Flat satellite dish
|Flat satellite dish|
Currently, satellite direct television reception (SNTP) as antennas most widely used two main paraboloid of rotation:
axisymmetric and offset. The complexity of manufacturing a parabolic reflector forced to look for alternative designs of antennas, more technologically advanced in production and self-production. Such designs include a flat Fresnel zone reflector (Fig. 6.17).
Auguste Jean Fresnel (1788–1828), a French physicist, one of the founders of wave optics, in the process of studying the diffraction of light used the method of splitting a wave front into annular zones, later named after him.
By the principle of operation, the Fresnel zone antenna (ZAF) differs significantly from the commonly used antennas containing a parabolic reflector. The description of the antenna and the method of its calculation were compiled by V. Nikitin (Moscow) and the author of this book.
Antenna Fresnel reflector is a conductive concentric annular surface located in the same plane. Under the influence of the incident wave of the electromagnetic field according to the principle of Huygens, each ring becomes a source of secondary radiation -
which is directed in different directions in contrast to the paraboloid of rotation, reflecting all the rays in the direction of the focus. It is possible to choose the width of each ring of the zone antenna and the distance between them so that the signals of the secondary radiation from the middle lines of each ring at a certain point in space coincide in phase. For this, it is sufficient that the distances between the middle lines of the rings and the specified point differ by the length
wave signal - l in . This point by analogy with a paraboloid can be called a focus. The focus, as in a parabolic antenna, is the feed.
In fig. 6.18 shows a section (side view) of the upper part of the central disk of the antenna and the first ring. If the focus is a point that is at a distance f from a plane with rings, then the signals emitted by the midpoints of the rings will coincide in phase at the focus with the following values of the distances between the edges of the rings and the focus:
The signals emitted by the middle of the rings are in phase with the signal emitted by the center of the disk. The skew between the signals emitted by the edge of the disk and its center, as well as the edges of the track and their middle, is only 1/4 of the wavelength.
Thus, the calculation of ZAF is reduced to choosing the location of the focus F on the imaginary axis of the antenna, i.e. the distance f from the antenna web, and calculating the inner and outer radii of the rings depending on the wavelength l , repeater according to the formula (6.2). The distance f is not critical
and it is chosen in the range of 500 ... 1000 mm (for antennas of large diameters).
The signals that emit the edges of the track are out of phase from the signals that the circle emits (located in the middle of the ring), which provides a phase response . Wide rings provide broadband antenna. Due to the fact that the radii of the ZAF track depend on the wavelength of the signal, it may seem that the antenna is narrowband and for each frequency (or wavelength) of the satellite transponder you need the appropriate size of the rings. However, calculations show that this is not the case.
If the radii of the rings are calculated for the average frequency range of 10.7 ... 11.7 GHz (wavelength
6.2, 6.3 shows the results of calculating the size of the ZAF for the specified frequency ranges.
In formula (6.2), consecutive radius numbers were substituted for n values (even numbers correspond to internal radii, odd numbers to outer radii, and r1 to radius of the central disk).
The distance f from the central disk to the focus F is chosen to be
If we calculate the radii of the gauge for the average wavelength of the entire broadcasting band Ki (10.7 ... 12.75 GHz), at its edges these "in-phase" circles extend beyond the surface of the rings. Therefore, at the edges of such a wide range of common mode summation of signals is not obtained.
As a result of the calculation, the radii of the "in-phase" circles are obtained, where n is the number of the ring.
The central disk corresponds to n = 1. The width is chosen arbitrarily.
In practice, you can make a central disk with a radius
The zone antenna is flat in shape, therefore it is much more technological in amateur manufacturing conditions. Such an antenna can be made of a large piece of foiled plastic or by etching, or by cutting out the gaps between the rings. It can also be made by sticking rings out of foil or flat tin on a sheet of getinaks , textolite, plexiglas, wood-fiber cloth (DVP). An arbitrary number of holes are drilled to reduce the wind load in the dielectric base of the antenna.
The main disadvantage of a zonal antenna as compared to a parabolic antenna of the same diameter is a lower gain, since not all of the signal energy that falls on the antenna web is directed to the feed. In conditions of a weak signal, a loss of even a 2 dB gain leads to signal damage by noise and loss of color. To compensate for the lack of gain, it is necessary to increase the antenna antenna diameter, although with sufficient satellite repeater power and high elevation angles (less affected by the thermal noise of the Earth) for a given reception point, this antenna provides good results.
The converter can be fixed in the ZAF focus in the same way as for a direct-focus parabolic antenna.