TV antenna switch with power supply and cable control. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television antennas

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When viewers use several television antennas that receive signals from different directions on different bands and channels, the task of transmitting them without loss to a TV set with the best quality appears. How to solve it, and the author of the published article tells.

The problem of high-quality reception of television programs has always been of concern to viewers, and at present it has not lost its relevance, but has acquired new aspects. Thus, in most cities television broadcasting is no longer limited to one or two channels. In connection with such a rapid growth in their number, some new features in the use of antennas also arise.

Since each television channel operates in its own band, the average frequencies of which can vary significantly, it is necessary to use several antennas for different ranges. Often TV signals come from different directions, and not only in rural areas, but also in cities. First, due to the fact that transmitters are sometimes located in different parts of the city or in different cities for rural areas. Secondly, sometimes the reflected signal is stronger than the direct signal (for example, if there is a large obstacle in the path of the direct signal). There are also multiple re-reflections of the signal. Therefore, the antennas need to be spatially oriented in different directions.

In addition, television signals from TV stations may have different polarizations. Therefore, the antennas must be placed in the appropriate planes (horizontal or vertical). In addition, the transmitters used often differ significantly in power (tens of times) or are located at significantly different distances.

These reasons lead to the need to use several antennas, each of which, in the ideal case, is tuned only to its own channel. In such conditions, indoor antennas often do not provide an acceptable reception quality for all channels, especially for residents of the lower floors of multi-storey buildings with dense urban development.

In turn, the problem arises of delivering the signals received by the antennas to the TV, which can be solved in several ways.

Firstly, they lay their own drop cable from each antenna and manually switch the plugs at the TV jack. This option is not economically profitable (a lot of cable is required) and inconvenient (frequent switching of plugs leads, in the end, to failure of the socket).

Secondly, crossover filters or adders are used, working on one drop cable. However, as the number of antennas increases, these devices become much more complex. In addition, the loss of the useful signal increases noticeably.

Thirdly, they switch the antenna cables with remotely controlled devices located in their immediate vicinity (on the roof), and send a signal through one drop cable. This method can be implemented in several ways. One of them is the use of electromechanical relays. However, with an increase in the number of antennas, the influence of the capacitance of switching contacts also increases. In addition, the relays consume quite a lot of current. Another way is to use an electronic switching device that does not have these drawbacks.

A variant of a relatively simple signal switcher for eight television antennas, fed and remotely controlled via a single drop cable, was assembled according to the circuit diagram shown in Fig. 1.

The basis of the device is the K561IE9 (DD1) microcircuit, which is a counter with a decoder for eight outputs. When the power is turned on with the SA1 toggle switch, through it, the SB1 button, the L11 choke located near the TV, the drop cable, the L10 choke and the VD1 diode, the capacitor C4 is charged, and the supply voltage is supplied to the microcircuit. At the first moment, the supply voltage passes through the capacitor C2 to the reset input R of the DD1 microcircuit and sets the counter to the initial (zero) state. At the same time, level 0 appears at its output 2 (pin 1), and level 1 appears at outputs 7 - 0. Transistor VT1 of the emitter follower of the first of the eight transistor-diode cells will be open, and transistors VT2-VT8 of other cells will be closed. An opening voltage is applied to the diode VD3, a current flows through it, the inductor L9 and the resistor R11. The closing voltage is supplied to the diodes VD4 - VD10.

In the open state, the diodes have a low resistance, equal to units of ohms, and in the closed state, they have high resistance and low capacitance, not exceeding units of picofarads. As a result, through the capacitor C5, the open diode VD3, the capacitors C14, C15 and the reduction cable, the high-frequency signal from the WA1 antenna comes to the input of the TV. Capacitors C5, C14, C15 are used for DC decoupling, and chokes L1 - L11 are used for high-frequency signal decoupling.

