Selective antenna amplifiers UHF

When receiving television signals in the UHF range, many TV owners are forced to use several different antennas, which can sometimes give rise to specific problems associated with summing the signals. Antenna amplifiers will help solve them, providing not only signal amplification, but also their filtering.

One of the problems that TV viewers have to deal with when watching television programs is the need to receive signals from different directions and at different levels. This forces them to use two or more directional antennas, and if the signal level is low - active antennas or antenna amplifiers, they have to turn on adders or TV signal splitters. Unfortunately, all this often does not provide the desired quality of reception. The reason for this does not necessarily lie in a bad feeder or unsuccessful coordination. If, for example, you have several antennas designed to operate in the same range, then reception of the same signal, especially a powerful one, will be possible with two or more antennas. However, in this case, due to different signal propagation times in the feeders, multiple contours or blurred images appear, although the signal level is quite sufficient for high-quality reception.

This disadvantage can be eliminated by using bandpass filters or selective amplifiers, which isolate one or more signals received by one of the antennas and suppress interfering ones. And so - after each antenna, while filtering different channels. Then all signals are summed. For the MB range, this problem is solved using amplifiers and filters discussed in. There are almost no descriptions of such structures for the UHF range. Therefore, options for selective amplifiers specifically for the UHF range are described here.

However, you should pay attention to the fact that the use of filters is not always advisable (although acceptable). The fact is that, firstly, filters introduce attenuation, and when receiving weak signals this can affect the image quality. Secondly, the frequency response of filters, especially narrowband ones, significantly depends on their coordination with the connecting cables. Therefore, even small changes in load resistance can greatly change the frequency response and reduce the quality of reception. To eliminate this undesirable effect, amplification stages must be installed at the input and output of the filter.

The schematic diagram of a selective amplifier for isolating one or more closely spaced signals is shown in Fig. 1.

The device uses a bandpass filter consisting of two connected circuits L2C7 and L3C9. At the input of the filter there is an amplification stage on transistor VT1, and at the output there are two stages on transistors VT2 and VT3. The overall gain reaches 20...23 dB, and the bandwidth is determined by a bandpass filter.

The signals received by the antenna are fed to the C1L1C2 filter, which suppresses signals with a frequency of less than 450 MHz. Diodes VD1, VD2 protect transistor VT1 from powerful signals and electrical interference from lightning discharges. From the input stage the signal passes to the first circuit L2C7. To obtain the required quality factor, partial switching is applied (to the tap of coil L2). For communication with circuit L3C9, capacitor C8 is included (capacitive coupling). The signal from part of the turns of coil L3 comes to the base of transistor VT2, and after amplification - to the base of transistor VT3. The frequency response of the output amplifier can be adjusted in order to further increase its selectivity by adjusting the L4C11 circuit in the feedback circuit.

Diodes VD3, VD4 protect the amplifier from electrical discharges from the TV. They can arise due to the fact that the switching power supply of modern devices is connected to a 220 V network through small capacitors. The amplifier is powered by a stabilized voltage source of 12 V and consumes a current of about 25 mA. The VD5 diode will protect the amplifier when a power source is connected to it in the wrong polarity. If it is planned to be powered via a separate wire, then the voltage is supplied directly to the diode VD5, and if via a reduction cable, decoupling elements L5, C16 are introduced into the amplifier.

All amplifier parts are placed on one side of the printed circuit board made of double-sided foil fiberglass, shown in Fig. 2.

The second side of the board is left almost completely metallized. It only has pads cut out for input, output and supply voltage (they are shown in the figure with a dashed line). The metallization of both sides is connected to one another along the contour of the board with soldered foil. After setting up the amplifier, the board is covered from the parts side with a metal cover and soldered to it.

The amplifier can use KT382A.B transistors, and if high sensitivity is not required, KT371A is also suitable; diodes KD510A, KD521A.

Capacitors C7, C9, C11 - KT4-25, the rest - K10-17, KM, KLS; resistors - MLT, S2-10, S2-33, R1-4. The leads of all parts must be of a minimum length.

Coil L1 is wound with PEV-2 0.4 wire on a mandrel with a diameter of 2.5 mm and contains 2.8 turns. Coils L2, L3 are made of PEV-2 0.7 wire on a mandrel with a diameter of 3 mm. Winding length - 7 mm. They have three turns with a tap from the middle of the first turn. Coil L4 is wound with the same wire and contains two turns, and coil L5 is wound with PEV-2 0.4 wire and has 15 turns, both on a mandrel with a diameter of 4 mm.

The design of capacitor C8 is shown in Fig. 3. It is made of two plates of tin or thick foil, which are soldered to the contact pads of the board. By changing the distance between the plates, the capacitance of the capacitor is changed.

Setting up the amplifier begins with installing and checking the necessary DC modes. By selecting resistor R1, a voltage of 4...5 V is achieved at the collector of transistor VT1. The mode of transistors VT2, VT3 is obtained automatically.

To adjust the frequency response of the amplifier, use a panoramic indicator. Capacitors C7 and C9 tune the circuits to the desired frequencies. With the specified ratings, the central frequency of the filter can be changed from 500 to 700 MHz. The bandwidth is set by adjusting the capacitance of capacitor C8. At the same time, the gain of the amplifier also changes within small limits. By adjusting capacitor C11, the maximum gain is obtained at the required frequency.

