On Youtube posted a sensible video explaining what a parabolic antenna. Made a screen, indicate the source www.youtube.com/watch?v=6Cku8eGomec, so that the authors get a share of advertising. Visible on the image occurs in the 50 seconds of the video. A parabola is a line-location of points equidistant from a given-focus and line, called a directrix. Physical feature of the parabola: rays coming from a single direction are collected in the focal plane( a plane parallel to the director and containing a focus on the surface).Moreover, if the direction is perpendicular, the point of reception will be the focus. Due to the above, a repeated amplification of the signal occurs, which is used in satellite television. A parabolic antenna, assembled by hand, was a dream of radio amateurs in the USSR - there was no such device on the free market. ..
The evolution of parabolic antennas
Paranoid-minded people fought the Cold War. An antenna with an incredible gain was required to communicate with the satellite; standard models did not fit. In order to spy, we decided to adapt parabolic antennas existing for locators. The meaning is already described above. Due to the peculiar shape of the surface, a thin beam is obtained with an incredible gain. Two difficulties arose, generated by one thing: how to keep track of a moving spacecraft, how a fixed geostationary satellite spy is useless.
So, the thin beam of the antenna is constantly aimed at a moving satellite. The device constantly changes the orientation for the observer's eyes: first it faces the first side, then the second. Consequently, linear polarization for transmission does not matter, would have to change the angle of the antenna, causing extra computational and energy costs. On spy satellites polarization was applied circular. No matter how you look, the signal is caught. NTV + use circular polarization, it is not necessary to turn the converter aside when setting up the equipment. And directly paraboloid to orient in a straight line to the satellite is required, otherwise the incoming rays in the focal plane will gather in addition to the focus.
By the way, this property is used to receive a list of satellites from a single plate with a set of converters. They are located in the focal plane on a straight segment of steel reinforcement, called a multifeed. It remains to add that the size of the plate is chosen arbitrarily. The store has a number of sizes, we recommend copying, making the antenna yourself.
Simply converters are designed to work with these devices, to act differently means to create difficulties. Together with readers we are going to study the requirements for the surface. Parabolic antennas are capricious to form factor, tolerances for deviations from nominal sizes are small( units and fractions of a millimeter for a coherent range).
The principle of a parabolic antenna is similar to a mirror; if the surface is uneven, the rays do not converge in the right place. We will not give exact tolerances on memory, we will plunge into the textbook.
This is interesting! The subject teacher told me that in the USSR, an unsuccessful batch of paraboloids was thrown into children's stores under the guise of sleds. Local radio amateurs instantly dismantled the goods.
Requirements for a homemade parabolic antenna
Let's make a parabolic antenna easier. Ideal for the method used in shipyards. According to the method, the figure is cut with planes along and across, then for a specific case, the generator is cut from steel. In the future, the hull sheets are placed on top. Boil, forming part of the bottom, the ship will go further. Sell:
- to get a book on anadireclicic geometry;
- find the paraboloid formula;
- solve the problem of cutting the planes, along and across;
- cut out the desired pattern from plywood;
- to collect the backbone of the boards intersecting at right angles;
- to glue in a single layer calico, impregnated with a typical adhesive sealant, it is important to withstand the smoothness of the transitions;
- wait until the mold is firm finally;
- carefully cover the surface with foil;
- test design( weighs a little) with a converter on any satellite;
- if the test is passed, it turned out an excellent mold, with which it is easy to assemble any number of parabolic antennas.
We won’t apply, who today needed a home-made parabolic dish antenna, if the excitement didn’t pass, we’ll start to realize the little things. Calico is chosen for its roughness and thinness of the fabric. Both characteristics are important. If you are sure that you will be able to smoothly glue the bones of other material, use the talent for its intended purpose. The parabolic antenna is calculated using the formula:
z =( x2 + y2) / a2;
As you can see, the difficulty in the manufacture of a homemade parabolic antenna in the search for the parameter a. The value is not too important, but we agreed to follow the factory models, which means that we must develop a methodology for determining the desired size. In the literature they write that the focal length is 0.2 - 0.4 times the diameter of the plate. The diameter of a typical dish is 90 cm with a focal length of 45 - 55 cm. This coincides with our numbers. Plus a self-made design: we can choose the diameter, focal length according to our own needs. We try to copy factory models.
How to connect the focal length with the parameter a to calculate the coordinates for plywood. We provide a link to the site http: //www.teleradio.ru/arials/part2/CHAPTER6/ 6-1.htm, where there is a ready calculation for a parabola with a diameter of 2 meters and a focal length of 75 cm.
Note. If it is interesting, on the indicated resource it is proposed to fabricate a mound of stones and cement using an antenna pattern. The paraboloid is cut at zero point. Then it is made of steel according to the coordinates of the piece. Suspended by the center rotates over the hill to give the desired shape of the stones. The result is a reusable pattern, which will produce parabolic antennas. In our opinion, such a parabolic antenna will be a pretty penny. The weight of the design is large, you will not make it at home.
It's easier to create a pattern from plywood, glue it with a thin cloth, then edit the putty as needed. Anyone who wants to do this kind of work at home or - for large copies - in the garage. The template is mobile and weighs relatively little. So, the relationship of focus and parameter a. .. The textbooks give scientific three-storyed formulas, but it is not written how to link the two simplest values together. There is a p-value, called the parabola parameter, and the focal length is half. We tie the formula with a. Simple math. Let's try to check on the data given on the site. Take note of the additional parabola equation:
z = x2 / 2p;
In this case, p = 2 x 0.75 = 1.5 meters, and in the zero sectional plane the formula is:
z = x2 / 3;
In order not to suffer, let's take a point from the table at a meter distance from the center: 1;0.333 and see that everything converges:
4f = a2;where we find the connection:
a = 2√f.
Now readers know how to relate both equations and calculate a series of planes to assemble a useful pattern from the crosshairs. We recommend not to mess with every millimeter so that the fight with veneer does not turn into hard labor. Instead, take key distances every 5 cm. You will then need to cover it evenly with a cloth smeared with glue.
How to make a parabolic antenna
using a pattern. It can be cut from two sides relative to the dish: external and internal. Readers attentive already understand how to act. They said that we would check the pattern with the foil, so the trimming was carried out on the outer parts, a concave bowl was obtained. Let's say more: you can take a ready-made parabolic antenna and sculpt your own in any quantity in the image and likeness. Those who are familiar with the manufacture of papier-mâché toys have already understood:
- A homemade parabolic antenna begins by wetting the bowl and lining the paper in a single layer.
- Thin cloth moistened with glue, lies in one layer. It is important not to fold and preserve the form factor.
- The frozen specimen is gently removed and is covered with layers on the reverse side until it reaches the specified strength.
- Finished blank pasted over with a thin foil to create a conductive layer and the formation of a mirror. The finished design is quite capable of receiving satellite broadcasting.
By the way, the geometric factor is maintained, not so that the waves fall into the aperture, but so that electromagnetic oscillations fall in one phase. Deviations from the form instantly violate the performance of the product. As for the opening of the converter, the position is not so critical. Making a parabolic antenna yourself is not easy. Try it, after some time you get a sensible design.