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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Simple broadband antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas When receiving television programs, radio amateurs often encounter difficult conditions when signals come from different directions, with different polarization, etc. The author of the article published below solved this problem by fairly simple means, using sets of half-wave vibrators for the purpose and optimizing all sizes to a minimum SWR in the MMANA program known to readers of the magazine. The emergence of accessible computer programs for calculating antennas has made it possible to take a fresh look at their development and combine experimentation with software optimization. Moreover, as it turned out, even the simplest of tuned television antennas can be improved and optimized - a half-wave vibrator, with which, at first glance, there seems to be nothing to do. We are talking about adding one or more vibrators to it. Options for improvement are discussed below. Their development, production and adjustment were accompanied by optimization of parameters using the MMANA program known to readers of the magazine. The main task was this: to achieve an expansion of the bandwidth of a half-wave vibrator with ease of manufacture and a minimum of material costs. The family of antennas developed includes two-channel designs for different channels and band designs that cover several channels. Antennas consist of tuned vibrators located at a certain distance from each other. Only one of them (with a gap between the halves) is active, i.e. it is connected to the feeder. A schematic representation of a two-channel antenna is shown in Fig. 1. The range of meter wave (MB) channels is divided into LF (channels 1-5) and HF (channels 6-12) sub-bands. The LF channel is determined by vibrator L1, and the HF channel is determined by vibrator L2. The distance between the halves of the active vibrator is about 40 mm. The dimensions of the vibrators for various MB channels are shown in table. 1.
Vibrators are made from pipes with a diameter of 15 mm made of copper or aluminum alloys. You can also use rods or strips of the same material. The input impedance of the antenna is determined by the length of the vibrators, the distance between them and their diameter. So, to obtain an input resistance of 75 Ohms, with the dimensions indicated above, the distance between the vibrators must be 53 mm. To roughly recalculate the antenna parameters to a different input impedance or when changing the diameter of the vibrators, you can use the formula
where h is the distance between the vibrators; K is the input impedance of the antenna or the diameter of the vibrators in the same units of measurement. The formula is valid for pipe diameters from 7 to 20 mm and input resistance from 25 to 300 Ohms. For example, if the diameter of the vibrators is 10 mm, the distance between them
If an input impedance of 100 ohms is required, the distance between the vibrators
Dimensions can be refined by optimizing for the minimum SWR in the MMANA program. It should be noted that for antennas with a low-frequency vibrator tuned to the second channel, the radiation pattern will be distorted: it becomes three-lobe. This is due to the fact that some frequencies in the HF subband are close to multiples of the frequencies of this channel. However, the second channel is mainly used as an output channel in UHF-MB collective antenna converters. So the loss is small. By the way, with such an antenna it is easy to carry out experiments in a wide frequency range by making replaceable half-wave or telescopic vibrators. In this case, the active vibrator must be at least twice as long as the passive one. To balance antennas, you can use the so-called TDL - transformer on long lines. To make it, you will need a ferrite ring, preferably high-frequency, with an outer diameter of more than 10 mm. The line is made of two copper wires PEV-2 or PETV. They are folded parallel and a soft insulating tube is tightly placed over them. The diameter of the wires is chosen as large as possible, but not more than 1,7 mm, so that four to five turns of the line fit on the ring. The transformer is connected between the halves of the vibrator (the beginnings of the line wires are connected to them) and a coaxial cable with a characteristic impedance of 75 Ohms (it is connected to the ends of the line wires). You can also use baluns from previously produced indoor antennas. UHF antennas can also have a similar design. However, as calculations and experiments have shown, on UHF it is better to use a band antenna that covers almost all channels from 21 to 60. A band antenna differs from a two-channel antenna in that it consists of three vibrators. Its design is shown in Fig. 2, and possible sizes are presented in table. 2. The diameter of pipes for channels 6-12 MB can be in the range from 10 to 15 mm. For a diameter of 15 mm in the frequency range from 172 to 228 MHz, the SWR does not exceed 1,3, and for a diameter of 10 mm in the range of 170...230 MHz - no more than 1,55. For an antenna for channels 3-5 (75... 100 MHz) MB with a pipe diameter of 20 mm, SWR is no more than 1,5. With a decrease in diameter to 10 mm, the SWR can increase to 2.
On UHF, with a pipe diameter of 7 mm for the active vibrator and 4 mm for the remaining vibrators, the SWR of the antenna for channels 21-57 does not exceed 2. On high-frequency channels 57-60, the SWR increases quite sharply. By adding a few more elements, you can significantly improve the directional properties of the antennas considered. As an example, in Fig. Figure 3 shows the antenna for channels 4 and 10. The dimensions are listed in the table. 3. Pipe diameter - 10 mm. The antenna gain is 5,7 dB, the front/rear ratio is 9,8 dB at the center frequency of channel 4. The same for channel 10 - 6,9 and 12,8 dB, respectively.
For other channels, an antenna of this design can be designed using the MMANA program in the following order. 1. The dimensions of the low-frequency vibrator (active) are determined by the minimum SWR. 2. The same is done in terms of the size of the HF vibrator and the distance to the active one. 3. At a distance of about 0,1 LF wavelength behind the active vibrator, a reflector with a length slightly greater than the length of the active vibrator is placed, and the size of the reflector and the distance from the LF vibrator are optimized for the minimum SWR and maximum front/rear ratio. 4. At a distance of about 0,1 HF wavelength, place the director in front of the HF vibrator and the HF reflector behind the active vibrator. Their initial length is the length of the HF vibrator. 5. Optimize the dimensions of the director, vibrator and HF reflector, as well as the distances between them in terms of minimum SWR and maximum front/rear ratio. Let's consider one of the options for a multi-channel receiving antenna, the elements of which are made according to the method described above. A plastic pipe with a diameter of 40 mm was used as a mast. The antenna consists of three tiers. This is due to the fact that in Donetsk it is possible to receive signals from several channels from three directions: in one of them - the sixth channel, in the other - the fourth, tenth and UHF channels, in the third - the twelfth channel with vertical polarization. Active vibrators of the sixth and twelfth channels for different polarizations are combined into one structure, forming the first (lower) tier. Their location in the vertical plane and dimensions are shown in Fig. 4. The distance between the halves of the horizontal vibrator is 20 mm. The vertical vibrator is located inside the mast. The parameters of the antenna for the sixth channel can be improved by adding a director 700 mm long and a reflector 850 mm long at a distance from the active vibrator of 270 and 900 mm, respectively. Calculated parameters at a frequency of 178 MHz: SWR - 1,3; gain - 7 dB; front/rear ratio - 15 dB. At a frequency of 226 MHz, the SWR is 1,1. The second tier is a two-channel antenna tuned to the fourth and tenth channels. On top of the mast (third tier) there is a broadband UHF antenna with an amplifier, which is located inside the mast. The SWA-9 amplifier is used, in which the input transformer is switched to matching device mode. To do this, the middle input terminal (two conductors) of the transformer is disconnected from the common wire. The single-color input terminals (there are now four of them) of the transformer are interconnected and connected to the halves of the antenna vibrator. The antenna in question has been operating for about three years, providing a normal image on ten channels (four on MB and the rest on UHF). The development, of course, does not claim to be exceptionally new, but such an antenna turned out to be quite sufficient for reception in difficult conditions. Antenna files described in the article and calculated in this program Author: G.Alekhin, Donetsk, Ukraine
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