By changing the capacitance of capacitor C8, you can achieve a minimum amplifier bandwidth of 10...12 MHz with a single-hump frequency response. This is necessary to isolate the signal of only one television channel. If you need to select two adjacent channels, then the bandwidth is increased to 40...50 MHz (the plates of capacitor C8 are brought closer together) with a double-humped frequency response with slight unevenness. In addition, the frequency response of the filter is also influenced by the location of the taps of the coils L2, L3.

However, the broadcast environment can be difficult. For example, in Kursk in the UHF range, broadcasting is carried out on channels 31 and 33 from one place and with high power, and on channels 26 and 38 - from another place and with less power. This option is quite typical for most cities in the country. Therefore, to receive and select signals from the 31st and 33rd channels, you can use the already described amplifier. Such an amplifier is not suitable for receiving signals from channels 26 and 38 (or two others with a large frequency separation). Here we need another one, which has two passbands, i.e. contains two filters.

The schematic diagram of such an amplifier is shown in Fig. 4.

The signal from the antenna through filter C1L1C2 is fed to the first amplification stage on transistor VT1. From its output, the signal is divided and sent to two independent stages on transistors VT2 and VT3, each of them loaded with its own bandpass filter: L2C10-C12L3 and L4C13-C15L5. The filters are connected to amplifier stages on transistors V4 and VT5, the outputs of which operate on the same load. The overall gain of this device is 18...20 dB, and the current consumption is approximately 40 mA.

This amplifier uses the same parts as those discussed above. A drawing of its printed circuit board with the placement of parts is shown in Fig. 5.

The setup is carried out in the same way. By selecting resistors R11 and R12, a constant voltage of about 5 V is established on the collectors of transistors VT4 and VT5. Filters are adjusted to the desired frequencies. By adjusting capacitors C6 and C7, maximum gain is obtained at selected frequencies.

If it is necessary to narrow the passband and increase the selectivity of the filter, increase the quality factor of the circuits by using thicker silver-plated wire in the coils and tuned capacitors with an air dielectric, or increase the number of circuits.

Literature

1. Nechaev I. Active antenna of the MB range. - Radio, 1997, No. 2, p. 6, 7.
2. Nechaev I. Active antenna MV-UHF. - Radio, 1998, No. 4, p. 6 - 8.
3. Nechaev I. Television antenna amplifier. - Radio, 1992, No. 6, p. 38.39.
4. Nechaev I. Combined amplifiers of TV signals. Radio, 1997, No. 10, p. 12, 13.
5. Nechaev I. UHF antenna amplifier on a microcircuit. - Radio, 1999, No. 4, p. 8, 9.
6. Nechaev I. TV signal adders. - Radio. 1996, no. 11, p. 12, 13.
7. Nechaev I. Correcting antenna amplifier. - Radio, 1994, No. 12, p. 8 -10.

UHF television reception has a number of features:

1. UHF practically does not bend around the earth's surface and have low penetrating power, so the area of ​​reliable reception is limited to the direct line of sight between the transmitting and receiving antennas.
2. At the same time, the UHF is well reflected from the earth's surface and from the ionized layers of the atmosphere. This makes reception possible at a considerable distance (300-500 km) from the television center. At the same time, the passage of UHF is quite stable and does not have fading characteristic of meter waves (MB).
3. A characteristic feature of UHF is the so-called wave propagation, in which the signal can be received at a distance of up to several thousand km from the television center. It occurs over the sea surface on clear days in the spring and summer months.
4. UHF receiving antennas have significantly smaller geometric dimensions than MB antennas. At the same time, their effective area is small, and therefore the signal power supplied to the input of the television receiver is small.
5. The sensitivity of television receivers in the UHF range is significantly lower than in the MB range, which is due to the poor noise parameters of the UHF selector.

Analysis of the listed features shows the fundamental possibility of long-range and ultra-long-range television reception in the UHF range and two main ways of its implementation. This is an increase in the efficiency of the antenna system and the real (noise-limited) sensitivity of the television receiver. The possibilities of increasing the gain of UHF antennas in practice are limited by the complexity of their design and coordination with the feeder.

Increasing the sensitivity of a television receiver requires altering the UHF selector and usually does not give the desired results. The fact is that in the UHF range the signal attenuation in the cable is high, and when using antennas with low gain it is not possible to obtain a significant gain in the signal-to-noise ratio at the input of the television receiver.

The most optimal way is to use a structurally simple antenna with an amplifier located in close proximity to it. In this case, it is possible to simultaneously increase the efficiency of the antenna and the sensitivity of the television receiver without modifying it.

The antenna amplifier must have a high gain, low noise figure, and a wide range of operating temperatures. At the same time, it should be simple in design, assembled from available parts, easy to set up and not prone to self-excitation.

