Features of building a digital television network. DMV therapy: indications and contraindications

Modern market offers a huge range of antennas for reception terrestrial television. There are two main types of these products that allow you to receive meter and decimeter radio waves. They can also be divided according to the place of use into outdoor and indoor. Fundamentally, they are not much different. Here, first of all, the emphasis is on size and maintaining the necessary parameters under the influence of weather conditions. In this article we will discuss existing species of these products, let’s look at what parameters they have and how to conduct testing. And for those who like to tinker, we’ll tell you how to make decimeter antenna with your own hands.

What's the difference?

Let's try to explain in a nutshell how to determine what type of product is in front of you. The UHF antenna looks like a ladder. Install them parallel to the ground. Meter ones are crossed aluminum tubes. The appearance of both types is shown in the photo below. There are also combined antennas, when both the “ladder” and cross tubes are combined.

The problem of choice

It would seem that everything is simple. However, the buyer is faced with the question of how to choose the right device and what parameters to pay attention to. In general, it is best to test TV antennas directly in the conditions in which they will operate. The passage of a radio signal is often individual for a particular area. Thus, a product shows the same results in laboratory conditions, but completely different results in the field. There are certain tactics that allow you to test both meter and decimeter TV antennas. However, when choosing such a product in a store, we do not have the opportunity to conduct full testing. No seller will agree to give us several different antennas to test. In this case, you have to trust the characteristics of these products. And hope that the selected antenna will perform its functions according to the passport data, and not real conditions.

Basic parameters

A decimeter antenna is characterized primarily by its radiation pattern. The main parameters of this characteristic are the level of the side (auxiliary) lobes and the width of the main lobe. The width of the diagram is determined in the horizontal and vertical planes at a level of 0.707 from highest value. So, according to this parameter (the width of the main lobe), diagrams are usually divided into non-directional and directional. What does this mean? If the main lobe has a narrow shape, then the antenna (decimeter) is directional. The next important parameter is noise immunity. This characteristic primarily depends on the level of the back and side lobes of the diagram. It is determined by the ratio of the power released by the antenna, subject to a consistent load at the time of receiving a signal from the main direction, to the power (with the same load) when receiving from the side and rear directions. First of all, the shape of the diagram depends on the number of directors and the design of the antenna.

What does the term “wave channel” mean?

TV antennas of this type are very effective directional receivers of radio signals. They are widely used in areas of clearly weak television airwaves. The antenna (decimeter) of the “wave channel” type has high gain and has good directivity. In addition, these products have relatively small dimensions, which (on par with high level reinforcement) makes it very popular among residents of holiday villages and other settlements remote from the center. This antenna also has a second name - Uda-Yagi (named after the Japanese inventors who patented this device).

Operating principle

A decimeter antenna of the “wave channel” type is a set of elements: passive (reflector) and active (vibrator), as well as several directors, which are installed on a common boom. The principle of its operation is as follows. The vibrator has a certain length, it is located in the electromagnetic field of the radio signal and resonates at the frequency of the received signal. In it, an electromagnetic field is induced on each passive element, which also leads to the occurrence of EMF. As a result, they re-emit secondary electromagnetic fields. In turn, these fields induce additional EMF on the vibrator. Therefore, the dimensions of the passive elements, as well as their distances from the active vibrator, are chosen such that the EMF induced by them due to secondary fields is in phase with the main EMF, which is induced in it by the primary electromagnetic field. In this case, all EMFs are summed up, which increases the efficiency of the design compared to a single vibrator. Thus, even an ordinary room can provide stable signal reception.

The reflector (passive element) is installed behind the vibrator 0.15-0.2 λ 0. Its length should exceed the length of the active element by 5-15 percent. Such an antenna produces a one-way directional pattern in the vertical and horizontal planes. As a result, the reception of reflected signals and fields that come from the back of the antenna is significantly reduced. If necessary, receive a television signal on long distances, as well as in difficult conditions, if there is large quantity interference, it is recommended to use a three or more element antenna, which consists of an active vibrator, one or more directors and a reflector.

