TV signal dividers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television, video equipment

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Nowadays, it is not uncommon for a family to have two or more televisions, each of which must be connected to an outdoor antenna. In the presence of a television antenna for collective use, the easiest way is to make an order for connecting the second or third subscriber outlet from a standard branching box installed in the stairwell. This eliminates the mutual influence between the TVs, ensures good matching of the input impedance of the TV receiver with the feeder and does not reduce the signal level at the inputs of the TVs. However, this requires a significant consumption of coaxial cable. If an individual outdoor antenna is used, then there is no junction box. However, the junction box can be made by yourself.

On fig. 8.1, a shows a schematic diagram of a branching box for two taps from the subscriber cable of the collective antenna, and in fig. 8.1, b - into three outlets.

The box is installed in a convenient place in the apartment, its input is connected to an existing cable from the splitter box of the collective antenna, and the outputs are connected by television cables to TVs. One TV is connected to each output. If for some reason the TV is not connected to one of the outputs (for example, it was taken to the country house or sent for repair), you need to connect a 75 Ohm resistor to the plug designed to connect to the antenna jack of the TV. Otherwise, a mismatch will occur, and the images on the screens of other TVs will begin to double. For the same reason, you should not make extra taps in the box, and if there are any, you need to connect a 75 ohm resistor to each of them.

As can be seen from the diagram, a two-output box contains three resistors, a three-output box contains four resistors. The number of resistors will always be one more than the number of outputs. All resistors in the box have the same resistance, which depends on the number of outputs and is determined by the formula:

where n is the number of branching box outputs. The exact values ​​of the resistances of the resistors found using this formula should not be selected, you can take the nearest standard value: 24 ohms for two outputs, 36 ohms for three, 43 ohms for four, etc.

The design of the splitter box is quite simple. The body of the box is made of sheet copper, brass or even tinplate from a can. The braids of all cables are soldered to the box body, and the central cores are soldered to the resistors. Resistors can be of the OMLT type with a power of 0 W. The box is closed with a lid made of the same metal, which is soldered to the body of the box at several points. After that, the box is attached to the wall.

The disadvantage of this method of signal branching is that the signal voltage at the input of each TV is reduced by the same amount as the number of devices connected to the box, due to signal energy losses in the box resistors. Typically, public antennas provide a sufficient signal level on the subscriber taps of the stair splitter box of the order of 1 mV. Therefore, when such a signal voltage is reduced by several times, the junction box remains at a sufficient level to obtain a good image on the TV screen.

The question often arises: why do we need resistors in the box, leading to a decrease in the signal level at its outputs. If, for example, the antenna jacks of three TVs were directly connected to the feeder, it would be very mismatched with the load: after all, the input impedance of each TV at the antenna input is 75 Ohms, the input impedance of three connected in parallel is 25 Ohms, or three times less than the resistance required for feeder approval. As a result, a significant part of the signal energy will be reflected back into the feeder and will not reach the TV input. Moreover, if the feeder is inaccurately matched with the antenna, which is usually inevitable, the reflected signal will again be reflected from the antenna and arrive at the TV inputs with a delay, which will lead to doubling and multi-contour images. If there are resistors in the box, it is easy to calculate that with any number of taps with TVs connected to them, the feeder remains consistent with the load.

It also happens that when using an individual antenna, one TV connected to it works well, but when a junction box with two or three TVs is connected to it, the signal level at their inputs is already insufficient, the image contrast on the screens becomes weak and noises are visible. In such conditions, an antenna amplifier should be connected between the antenna feeder and the junction box. It should be recalled that in the event that the signal level is insufficient even for the operation of one TV, a more efficient antenna must be used, and with a long feeder, the antenna amplifier must be installed on a mast near the antenna.

Another version of the junction box, similar to those used in public antenna networks, is shown in Fig. 8.2.

This box can be used to connect two TVs to an individual outdoor antenna. A box of this type ensures the matching of the feeder with the load, regardless of whether TVs are connected to both outputs, and also slightly attenuates the signal at the outputs relative to the signal level at the input of the box. The box contains two identical coils of copper bare silver-plated wire with a diameter of 1 mm. Coils are wound on a mandrel with a diameter of 4 mm and contain 3 turns. After winding, the coils are removed from the mandrel and stretched into a spiral with a step of 2 mm (the distance between the turns is equal to the diameter of the wire).

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