As a result of many years of theoretical and experimental research, we were able to create an optimal circuit and design for the UHF amplifier according to the listed requirements. has no industrial or amateur analogues

1. UHF antenna amplifier.

Amplifier parameters and circuit:

The amplifier has the following parameters:

Gain coefficient Ku and noise figure Fsh in the range
470-630 MHz (21-40 channels) - Ku? 30 dB, Fsh? 2.0 dB;
630-790 MHz (41-60 channels) - Ku? 25 dB, Fsh? 2.5 dB;
790-1270 MHz (61-100 channels) - Ku? 15 dB, Fsh? 3.5 dB.

Input and output impedance - 75 Ohm
- supply voltage - 9-12 V
- operating temperature range - (-30...+40) °C.

The amplifier circuit is shown in Fig. 1. It contains two cascades on transistors VT1 and VT2, connected according to a circuit with a common emitter. To obtain maximum gain, the emitters of the transistors are connected directly to the common wire. The loads of the cascades are broadband circuits L2, R2, L3, C4 and L4, R6, L5, C10, which ensure matching of their input and output impedances. Circuit L1, C1 is a high-pass filter (cutoff frequency 400 MHz), which is used to eliminate interference from MB band TV transmitters. Capacitors SZ, C5, C7, C8 are blocking. The amplifier is powered via a coaxial cable connecting it to the TV, through a low-pass filter L6, R8, C11. Directly in front of the TV, the UHF signal and the supply voltage are separated by filter C12, L7, C13.

The DC modes of the transistors are set by resistors R1 and R5 so as to obtain the optimal values ​​of the collector currents I1 and I2 of transistors VT1 and VT2. Current I1 is selected from the condition of obtaining the minimum noise figure of the first stage, and I2 - from the condition of obtaining the maximum gain of the second stage.

Amplifier parts and design.

All amplifier resistors are MLT-0.125. Capacitors C1, C2, C4-C7, C9, C10 - small-sized disk capacitors (types KD, KD-1, etc.); SZ, S8 and S11 - type KM-5b, KM-6, etc.

All amplifier coils are frameless. Coil L1 contains 2.75 turns of silver-plated wire with a diameter of 0.4-0.8 mm, its outer diameter is 4 mm, the interturn distance is 0.5 mm. Coils L2-L5 are the leads of resistors R2 and R5, wound on a mandrel with a diameter of 1.5 mm, so that the interturn distance is 0.5 mm, and contains 1.5 turns each. The directions of windings L2, L3 and L4, L5 must be the same (i.e., for example, L2 and L3 are a coil of 3 turns, in the gap of which resistor R2 is connected). Coil L6 contains 15-20 turns of enameled copper wire with a diameter of 0.3 mm, wound turn to turn on a mandrel with a diameter of 3 mm. Choke L7 is a standard type DM-0.1 with an inductance of more than 20 μH. Zener diode VD1 - any with a stabilization voltage of 5.5-7.5 V.

The amplifier can use microwave low-noise transistors with cutoff frequency fgr. more than 2 GHz. If the amplifier operates in the range of 21-60 channels, then transistors with fgr can be used. more than GHz, and if - only in the range of 21-40 channels, then - with fgr. more than 800 MHz. in this case, it is necessary to install a transistor with a lower noise figure in the first stage, and in the second - with a higher gain. In table 1 shows the parameters of transistors that can be used in the amplifier. Transistors are arranged in order of worsening parameters.

It is not recommended to use transistors KT372 due to their tendency to self-excitation and GT346 due to poor noise parameters. If pnp transistors are used, then it is necessary to change the polarity of the amplifier's power supply.

The amplifier is assembled on a printed circuit board made of foil fiberglass laminate with a thickness of 1-1.5 mm. A drawing of the printed circuit board and a diagram of the installation of parts on it are shown in Fig. 2. The board is designed to use transistors with planar leads (KT3132, KT3101, KT391, etc.), which are soldered directly to the contact pads on the foil side. However, it also allows the installation of transistors with a different pin arrangement (KT399, KT3128, etc.), but from the installation side, for which it is necessary to drill the corresponding holes in the board for the pins (see below).

The transistor leads must have a minimum length, especially the emitter lead, which should not exceed 4 mm. The terminals of capacitors C4, C5, C7 and C10 should be no more than 4 mm, and capacitors C1, C2, C6 and C9 should be 4-6 mm (they are additional inductances included in the circuits). Some of the terminals of capacitors C1 and C2 are soldered into the board, while others are soldered directly to the central core of the input coaxial cable. Capacitors C6 and C9 are soldered at one end to the heads of resistors R2 and R6, cleared of paint. The other end of C6 is in the board, and C9 is soldered to the central core of the output coaxial cable. Capacitor C2 is soldered at one end to the board, and at the other end it is soldered to coil L1 at a distance of 3/4 of a turn from the top end according to the diagram. Resistors R3, R4, R7 and R8 are installed vertically.

The printed circuit board is placed in a rectangular sealed case, divided into 4 parts by shielding partitions (Fig. 2, 4). Drawings of the housing parts are shown in Fig. 3. It consists of a side wall 1, a sleeve 2, a partition 3, 4 and covers 5. Parts 1, 3, 4 and 5 are made of sheet brass (it is convenient to use a photo-glazing plate annealed over a gas burner), parts 2 are machined from a brass rod. Bushings 2 are designed so that the input and output of the amplifier are made of a 75-ohm coaxial cable with an outer insulation diameter of 4 mm. You can use another 75-ohm cable, but in this case it is necessary to change the diameters of bushings 2 and holes in the housing wall 1 accordingly.

