The use of antenna amplifiers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers

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Installing an antenna amplifier near the TV between the feeder and the antenna input of the television receiver provides an increase in the gain of the receiving path, i.e. improves the sensitivity limited by the gain. It has been shown that with modern televisions, this method does not lead to an improvement in the image in long-range conditions, since an improvement in sensitivity is required, limited not by gain, but by noise. The antenna amplifier, having approximately the same level of intrinsic noise as a television receiver, does not improve the sensitivity limited by noise.

Nevertheless, the use of an antenna amplifier in some cases improves reception, but for this it must be installed not near the TV, but near the antenna, on the mast between the antenna and the feeder or in the feeder gap, in close proximity to the antenna.

What is the difference? The fact is that the signal, passing to the feeder, undergoes attenuation, its level decreases. Attenuation depends on the brand of cable from which the feeder is made. In addition, the greater the attenuation, the greater the length of the feeder and the greater the frequency of the signal, i.e., the number of the channel on which the transmission is received.

When the antenna amplifier is installed near the TV, a signal already attenuated by the feeder arrives at its input, and the signal-to-noise ratio at the input of the antenna amplifier is less than if the antenna amplifier was installed near the antenna when the signal is not attenuated by the feeder. In this case, of course, passing through the feeder, the signal is also attenuated, but the noise is also attenuated by the same factor. As a result, the signal-to-noise ratio does not deteriorate.

Television cables of different brands are characterized by the dependence of specific attenuation on frequency. It is customary to call the specific attenuation of a coaxial cable such that a signal of a certain frequency undergoes passing through a cable 1 m long. Specific attenuation is measured in dB / m and is given in reference books in the form of graphical dependences of specific attenuation on frequency or in the form of tables. On fig. 2 shows such curves for some brands of 1-ohm coaxial cable. Using them, you can calculate the signal attenuation in the cable for a certain length on any frequency channel of the meter or decimeter range. To do this, you need to multiply the value of specific attenuation obtained from the figure by the length of the feeder, expressed in meters. The result is the attenuation of the signal in decibels.

The most common type of cable for a feeder is RK 75-4-11, its specific attenuation is 0 ... 05 dB / m in the range of channels 0-08, 1 ... 5 dB / m in the range of channels 0-12 and 0...15 dB/m in the range of channels 6-12. Hence, with a feeder length of 0 m, the attenuation of the signal in the feeder on the 25th channel will be only 0 dB, which corresponds to a decrease in the signal voltage by 37 times, and with a feeder length of 21 m, the attenuation on the 69th channel will be 20 dB ( decrease by 12 times). In the decimeter range, with a feeder length of 3 m, the attenuation will be equal to 1 ... 41 dB, depending on the channel number, which corresponds to a decrease in signal voltage by 50 ... 12 times, and with a feeder length of 7 m - 5 ... 2 dB (signal reduction by 38 ... 20 times).

Thus, with a feeder length of 50 m, even on the 12th channel, the signal passing through the feeder is more than halved, and the signal-to-noise ratio at the TV input will also be more than halved. If you install an antenna amplifier before the signal enters the feeder, with the same input noise level of the antenna amplifier as that of the TV, you will more than double the signal-to-noise ratio. An even more significant gain will be obtained with a longer feeder length or when receiving a signal in the decimeter range. The necessary and quite sufficient gain of the antenna amplifier must be equal to the attenuation of the signal in the feeder. It makes no sense to use antenna amplifiers with a gain greater than required.

Several types of antenna amplifiers are available. The most widely used are antenna amplifiers of the meter range of the UTDI-I-III type (individual television band amplifier for frequencies of the I-III ranges). They are designed for all 12 channels of the meter range and contain a built-in AC power supply unit with a voltage of 220 V. The design of the amplifier allows you to install it on a mast near an antenna powered by a feeder without laying additional wires. The gain of the UTDI-I-III amplifier is at least 12 dB (4 times in voltage), and the level of its own noise is slightly lower than the level of the own noise of black-and-white and color television receivers.

