeding two heads or a cast solid head bisatellite. Parabolas with multi-hearth are available, covering an orbital segment going from 28°Est (Astra 2) to 5°Ouest (Atlantic Bird 3).

For a given frequency, the profit of the antenna increases with the increase in the diameter of the parabola while the aperture decreases.

For the frequencies lower than 1 GHz, the parabolic aerials, more cumbersome, are generally replaced by antennas Yagi, quad, etc, but in delicate cases encountered in TV-UHF, the parabola is sometimes preferred with the “rake”.

Alternatives

• an ellipsoidal antenna can be used to collect two or three satellites
• a toroidal antenna uses two reflectors (one opposite the other) so that the waves converge in a line instead of a single point; what allows the reception of several satellites in an arc going until 40°

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