Direct and reflected signals

In an article devoted to a wave receiving device (“Tele-Sputnik” No. 11 for 1998), it was noted that in the case when the signal source is not a standard (that is, not laboratory) generator and emitting antenna, and the signal is broadcast TV tower, weather conditions play a significant role, as does the location where the receiver is installed. This especially affects the operation of UHF products. This is explained by the fact that in the decimeter range there is less, and accordingly, rounding of obstacles is much worse, and any reflections of the signal play a role important role as the received image. In particular, even the wall of a house can be a wave reflector. So, in conditions where there is no direct visibility, this property can be used - to receive the reflected signal. However, its quality will be lower than that of the direct one. If the level of the transmitted signal is high, but there is no line of sight, then you can use the reflected wave. In fact, an indoor decimeter antenna works precisely on this principle. After all, it is difficult to catch a direct wave in a room if the windows face reverse side. Therefore, if you try, you can always find a point where the received signal will be higher. But in the case of direct visibility, any reflected interference will spoil the received picture.

A technique that allows you to compare antenna parameters

In order to test receiving devices, they need to create the same conditions:

1. Select the installation location where your antenna will operate. You can use a balcony, roof or mast. The main thing is that both the height and the location are the same for all products.

2. The direction to the source of the broadcast signal should be maintained with an accuracy of three degrees. To do this, you can make a special mark on the mounting pipe.

3. Measurements should be carried out under the same weather conditions.

4. The cable connecting the antenna and the TV must have the same resistance and length. It is best to use one wire, changing only the receivers.

Testing should only be carried out on products of one type. For example, indoor antenna The UHF range should not be compared with outdoor or meter receivers. It should be understood that field tests may produce results that differ significantly from laboratory tests.

UHF antenna for digital television

Recently, the media have been increasingly talking about the need to switch to digital television. Many have already done this, and some are still thinking about it. So far, the signal is broadcast in both modes. However, the quality leaves much to be desired. In this regard, people are interested in what decimeter antennas can be used for T2. Let's look at this issue. Essentially, digital television broadcasts on a UHF channel. So a standard UHF antenna may be suitable for receiving it. You can often see receivers in stores that indicate that they are intended for digital television. However, this is a marketing ploy that allows you to sell a standard decimeter antenna for more than it costs. When purchasing such a product, you will not have a guarantee that it will provide better reception than what you already have in your home and has been working for more than one year. As we said earlier, the quality depends mainly on the level of the broadcast signal and line of sight conditions. However, it should be borne in mind that in most cities, significantly more powerful generators are used for transmitting digital television than for analogue. This is done in order to speed up the transition to the new standard. After all, the audience wants to see clear image, not “snow” on the screens. Therefore, if there is a receiver in the window on which it is written “UHF antenna for DVB T2”, know: this does not mean at all that this is some kind of special product. It’s just that a not entirely honest seller wants to profit from an uninformed buyer. You should also know that the transition program to the new standard provides for the creation of advisory centers. In them you can get comprehensive information on any issue related to digital television. All consultations are provided free of charge. In some cities, this equipment is in test mode, so the signal may be unstable or weakened. Don’t worry, the center staff will always tell you how to solve the problem with signal reception quality.

DIY decimeter antenna

The length of UHF waves falls within the range from 10 cm to 1 m. Their name comes from this feature. At this frequency they propagate predominantly in a straight line. They practically do not bend around obstacles and are only partially reflected by the troposphere. In this regard, long-distance communication in the UHF range is very difficult. Its radius does not exceed one hundred kilometers. Let's look at a couple of examples of how to make a decimeter antenna at home.

First option homemade receiver television broadcasting will, so to speak, be assembled on the knee from scrap materials. UHF channels are located in the range from 300 MHz to 3 GHz. Our task is to produce an antenna that will operate precisely at these frequencies. For this we need two 0.5 liter beer cans. If you use a larger capacity, the received frequency will decrease. For installation you will need some kind of frame; you can use a board 10 cm wide. You can also use a regular wooden hanger, in which case the resulting antenna can be hung on a nail in any convenient place in the room. In addition to the frame and cans, you need to prepare a couple of self-tapping screws, tools, coaxial cable, connector, terminals, insulating tape. We put a television connector on one end of the cable and solder it. We insert the second end into the terminal block. Next, we attach the terminals to the necks of the cans with screws. The wires should fit snugly to the metal. Now let's start assembling the antenna itself. To do this, we secure the jars on a horizontal crossbar with their necks facing towards each other. The distance between them should be 75 mm. You can use insulating tape to secure the cans. That's it, the antenna is ready! Now you need to find a place for stable reception TV signal and hang our “hanger” in this place.

