Plan:

1. Satellite dishes.

2. Classification of advantages and disadvantages.

3. Modulation and coding.

Satellite antennas are of two types: direct focus satellite antennas and offset antennas. Direct focus antennas are when the antenna is circular, and the focus is where the beams gather, that is, it is in the center. These antennas are large diameter, and they are produced like that. They start from 120-150 cm. They are easier to set up, but a special converter is needed.

The disadvantage of a direct focus antenna is that the antenna looks into the satellite, and snow accumulates on it even in winter. Direct focus antennas were widely spread some time ago, with a diameter of 180 cm, the production was Chinese, they were collected from individual petals. And one more drawback of a direct focus antenna is very poor quality. For a direct focus antenna, the mirror area is most efficiently used. The straight focus antenna rises to an angle and is a bowl.

Offset satellite dishes have a round shape, their focus is shifted from its center. Offset antennas are available in all sizes. Another plus of the antenna is that snow does not accumulate on them in winter, it is located vertically. Inexpensive converters are suitable for it. Offset antennas are of two types: long-focus and short-focus. For long-focus antennas, the focus is farther from the antenna. Converters for offset antennas are designed for long-focus antennas. You can only lose efficiency if you install a tapered bell. This happens very often, since the short-focus antennas of the Supral plant are very common. The diameters of these satellite dishes are 60, 90 and 120 cm. However, offset antennas are used for the C and Ku signal. But usually Ka-band reception is also available.

A satellite antenna is a receiver (or transmitter of a signal) from an artificial earth satellite and is a mirror (reflector) to which a device (converter) is connected, which receives and converts the received high-frequency signal into a lower intermediate frequency.

Exists different types satellite dishes: mirror, flat, spherical, horn and others. They have many valuable qualities, but due to high cost and difficulties in mass production, they are produced in limited quantities. Therefore, the most common type of satellite dish is the "dish".

Since the satellite broadcast signal has a very high frequency and by its physical properties it is close to the properties of light, i.e. reflected from the mirror surface and split at the border of different physical environments(air humidity, etc.). Therefore, in order to amplify the signal, it must be focused at one point using a large lens - a reflective parabolic antenna. After this amplification, the signal goes for further conversion.

A satellite dish of the "dish" type has two forms of reflector: long-focus ("shallow") and short-focus ("deep").

In systems satellite reception most often, long-focus reflectors are used (as described above, this shape of the reflector is described by the term “small”).

Parabolic reflector antennas are divided into two types: Direct focus (axisymmetric) and offset (the feed is out of focus).

Direct focus antennas focus the entire signal in the center of the reflector, represent the classic type of paraboloid of revolution, and this contributes to more accurate orientation to the selected satellite. Typically, such antennas are used to receive a signal in the C-band as weaker, but this is possible in the Ku-band, as well as combined.

Offset antennas focus the received signal away from the center of the reflector, i.e. the focal segment is located below the geometric center of the antenna - this eliminates the shading of the antenna useful area by the feed and its supports, which increases its efficiency with the same mirror area with an axisymmetric antenna.

Each type has its own advantages and disadvantages:

Direct focus antennas.

Advantages:

the manufacturing process is very simple;

low price.

Flaws:

it is impossible to mount on the walls of houses without a long external mount (to provide the desired angle of elevation), otherwise the edge of the antenna mirror will simply rest against the wall;

complex setup and installation;

the converter is installed in the path of the signal from the satellite, thereby shading part of the reflector, and this reduces the overall gain of the received signal;

atmospheric precipitation accumulates in the antenna: rainwater and snow.

Basically, a direct focus antenna is used for professional reception of signals from a satellite, and when the reflector size is more than 1.5 ... 2.0 meters.

Offset antennas.

A toroidal satellite dish, in addition to two reflectors, can be equipped with several converters. Thanks to this production technology, a toroidal satellite dish is able to receive signals from several satellites at the same time, while each converter will be directly in the focus of the satellite to which it is directed - this is a big plus, since there is no need to install rotary devices to capture the signal from the satellite.

The outer main mirror of a toroidal antenna differs from a conventional offset parabolic antenna, since it is most likely not an oval, but an egg-like shape. Due to such a complex profile and reflection from the auxiliary mirror, not a separate point focus is formed in space, but an extended curve, the focus of any visible point of the geostationary orbit.

A toroidal satellite dish is very difficult to manufacture and configure - this is probably the only drawback of this type of satellite dish.

Note; "Dish" is a mirror on which reflection occurs at a certain angle (the angle of reflection is determined by the manufacturer), that is, the offset antenna has a certain angle of signal reflection (19 ... 27?)

Interesting fact.