The power supply filter L7, C12, C13 is mounted in a separate box of any design, on which the input antenna socket and output antenna plug are installed.

The amplifier can be powered from any stabilized 9-12 V source, for example, from commercially available power supplies for transistor receivers BP9V, D2-15, etc.

You can also mount filter elements inside the TV next to the UHF antenna input, and use 12 V voltage from the UHF selector to power the amplifier.

Installation and configuration of the amplifier.

The amplifier is assembled in the following sequence. Mount all elements on the board except resistors R1 and R5. If transistors with non-planar terminals are used, then holes are drilled in the board for them, and rectangular cutouts are made in partitions 4 (shown with a dashed line in Fig. 3). The partitions 3 and 4 are soldered into the board with the corresponding protrusions. The side wall of the housing 1 is bent and soldered. The bushing 2 is hermetically sealed into it. Input 7 and output 8 coaxial cables 80 cm long are inserted into the holes of the bushings, the braid is divided into 2 parts and soldered to the housing from the inside. The central core of the cables should protrude 3-4 mm into the housing. Insert the board into the case so that the edges of the partitions 3, 4 and the edge of the wall 1 lie in the same plane (Fig. 4), and solder the joints of the partitions between themselves and the case. In addition, the odd board is soldered to wall 1 at 10 points. The soldering locations are shown in Fig. 2 and fig. 4. Elements C1, L1 and C9, L6 are soldered to the central cores of the cables. Check the rice carefully. 1, 2 and 4 correct installation.

Next, the amplifier is configured. To do this, power is supplied to the amplifier via output cable 8. By measuring the voltage U1 on resistor R3 by selecting resistor R1, set the value of current I1 (I1 = U1/R3) in accordance with table. 1 for the transistor of the first stage. Solder the selected resistor R1 into the board. A similar procedure is performed for the second stage, measuring the voltage U2 across resistor R7 and setting the current I2 = U2/R7 in accordance with table. 1. Solder in resistor R5. In Fig. 1, the values ​​of R1 and R5 are approximate; in reality, they may differ significantly from those indicated. Check the absence of self-excitation of the amplifier. To do this, connect a voltmeter in parallel with R3 and touch the collector output of transistor VT1 with your finger. If the first stage is not excited, the voltmeter reading will not change. The second cascade is checked in the same way. Self-excitation can be eliminated (its presence is indicated by a sharp decrease in the transistor current when it is touched with a finger) only by replacing the transistor. It should be noted that the amplifier is not prone to self-excitation - out of several dozen amplifiers manufactured, only one, assembled on KT372A transistors, was excited. Check the current consumed by the amplifier, which should be equal to: I1 + I2 = 10 mA; if necessary, select resistor R8 so that the current through the zener diode VD1 is about 10 mA. The final operation is to seal the amplifier. To do this, covers 5 are soldered around the perimeter of the case, and the places where the coaxial cable is inserted are additionally coated with some kind of sealant, waterproof glue, etc. The amplifier is then attached to the antenna mast.

2. UHF antenna

As mentioned above, it makes no sense to achieve a very high gain of the UHF antenna, since this leads to unjustified complication of its design. However, you can’t count on long-range reception with an ineffective antenna either.

Experience in the design and use of UHF antennas shows that the simplest and at the same time very effective is the Z-antenna with a reflector. Its distinctive features are wide-bandwidth, high gain, good matching directly with a 75-ohm coaxial cable and non-critical dimensions.

The antenna design for 21-60 channels is shown in Fig. 5. If the antenna will be used in the range of 61-100 channels, then all its dimensions must be reduced by 1.5 times. The active canvas 1 of the antenna is made of aluminum strips and is fastened “overlapping” with screws and nuts. There must be reliable electrical contact at the points of contact between the plates. At match 6 (it can be metal or wood), the canvas is fixed with the help of support posts 2 at points C and D. Since these points have zero potential relative to the ground, the posts 2 can be metal. Cable 3 is connected to points A and B (the braid to one point, and the core to the other) and is laid along the fabric along the lower post 2 and along match 6 to amplifier 7. The cable is secured with wire clamps. The web 1 can itself be used as an antenna. Its gain is 6-8 dB. However, it is better to equip the canvas with a reflector.

The simplest reflector 4 (Fig. 5b) is a flat screen made of tubes or pieces of thick wire. The diameter of the reflector elements is not critical and can be 3-10 mm. An antenna with a flat reflector has a gain of 8-10 dB. The gain factor can be increased to 15 dB (equivalent to a 40-element “wave channel” antenna) using a complex “dilapidated box” type reflector (Fig. 5c). The design of such a reflector can be very different, depending on your capabilities.

The spatial orientation of the antenna, shown in Fig. 5 corresponds to receiving signals with horizontal polarization. To receive vertically polarized signals, the blade and reflector must be rotated 90°.

