UHF antenna - per hour of work. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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UHF antenna - per hour of work. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Television antennas

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With the independent manufacture of the UHF antenna, you can use the simple design described here and get by with improvised materials.

The relatively good transmission of decimeter radio waves through reinforced concrete building structures makes it possible to receive television programs in this range on a simple indoor antenna and often with very good quality, even outside the direct line of sight of the transmitting antenna. The proposed double triangular antenna design can be made fairly quickly. The main materials are sheets of corrugated cardboard from packaging boxes, household aluminum foil of suitable sizes, as well as a coaxial cable with polyethylene insulation (only not fluoroplastic) and a characteristic impedance of 75 ohms.

The manufacture of the antenna begins with drawing the contours of its web on sheets of foil and cardboard according to Fig. 1 (sharp corners at the edges can be cut off a little, this is shown in the figure by dashed lines). Then, carefully cutting the antenna sheet from a sheet of foil, water-based glue (for example, Bustilat, PVA, etc.) is applied to the sheet and cardboard. Combining with the applied contour, glue them to each other. The canvas is cleaned of glue that has fallen on it, especially in places where cables and electrical connections are laid.

UHF antenna - per hour of work

While the glue dries, form the cable according to the laying diagram shown in fig. 1. To give the cable the required shape at the bends, it is carefully heated, followed by cooling. At the same time, they try to make the formed cable flat. Next, the braid and the central conductor of the cable are exposed at the points of electrical connections with the antenna sheet and, making sure that the glue does not get on them, glue the cable to the antenna sheet (and, if necessary, to the cardboard) with glue "88", "Moment "," Contact.

After the cable is glued, having previously wrapped its braid and the central conductor with foil strips for reliable contact, they are connected to the antenna web by sewing (three stitches for each connection). At the same time, so that the thread does not cut the cardboard, spacers made of wood or plastic are glued on the reverse side. The ends of the threads are also fixed with glue. The antenna stand can be glued from cardboard.

The dimensions of a double triangular antenna are calculated according to the formulas published in the book by G. I. Boriychuk and V. I. Bulych "A radio amateur about television antennas" (M .: DOSAAF, 1977):

L=0,27λmax (without screen);

L=0,25λmax (with screen);

Δ=0,09f;

m=0,72f;

∂=10 mm,

where λmax is the maximum length of the received carrier wave in millimeters.

So, for example, for the interval of television channels 21-49 with λmax=636,6 mm, the dimensions of the antenna without a screen turned out to be: L=172 mm, Δ=15,5 mm, m=124 mm, and antennas with a screen - L=159 mm , Δ=14,3 mm, m=115 mm.

The antenna has an overlap ratio of the specified interval of about 1,45 with KBV>0,48 and a gain of about 5 dB. With a further increase in the frequency of the received signal, the efficiency of the antenna decreases.

A reflector screen is an effective remedy for strong reflected signals (although, under the reception conditions under consideration, there is still no special need for it). A shield is made from a solid sheet of foil, which is glued to a piece of cardboard measuring 2L (width) x 1,75L (height) and fixed in parallel at a distance of 0,7L from the antenna web using U-shaped bent strips of cardboard. For the convenience of subsequent assembly, it is recommended that the dimensions of the cardboard sheet with the antenna be the same as those of the sheet with the screen.

If the signal level at the receiving location is low, it can be increased by combining several (N) antennas into a simple antenna array. For example, for four antennas (N=4) this is shown in Fig. 2. Such connections are especially convenient for antennas with an internal impedance close to the wave impedance of the feeder (in our case, 75 ohms). Polarities (+ and -) are shown symbolically to indicate the phase of the signal. In order to exclude the influence of the antennas of the array on one another, the distance between their webs must be at least 90 mm, and to ensure common phase when adding signals, the lengths of the cables from the antennas to the place of their connection must be equal.

UHF antenna - per hour of work

Before deciding to manufacture an antenna of complex design or before purchasing an expensive industrial antenna, it is recommended to make the simplest one with your own hands in order to assess in advance the possibilities of satisfactory reception in a particular case.