When you press the SB1 button, the supply voltage to the switch and, therefore, to the collectors of transistors VT1 - VT8 is not supplied. In this case, the VD1 diode does not allow the capacitor C4 to discharge through the button connecting the switch power circuit to the common wire. In this state, the discharge current of the capacitor C4 is determined by the consumption current of the microcircuit and the current through the emitter junction of one of the transistors (in our case, VT1). At the input CN counter DD1 is formed by the front of a negative pulse.

When the SB1 button is released, the supply voltage will again be supplied to the switch and a negative pulse will drop at the CN input of the counter, which causes a change in its state. Now, output 1 (pin 1) will have level 1, and outputs 0, 2 - 7 will have level 0. The WA2 antenna is connected to the TV input, and the rest of the antennas are turned off. Capacitors C1, C13 eliminate the influence of the "bounce" of the contacts of the button SB1.

Therefore, by briefly pressing the button, you can remotely control the connection of antennas around the ring.

In the event that it is required to switch not 8, but 10 antennas, the K561IE8 microcircuit is used, increasing the number of transistor-diode cells accordingly. If there are less than eight antennas, for example, five, the output of the microcircuit, the number of which corresponds to the number of antennas (in our example, the fifth), is connected through the node, the diagram of which is shown in Fig. 1 at the bottom right, and unnecessary transistor-diode cells are removed (from outputs 5 - 7). Then, when level 1 appears at output 5, the counter is set to the initial (zero) state and the WA1 antenna will be connected.

The device is placed in a metal case (the author used a metal can with a diameter of 70 and a height of 23 mm). Parts C5-C14, L1 - L10, VD3-VD10 are hinged mounted, the rest are mounted on a small printed circuit board, the drawing of which is not presented due to simplicity. It is desirable that the leads of the elements through which the high-frequency signal passes are of the shortest length.

The device uses MLT-0,125 resistors, capacitors KD-1 or KT-1 (C5 - C12, C14, C15), K50-16 or imported (C4) and K10-17 (C1 - C3, C13). Chokes L1 - L11 - DM-0,1.

Diodes VD1, VD2 - KD521 with any letter index or similar, VD3 - VD10 - diodes with the lowest junction capacitance in the closed state, for example, except for that indicated on the diagram, KD420A, KD407A, KD409A, KD413A, KD514A or KA517A (having open resistance 1 ... 2 Ohm and having a capacitance in the closed state in fractions of picofarads). Transistors VT1 - VT8 - KT315 with any letter index. These microcircuits can be replaced with similar ones from other series, for example, 564.

If the drop cable is long and the signal is strongly attenuated, an industrial-made broadband RF amplifier common to all antennas or an amplifier assembled according to one of the schemes described in the magazine is added to the device. You should select an amplifier with a supply voltage close to the nominal one for the switch (10 V ± 10%), although for K561 series microcircuits, a supply voltage of 3 ... 15 V is allowed. The connection point of the amplifier in the switch is marked with a cross. The amplifier is turned on according to the diagram in fig. 2 in a chain break.

Diode VD11 and capacitor C16 are optional, they are used for a more gentle operation of the amplifier when manipulating the SB1 button. In addition, the amplifier can be used both for any one antenna and for each antenna. The connection is shown in fig. 3.

It is possible to switch a larger number of antennas, increasing the number of counters accordingly, but the device will be more complex. It should also be borne in mind that with an increase in their number, the influence of the total capacitance of closed diodes increases.

An increase in the number of switched antennas is also possible due to the use of crossover filters and adders switched on at the inputs of the switch. For example, this is how you can connect antennas of the MB (WA1a) and DM V (WA1b) bands according to the diagram in fig. 4.

In addition, instead of signals from television antennas, cable television signals or signals from antennas of VHF broadcasting bands can also be fed to the switch.

Such a switching system can be supplemented with an LED indicator of the number or name of the included antenna. It is mounted in the immediate vicinity of the control button. It works on the same principle as the antenna switch. The schematic diagram of the indicator is shown in fig. 5. It switches synchronously with the switch.

With a smaller number of antennas (less than eight), the indicator is changed in the same way as the switch.

Author: O.Bobrov, Voronezh

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