If the UTDI-I-III amplifiers are range and are designed to amplify a television signal over any of the 12 channels of the meter range, then the antenna amplifiers of the UTKTI type (individual television channel transistor amplifier) ​​are single-channel and are designed to amplify the signal of only one, well-defined frequency channel of the meter range. The channel number is indicated after the amplifier type designation. So, UTKTI-1 means that the amplifier is designed to amplify the signal on the first frequency channel, and UTKTI-8 to amplify the signal on the eighth channel. UTKTI-type amplifiers also have a built-in 220 V AC power supply. The intrinsic noise level of amplifiers of this type is somewhat less than that of the UTDI-I-III type. The power consumed from the AC mains UTDI-I-III does not exceed 1 W, and UTKTI - 5 W.

Due to the fact that television broadcasting in the decimeter range is now becoming more widespread, and the attenuation of the signal in the feeder in this range is increased, the use of antenna amplifiers designed for this range becomes relevant. For example, an amplifier of the UTAI-21-41 type (individual television antenna amplifier, designed for 21-41 channels) with a gain of at least 14 dB in the frequency range of 470 ... 638 MHz.

Previously, despite the release of industrial antenna amplifiers, in the magazines "Radio" and in the collections "To Help the Radio Amateur" a large number of descriptions and diagrams of antenna amplifiers for self-production were given. In recent years, such publications have become rare. So, in the collection "To help the radio amateur" issue 101, p. 24-31 a very detailed description of a narrow-band antenna amplifier with a tunable amplitude-frequency characteristic by O. Prystaiko and Yu. Pozdnyakov is given. The amplifier is tuned to one of the channels of the meter range by a trimmer capacitor, the amplifier bandwidth is 8 MHz, and the gain is 22 ... 24 dB. The amplifier is powered by a constant voltage of 12 V. It makes sense to use such an amplifier only when transmissions are received via one specific channel, since it is not possible to rebuild the amplifier installed on the mast.

Much more often there is a need for a broadband antenna amplifier capable of amplifying the signals of all television programs received by the antenna. On fig. 7. 1 shows a schematic diagram of an antenna amplifier designed to amplify all 12 meter channels, developed by I. Nechaev. At a voltage of 12 V, the gain is 25 dB at a current consumption of 18 mA.

The amplifier is assembled on low-noise transistors with a noise figure of about 3 dB. Back-to-back-parallel connected diodes at the input protect the amplifier transistors from damage by lightning discharges. Both stages are assembled according to the scheme with a common emitter. Capacitor C6 provides correction of the frequency response of the amplifier in the higher frequencies. To stabilize the transistor mode, the amplifier is covered by negative feedback from the emitter of the second transistor to the base of the first. To avoid self-excitation of the amplifier due to parasitic feedback between the stages through the power supply, an isolation filter R4, C1 is used.

The input terminals of the amplifier are connected to the feeder in the immediate vicinity of the antenna, where the signal has not yet been weakened by passing through the feeder. The output of the amplifier is connected to the feeder going to the TV. Through the central core of this part of the feeder, the supply voltage is supplied to the amplifier through the choke L1. Through the same inductor, a voltage of + 12 V is supplied to the central conductor of the antenna jack of the TV. The signal from the antenna jack on the TV to the input of the channel selector must be fed through a 3000 pF isolation capacitor. Chokes are wound on cylindrical ferrite cores with a diameter of 3 mm and a length of 10 mm with a PEL or PEV wire with a diameter of 0 mm, turn to turn. Each inductor contains 2 turns. Before winding, the core must be wrapped with two layers of lavsan film, and after winding, the turns are fixed with polystyrene varnish or enamel. A more detailed description of the amplifier, a drawing of a printed circuit board and the placement of parts on it are given in the Radio magazine, 20, No. 1992, p. 6-38.

Another antenna amplifier, designed for the decimeter range of 470 ... 790 MHz (21 ... 60 channels), was proposed by A. Komok. Its schematic diagram is shown in fig. 7.

