Zigzag active UHF antennas. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television antennas

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To receive television signals in the UHF range, especially in unfavorable conditions, it is necessary to use good antennas with antenna amplifiers, i.e. active antennas. The author of the published article talks about the experience of building such antennas.

In the UHF range, the use of efficient antenna-feeder systems (AFS) for receiving signals in difficult conditions has not lost its relevance. The relatively short length λ of these waves makes it possible to create highly efficient antennas with relatively small sizes.

After lengthy experiments with different antennas, the well-known zigzag antenna [1], shown in Fig. 1, was taken as a basis. 180. Structurally, in its classic form, the antenna sheet consists of two identical diamond-shaped parts, rotated 2° relative to each other. Therefore, such an antenna is symmetrical. This feature allows the use of antenna amplifiers (AU) with a symmetrical input and high gain, for example, plate amplifiers (PAA) SWA, etc. [3, XNUMX].

The gain of a zigzag antenna depends on the l/λ ratio, and its input impedance depends on the l/d and l/λ ratios. The maximum gain is achieved at a length l = 0,375λ, but it strongly depends on the diameter of the wire.

At l = 0,25λ, the gain is, of course, less, but the dependence on the wire diameter also decreases.

When the angle α changes, the dimensions of the web change. So, if α = 90°, then SH = 2√2l = 2,83l; SE = l√2= 1,41l, and if α = 120°, then SH = 2l; SE = 1,73l. This must be taken into account when creating complex APIs (more on this later). The main dimensions of the antenna fabric, for example, for channel 29, are summarized in table. 1. It should also be borne in mind that as the diameter of the wire decreases and the perimeter of the fabric increases, the gain increases. In addition, when choosing a thinner wire, the windage of the antenna is reduced.

Different antenna designs have different input impedances (Table 1). Consequently, different methods are needed to match the symmetrical input of the canvas with the symmetrical input of the AU, which has an input impedance of 300 Ohms. They are shown in Fig. 2 [4].

With an input impedance of the web of 300 Ohms, the AU, of course, can be connected directly to points a - a. However, to increase the gain and directional action of the antenna, the canvas is usually used together with a reflector (which will be discussed below). Therefore, it is better to install the AC behind the reflector, connecting it to the canvas with a symmetrical line with a characteristic impedance of 300 Ohms, as shown in Fig. 2,a - for an overhead line, in Fig. 2,6 - for CATV cable or in Fig. 2,v - for cable RK-150. In the latter case, the braids of two cable sections are soldered to one another at the ends.

In all cases, it is necessary to take into account the line shortening coefficient K. For an overhead line made of wires (Fig. 2, a) - K = 0,975, for CATV (Fig. 2,6) - K = 0,8, for cable RK-150 (Fig. 2,c) - K = 0,75...0,86 depending on the type of cable.

It is most convenient (according to the author) to use a blade with an input impedance of 75 Ohms. In this case, for matching, you can use a quarter-wave matching transformer from a line with a characteristic impedance of 150 Ohms, as shown in Fig. 2, g. It is formed by two sections of cable RK-75 with a length of 0,25λKn, where n is an odd number. The K coefficient is 0,65789 for a cable with polyethylene insulation. The dimensions of the transformer are given based on the braids soldered at the ends.

The formula for calculating the transformer is known:

Ztr = √Zin Zout,

so it turns out

Ztr = √75 · 300 = 150 Ohm.

The open matching loop shown in Fig. 2, d, and a quarter-wave transformer (Fig. 2, f) make it possible to match the amplifier and antenna with an input impedance of less than 300 Ohms. To make a cable, use the graphs in [4]. Approximate coefficients for calculating the loop and parameters of the quarter-wave transformer are indicated in table. 2. The main requirement for the loop is Zl = Zsh = 300 Ohm. The dimensions of the cable and connecting line are related by the ratio A = B + C.

In Fig. Figure 2d shows a method for connecting a blade with Rin = 100 Ohm to an AC with Rin = 300 Ohm, with B = 0,13λK, and C = 0,09λK. For connection, use a symmetrical CATV cable (SLX-300) or an overhead line with a characteristic impedance of 300 Ohms. For the second case, the ratio (D/d) = 6,11. When using a wire with a diameter of 3,569 mm, the distance between the axes of the wires is D = 21,8 mm. To maintain a fixed distance between the wires, several transverse spacers made of high-quality insulating materials that do not deteriorate when exposed to the environment (fluoroplastic, polyethylene, organic glass) are placed along the line. It should be borne in mind that by moving the cable at points b - c and thereby changing size C, you can achieve a clearer image on the TV screen.

A quarter-wave transformer can be made from tubes with a diameter of more than 10 mm, as in Fig. 2, e. With a smaller diameter, the gap between the tubes will be very small, which will complicate the manufacture of the transformer.