The main parameter for satellite dishes is gain. The larger the surface of the reflector, the more radiation it will collect in focus. Accordingly, the dimensions of the reflector will be determined by the signal supplied from the satellite to this reflector: if strong signal from the satellite to the antenna, then you can choose a smaller size of the reflector, and if the signal is weak, then the reflector should be the largest, respectively. The main attention should be paid to the fact that for reliable reception in any weather, the size of the reflector must be chosen the largest to obtain a power reserve (during precipitation, part of the signal will be absorbed by water droplets, therefore, the signal power is lost and for reliable reception you need to use an antenna with large reflector).

Calculation of angles of rotation and position satellite dish using the program " Satellite antenna Alignment ".

Immediately I want to bring you up to date. Calculating the angles and determining the direction to the satellite is not a sufficient condition for tuning the antenna to the satellite. The beam width of the antenna, its directional pattern, is, on average, 1.5-2.5 degrees, depending on the size of the dish, so "catch" desired satellite not so easy, especially with a plate size of 0.9m or more. For tuning it is desirable to use SatFinder, an inexpensive satellite tuner indicator.

Before starting to determine the angles and directions to the satellite, you can use the program to accurately determine the geographic coordinates of your settlement, this may be needed for further calculations, this program can be found on the website: http://www.tour-info.ru/maps/locate_geo.html

Using the program "Satellite Antenna alignment»You can calculate the angles required when installing and tuning a satellite dish to a satellite. With its help, the azimuth and elevation angle for any geostationary satellite at a given geographic receiving point are determined. Its main difference from similar programs- the ability to make a calculation for everyone at once geostationary satellites, which gives an idea of ​​their location on the "arc" and their availability for signal reception. The Satellite Antenna Alignment program also has a Russian interface.

To work, you need to download latest version the Satellite Antenna Alignment program at the link: http://www.al-soft.com/saa/saa.exe

The program remembers the list geographic points for which the calculation was made. Subsequently, you will not need to re-enter the coordinates of these places, you just need to select them from the table.
The work with the program "Satellite Antenna Alignment" begins with entering the geographic coordinates of the installation site of the satellite dish, for this you need to enter the coordinates of the installation in the section "Coordinates of the installation site of the antenna".

Designations: north latitude - "N", south latitude - "S" and, similarly, east longitude - "E", west longitude - "W". After entering the coordinates, on the left side of the table you will receive the calculated angles for all satellites at once. The program calculates the azimuth and elevation of the antenna (elevation). The resulting azimuth is the direction to the satellite in degrees, defined as the angle from clockwise north to the satellite. Elevation is the angle (in degrees) between the direction to the satellite and the imaginary tangent plane to the earth's surface at the receiving point. If the elevation angle is negative, then the satellite is hidden behind the horizon line and reception of a signal from it is, in principle, impossible. Thus, from the installation site of the antenna, theoretically all satellites available for reception are visible. Having determined the azimuth and elevation angle, you can quickly navigate and determine the direction to the satellite on the ground in order to assess the possibility of signal reception if there are obstacles on the way to the satellite (houses, buildings, mountains, trees, etc.).

The above calculations are based on the compass readings, but if you do not have one at hand or you don’t trust its readings, then you can use the orientation by the sun.

The program allows you to calculate the azimuth in the sun. The calculation is performed for the same place for which you have already specified the geographic coordinates when calculating the azimuth to the satellites. Height above sea level is assumed to be 0 meters.

To calculate the movement of the sun with an accuracy of a minute, you must specify the date (the default is taken The current date). The calculation results are formed on the left side of the table. For the sun, both the azimuth and the elevation are determined at the current time. This calculation allows you to do without a compass when installing the antenna.

How to work: first, the azimuth to the satellite you need is determined, and then the azimuth in the sun is calculated on the day the antenna is installed. Then, in the table, we find the azimuth of the sun closest to the azimuth to the satellite, and determine the time (and date) when the sun will be in the same direction as the satellite. At this moment in time, we turn the antenna directly to the sun, the azimuth of the sun must coincide with the azimuth of the satellite. You can simply mark this position and rotate the antenna later.

When calculating, it is necessary to indicate your time zone (for Moscow it is +3 hours from Greenwich).

Additionally, the program calculates the azimuth of sunrise and sunset, as well as the time and angle of elevation when the sun is strictly in the south.

When switching to the summer time period, you need to add 1 hour to the obtained results of calculating the azimuth in the sun.

The program displays a simple diagram showing the sides of the horizon. The yellow sector denotes daylight hours, on the eastern part of the sector - sunrise, on the western part of the sector - sunset. On this diagram, you can also display the direction to the satellite you need, for this select the satellite in the list on the left, the direction to it (azimuth) is drawn with a red line. If the elevation angle to the satellite is negative, then the red line is not drawn, because the satellite is not visible.