It should be noted that for an antenna array with N=4, the output voltage will be twice as high (+6 dB) as compared to a single antenna (N=1).

The desirability of using antenna arrays is also indicated by the fact that the use of antenna amplifiers to increase the level of the received signal in combination with modern televisions with high sensitivity (when the antenna input is closed, their own noise such as falling snow is visible on the screen) may not improve signal-to-noise ratio, since antenna amplifiers use low-noise transistors, similar to those used in TV channel selectors. In such cases, the noise will only look more contrast - that's all.

Of course, antenna arrays are more suitable for open spaces. In addition, for the conditions indicated by us, the probability of the arrival of an antiphase signal along with an in-phase signal is quite high, and it grows in proportion to the area of the grating. An effect opposite to addition is observed.

It is quite easy to determine the presence of an antiphase signal: it is enough to close the braid and the central conductor of the cable on the antenna under test with a short conductor. If the signal has increased (it is controlled by the image on the TV screen), the signal is in antiphase, if it has decreased, it is in phase. So a simple increase in the number of antennas may not improve reception.

For N=2, in order to match the impedances of the antennas (150 ohms) and the TV input (75 ohms), you can use the widely used method of broadband switching through various LC filters. On fig. 3 shows a diagram of such matching by an RF filter. The matching band is 410 ... 914 MHz, the frequency response unevenness is 0,044 dB. In addition to matching, the filter reduces the MB signals.

UHF antenna - per hour of work

The calculated values of the parameters of the filter elements are as follows: C1=2,32 pF, C2=4,33 pF, L1=48,7 nH, L2=26,1 nH. They are given in three significant figures for reference only - an accuracy of ± 5 ... 10% of those indicated is quite sufficient.

Coils L1 and L2 are frameless. Their structures are shown in Fig. 4. They have 4 and 2 turns, respectively, with inner diameters of 4,8 and 3 mm. Winding pitch - 2,4 mm, wire - PETV-2 or similar with a copper diameter of 0,8 mm.

UHF antenna - per hour of work

Tubular capacitors - KT-2. If there are no capacitors of suitable capacity, you can easily make them yourself. Taking a capacitor with a slightly larger than necessary capacity, it is reduced with a razor to the required one by evenly removing the outer metallization layer (the diameter of the capacitor in this place is slightly larger), then polished with a piece of emery stone. The capacitance of the capacitor is measured in an already molded form, not forgetting to compensate for the additional capacitance of the supply conductors.

UHF antenna - per hour of work

Foil fiberglass - always one-sided! The cable braid is tinned and soldered along the entire length of the printed conductor. The dimensions of the printed conductors should be more precisely observed, since their own inductance was taken into account when calculating the dimensions of the coils.

If it is necessary to increase N to 4 (300/75 ohm matching), the left side of the printed circuit board is changed to connect four cables as shown in fig. 6. Such inclusion (using a filter) is sometimes necessary due to the interfering effect of too powerful MB transmitters on the UHF selector. You can place the matching filter both directly at the TV, and at the junction point of the lattice cables according to fig. 2. In both cases, it is necessary to connect the antenna cables to the filter in accordance with the phasing shown in fig. 2 and 6 (the antenna cables of the right side of the array are crossed).

UHF antenna - per hour of work

The values of the calculated ratings of the filter elements in this case: L1=66 nH, L2=31,6 nH, C1=1,41 pF, C2=2,95 pF. The inner diameters of the coils L1 and L2 are 6,1 and 4,3 mm, respectively, otherwise they do not differ from those described above. The filter matching frequency band is 410 ... 897 MHz, the frequency response unevenness is 0,18 dB.

To a TV with one antenna input, the UHF antenna is connected through the MB and UHF signal adder. Recommended domestic - STS-1 or similar. It has an isolation between the MB and UHF channels of at least 20 dB and a good frequency response in the passbands. Its cost is quite low, so making such an adder on your own does not make much sense.

When forming the cable, great care must be taken, since when heated and kinked, its center conductor can easily close with the braid. In addition, it must be borne in mind that the wave impedance of the cable also changes with all the ensuing consequences. It is best to skip this process if possible.

Author: V.Mikhailov, Moscow

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