The UHF amplifier is located in close proximity to the antenna (see Fig. 5). The amplifier input is connected to the antenna surface using the same cable that is embedded in the amplifier. The amplifier input cable is extended with a reduction cable. It is desirable that it be as large in diameter as possible (losses in the cable depend on this); a cable with a diameter of 4 mm can be used only if its length does not exceed 10 m.

Cable connections should be made “vetically”, so that the coaxial structure of the feeder is minimally disrupted.

If it is not possible to produce the antenna described, then the amplifier can be used with slightly worse results with industrial outdoor broadband UHF antennas, for example, type ATNG(V)-5.2.21-41 (trade name “GAMMA-1”).

Antenna installation is determined by what type of UHF transmission you are counting on. If it is necessary to receive reception directly outside the service area of ​​the television center (60-200 km), then the antenna should be installed so that in the direction of arrival of the signals there are no obstacles between it and the horizon line (houses, hills, etc.). If you are focusing on ultra-long-range reception with tropospheric or wave propagation (in this case, the signal comes “from the sky” at an angle of 5-10° to the horizon), then obstacles that are not very close are usually not an obstacle.

CONCLUSION

In conclusion, a few words about the practical results of taking UHF. An antenna with an amplifier manufactured according to the attached description was used for several years in Odessa for regular reception of signals from the Chisinau television center (distance - 160 km). Outside the city, in the radio shadow zone for the MB television center, signals from low-power UHF repeaters located on the opposite side of the Odessa Bay (distance - 60-80 km) are confidently received. On clear days in the spring and summer months, the Bulgarian BT2 program from Varna (distance - 500 km) and the Turkish TV2 program from Istanbul (distance more than 600 km) are received with good quality.

It was already noted above that installing an antenna amplifier near the TV between the feeder and the antenna input of the television receiver increases the gain of the receiving path, i.e. improves the sensitivity limited by the gain.

It has been shown that when using modern televisions, this method does not lead to improved images in long-distance reception conditions, since it requires an improvement in sensitivity, which is limited not by gain, but by noise. The antenna amplifier, having approximately the same noise level as a television receiver, does not improve noise-limited sensitivity.

Nevertheless, the use of an antenna amplifier in some cases can improve reception, but for this it should 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 the immediate vicinity of the antenna. What's the difference?

The fact is that the signal, passing to the feeder, undergoes attenuation and 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 through which the transmission is received.

When an antenna amplifier is installed near the TV, its input receives a signal that has already been weakened by passing through the feeder, and the ratio of the signal level to the noise level 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 weakened, but by the same amount. noise is also reduced. 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. The specific attenuation of a coaxial cable is usually called the attenuation that a signal of a certain frequency experiences when 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. In Fig. 1 shows such curves for some brands of 75-ohm coaxial cable.

Using them, you can calculate the signal attenuation in a cable for a certain length, on any frequency channel in the meter or decimeter range. To do this, you need to multiply the specific attenuation value obtained from the figure by the length of the feeder, expressed in meters. The result is signal attenuation in decibels.

Rice. 1. Specific attenuation curves of coaxial cables.

The most common type of cable for the feeder is RK 75-4-11, its specific attenuation is 0.05...0.08 dB/m in the range of channels 1-5, 0.12...0.15 dB/m in the range of 6-12 channels and 0.25...0.37 dB/m in the range of 21-69 channels. Hence, with a feeder length of 20 m, the signal attenuation in the feeder on the 12th channel will be only 3 dB, which corresponds to a decrease in signal voltage by 1.41 times, and with a feeder length of 50 m, the attenuation on the 12th channel will be 7.5 dB (decrease I 2.38 times).

In the decimeter range, with a feeder length of 20 m, the attenuation will be equal to 5.0...7.4 dB V, depending on the channel number, which corresponds to a decrease in signal voltage1 by 3.78...2.34 times^, and with a length feeder 50 m - 12.5... 18.5 dB (signal reduction by 4.22...8.41 times).

Thus, with a feeder length of 50 m, given to channel 12, the signal passing through the feeder is reduced by more than half, and the signal-to-noise ratio at the TV input will also be reduced by more than half. If you install an antenna amplifier before the signal enters the feeder, at the same level of input noise of the antenna amplifier as the TV, the gain in signal-to-noise ratio will be more than doubled.

An even more significant gain will be obtained with a longer feeder length or when receiving a signal in the UHF range. The necessary and sufficient gain of the antenna amplifier must be equal to the signal attenuation in the feeder. There is no point in using antenna amplifiers with a gain greater than required.

Several types of antenna amplifiers are available. The most widely used antenna amplifiers for the meter range are the UTDI-1-Sh type (individual range television amplifier for frequencies 1-1II bands).

They are designed for all 12 channels" of the meter range and contain a built-in power supply from an AC mains voltage of 220 V. The design of the amplifier allows it to be installed on a mast near the antenna with power supply through the feeder without laying additional wires. The gain of the UTDI-1-Sh amplifier is no less than 12 dB (4 times the voltage), and its noise level is slightly lower than the noise floor of black-and-white and color television receivers.

