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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Outdoor TV antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas Due to the rapid growth in the number of television broadcasting channels, it is very difficult to ensure high-quality reception of all television programs. First of all, it depends on the antennas used. Therefore, their main designs, tested by the author in various conditions, are considered below. But first, let's recall the basic information about television frequencies, ranges and channels. Television frequencies cover the interval 48,5 ... 790 MHz. They are divided into meter (channels 1-12, frequencies 48,5 ... 230 MHz) and decimeter (channels 21-60, frequencies 470 ... 790 MHz) ranges. One television channel occupies a frequency band of 8 MHz. To calculate the operating wavelength of antennas, it is recommended to select the carrier frequency of the image, since the image signal is amplitude-modulated, more susceptible to interference and requires more amplification than a frequency-modulated audio signal. The image carrier frequency for the 1st and 2nd television channels is 49,75 and 59,25 MHz, respectively. For the 3rd-5th channels, it (in megahertz) is calculated as follows: fn.iz \u77,25d 3 + (N-8)x6, where N is the channel number; for the 12-175,25th: fn.out=6+(N-8)x21, and for the 60-471,25th: fn.out=21+(N-8)xXNUMX The channel band center frequency can be obtained by adding 2,75 to the image carrier value. The audio carrier frequency is 6,5 MHz higher than the picture carrier frequency. The operating wavelength L (in meters) in air depending on the operating frequency f (in megahertz) is determined by the formula l=300/f. In other dielectrics, the wavelength is shorter (for example, in polyethylene by 1,52 times). This fact must be taken into account in the manufacture of resonant elements of matching devices from a coaxial polyethylene cable. Now a little about the design of the antennas. The material for their manufacture is desirable to use tubes, rods, strips, corners, wires made of metals and alloys with good electrical conductivity (copper, aluminum, brass). The broadband of the antenna depends on the surface area of the active elements: the larger the area (the larger the diameter of the tubes or the width of the strips), the broader the antenna will be (but also heavier). It is not recommended to choose the transverse size of the elements (tubes, rods) of the antenna less than 1/200 of the wavelength at which it operates, since this significantly deteriorates the electrical parameters and mechanical strength. The width of the strips is chosen 1,5 ... 2 times the recommended diameter of tubes or rods, the thickness is 2 ... 3 mm. The surface of the elements must be even and smooth. For the UHF range, the best results are obtained if elements made of a material with a polished surface are used, since RF currents are induced only in their surface layer. If it is required to bend the antenna elements, this is done carefully by placing pieces of textolite or wood under the vise jaws so as not to damage the surface. Before this, the tubes must be tightly filled with sand and plugged with wooden plugs. When installing antennas outside the action of lightning rods, they provide reliable lightning protection [1]. In addition, it is necessary to carefully seal the electrical connections and the places where the cable braid comes out of the outer insulation, filling them with water- and heat-resistant dielectric varnishes or resins. To accurately orient the antenna in the desired direction, the signal at the TV input should be attenuated several times with an attenuator. At the same time, the AGC system of the TV ceases to operate and the maximum of the received signal becomes more noticeable. And finally, about the antennas themselves. Let's start with narrowband. They are designed to receive one or more television channels, provided that their frequencies differ by no more than 5 ... 10%. In this case, the antenna is calculated for the geometric mean frequency, calculated as the square root of the product of the frequencies of each channel, or for the signal frequency of the weaker channel. Antennas are also divided into simple and complicated ones [2]. The latter may contain, in addition to one active element, a reflector, directors, and even additional active elements. The simplest antennas equally receive both a direct signal and a signal coming from the opposite direction from the television center. They have the lowest gain (and directivity). Therefore, their use, as a rule, is limited to a small distance from the transmitting antenna in the absence of reflected signals (visually, they appear on the TV screen as a multi-contour or blurring of the image). The simplest antennas include a "linear half-wave split vibrator" [1, 2], shown schematically in fig. 1. It has an input impedance at resonant frequency of about 75 ohms. It is its gain that is usually conditionally taken as 0 dB. The vibrator is made from tubes, rods or strips. The diameter d of the tubes is chosen equal to 20 ... 30 for MV, and 6 ... 12 mm for UHF. The distance between the ends of the tubes l should be equal to half the working wavelength of the antenna, multiplied by the velocity factor, which depends on the ratio of the diameter of the tubes to the working wavelength. For ratios of 0,001 and 0,003, the coefficient values are 0,96 and 0,95, respectively. If it rises to 0,005 or more, the coefficient is reduced to 0,94. The distance L is chosen within 50 ... 80 for MV and 20 ... 30 mm for UHF. To obtain the maximum signal level, the vibrator is placed horizontally in a plane perpendicular to the direction of reception (with horizontal polarization of the transmitter waves).