Currently, offset satellite dishes are widely used. Such an antenna, installed strictly vertically, already has a certain elevation angle (20 ... 25 degrees). The program allows you to accurately calculate the elevation angle of the satellite, and the real angle of the antenna (in degrees from the plane of the earth), for this you need to enter the dimensions of your antenna in millimeters (height and width) in the program. The calculation is performed only for antennas whose height is greater than the width, i.e. offset plates.

This program has a wonderful ability to calculate the angle between the obstacle in the path of the satellite dish and the conditional horizon on which the antenna is located. By specifying the height of the obstacle and the distance to it, you will determine this value. If this angle is greater than the elevation angle of the satellite you selected, reception is not possible under the given altitude and obstacle distance conditions.

The program has one more useful function: by selecting the satellite you need and activating the "Transponders" tab, the program downloads from the Internet all active transponders operating on this satellite.

Finishing necessary calculations the program has the ability to save them in text file, in the buffer Windows sharing, or in a printout on a printer. The calculation table can be exported to MS Excel, MS Word, HTML and CSV files.

For modern world characteristic development information technologies at a fast pace. Therefore, the means of transmitting information are also significantly improved. After all, they should provide more high quality and processing speed. One of these areas, which have become widespread, has become satellite connection... It is used in many fields of human activity: from large industrial developments to small households.

One of these developments is satellite television. Now it is already very difficult to find at least one apartment building in which at least several would not have been installed.This is not surprising, because for enough small fee you can get great quality broadcasts television channels from all over the world.

There are two main types of these devices: direct focus and offset antennas. Their main difference from each other lies in the location of the place of the received signal. Also, the dimensions are not the same.So, an offset antenna has small size, and the point where the converter is placed (it is reflected into it) is shifted relative to the geometric axis. A direct focus antenna is characterized by large dimensions and, accordingly, reflection to a point located on the geometric axis.

Offset antenna works on the principle of signal reflection from the front of the parabola into the converter. In this case, only a part of the entire branch of the parabola is used. A large offset antenna has a full-face look of an egg, those that are smaller - a circle. Large devices include devices with a minimum diameter of more than one meter. To small ones - less than a hundred centimeters.

But it should be noted that the offset antenna cannot be used to receive professional television signal... This is due to the fact that when reflected on the converter, an uneven spot is formed.

To install offset ones does not require a significant investment of time and energy, since these devices must be fixed almost vertically, and this greatly simplifies their mounting on a balcony or simply on a wall of a house.

Also, this device has one important advantage: any do not linger on the reflective dish. All these parameters contributed to the widespread use of similar devices to connect satellite TV.

Offset antennas can be made of different material: plastics, steel, mesh, aluminum - all these can form the basis for the production of reflective surfaces. Each of these devices has its own disadvantages and advantages. So, for example, plastic antennas are highly susceptible to the influence of temperatures, but they are much lighter in weight and more convenient to install. Steel, on the contrary, is heavier, but much stronger and more reliable. Although antennas made of this material have one property that negatively affects reception - they rust over time.

The modern market will provide you with a wide range of goods, so you can always choose exactly what is right for a particular case.

V currently an unusual situation has developed on the market of radio and television products in Ukraine: there are no cheap parabolic antennas small standard size. Parabolic imported antennas "pizza-size" ("the size of a pizza", ie with a diameter of 20-40 cm) at the "electronic" flea market (Karavaevy Dachi) cost up to $ 15. For this money in a store or from a reputable company, you can buy an antenna significantly bigger size- 0.6.0.8 m. The noticeable price also testifies to the popularity of this product. Indeed, small antennas are used in MITRIS systems, modern systems UHF retransmission of television in big cities. Today MITRIS works in Kiev, Odessa, Lugansk, Zaporozhye, Chernivtsi, conquering more and more regional centers and their suburbs, and there is no doubt that it will seriously compete even with cable and satellite TV... There is nothing to replace a small antenna with: the reception of MITRIS on a "bare" converter is often uncertain, and the use of antennas with a diameter of 0.6 m can give a too high signal level, which creates mutual interference between the channels.

So, the demand for small antennas is and will continue to grow. Why are there no antennas? Because their production is considered complex, even knowledge-intensive, and it focuses exclusively on large enterprises, unfortunately, specializing in the production of only large batches of products. Where is the exit? It is obvious that small businesses can fill this niche in consumer demand. Equipped and working hard small number specialists are able to saturate all of Ukraine with cheap and high-quality antennas. And if this has not happened so far, it is only because skilled technicians are still shy about business, and businessmen - about technology. In this article, we will try to alleviate the fate of those who nevertheless decide to take up this business by talking about the device and methods of designing parabolic antennas of small size and accessories for them.

