Multi-storey TV antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television antennas

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To make a good television antenna, you need to determine the dimensions of its elements and choose a design. The dimensions are simple - they are selected according to the reference book. Finding a description of a suitable design is quite difficult, because you want something that is not very laborious. The simplest antenna design is made of wire frames, which are bent from one piece of wire and soldered. To increase efficiency, you can make it into several floors (Fig. 1).

Fig.1. Wire Frame TV Antenna

If a decimeter antenna consists of only three quarter-wave frames and a carrier boom made of copper wire with a diameter of 2,5 mm, then it turns out to be quite rigid. The TV cable goes straight to the TV and does not bend the carrier boom. But such an antenna receives weakly, because the room where the experiments were carried out is located on the ground floor and is closed from the signal source by multi-storey buildings. Only a weak reflected signal penetrates through the barriers.

I had to increase the number of frames to six, soldering 3 more in front, the same as the first one. The seventh frame no longer increased the gain, but reduced it. Apparently, the dimensions of the antenna began to go beyond the antinode zone of the signal. The use of the same wire and fastening of the elements by soldering simplified the experiments. After all, it is easy to bend additional frames and replace the carrier boom using a longer piece of the same wire.

When the final length of the antenna was obtained, the cantilevered wire carrier boom AB easily bent down under the weight of the frames and cable. I had to solder the AC extension. During the experiments, the frames were shifted one relative to the other from bottom to top. Reception improved unexpectedly. It also turned out to be optimal to mount the carrier boom at an angle to the upright (the AED angle is ~60°). The frame planes remained vertical. AC stretching is needed regardless of the size of this angle.

Then two more of the same antenna sheets were made, and a multi-storey antenna was built. However, an increase in the gain was observed only at a small distance between them (half the frame length). It seems that a greater distance brought the antenna vertically beyond the size of the standing wave zone. Supports AF and GH are used to prevent the KD and GC carrier arms from bending. They are zero potential, so no current flows through them, and they do not interfere with reception. Now all antenna sheets are based on the triangle ABC. In order not to bend the vertical support rod 4, twisted from the same wires as the frame, it is curved in a meander and put on a metal pin 5 with loops 6 formed by pushing the wires of the rod 4 with a screwdriver. The support booms KD, GC, AB are soldered in the corners B, C, D rods 4, where it is more convenient to solder.

The antenna is held on the pin 5 with the help of two nuts 7. For them, a thread is cut in the upper part of the pin 5. The distance from rod 4 and pin 5 to the reflector frames does not affect the signal amplification. Reception does not deteriorate even when the matching couplers 8, 9,10 are bent from the vibrator frames 11, 2, 12. So, the matcher 10 is bent by an arc, the coupler 8 is directed into the gap between the reflector frames 13, 3, and the matcher 9 is located inside the reflector frame 3 GC antenna sheets. Such bends of the matching couplers from the frames-vibrators 11, 2, 12 are needed to position the pieces of the connecting cable 14, 15, 16, 17 closer to pin 5. The cable is screwed with pieces of wire 18, 19, 20, 21 to pin 5. Each frame is the vibrator (for example, 12) and its matcher (10) are bent from one piece of wire. Such a quarter-wavelength matching branch is soldered with its end to the cable braid at point M. The end of the cable segment 17 is connected by a braid to point L. The core of the cable 22 is soldered to the point N of the wire bend, where the vibrator 12 goes into the matcher 10.

To solder a thick wire to the cable sheath at point M, you must first remove the upper insulation from the cable in the form of a ring in this place. Then move the upper insulation together with the braid in the LM section towards the point M, where a braid protruding from the cable is formed. Then there is a place on the cable sheath where it is possible to solder the wire to the sheath if you use an adapter made of thinner wire.

Pin 5 on plywood stand 25 holds bar 24. Bracket 23, fixing the cable, fixes the position of frame 12 and coupler 10. Bar 24 and plywood 25 are fastened together with glue and screws. Pin 5 is inserted into a hole in bar 24. The dimensions of parts 24 and 25 depend on the dimensions of the antenna. They provide structural stability. The cable extending from the antenna is attached with a bracket 26 to the stand 25. Numerous cable fastenings prevent the antenna from rotating on the rod 5.

The lengths of the cable segments 14, 15, 16, 17 must be such that the current from the three antenna sheets is summed up and not subtracted. They are selected experimentally. For this, several pieces of cable with a length of 1/8, 1/4 and 1/2 wavelengths are used. Combinations are made of them so as to obtain a more contrasting image. The resulting length is averaged. In this way, antennas are selected for any television channel. The figure shows the dimensions for the antenna on the 37th channel, the frequency of which is 600 MHz, and the wavelength is 50 cm. The frames are with a quarter wavelength side. Supports HG and AF have a length of 6,5 cm. The height of pin 5 from the stand is 25 - 64 cm. The length of the LM cable is 12 cm (similarly for the other two matchers), the rest of the cable section is 5 ... 17 cm. Length PR cable - 22 cm. Cable segment 8 from the wire soldering point 14 to the cable connection point P - 8 cm. For the rigidity of the connection of cable segments, at point P, textolite strips are superimposed over the insulation and wound with electrical tape. At point R, the textolite lining has a T-shape. The entire antenna, except for pin 21, is soldered from copper wire d5 mm with a 2,5 W soldering iron.

For other television channels, the dimensions of the antenna must be changed in proportion to the change in wavelength.

Author: V. Solonin, Konotop; Publication: radioradar.net

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