When connecting the antenna to the feeder, a "quarter-wave loop" matching device is used according to the scheme shown in fig. 2, which is made of a short-circuited piece of cable with a length lsh equal to a quarter of the wavelength to which the antenna is tuned (taking into account the shortening factor). Distance D for the MV range is chosen 50 ... 80, and for the UHF range - 20 ... 30 mm.
Another of the simple antennas is the "half-wave Pistohlkors vibrator" [1, 2], shown in Fig. 3 has an input impedance at the resonant frequency of 295 ohms. Just like a split vibrator, the antenna is made from tubes, rods or strips. The bend radius does not matter, bends can be made at right angles. The main advantage of the Pistohlkors vibrator over the split vibrator is that at the point of symmetry it has zero potential, and at this point it can be attached to the mast without insulators. The vibrator is also placed horizontally in a plane perpendicular to the receiving direction.
The antenna has a wider bandwidth than a split vibrator and has better noise immunity. Dimensions l, L, d are chosen in the same way as for a split vibrator. However, when calculating the coefficient of shortening of the Pistohlkors vibrator, instead of the diameter of the tubes, a value is taken equal to twice the square root of the product of the diameter of the tubes d and the size S in the vibrator. The latter is 80 ... 100 for MV and 40 ... 50 mm for UHF. To connect the antenna to the feeder, use the one shown in Fig. 4 matching device "U-elbow", made of a piece of coaxial cable with a characteristic impedance of 75 ohms. The length lsh is equal to half the wavelength at which the antenna operates, divided by the shortening factor for a polyethylene cable (1,52). The device is connected to the vibrator at points A and B.
It is possible to reduce the influence of the reflected signal and slightly increase the gain of the simplest antennas by complicating them, for example, by placing a reflector behind the active vibrator (in the direction from the television center) as shown in Fig. 5 for the "wave channel" antenna, which will be discussed below. The length of the reflector elements should be greater than the length of the vibrator l by 5...15%, and the distance from the vibrator to the reflector should be chosen within 0,15...0,2 of the operating wavelength.
Loop antennas [1], shown in fig. 6 and 7 have good parameters with the relative simplicity of the device. Their input impedance at the resonant frequency is 73 ohms, the gain is 3,5 dB. They are positioned in the same way as the Pistolkors vibrator to obtain the maximum signal level.
For an incomplete zigzag antenna (Fig. 6), the distance a is chosen equal to a quarter of the operating wavelength. In a ring antenna (Fig. 7), the circumference l is equal to the wavelength at which it operates. For both antennas, the distance L is 10 ... 15 for MV and 7 mm for UHF. Loop antennas are connected to the feeder through a "quarter-wave short-circuited loop" (see Fig. 2). If a strong reflected signal interferes with reception from the side opposite to the direction to the television center, then its influence can be significantly reduced by placing a reflector screen behind the antenna as shown in Fig. 8. This also increases the antenna gain by approximately 3 dB.
Structurally, the screen is made of the same elements as the antenna web itself, but thinner conductors can also be used. The width a and height b of the screen are 5...10% larger than the corresponding overall dimensions of the antenna. The distance D between the elements of the screen is no more than 0,1 of the working wavelength, and C between the antenna sheet and the screen is 0,21 ... 0,27 wavelengths. Screen elements are attached to the mast only in the center. The complete zigzag antenna shown in Fig. 9 is also not difficult to manufacture [1]. It is composed of two incomplete ones (see Fig. 6). It is made of tubes, rods, strips or two or three copper wires 2 ... 3 mm thick, placing them in parallel at a distance of 5 ... 10 for UHF and 20 ... 50 mm for MV. The input impedance of the antenna at the resonant frequency is 73 ohms. Gain - 6 dB.
The antenna is connected to the drop cable without matching devices directly at points A and B. The feeder is laid on one side of the antenna. If it is necessary to increase the gain and reduce the influence of reflected signals, a reflector screen is installed in the same way as for loop antennas. An increase in the gain of a zigzag antenna is achieved using multi-element systems, antennas with open extreme elements and with an angle greater than 90° [3]. The "triple square" antenna [4] refers to complicated designs and is a hybrid of a loop antenna and a "wave channel". It is shown in fig. 10. Its input impedance is 70 ohms, the gain is 8 dB. The antenna consists of three square elements: a reflector (P), an active vibrator (B) and a director (D). The elements are made of rod, wire, tubes or strips with a transverse dimension of at least 3 mm for DMV and 10 mm for MV. The sides of the squares P, B and D are equal to 0,32, 0,25, 0,22 of the working wavelength, respectively. The distance a between the reflector and the vibrator is 0,16, and between the vibrator and the director b is 0,11 of the working wavelength.