Let's give an example of calculating the canvas for channel 29. With Fiz = 535,25 MHz we find λiz = 300/Fiz = 000 mm. If Rin = 560,48 Ohm and α = 75°, the side size of the diamond-shaped part (see Table 90) is equal to l = 1λ = 0,29 mm, α (l/d) = 162,5...32. Therefore, the diameter of the web wire is 75...2,1 mm. You can use strips 5,1d wide, i.e. 2...4,2 mm, made of copper or duralumin.

Note that in all subsequent figures the dimensions are given for the 29th channel. Converting to other channels is not difficult: knowing the ratio of the frequency of the 29th channel to the frequency of the channel being determined, the known dimensions are multiplied by this ratio.

Of course, the antenna fabric, in addition to the diamond-shaped parts, can be of other shapes, for example, a zigzag ring with solid metal sectors, as shown in Fig. 3.

Depending on the angle β, the blade has a different input resistance. For example, at β = 90° it is equal to Rin = 100 Ohm, and at β = 140° - Rin = 75 Ohm. This also determines different ways of matching the canvas with the AU. Thus, the blade at β = 90° is more broadband and is consistent with the train in accordance with Fig. 2, d. At β = 140°, the antenna will be more narrowband due to the need to use a quarter-wave matching transformer according to Fig. 2, g.

To make such a sheet, brass plates 0,3 mm thick are used. In order to reduce the windage of the canvas, 15-20 holes with a diameter of 5 mm are drilled in each sector with a uniform distribution over the area.

Cable dimensions for coordination according to Fig. 2, d are as follows: B = 60 mm, C = 40 mm, sections in - c of the CATV cable can be 224n mm long, where n = 1,2,3.... Quarter-wave transformer from RK-75 cable when coordinated according to Fig. . 2, g can have a length of 92,18n mm, where n = 1,3,5,7....

According to the table 1, you can choose any canvas from the 25 offered based on the availability of materials or other characteristics.

The directional pattern of the antenna sheet (without a reflector) is a two-lobe figure-of-eight, so the use of a reflector in all cases is advisable and effective, as it improves the directional properties and increases the antenna gain by about 3 dB with a reflector design similar to the sheet. However, a more effective way to increase the antenna gain by about 7 dB is to install a reflector array or a fine-mesh grid. The grille/mesh must be welded and have an anti-corrosion coating. The dimensions of the grid/mesh should be 5...10% larger than the vertical (Sн) and horizontal (SE) dimensions of the web.

The grating/mesh is placed at a distance h=100...50 mm behind the web, depending on the received channel (21-69). The value of h affects the input impedance of the web and can serve as an additional way to improve the matching of the entire APS. By changing h when placing the grille on the threaded rods, we achieve a clearer image with the least amount of noise ("snow") on the TV screen.

The use of a reflector array/mesh changes the antenna's radiation pattern, turning it into a narrow single-lobe. As a result, reception from the reflector is significantly weakened, which increases the noise immunity of the APS.

An even greater increase in the directional action and gain of the antenna can be achieved by using in-phase connection of two or more blades - in-phase arrays. This allows you to receive transmissions over long distances and in difficult conditions. Such antennas are several parallel-connected canvases, spaced horizontally and/or vertically in the same plane.

For example in Fig. Figure 4 shows the in-phase connection of two panels with an input impedance of 150 Ohms, spaced vertically. The canvas shown in the figure can be considered a modification of a zigzag ring antenna with an angle β = 0 or a type of ring antenna. The antenna works well in the UHF range with a wire diameter of only 1,5 mm.

The methods for matching such an antenna with the AU may be different. So, in Fig. Figure 4 shows an option for connecting two panels located at an optimal distance of 0,7λ vertically, with a power line connected to the lower panel (floor). For communication between floors, a two-wire line of length λK is used. The line is formed by two sections of RK-75 cable (K=0,65789). It is symmetrical and has a characteristic impedance of 150 Ohms, which ensures good matching with the canvas.

As a result of such a parallel connection of two identical canvases, the input resistance of the entire APS at points a - a1 is equal to 75 Ohms. Coordination with the AU is made using a quarter-wave matching transformer according to Fig. 2, g. formed by two sections of RK-75 cable.

However, another option is more preferable (in the opinion of the author) - central power supply. It has a wider bandwidth. Moreover, the canvases can be spaced both vertically and horizontally by (0,7...0,75)X between their centers.

To combine the canvases with central power supply, two series-connected symmetrical lines are connected between them according to Fig. 2, 0.5ХК in length (184,4 mm along the soldered braids at the ends), but formed by sections of RK-75 cable. In this case, at the central points in - in, the input impedance of the antenna is 75 Ohms. The same quarter-wave matching transformer is connected to them as in Fig. 4.

The canvases shown in Fig. are used in the same way. 1 with angle α = 120°. If such canvases are used with an angle α = 90°, then it is better to space them horizontally.