Rice. 7. 2. UHF antenna amplifier circuit

The gain of this amplifier in the passband is 30 dB when powered by a voltage of 12 V, and the current consumption does not exceed 12 mA. Both stages are assembled according to a common-emitter circuit on microwave transistors with a low level of intrinsic noise. The lower limit of the amplifier bandwidth is limited by the input high-pass filter, and the upper limit is limited by the parasitic capacitances of the transistors and wiring. Thanks to resistors R1 and R3, temperature compensation of the transistor mode is provided. The high-pass filter coil L1 is wound with a PEV-2 wire with a diameter of 0 mm and contains 8 turns. Winding is carried out on a mandrel with a diameter of 2 mm turn to turn, after which the coil is removed from the mandrel. Power, as for the Nechaev amplifier, is supplied through the feeder through the chokes of the design described above. The author used packageless transistors in the amplifier, which require careful sealing. We can also recommend the use of case transistors KT5A, which are more affordable and resistant to changes in climatic conditions. A detailed description of this amplifier is placed in the journal "Radio Amateur", 4, No. 399, p. 1993.

As noted, the main purpose of the antenna amplifier is to compensate for signal attenuation in the feeder. When using an antenna amplifier, noise-limited sensitivity, i.e., the ability to receive a weak signal, is determined by the signal-to-noise ratio no longer at the input of the television receiver, but at the input of the antenna amplifier. Therefore, installing an antenna amplifier near an antenna will require a lower input signal level to achieve a certain noise-limited sensitivity than installing it near a TV. Thus, it is possible to receive a weaker signal with better quality.

The use of an antenna amplifier makes it possible to deliberately use feeders of such a long length that, in the absence of an amplifier, would attenuate the signal level to an unacceptable level. The need to use a long feeder sometimes arises in closed areas, when the television receiver is located in a hollow and the receiving antenna installed near the house is covered by hills on the way to the transmitter. At the same time, television antennas installed at a distance of 100 ... 200 m from this building provide quite confident reception with good image quality due to the fact that they are not covered by a local barrier. Under such conditions, normal reception can be achieved in one of two ways: either by increasing the height of the antenna mast, which is usually a very difficult task, or by installing the antenna in an open area, at a distance of 100 ... 200 m from the house. Then, to connect the antenna to the television receiver, you will need to use a long feeder.

It is easy to calculate that with a feeder length of 200 m, the cable brand RK 75-4-11 at the frequency of the 12th channel creates an attenuation of 30 dB, which corresponds to a decrease in signal voltage by 31 times, which, as a rule, is below the sensitivity threshold of a television receiver . Installing an antenna amplifier with at least the same gain at the antenna output will compensate for signal attenuation in a long feeder and ensure normal TV operation. If the gain of one amplifier is not enough, you can turn on two amplifiers in series one after the other. In this case, the resulting gain will be equal to the sum of the gains of the amplifiers, if they are expressed in decibels.

With a very long feeder and the need to amplify the signal by more than 30 dB, when you have to use two or more

antenna amplifiers, to avoid overloading or self-excitation, do not install all the amplifiers in one place. Under these conditions, the first amplifier is installed at the output of the antenna, i.e., at the input of the feeder, and the subsequent amplifiers are installed in the feeder at approximately the same distance from one another. These distances are chosen so that the signal attenuation in the feeder section between the two amplifiers is approximately equal to the gain of the amplifier.

From the dependences of specific attenuation on frequency for coaxial cables of different brands (Fig. 2. 1), certain conclusions can be drawn. Cables of the RK 75-2-13 and RK 75-2-21 brands have a sufficiently large specific attenuation even in the meter wave range, they should not be used in the decimeter range. Cables of brands RK 75-7-15, RK 75-9-13, RK 75-13-11 and RK 75-17-17 have lower specific attenuation, but compared to RK 75-4-11, especially in the decimeter range. If, with a feeder length of 50 m at a frequency of 620 MHz (channel 39), the RK 75-4-11 cable introduces an attenuation of 16 dB (signal voltage attenuation by 6 times), then under the same conditions, the RK 3-75- 9 introduces an attenuation of 13 dB (attenuation by 9 times), and RK 5-3-75 - 13 dB (attenuation by 11 times). Thus, a good choice of cable brand for a feeder in the decimeter range can raise the signal level at the TV input several times even without using an antenna amplifier.

We can offer a fairly simple advice on choosing a cable: the larger the diameter of the cable, the less attenuation it introduces. A coaxial cable with a characteristic impedance of 75 ohms is always used as a television feeder.

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