If the UTDI-1-III amplifiers are band and are designed to amplify a television signal on any of the 12 channels of the meter range, then antenna amplifiers of the UTKTI type (individual channel transistor television amplifier) ​​are single-channel and are designed to amplify the signal of only one, very specific frequency channel of the meter range.

The channel number is indicated after the amplifier type designation. Thus, UTKTI-1 means that the amplifier is designed to amplify the signal on the first frequency channel, and UTKTI-8 is designed to amplify the signal on the eighth channel. Amplifiers of the UKTI type also have a built-in power supply from an alternating current network with a voltage of 220 V.

The gain of UTKTI-1 - UTKTI-5 is not less than 15 dB, and UTKTI-6 - UTKTI-12 is not less than 12 dB. The self-noise level of amplifiers of this type is somewhat lower than that of the UTDI-1-Sh type. The power consumed from the alternating current network UTDI-1-Sh does not exceed 7 W, and UTKTI - 4 W.

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

Previously, despite the production of industrial antenna amplifiers, in the magazines "Radio" and in the collections "To Help the Radio Amateur" there were a large number of descriptions and diagrams of antenna amplifiers for self-production. In recent years, such publications have become rare. So, in the collection “To help the radio amateur,” issue 101, p. 24-31 provides a very detailed description of a narrowband antenna amplifier with a tunable amplitude-frequency response by O. Prystaiko and Yu.

Pozdnyakova. The amplifier is tuned to one of the channels of the meter range using a tuning capacitor, the amplifier's 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 in the case when transmissions are received via one specific channel, since it is not possible to rebuild the amplifier installed on the mast.

Wideband antenna amplifier MV

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. In Fig. 2 shown circuit diagram of antenna amplifier, designed to amplify all 12 meter channels, developed by I. Nechaev.

Rice. 2. MV antenna amplifier circuit.

At a voltage of 12 V, the gain is 25 dB with a current consumption of 18 mA. The amplifier is assembled using low-noise transistors with a noise figure of about 3 dB. Back-to-back diodes connected at the input protect the amplifier transistors from damage by lightning discharges. Both cascades are assembled according to a common emitter circuit.

Capacitor C6 provides correction of the frequency response of the amplifier in the higher frequencies.

The amplifier output is connected to the feeder going to the TV. The central core of this part of the feeder supplies the amplifier with supply voltage through inductor N. Through the same inductor, a voltage of +12 V is supplied to the central conductor of the antenna socket of the TV. The signal from the antenna socket on the TV to the input of the channel selector must be supplied through an isolation capacitor with a capacity of 3000 pF.

The chokes are wound on ferrite cylindrical cores with a diameter of 3 mm and a length of 10 mm using PEL or PEV wire with a diameter of 0.2 mm turn to turn. Each inductor contains 20 turns. Before winding, the core must be wrapped in two layers of lavsan film, and after winding, the turns are secured with polystyrene varnish or enamel.

A more detailed description of the amplifier, a drawing of the printed circuit board and the placement of parts on it are given in the magazine "Radio", 1992, No. 6, p. 38-39.

Another antenna amplifier, designed for the UHF range 470...790 MHz (21...60 channels), was proposed by A. Komok. Its circuit diagram is shown in. rice. 3. The passband gain of this amplifier is 30dB when powered at 12V, and the current consumption does not exceed 12mA.

Rice. 3. UHF antenna amplifier circuit.

The high-pass filter coil L1 is wound with PEV-2 wire with a diameter of 0.8 mm and contains 2.5 turns.

Winding is carried out on a mandrel with a diameter of 4 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 unpackaged transistors in the amplifier, which require careful sealing.

We can also recommend the use of KT399A packaged transistors, which are more affordable and resistant to changes in climatic conditions. A detailed description of this amplifier is published in the magazine Radio Amateur 11, 1993, No. 5, p. 2.

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 not at the input of the television receiver, but at the input of the antenna amplifier. Therefore, when installing an antenna amplifier near an antenna, to obtain a certain sensitivity value limited by noise, a lower input signal level will be required than when installing it near a TV. Thus, it is possible to receive a weaker signal with better quality.

Application of antenna amplifier allows the deliberate use of feeders of such great length that, in the absence of an amplifier, would weaken 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 obscured 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 reliable reception with good image quality due to the fact that they are not covered by a local obstacle. In 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, a cable of the RK 75-4-11 brand at the frequency of the 12th channel creates an attenuation of 30 dB, which corresponds to a decrease in signal voltage by 31.6 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 operation of the TV.

If the gain of one amplifier is not enough, you can connect 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.

If the feeder length is very long and the signal needs to be amplified by more than 30 dB, when it is necessary to use two or more antenna amplifiers, in order to avoid overload or self-excitation, all amplifiers should not be installed in one place. Under these conditions, the first amplifier is installed at the antenna output, i.e., at the input of the feeder, and the subsequent ones are installed in the feeder gap at approximately equal distances 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. 1), certain conclusions can be drawn. Cables of brands RK 75-2-13 and RK 75-2-21 have a fairly high specific attenuation even in the meter wavelength range; they should not be used in the decimeter wavelength 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 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 (attenuation of the signal voltage by 6.3 times), then under the same conditions the RK 75-9 cable -13 introduces an attenuation of 9.5 dB (attenuation by 3 times), and RK 75-13-1.1 - 7.25 dB (attenuation by 2.3 times). Thus, a successful choice of cable brand for a feeder in the UHF range can increase the signal level at the TV input several times, even without using an antenna amplifier.

