Collective filter for TV antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas
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When receiving TV or VHF stations, a separate antenna is required for each of the frequency bands. By combining the signal from antennas located, for example, on the roof or in the attic, in a special filter, they can be connected from the antenna inputs to the receiver with one common cable. In practice, the simplest filter is most often used, having VHF and UHF inputs, as well as a common output. The VHF circuit uses a low-pass filter (high-cut), and the UHF circuit uses a high-pass filter (low-cut). In the past, manufacturers included such a small box with each TV receiver. The most important accessory of serious receiving stations is a collective filter with many inputs. It makes it possible to combine signals from individual high-power antennas, including VHF broadcast signals (CCIR). One of the possible solutions to this problem is shown in Fig.1.
The top input labeled "VHF", which is wideband, can be used for TV-dx, for example; you can connect a rotary antenna to it. The described filter is not designed to receive the "half-dead" OIRT-VHF band (66 MHz ... 73 MHz). If there is no need for a CCIR-VHF input, one can simply remove all L- and C-elements to the right of L9, while increasing the number of turns of L9. This will then be the VHFI input. The printed circuit board of the filter and the layout of parts on it are shown in Fig.2.
Coils are made of copper wire (preferably silver plated) with a diameter of 0,5 mm and have a diameter of 3 mm. L1 contains 2 turns; L2 - 5,5 turns; L3 - 3,5 turns; L4 - 6 turns; L5 - 4,5 turns; L6 - 4,5 turns; L7 - 7,5 turns; L8 - 4 turns; L9 - 4,5 turns; L10 - 11,5 turns; L11 - 15,5 turns; L12 - 6,5 turns; L13 - 10,5 turns. All parts are installed on the printed circuit board from the foil side. The frequency response from the inputs of the collective filter to its total output is shown in the graph.
As you can see, the bandwidth in relation to the nominal frequencies is quite large, so, most likely, there will be no need to adjust the coils. We also note that, as it is easy to verify from experience, the maximum attenuation at a frequency of about 50 MHz in the VHF radio band is due to the chain C7-C8-C9-L10 (one of the polar frequencies). The lowest point at about 85 MHz on the VHF I band curve (second pole frequency) is due to the chain L11-L12-L13-C10. The nominal impedance of the connection points for coaxial cables is 75 ohms. Inputs that are not used must be terminated with 75 ohm non-inductive resistors.
Literature
- Radiomir, 9 2001
Publication: cxem.net
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