Which parabolic antenna to choose for the transmission of MITRIS-type systems: offset or direct focus? Better - offset. For this antenna, the converter does not shade the mirror, as in the axisymmetric one. With small antenna sizes, the shadow from the converter is commensurate with the area of ​​the mirror, and this becomes significant disadvantage axisymmetric antennas. Their second serious drawback is the ability to accumulate snow in winter, which is not radio-transparent in the microwave. Even with horizontal arrangement the main lobe of the radiation pattern (BP), the lower part of the mirror surface of the axisymmetric antenna is tilted at a positive angle to the vertical, which contributes to the adhesion of snow. Considering the size of the antenna, it takes quite a bit of snow to cover half of the mirror. If the offset antenna has the main lobe of the antenna pattern parallel to the horizon, then the opening of the mirror "looks" at the ground, the angle between the plane of the opening and the vertical is negative, and snow does not stick. To be fair, it should be said that at a positive angle to the vertical the plane of the plastic cover of the converter horn (waveguide) is located, which is installed on the offset antenna, however, snow, as a rule, does not stick to the plastic.

So, we choose an offset mirror. Figure 1 explains how offset and axisymmetric mirrors are "cut" from a primary paraboloid. This drawing is also necessary in order to understand how the tooling for production should be designed and manufactured. The primary paraboloid is the surface of rotation of the parabola y = x2 / 4F, where F is the focal length. The parabola as a generator rotates around an axis, producing a paraboloid of revolution. The focal point is located on the y-axis at a distance F from the origin. The parabolic mirror of the satellite dish is cut from the primary paraboloid with a secant cylinder, the axis and generatrix of which are parallel to the y-axis of the primary paraboloid. If the secant cylinder is located symmetrically to the axis of the primary paraboloid, then an axisymmetric mirror is obtained. Usually, an offset mirror corresponds to the variant in which the generatrix of the secant cylinder coincides with the axis of the primary paraboloid. Then, as can be seen in Fig. 1, the paraboloid axis passes through the edge of the mirror. The focal point F and the direction to the perceived signal remain, of course, unchanged, therefore, in the standard position on the MITRIS repeater, opening the offset antenna will "look" at the ground. An offset antenna resembles a cross-eyed person: it seems to us that it is "looking" in the wrong direction. The direction of maximum reception at the offset antenna almost coincides with the console that holds the converter. The diameter of the secant cylinder will be the diameter of the axisymmetric mirror and the minor axis of the ellipse of the offset mirror aperture. This minor axis is also called the "conventional diameter" of the offset mirror: from the side of the satellite or the MITRIS microwave repeater, the offset mirror is represented as a circle with a secant cylinder diameter. If you look at the opening directly, you get an ellipse: it is formed by the line of intersection of the paraboloid of revolution and the cylinder parallel to the axis of rotation.

Considering further Fig. 1, it is appropriate to discuss the question of where the axis of the converter should be directed, which is set at focus F: if the mirror were direct-focus, the converter would obviously be oriented to the bottom of this mirror at the origin of coordinates, therefore, to offset he must "look" along the bisector of the aperture angle, i.e. the angle at which the offset mirror is seen from focus F. However, there is one caveat. An offset mirror is "illuminated" by a radio wave unevenly: the radio flux density is higher near the origin and somewhat less at the offset edge distant from it — a change in the angle of inclination of the surface to the radiation flux affects. Bottom part The offset is most "loaded" with radiation and, accordingly, it most of all re-radiates energy into the converter. I would like to make such a comparison: in spring, snow melts much faster on the slopes of ravines, on which sunlight falls almost perpendicular to their surface, and where the radiation density is highest. Because of this, the "aiming point", i.e. the place on the mirror, where the converter axis is directed, is moved slightly below the sight along the bisector.

Now it's time to select the initial offset parameters. I propose to accept the nominal offset diameter of 33 cm. It will be a big pizza! If your requests differ from mine, then, following the calculations below, you can design a different "pizza". So, D = 33 cm. Choosing focus F, it should be remembered that here the range of our "arbitrariness" is already small, since we are limited by the F / D ratio: for the converter to "see" the entire offset well, the F / D ratio must be large enough , for example, 0.5-0.6. This value is traditional for offsets (offsets are long-focus), while for straight focus antennas, a different F / D ratio is characteristic - 0.3.0.4. We choose a ratio of 0.5. Then F = 16.5 cm. Immediately calculate the aperture angles: the angle to the major axis of the aperture ellipse from the focal point arctan (4FD / (4F2-D2)) = 90 °, and the angle to the minor axis of the aperture ellipse (by the conventional offset diameter 33 cm) 2arctg ((0.5D / [(0.5D) 2+ (F-D2 / 8F) 2] 1/2) = 83.6 °.