In-phase connection of three identical panels according to Fig. 1 with central power is shown in Fig. 5. The grille is equipped with a reflective grid. The input resistance of each blade is about 100 Ohms and depends slightly on the diameter of the wire. For testing, wires with a diameter of 1,2 [(l/d) = 117] and 2,76 [(l/d) = 51] mm were used. The dimensions of the connecting lines λK will remain the same if you use other fabrics with Rin = 100 Ohm (according to Fig. 1 at α = 120° or according to Fig. 3 at β = 90°).

(click to enlarge)

The canvases are connected to each other in parallel by symmetrical lines with a characteristic impedance of 100 Ohms, formed by sections of RK-50 cable with a length (along the soldered braids) equal to λK (this condition is mandatory!). At points c - c, the total input impedance of the antenna is 33,3 Ohms. Coordination with the AC is ensured by a quarter-wave transformer made from sections of RK-50 cable (according to Fig. 2d) with a length of 277 mm.

All canvases are fixed on a 5 mm thick organic glass strip. The strip is secured to the reflector and the mast with four threaded rods at points 0. The reflector grid (cells with dimensions of 18x18 mm) is removed from the antenna sheet at a distance of h = 105 mm, changeable by ±15 mm.

As mentioned above, the AU is installed behind the reflector on the mast and connected to the canvas at points c - c. The power supply unit (PSU) of the AC is placed next to the TV or on its rear wall as shown in Fig. 6.

A constant voltage of 12 V from the power supply is supplied via the reduction cable RK-75 through an isolating device (RU), connected in accordance with Fig. 7. The switchgear consists of inductor L1 and capacitor C2.

Typically, PAHs of types SWA, GPS, etc. are powered by low-power power supplies, which have different circuit designs, but most often are not protected from short circuits in the load. And such protection is necessary. In addition, if television signals are received from different directions, for example, on two antennas, then switching cables from the antennas at the TV input introduces a number of inconveniences, and the connectors quickly wear out. Therefore, it is advisable to provide for their automatic switching.

To eliminate these shortcomings, various power supply units have been developed. A schematic diagram of one of the power supply options using a relay for automatically switching antennas is shown in Fig. 8. Reception of strong UHF signals is ensured by antenna A1 without AC, connected to socket XW2, and the power supply is turned off in this case. To receive weak signals, antenna A2 (XW3) is connected to the AC, which occurs when the power supply is turned on.

(click to enlarge)

The power supply turns on when you press the SB1 button. In this case, relay K1 is activated and its contacts K1.1 block the SB1 button, keeping the power supply switched on. Contacts K1.2 disconnect antenna A1 and connect antenna A2 to the TV. The rectified voltage, indicated by the HL2 LED, passes from the PSU output to the AC.

In the event of a short circuit in the AC or feeder, the voltage at the power supply output and the current through the relay winding K1 will drop. The relay will release contacts K1.1, which will turn off the power supply. LED HL2 and lamp HL1 will go out.

Resistor R1 is selected so that, at a stabilized voltage of 12V, it ensures clear operation of the relay with a minimum current through its winding. The relay can be any, for example, RES47 (RF passport 4.500.409). Lamp HL1 (6,3 V x 0,28 A) indicates that the power supply is turned on via the network and at the same time serves as a fuse in the primary circuit of transformer T1. Transformer - any with a voltage on winding II - 9...11 V. Choke L1 - also any, for example, DM-0,6. The KR142EN8B microcircuit provides a maximum current of 1,5 A and has overcurrent protection. However, the power supply consumes no more than 0,1 A, so you can use a less powerful microcircuit, for example, 78L12.

To receive signals in the UHF range, several AU are considered in the journal, for example, [5]. All of them have an input impedance of 75 ohms. They can also be used with the described antennas with balanced input. To do this, you need to use a well-known matching balun (BDU) on a ferrite ring, connected according to the diagram in Fig. 9a. But you can install the SSU in the form of a U-loop according to Fig. 9, b. The cable going to the AC should be short and preferably 0.5λK long.

When choosing a location for installing the antenna, you must remember that every extra meter of reduction cable will weaken the signal in the UHF range by 0,16...0,4 dB. The thinner the cable, the greater the loss. During the final installation of the AFS, it is advisable to install a new cable, since by the end of its shelf life (it is defined as 12 years), the attenuation coefficient increases by 30...60%. It is better to choose a cable with a higher frequency, with a larger diameter of the central conductor. It is also necessary to ensure reliable waterproofing in the soldering areas.

Literature

1. Kharchenko K. Zigzag antenna. - Radio, 1961, No. 3; 1999, no. 8.
2. Pakhomov A. Antenna amplifiers SWA. - Radio, 1999, No. 1, p. 10-12.
3. Pakhomov A. New antenna amplifiers. - Radio, 2000, No. 7.
4. Rothammvl K. Antennas. - M.: Energy, 1969.
5. Nechaev I. UHF antenna amplifier on a microcircuit. - Radio, 1999, No. 4, p. eight.

Author: Yu.Filichev, Vilnius, Lithuania

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