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

Nikitin V.A., Sokolov B.B., Shcherbakov V.B. - 100 and one antenna designs.

Make your own subwoofer filter

Making your own filter for a subwoofer is not as difficult as it seems at first glance. The decision to make it yourself does not come easily.
Sooner or later, all car audio enthusiasts become professionals and try in every way to improve the audio system. The simplest low-pass filter for a subwoofer and its manufacture will become one of the modernization solutions.

Purpose

Beyond the boundaries of the “native” band (effectively reproduced), the sound pressure coming from the speaker noticeably decreases and at the same time the level of distortion increases.
In this case, it is simply stupid to talk about some kind of sound quality and, therefore, in order to solve the problem, you have to use several speakers in the audio system (see).

This is the reality: this happens in both home acoustics and car audio. That's not news.

Typical speaker layouts in cars and the role of filters
Regarding car acoustics, I would like to highlight two typical schemes for constructing a sound system, which are probably familiar to everyone who is not very familiar with car audio.

• We are talking about the following schemes:

The most popular scheme involves three speakers. This is a woofer (aimed exclusively at the lows), a mid- and low-frequency speaker (midbass) and a tweeter responsible for high-frequency reproduction.

• Note. This circuit is used mostly by amateurs and can be found in any car where the acoustic circuit is properly used.

Note. Despite the significant differences, both schemes obey the same rule: each speaker is responsible for reproducing its own frequency band and does not affect others.

It is precisely in order not to violate this requirement that electrical filters are designed, the role of which is to isolate specific “native” frequencies and suppress “foreign” ones.

Filter types

• A notch filter is the exact opposite of a bandpass filter. Here, the band that the PF passes through without changes is suppressed, and bands outside this interval are enhanced;
• FINCH or infra-low frequency suppression filter stands apart. The principle of its operation is based on the suppression of high frequencies with a low cutoff rate (10-30Hz). The purpose of this filter is to directly protect the bass player.

Note. The combination of several filters is called a crossover in acoustics.

Options

In addition to the types of filters, it is customary to separate their parameters.
For example, a parameter such as order indicates the number of coils and capacitors (reactive elements):

• 1st order contains only one element;
• 2nd order two elements, etc.

Another, no less important indicator is the slope of the frequency response, which shows how sharply the filter suppresses “alien” signals.

For subwoofer

In principle, any filter, including this one, is a combination of several elements. These components have the property of selectively transmitting signals of certain frequencies.
It is customary to separate three popular schemes for this separator for the bass player.
They are presented below:

• The first scheme involves the simplest separator (which is not difficult to make with your own hands). It is designed as an adder and uses one transistor.
  Of course, serious sound quality cannot be achieved with such a simple filter, but due to its simplicity, it is perfect for amateurs and novice radio enthusiasts;

• The other two schemes are much more complex than the first. Elements built according to these circuits are placed between the signal output point and the input of the bass amplifier.

Whatever the separator, simple or complex, it must have the following technical characteristics.

A simple filter for a 2-way amplifier

This separator does not require any special setup and assembly is as easy as pie. It was performed using available op-amps.

Note. This filter circuit has one slight advantage over the others. It lies in the fact that when the low-frequency channel is overloaded, its distortions are well masked by the mid/high-frequency link and, therefore, the negative load on hearing is noticeably reduced.

Let's get started:

• We apply the input signal to the input of the operational amplifier MC1 (it performs the function of an active low-pass filter);
• We also feed the signal to the input of the MC2 amplifier (in this case, we are talking about a differential amplifier);
• We now apply the signal from the output of the low-pass filter MS1 to the input of MS2.

Note. Thus, in MS2, the low-frequency part of the signal (input) is subtracted from the spectrum, and the high-frequency part of the signal appears at the output.

• We provide the specified cutoff frequency of the low-pass filter, which will become the crossover frequency.

The process of making a filter with your own hands will require familiarization with the thematic video review. In addition, it will be useful to study detailed photos - materials, diagrams, other instructions and much more.
The cost of making and installing a filter yourself is minimal, because there are practically no expenses required.

Ph.D. ROSOV Andrey Valentinovich

(LLC "Technical Center ZHAiS")

Today you can find a fairly large number of different antenna amplifiers on sale. If you look at their passports, everything looks quite convincing, and most importantly, they claim quite good characteristics. However, when it comes to the practical use of these “toys”, there is either no effect, or vice versa - the use of an amplifier only worsens the quality of the television image. The fact is that the development of a truly high-quality antenna amplifier is a rather serious matter and requires the simultaneous solution of many problems: minimizing the noise figure, ensuring the required gain in the operating frequency band for a given frequency response unevenness, the required dynamic range of the input signal, high temperature stability (in the case of , if the amplifier is directly located on the antenna. And this is where it should be for normal and efficient operation), high manufacturability and repeatability of parameters, and many others.

So let's go back to the amplifier. In Fig. 1 shows its schematic diagram.