As you can see, these angles are almost the same because the offset mirror is long focus. Offsets match perfectly with classic converter horns designed for this type of mirror. Such horns represent a cone with a solid angle of 45 °, the width of the main lobe of its BP at the 1/2 power level is 80-90 °. I will note one important detail: The area of ​​maximum sensitivity of the converter is directed forward towards the center of the mirror. The edges of the mirror the converter matched to it "sees" worse, and effective area mirrors, which creates the radiation flux perceived by the converter, is only about 0.6 of the total aperture area. This parameter q = 0.6 is called the surface utilization factor. Now we start defining other design and analytical characteristics of our antenna. The major axis of the aperture ellipse is B = D (16F2 + D2) / 4F = 36.9 cm.The maximum depth of the mirror, measured from the plane of the aperture to the paraboloid, H = 0.25D2 / (16F2 + D2) = 3.7 cm. These characteristics will be required to preliminary estimate the metal consumption for the production of the mirror and the manufacture of tooling. The offset mirror has a symmetrical elliptical opening and an asymmetrical profile: in the lower part, on the side of the fixed converter, it gains depth faster. The angle between the tangent to the generatrix of the paraboloid and the major axis of the aperture ellipse at the bottom and top of the mirror is, respectively, arctan (D / 4F) = 26.6 ° and arctan (D / 2F) - arctan (D / 4F) = 18.4 °. For this reason, the point of maximum depth is located closer to the bottom of the offset. The difference in these angles is only 8.2 °, and this small value will be the only indicator of the correct orientation of the mirror in the "up-down" direction, therefore, design and technological measures will be required so that this orientation never gets lost during the production and assembly of mirrors.

Let's determine the expected gain of our antenna. The gain of a reflector parabolic antenna is highly dependent on the wavelength of the radio emission, so the operating frequency and wavelength range should be selected. Kiev MITRIS operates in the range of 11.7-12.5 GHz, so we will assume that the characteristic frequency of the operating range is f = 12 GHz, and the characteristic wavelength is 2.5 cm.The calculated gain of an ideal antenna with a diameter of 33 cm is G = 20lg (nDq1 / 2 / l) = 30.1 dB.

Note that an ideal antenna, i.e. the antenna, the gain of which corresponds to the calculated one, must have a deviation from the parabolicity of no more than 1/32 = 0.8 mm. Manufacturers know that this is a rather stringent requirement, but on small diameters it can be achieved without big problems... The next quality class is a deviation of no more than 1.6 mm. It is quite easy to keep within this ratio even with large diameters of mirrors, but the antenna gain with such a ratio will already be slightly lower than the calculated one. Since the antenna gain includes the surface utilization factor q, the gain is, as it were, tied to the horn that uses the mirror for irradiation during transmission and for the perception of a radio wave during reception with the standard value of the parameter q = 0.6.

Therefore, the amplification of a satellite dish is a kind of "thing in itself". Specialized irradiators, matched to different F / D ratios, are stored at equipped test sites. It is unlikely that such a landfill should be owned by a small enterprise producing "pizza-size" antennas. The opinion of the author as an "old antennae" is as follows: all satellite dishes household use designed for the use of arbitrary irradiators should only be metal structures in which the manufacturer guarantees only correct shape mirrors. For an individual parabolic antenna, only the verified geometry is important, that's all. Experienced manufacturers know what this is about.

Next, let's design a measuring triangle for our antenna. Of course, it will not be required now, but in production, but its design will add information and confidence that you own your antenna. Fig. 2 shows the shape of the measuring triangle and helps to understand its function. The measuring triangle will help you always find the exact focus of your dish and the position of the converter. The sides of this triangle a, b, c calculated as follows:

a = B = 36.9 cm;

b = F + D2 / 4F = 33 cm;

c = F = 16.5 cm.

In practice, you can make the lower side a together with a frame, the curved part of which is a forming paraboloid, i.e. parabola. This alignment is convenient because the installation of the triangle on the mirror will always be unambiguous, and sharp corners at the ends of side a will not scratch the painted surface. In fact, the measuring triangle can be further improved. After adding a parabolic frame, side a can be slightly extended, it will lie on the edge of the mirror, which will make the triangle more comfortable. From the focal point, an aiming direction should be drawn to orient the converter. It has already been mentioned that the bisector of the angle bFc is not quite suitable for this purpose. It is better to orient the converter to the point of maximum depth of the mirror. It is located at the intersection of the axis of the generating cylinder with the paraboloid.

This point is very easy to find, and the accuracy of its determination will be even higher if you do not measure the maximum depth in general, but proceed as follows: divide the side b, equal to the nominal diameter of 33 cm, in half and from its middle parallel to the paraboloid axis, i.e. parallel to side c of the triangle, draw a straight line, it intersects the paraboloid at point P. This point is the point of maximum depth, and we choose it as an aiming point, and the axis of the converter should be located on the straight line PF. The PF line can be highlighted with paint, but it is even better to fix a removable cylindrical mandrel on it, which should fit into the suspension clamp, designed to secure the converter. The diameter of this mandrel should be equal to 40 mm, this is already an established standard for converters. There is no longer any other diameter of the necks of offset converters. But another important connection size of the converters - the distance from the neck to the end of the horn (plastic cap) has not yet taken shape. Most often, the geometry of the converter corresponds to the dimensions shown in Fig. 3.

