It is believed that the spiral antenna is characterized by circular polarization, but the opinion is wrong. In fact, the structure of the turns is such that waves are accepted with linear polarization. This is convenient when there is an opportunity to work on any wave structure. And helical antennas are used as an irradiator of mirrors on the satellite. For radio amateurs, the disadvantage is that the wave with linear polarization is attenuated by three decibels, as is well known, in radio and television broadcasting the other is not used. In the country, a spiral feed is appropriate only for catching NTV + from a satellite, where the method is not used. A number of special applications of these antennas will not be discussed. However, requests on the subject are found on the network. To whom the spiral antenna useful from a wire and dressed on a piece of pipe comes in handy, we do not undertake to answer, even in the collection of works of radio amateurs this class of products is completely absent.
How to assemble the spiral antenna
The spiral antenna resembles an infrared heater of a specific design. In the USSR, military factories produced household appliances. Hence the similarity of parabolic plates and heaters. For the assembly you will need to know the diameter and the step of winding the wire, the number of turns. From the materials needed:
- Steel sheet for the screen, arbitrary thickness, so as not to bend in the wind and other collisions.
- Cut the wire to have enough to wind the coils with a margin.
- Power Cable: 75 Ohm Television, 50 Ohm Radio.
- Plastic pipe of the desired diameter.
Spiral antennas belong to the class of traveling wave, the resistance of the devices is high, so that, having correctly calculated the device, connect without coordination. First, the pipe is laid out, with a margin, so that it is possible to stick it into the screen and glue it. A winding step is marked along the axis( preferably from both sides).In the future, risks are used for leveling. Step back in front a couple of centimeters, start working with a marker. Note that on the back of the coil is shifted exactly half a step.
The spiral is wound onto the pipe without taking into account the pitch, with the required number of turns. In the future, starting with the first risk, you need to stretch the wire in the right way. To prevent further displacement, the correct position should be fixed with drops of glue. About three or four per turn. Meanwhile, we will make a screen.
Choose a square with a side on the order of five diameters of the winding tube. There is no difference, what is the thickness of steel, withstand the strength characteristics. The assembled screen is perpendicular to the pipe.
For the electrical assembly, in the area of the end of the helix( pipe base), drill a hole and pass the wire inwards. Behind the screen in the sidewall we make an additional hole, where we pass the braid of the cable supplying it. Electrically the central core is connected to the helix, the feeder screen with the antenna screen. Formed structure for receiving and transmitting waves. The pipe with the steel screen is connected by glue-sealant at the corner to ensure strict perpendicularity of the parts. Key points:
- The spiral and screen are made of a conductive material, such as copper.
- Pipe from dielectric.
Spiral Antenna Calculation
Spiral antennas are good at capturing any type of wave used in terrestrial broadcasting. However, to catch the radio, the axis should be directed up, the screen will be located horizontally. The device has inherent pronounced directional properties, do not expect that you will be able to cover a number of towers from one point. Not so easy. The radiation pattern depends on the dimensions of the helical antenna and strongly:
- If the length of the coil is much smaller than the wavelength, lateral radiation prevails across the antenna axis. Moreover, the polarization is not circular.
- In the ideal case, the length of the coil is within the frame of 0.75 - 1.3 wavelengths. In this case, we observe the main lobe of the radiation pattern, looking forward. Of course, you need a screen.
- If the length of the spiral is more than 1.5 wavelengths, two petals are formed, directed into the front half-plane. More precisely, it turns out something resembling a conical surface.
Indirectly( in the second paragraph), readers have already made up an idea of the range. Twice we will expand a strip, applying not a cylindrical, but a conical spiral( a conical spiral antenna).We recommend online calculator on the site http: //aerial.dxham.ru/ online-raschety / raschety-antenn / raschet-spiralnoj-antenny. Here it is proposed to set the frequency, the step of winding the spiral and the length of the radiator:
- The width of the main lobe of the radiation pattern depends on the length of the spiral winding. Vary the number of turns and watch for the parameter( located at the bottom of the calculator page).The diameter of the winding of the helix barely changes. There is no explanation for this, the creators of the calculator know better. Of course, you will need more copper, which is reflected in the corresponding parameters.
- Add that with increasing length increases and gain. This is a typical effect: the petal narrows - the gain increases. The area of the radiation pattern is a constant value. As Lomonosov used to say, if in one place of which he arrives, in the other he will certainly have to go down. Notice that as the coils grow, the bandwidth barely drops.
- The gain depends on the winding step: the larger the digit, the lower the gain, the narrower the radiation pattern. In our opinion, this is a mistake of the authors, because it turns out that it is more profitable to wind tightly. In addition, the wire will leave less. Only the advantages are shown, in practice this looks doubtful.
Of the useful properties of this online calculator, I would like to mention the calculation of the minimum screen size. And about the step check in the directories, and what we do. By the way, the curious fact is that by default, the site immediately has a WiFi frequency of 2.45 GHz. Here today, spiral antennas are often used.
Found: the gain depends only on the number of turns. The winding step is recommended to choose 0.22 - 0.24 wavelength. On the website, we set this value over a wide range. We suggest that readers choose a step by varying the number of turns. It happens that there are errors in separate calculators, only web-programmer has exact information.
By the way, in a new source of information given that the screen is placed behind the spiral at a distance of 0.12 wavelength. It is added that if the diameter of the screen is chosen equal to 0.8 wavelengths and more, the side of the square is even larger: 1.1 λ.The situation is not so obvious, but imagine that the circle must fit in - everything gets in place.
As for matching, the resistance of the helical antenna strongly depends on the thickness of the wire and decreases with increasing. It is possible to achieve figures equal to 75 and even 50 ohms. In this case, approval is not required, which simplifies operation. At high frequencies it works. For example, the characteristic impedance will be 75 Ohms with a wire thickness of 5% of the wavelength. Getting 50 ohms, you should take a wire thickness of 7% of the wavelength. You see that at WiFi frequencies this is real, which means that we will calculate the parameters in such a way, avoiding matching.
Please note that the calculator does not give an opportunity to set the wire thickness, and with the existing characteristic impedance is 140 Ohm. This is probably a professional trick, to our knowledge the cable should be 50 Ohms on WiFi frequencies. But it is easy to check whether the dependence on the thickness of the wire. We give a table and compare the result.
So, the frequency is 2450 MHz, we find the wavelength by the simple formula:
λ = 299 792 458/2450 000 000 = 0.1223 meters.
Find the desired wire diameter for a resistance of 140 Ohm:
0.1223 x 0.02 = 2.45 mm, check if it matches the online calculator! Look and see: 2.4.Well, if we take into account that without rounding it turned out to be 2.447 mm, then we will assume that the two sources repeat each other, which means that the instructions for choosing the winding pitch( see above) can be believed. At this point we believe that the homemade spiral antenna is ready, and we will also find the wire thickness at which the resistance will become 50 Ohms: it turns out 8.5 mm. Moreover, it is difficult to ensure the required conditions at the indicated high frequency. Therefore, the goal of making a spiral antenna yourself is often set by computer scientists.
For inconsistencies in the calculator, check the technical information readable on the Internet several times. We believe that we answered the question of what a spiral antenna is and how to make a spiral antenna. Plus the design is simple to manufacture, if the patches need to be calculated, coordinated, and not the fact that it will work out, there is a good device here that satisfies the specified conditions, eliminating a lot of noise. On both sides( for reception and transmission) there are identical antennas to work with circular polarization, otherwise the result will be mysteriously unpredictable. A spiral antenna assembled by itself is a reality.