Receiver for digital television

This section is intended for people who do not want to use a regular (analog) product, but want a special UHF antenna to be used for the new format. It is also easy to assemble such a receiving device with your own hands. To do this, we will need a square wooden (or plexiglass) frame with a diagonal of 200 mm and regular cable RK-75. The option presented to your attention is a zigzag antenna. It has proven itself well when working in the digital television reception range. Moreover, it can be used in places where there is no direct visibility to the signal source. If your broadcast is weak, you can connect an amplifier to it. So let's get to work. We strip the end of the cable by 20 mm. Next, we bend the wire into a square shape with a diagonal of 175 mm. We bend the end outward at an angle of 45 degrees, and bend the second stripped end to it. We connect the screens tightly. The stripped central core hangs freely in the air. On the opposite corner of the square, carefully remove the insulation and screen over a 200 mm area. This will be the top of our antenna. Now we connect the resulting square with a wooden frame. At the bottom, where the two ends are connected, copper staples made from thick wire should be used. This will ensure better electrical contact. That's all, the decimeter antenna for digital television is ready. If it will be installed outside, you can make a plastic case for it, which will protect the device from precipitation.

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In Fig. Figure 5.27 shows an example of a circuit diagram of the reactive frequency measurement method applied to the decimeter and centimeter wave range.  

The need for radio frequency cables has increased significantly due to the use of ultra-high frequencies - the range of decimeter and centimeter waves.  

Magnesium-zinc ferrites A-34, A-1331 and others and magnesium-manganese are designed to work in the decimeter and centimeter wavelength ranges.  

Using conventional vacuum tubes(diodes, triodes, pentodes, etc.) in the range of decimeter and centimeter waves is limited mainly by the inertia of electrons and the influence of distributed reactivity: interelectrode capacitances and input inductances. The main features of electronic devices in the microwave range are associated with eliminating the influence of the listed factors.  

Repeating the same frequencies at RRL intervals is permissible, since in the decimeter and centimeter wave ranges, in the absence of direct visibility between the antennas of radio relay stations located at three intervals, the signal attenuation, as a rule, is quite large. However, under certain conditions of radio propagation, for example, with increased refraction, it is possible to receive a signal from a station located three intervals away (bypassing two stations), which leads to significant distortion of the transmitted signals.  

Resonant wavemeters, due to their design and simplicity of reading, are the most convenient instruments for measuring wavelength in the range of decimeter and centimeter waves and, thanks to this, have been widely used in radio measuring technology.  

Ceramics are most widely used in the so-called metal-ceramic lamps, which were originally developed to operate specifically in the decimeter and centimeter wavelength ranges. In all cases, special ceramics with low dielectric losses and alloys or metals with a coefficient of thermal expansion close to the same coefficient for ceramics are used.  

Thus, conventional waveguides in the lower millimeter range and especially in the submillimeter range have almost the same disadvantages that are inherent in conventional long lines in the decimeter and centimeter wavelength ranges. The problem of transmission lines in the millimeter and submillimeter ranges, suitable for transmitting high powers and having low losses, has not been completely solved to date.  

Trunk radio transmitters are included in radio relay lines line of sight (RLS), space (satellite) communication lines and communication lines that use the phenomenon of scattering of radio waves by inhomogeneities in the troposphere - tropospheric lines. Such lines operate in the decimeter and centimeter wavelength ranges, providing multi-channel radiotelephone communications (with the possibility of secondary multiplexing of telephone channels with telegraph ones) and transmission of television programs. Transmitter-relays located on satellites have a power of about several tens of watts.  

The noise figure for the mixer is calculated if there are no amplification stages before it. This usually occurs in the decimeter and centimeter wavelength range. In the meter range, the mixing stage is usually preceded by an RF amplifier.  

The decelerating system along which the traveling wave moves is ring-shaped. It is used as a power amplifier in the range of decimeter and centimeter waves and as a self-exciting generator.  

Before moving on to the description of individual elements of the receiving path, we note that the devices used as UHF have already been discussed in the previous chapter. Of these devices, TWTs have become most widespread in the decimeter and centimeter wavelength ranges, and more recently - parametric and quantum mechanical amplifiers, which are also used at shorter waves.  

Content:

Electromagnetic waves are formed when electric field. And it changes when they move electric charges. In order for the electromagnetic field to be formed constantly, the change in charges must occur continuously. The most common movement of charges is movement in a circle. And in this case, the electromagnetic field becomes periodic, sinusoidal, and around it it will spread in the form of waves, like ripples on a water surface.

What dangles in the middle is usually called an oscillator, this is if you take a small material object and give it an oscillatory movement on the water surface. Then you will get something like this picture of the waves.