Such or approximately the same geometry is now possessed by converters Gardiner, Cambridge, FTE, Strong, etc. The focal point should be located slightly deeper than the cap (ie inside the horn of the converter) by approximately W4 = 6 mm. Therefore, the sharp point F of the measuring triangle can be cut off by this small amount, or, if a mandrel is made that simulates a converter, the mandrel can be moved closer to the mirror. This last procedure almost completes the triangle. Why "almost"? Because there is also the effect of under-pressing the mirror, which slightly removes the focus.

Looking ahead, let's say this: if underpressure of mirrors, measured as incomplete, not corresponding to the calculation, the maximum depth of the mirror, has become a mass phenomenon, then I recommend moving the converter clamp aside due to the actual distance from the focal point. This distance in in this case can be calculated using the formula: 5F = -4.55H, where 5H is the difference between the calculated and actual maximum depth of the mirror; 5F - change the focal length. The minus sign in the formula is because a decrease in the depth of the mirror corresponds to an increase in the focal length.

Let's start designing the equipment. To do this, we need to know which pressing technology we can focus on. Usually mirrors are of medium size, i.e. from 0.6 to 2.2 m are pressed using pneumatic or hydraulic pressure: a thin sheet aluminum or steel billet is hermetically crimped along the perimeter (along the contour) between the matrix and the lid, then it is poured under the lid compressed air or water under pressure of several atmospheres, and the workpiece is stretched, pressing into the matrix and acquiring its parabolic shape. The workpiece must be made of a plastic material, for example, aluminum grade A5, A6 or steel grade 08KP. Known alternative technologies production of mirrors: axisymmetric mirrors can be rolled out, sequentially squeezing the workpiece with a roller, clamped at the top of the punch. The punch is mounted on a carousel and rotated, while the roller remains stationary. Small-size axisymmetric mirror can be rolled out to lathe... Large antennas, for example, from 3 to 5 m or more, are made from petals, collecting them on a slipway. The petals themselves are performed on a tightening press, pulling the workpiece onto a parabolic block. Also known unique technology pressing by explosion: first, the mirror is pressed by the hydrostatic pressure of water, and then a small charge is exploded in water, and the blast wave perfectly compresses the mirror made of elastic alloys, for example, from AMTs-M. This ensures additional qualities such mirrors: they are durable, precise and lightweight. Recently, cast (lost-wax) pizza mirrors have appeared on the market. Maybe you have some other new technology? Go for it!

This article describes the device and methods of designing small-size parabolic antennas and accessories for them. The beginning of the article is in the previous issue of the journal.

Small mirrors can be pressed in the classic way, for example, pneumatic pressure, while measuring the magnitude of the pressure with the thickness of the metal and the size of the mirror: the pressure is proportional to the thickness of the metal and the length of the contour (perimeter) of the workpiece and inversely proportional to the area of ​​the workpiece. The length of the perimeter L and the area of ​​the elliptical workpiece S are related by the ratio

Therefore, the pressure P, the thickness of the metal and the length of the perimeter L (or the average diameter) are related by the similarity relation P-Ld / S-d / L-d / D ^.

The average and nominal diameters are close, and for estimation calculations, their difference can be neglected. It is known that an offset mirror with a nominal diameter of 0.9 m from a steel billet 08KP with a thickness of d = 0.8 mm can be reliably pressed at a pressure of 6 atm. What air pressure is required to press a mirror with a nominal diameter of 0.33 m from a steel sheet 0.5 mm thick?

Answer: P = 6.0.9.0.5 / /(0.8.0.33)=10 atm.

If your compressor and the quality of the workpiece crimp can cope with this pressure, then you will not have any problems. You can switch to a thinner sheet if you have problems, but not thinner than 0.35 mm (on steel): the strength of the mirror and the durability of your pizza antenna will decrease.

There is a radically different way of pressing - drawing. This is how the dishes are pressed: the workpiece is compressed along the contour, and the shaping is carried out by the lid, transformed into a movable punch, which moves onto the clamped sheet of metal and pulls it over itself. The metal takes on the shape of a punch. No pneumatics or hydraulics are required, but the press must be two-way (crimp plus stretch). In addition, punch wear is a problem: if punch wear is not critical for pressing dishes, then it is important for the production of mirrors. A worn out punch should be repaired or replaced. There is practically no wear of the forming matrix during pressing by blowing, it is an "eternal" tooling. For this method, a small hydraulic press with a pressing force of several tens of tons is ideal, which is necessary for flanging the mirror and clamping the blank sheet during the inflation of the mirror. Drawing requires a two-way mechanical or hydraulic press with approximately the same pressing force. To draw out our mirror, a small force PS = 10 tons will be required. Flanging, depending on its design, will require the same 10-20 tons. These efforts are determined by the sectional area of ​​the metal being pulled.