Rice. 1 Schematic diagram of the UHF antenna amplifier.

On elements C1, L1, C2 a third-order high-pass filter (HPF) is made, having a cutoff frequency of 360...400 MHz. This high-pass filter performs the following functions: it ensures matching of the input impedance of the amplifier stage on VT1 with the characteristic impedance of the antenna, reduces the effective noise bandwidth of the amplifier and largely eliminates the effect of “clogging” the amplifier with powerful stations operating in the meter wavelength range. The amplifier consists of three amplification stages made of microwave transistors VT1...VT3, connected according to a circuit with an OE. Stabilization of the operating modes of transistors for direct current is carried out through negative feedback (NFC) through resistors R1, R3, R5. This stabilization circuit allows the emitter terminals of the transistors to be directly grounded, which ensures a high stable gain of each stage. The load of each of the stages is the corresponding inductances (L2, L4, L6). The inductive nature of the load makes it possible to increase the cascade gain in the high-frequency region by compensating for the frequency dependence of the transistor transconductance. The high transmission coefficient of each stage is also achieved due to the elimination of negative feedback at high frequencies by installing blocking capacitors C4, C7, C10. The required amplitude-frequency response of the amplifier is formed by high-pass filter elements, inductances L2, L4, L6 and capacitors C5 and C8, which perform the function of coupling between stages. Capacitor C11 provides output matching.

The amplifier can be powered in two ways: either from a separate external power supply, or through a reduction cable from the corresponding supply voltages of the TV. The supply voltage must be within +8...16V. The amplification stages are directly powered from an external stabilizer with a voltage of +4.7V, made using a zener diode VD1 and a quenching resistor R7. All amplifier stages are isolated from each other via power circuits using filters L3C3, L5C5, as well as elements R2C4, R4C7, R6C10. All this allows us to ensure high stability of the main parameters of the amplifier under the influence of various destabilizing factors.

Diode VD2 prevents DC voltage from entering the input of the television receiver when using a separate power supply. The first stage of the amplifier (on transistor VT1) is optimized to minimize noise figure and its emitter current is 2...3 mA, which is achieved by appropriate selection of R1. The current consumption of the second and third cascades (on VT2 and VT3) is about 5...7 mA, which makes it possible to achieve maximum cascade gains. A typical amplifier frequency response is shown in Fig. 2.

Rice. 2 antenna amplifier frequency response

Structurally, the amplifier is made on a printed circuit board made of single-sided foil fiberglass laminate with dimensions of 48x60 mm (in microwave technology, standard sital substrates with the same dimensions were used) with a thickness of 1.5 mm. A distinctive feature of the printed circuit board is the installation of all attachments on it according to option U 1. b. (OST 4GO.010.030-81), i.e. from the side of the current-carrying tracks, which eliminates drilling holes in the board and increases the manufacturability of the amplifier as a whole in small-scale and mass production. High-frequency inductors are made by printing, which also makes it possible to improve the manufacturability of the amplifier and the stability of the parameters of these coils both within one amplifier and within a production batch. The developed amplifier topology allows you to completely get rid of tuning elements and achieve high repeatability of the main amplifier parameters from instance to instance. An amplifier assembled from known good parts immediately provides output characteristics after power is applied.

The circuit and topology of the amplifier allows the use of many microwave transistors (KT372, KT3115, etc.) that have the same pinout.

Rice. 3 PCB topology

Figure 3 shows the printed circuit board of the amplifier. The area marked in black is the tinned foil layer, white is the etched part. Board dimensions - 48x60mm. The printed circuit board in Fig. 3 is made on a scale of 1:1.

The arrangement of elements is shown in Fig. 4

Fig.4 Location of elements

The amplifier housing at home can be easily made from double-sided foil fiberglass laminate with a thickness of 1.5-2 mm.

In Fig. Figure 5 shows the appearance of such an amplifier (without the top cover).

Rice. 5 Appearance of the antenna amplifier. Rice. 6. Fragment of inductor L1

Now a little about the details. Resistors are the most affordable: either C2-33 or MLT-0.125. The only requirement is that during installation the resistor leads should be as short as possible. Blocking capacitors are preferably unframed ones (they take up less space. Well, if you don’t have them at hand, use the ones you have. Just make your conclusions shorter!). There are quite a wide variety of them now available. Capacitors C1, C2, C5, C8, C11 are high-frequency, and their capacitance must be exactly the same as indicated on the circuit diagram. Inductor L1 - 3-4 turns of PEV -1.0 wire. The inner diameter of the winding is 4 mm. Chokes L3, L5 - either standard type DM-0.1, for example, with an inductance of 50 μH, or 18-20 turns of PEV-0.1 wire with the same internal winding diameter as L1. After installation, you need to check the functionality of the amplifier (if you did everything correctly and used known-good radio components, then there will be no problems). To do this, it is necessary to measure the voltage drop across resistors R2, R4, R6, and then, using the well-known Ohm’s law, calculate the collector current of transistors VT1...VT3. If they correspond to the numbers indicated above, then everything is fine and you can safely solder the top cover to your amplifier, thereby ensuring its complete tightness.