In the manufacture of the antenna, the planes of the squares must be parallel, and their centers must be on the same axis. You can increase the rigidity of the antenna by installing, in addition to the upper metal crossbar, dielectric spacers between the squares. Distance L for MV is 40, and for UHF - 15 mm. The antenna is connected to the feeder through a "quarter-wave short circuit" matching device (see Fig. 2). The worst results when simplifying the antenna can be obtained by abandoning the director (antenna "double square"), but at the same time changing the side of the reflector P and the distance a to 0,31 and 0,18 wavelengths, respectively. The input impedance of such an antenna is about 100 ohms, and the gain is 3 ... 4 dB worse than that of the "triple square". Even more complex narrow-band designs include the "Spindler wave channel" antenna [5], shown in Fig. 5. Its input impedance at the resonant frequency is 280 ohms. The gain depends on the number of elements (see table).
Such a multi-element antenna, in addition to an active vibrator, which is usually made in the form of a Pistohlkors vibrator, consists of several passive director vibrators, with decreasing length located in front of the active vibrator (in the direction of the television center), and a reflector screen placed behind, in the opposite direction from telecentre. It works on the principle of "traveling wave" and is considered the most efficient narrowband antenna. However, it is difficult to calculate and requires precision in manufacturing. The purpose of the directors is to amplify the useful signal coming from the main direction, and the reflector is to attenuate the reflected and other interfering signals. Structurally, the antenna elements are mounted on a metal or dielectric traverse, which has the necessary mechanical strength. When using a metal traverse, the length of the elements is increased by half the transverse dimension of the traverse. To calculate the dimensions of the antenna, complex formulas or ready-made computer programs are used. One of these programs was developed by the author and is located on the website of the Radio magazine. In the manufacture of the antenna, special attention should be paid to the observance of the exact dimensions of the elements, the distances between them and the symmetry of the antenna. The drop cable is connected through the "U-elbow" matching device (see Fig. 4) to points A and B of the Pistohlkors vibrator. Broadband antennas are designed to receive television signals that differ significantly in frequency. They work well without tuning, sometimes completely covering the MV or UHF bands and even all the MV and UHF television channels. The simplest of these broadband antennas are gossamer and zigzag antennas. The design of the web antenna is shown in fig. 11. A similar antenna is described in [2]. Its gain is 1,5 dB, the input impedance is 73 ohms. Such an antenna has a wide operating frequency range. However, due to its low gain, its use is limited to the MW range. Orient the antenna in the same way as any simple antenna.
Antenna elements are made of copper wire or brass rod with a thickness of at least 3 mm. At the junctions of the wires, reliable electrical contact is ensured. Antenna dimensions are chosen for the lowest frequency of the range, as for a half-wave split vibrator. The opening angle a is chosen from 90 to 120°. The antenna does not require the use of matching devices - the feeder is connected directly to points A and B. The zigzag antenna turns out to be small-sized if it is used on the UHF. However, as the studies described in [6] showed, it is also possible to expand its operating frequency band to a lower frequency region if additional elements shown in Fig. 9 are used in the design. XNUMX dashed line. In this case, the broadband zigzag antenna is designed for the highest frequency of the received signal. Very often (especially in areas remote from transmitting stations) the gain of one antenna is insufficient for reliable reception. In this case, either antenna amplifiers or antenna arrays are used [4]. Moreover, the use of the latter is more preferable, since any amplifier introduces its own noise and distortion into the useful signal, and requires careful tuning with rather complex measuring equipment. The simplest two-story array consists of two antennas of the same type, the active elements of which are located in the same vertical plane. The antennas must be spaced apart from one another (usually vertically) by a distance H equal to the operating wavelength. The gain of such an array is about 3 dB higher than the gain of a single antenna. Best results can be obtained by using a four-antenna array, called a double-deck double-row array, as shown in Fig. 12. In this case, the gain increases to 6 dB compared to a single antenna. The distance H is also chosen equal to the working wavelength. Often the array is made up of "wave channel" antennas, less often loop antennas are used.
To sum up the signals of individual antennas of the array, the cables from them are connected through matching systems consisting of segments of a coaxial cable with different wave impedance of length T equal to half the operating wavelength (taking into account the shortening factor). An array of two antennas is connected to the drop cable through a piece of cable with a characteristic impedance of 50 ohms as shown in Fig. 13.
If you use a cable with a wave impedance of 75 ohms, then connect the two antennas according to Fig. 14.
In the case of an array of four antennas, the connection is made with a PK-75 cable in accordance with Fig. 15.
Literature
Author: V.Portunov, Bryansk
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Comments on the article: Jury Great article for beginners. Viktor Excellent technically competent short clear article.
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