How to make a rig? I would not like to go into details, if only because the tooling is structurally tied to specific presses, to the technological traditions of the press section and the possibilities of tool production. I would like to draw your attention more to the specific requirements for the tooling, the main of which is taking into account the thickness sheet material... If you use the inflation method, then the forming matrix should not have a parabolic profile, but the equi-distance of the paraboloid; its surface should move away from the paraboloid of revolution by the thickness of the metal being pressed. If you hope to use two types of material of different thicknesses (aluminum and steel), then you can bore the matrix to the equidistant of the average metal thickness, for example, if for aluminum sheet 5 = 1 mm, and for steel sheet 5 = 0.5 mm, then we choose the equidistant s 5 = 0.75 mm. The boring of the matrix (and of the punch too) is usually carried out on a CNC carousel machine. The technologist-programmer must enter a program, for the compilation of which a tabular or analytical task of the tool tip path is required. If the equidistance is not taken into account, i.e. if we neglect the thickness of the sheet material, then the programmer needs to specify the parabolic generator y = x2 / 4F.

Taking into account the thickness will give such an analytical function

y = x2 ^ + d - d ((x / 2F) 2 + 1) 1/2, where the origin is chosen on the surface of the matrix.

Figures 4 and 5 show the process of making a die and a punch from forgings. The carousel rotates around the y-axis. Both the matrix and the punch of a pizza mirror can be bored not according to a program, but according to a template that has been previously made by a neat toolmaker. Making a paraboloid surface is difficult, but only half the battle. The matrix after this operation is sent to a coordinate milling machine with CNC for boring the flange profile. A cover must be made on the same machine. If you have chosen the method of drawing the mirror and on a carousel machine you performed a boring of a paraboloid on a punch, then after that it can be reinstalled on the same machine and bored into a cylinder with an outer diameter of 33 cm.It will enter the mandrel where the sheet of metal intended for drawing is clamped exactly like a cylinder, despite the fact that in the opening it is a strict ellipse. Entry angle a = arctan (D / 4F) = arctan 0.5 = 27 °.

If this story seemed too complicated for you, do not be embarrassed and try to perform calculations yourself or mold a model of a rig from a piece of plasticine. Please note that modern design parabolic antennas sometimes assume a circular aperture rather than an elliptical one, or constrains the aperture ellipse to a square, or slightly flattens the ellipse by limiting it in width or height. The complexity of the design and manufacture of tooling then increases enormously. It should be noted that the radio-technical properties of the mirror also deteriorate.

Now let's discuss the converter suspension and holder devices. If you want to make a plastic converter holder (clamp), be sure to choose a material with a high guaranteed climatic resistance. The converter weighs up to a kilogram and costs decent money. Destruction of the holder during the service life (10-15 years) must be completely excluded. Attaching the converter holder assembly to the mirror is economical and reliable, but less aesthetically pleasing than a cantilever hanger attached to the rear of the mirror.

Antenna suspension should ensure its alignment in azimuth and elevation and fixation in the chosen direction. It is very important that the range of motion of the mirror in the declination angle corresponds to the operating standards: the axis of the paraboloid should be directed along the surface if the antenna is intended for MITRIS. If you intend to use an antenna for receiving satellite TV, then the range of movement of the axis relative to the horizon, i.e. the elevation should be increased. The elevation angle of the vertex satellite depends on the latitude of the terrain in accordance with the formula φ = arctan ((cos ^ -0.1511) / sin ^), where φ is the latitude angle of the terrain. Suspension design should provide angle variation to the benefit of all buyers. Please note that the elevation adjustment of the antenna must be carried out from a vertical support, for example, from a long vertical pipe. It is good if you find this requirement obvious, but it is not obvious to everyone. You have probably seen Polish MABO antennas, beautiful in every other sense, which are frozen on our rooftops in the most exotic poses. They do not allow the beam to be aligned parallel to the Earth's surface if the antenna is mounted on a long vertical pipe.

The suspension unit must be simple and reliable. When you make a breadboard or prototype of the antenna, do not forget to invite an experienced antenna engineer: he will make the correct conclusion regarding the success of the suspension design. Choose a material for the suspension that is thicker and harder than for a mirror; if you like stamping, then this material will still be reinforced with zigs, while you must make sure that the rigid attachment legs are tangent to the mirror at the point of their connection, otherwise the mirror deformation is inevitable: it is lost appearance and the gain deteriorates. Unfortunately, the products of many companies have this obvious drawback. The tangency of the legs on the small antenna can be selected practically, but this must be carefully carried out in the documentation and performed in the rig. If the antenna is larger than the pizza, then it is better to calculate the geometry of the legs first.

Imagine that on large table we plotted a rectangular coordinate grid (x ^) and put our mirror on it with the aperture down, and put it so that the x1 axis coincides with the major axis of the aperture ellipse, and the point x1 = z1 = 0 falls on the origin of the major axis in the lower part of the aperture. We will assume that the y1 axis is directed upwards, it will be the scale of heights on a parabolic surface. This situation is depicted in Figure 6. Suppose the antenna hanger contains four legs and you need to determine their inclination to the plane of the table. Since the paraboloid is curvilinear, then for each attachment point it is necessary either to specify two angles - along the x1 axis and along the z1 axis, or indicate the direction of the greatest curvature and give the angle of inclination in this direction. The paraboloid is symmetric about the x1 axis, so it is enough to solve the problem for two points A and B. We will work out the method for calculating the angles using the example of point (hole) A. This method is completely based on calculating the height y1. To calculate the height of point A above the table surface, use two formulas y1 = (Dt-t2-z12) (16F2 + D2) -1/2, where the auxiliary parameter t is defined as t = -8F2 / D + 1/2. These formulas are given in general view so you can use them whenever you want. In the case of our antenna, F = 16.5 cm and D = 33 cm, so the formulas are simplified: y1 = (33t-t2-z12) / 73.8; t = -66+ (43.56 + 147.6x1-z12) 1/2. It remains to be hoped that the numerous formulas did not strain your memory very much with concepts from analytic geometry and mathematical analysis... Let them finally work for a small business! In conclusion, I would like to remind you what you already know: honor must be protected from the youth, and quality - from the first samples. Raise the quality bar as high as possible and hold on with all your might, because provocations to a drop in quality will arise every day. The greatest problems will arise regarding the quality of paint and varnish and galvanic coatings. The preparation of the mirror surface for painting should be better than "according to technology". Of course, the painted parts must be protected during transportation and storage. This is your problem, not the customer's, since the damaged appearance of the antennas can ruin your reputation. If you can do electroplating somewhere in the defense industry, then you are in luck. If you do hot-dip galvanizing, you will bypass all competitors. In order not to forget about competitors, hang a Polish MABO on the site, for example, with a diameter of 0.6 m offset (it is small), and next to it - your antenna and every day look at this pair through the eyes of a buyer.

M.B. Loshchinin, Kiev

At this stage, we need to determine the approximate tilt of the mirror of our satellite dish in the vertical plane.

You can of course not calculate this parameter... But, knowing the correct slope satellite dish, will save a beginner, in case of an unsuccessful signal search, from unnecessary assumptions "is the antenna correctly positioned?" For example, if, when tuning, you tilt the antenna mirror up (or down) quite strongly. After all, you will already have visual presentation, about how it should stand, and in any case, return the mirror satellite dish to the original state, after which, continuing the search.

Although, of course, this will not get rid of manual setting, but still, it will significantly facilitate the whole process satellite dish settings(while also saving time).

Now let's calculate the tilt of the offset mirror satellite dish.

Satellite Dish Tilt - Calculate Tilt Angle

Unfortunately, since the slope satellite dish, directly depends from her diameter and shape, in order to calculate this slope, alas ... you need to make a calculation using special formulas.

I don't want to overload your head, and that's enough already large quantity information. Therefore, here, I will offer three ways:

The first way. Don't do any calculations now. When adjusting the vertical position of the antenna, first place it in the vertical position. Then, gradually lower the mirror down (or raise) until a signal from the satellite appears. In principle, all experienced tuners do this.

Second way. Pay attention to the angle of inclination satellite dishes installed in the neighborhood, for example, on the same house, or balconies and roofs of neighboring buildings.

Third way. To calculate the angle of inclination satellite dish, for this, use some kind of computer program.

To make it clearer, to determine the antenna tilt, I will use the same program "Satellite Antenna Alignment".

To do this, run this program, and go to the tab " Offset antenna"(Photo 1).

In the satellite selection window, set the one to which it will be conducted. In this case, I chose the Express AM 22 satellite (Photo 2).

Photo 2 Choosing a satellite to which it will be held setting up a satellite dish.

Let's set the dimensions of your antenna mirror in the “Antenna Width” and “Antenna Height” cells (Photo 3).

Photo 3 Specify the dimensions of the offset mirror satellite dish.

As soon as we enter the dimensions of the antenna, next to the image below, the numerical indicator “ Antenna tilt required"Will change its meaning. In my case, it was equal to 73.20 ° (Photo 4).

Photo 4 Tilt satellite dish.

Being at the installation site of the antenna, based on the fact that we will only visually insert the initial position, the tilt of the mirror satellite dish, it is easier to measure not from the horizontal, as it is done in the program, but from the polar axis. To put it more with the right words, this will be the Declination angle - that is, the angle of declination of the mirror satellite dish(Figure 1).