Even if you throw a stone into the water, that is, perform a single impact, not just one wave, but a whole package of waves will spread around. It follows from this that the very nature of the waves is precisely oscillatory, and this is how the waves spread around - attenuating, but without changing their oscillatory nature.

Properties of waves

When encountering waves of objects in our material world, several phenomena are observed at once:

• reflection of waves from obstacles;
• passing through an obstacle;
• wave absorption by the transmission medium;
• bending waves around obstacles.

The last phenomenon relates to the interaction of waves with each other. When waves meet other waves, they overlap and add and subtract. This is called wave interference.

But a wave can interfere not only with another wave - a wave from another source - it can also do the same with itself when some obstacle splits one wave into two streams. When passing an obstacle, the wave unites again and gradually “forgets” about the obstacle, when the bands of strengthening and weakening behind the obstacle fade and come to naught.

All these phenomena are inherent in all waves, both mechanical, such as on the surface of water or like acoustic waves in the air, and electromagnetic, penetrating both air and airless space.

Electromagnetic waves and us

We are accustomed to attributing to electromagnetic wave phenomena phenomena that are completely different for us and our perception. With our eyes we feel visible light, with our skin - heat from infrared radiation, our skin can tan almost without sensation from ultraviolet radiation, and we don’t feel x-rays at all, but it is their work that we see on an x-ray of our body, which they can take for us in a hospital . We know radio waves from the work of many different technical means.

The difference between them is very simple - these are all different wavelength ranges, or frequency ranges of emitters, which vary over a very wide range. The frequencies themselves are generated by the physical dimensions of the radiating bodies and the speeds of the electrical processes occurring in them. And the lengths of the resulting waves, when propagating, interact with the objects they encounter, also according to the principle of the proximity of the wavelengths to the physical dimensions of the obstacles. Of course, not only this. The material with which the wave encounters also affects the material of the environment and obstacles. Since the waves are electromagnetic, it is the electrical properties that play a role. More or less electrically inert media - dielectrics - interact weakly with electromagnetic waves, while other media that conduct electricity interact strongly. Hence, dielectrics are often transparent, but metals are all opaque and strongly reflect light, which is why they shine with a metallic sheen.

They actively reflect and absorb waves, and can also create secondary electrical phenomena within themselves. Our whole science of radio waves is based on this, as well as the technology of using radio, television, communications and all that other stuff.

Radio waves

It is enough to imagine that both processes are symmetrical: when waves are emitted and when they are captured and converted into an electrical signal. To emit waves, a source is used, and to receive, a receiver is used. And in both cases, an antenna is used, a material, geometric part of a radio device. When emitted, it imparts certain spatial properties to the wave, and in the case of a receiver, it “removes” the electromagnetic wave from space, forming a signal “ confident reception”, that is, such that it can be separated from the rest of the radiophone. Separate and strengthen.

In this case, the dimensions of the antennas or their parts are precisely dependent on the lengths of the received waves. Often antennas look like some compositions of conductors repeating in space. This is done for the resonant interaction of waves in them with the resulting alternating electric current, which is done to amplify the radio signal of specific wavelengths.
Another characteristic of an antenna is directivity. It either emits or receives signals predominantly from a certain direction, which also helps to isolate this particular signal from a specific emitting device.

Electromagnetic wave bands

In general, it is useful to imagine the entire spectrum of electromagnetic wave ranges and be able to compare the waves with objects of our material world.

The radio range is divided into several others according to wavelengths.

As we see, the ranges of radio waves precisely cover all of our everyday life, from distant stars to the person himself and his organs. And also all the items of our everyday life.

For example, would you like a hot sandwich? - one minute in the microwave.

But VHF is also divided into:

Each of these sub-bands is interesting in its own way, but we need decimeter waves.

Decimeter waves

Decimeter waves, unlike all others, work only in line of sight. They are not reflected by the ionosphere as short waves - the ionosphere is transparent to them; they do not bend around obstacles like long waves. The obstacles that they can bypass using their diffraction are comparable to our ordinary objects, that is, they will bypass a person or a stool, but it’s already difficult to bypass a house. But they are reflected from objects that are large for them and can enter, for example, through a window, reflected from a neighboring house. That is, they behave almost like people with hooligan tendencies. What is close and dear to us in its own way.

Self-production

To receive waves whose length is quite commensurate with the objects in our environment, the antenna will be such that it will fit into our environment. Consequently, in this regard, it is possible to manufacture not just an undoubtedly useful item, but even a detail that says a lot about the character and tastes of the owner. And which can often be called an architectural detail, and sometimes even a feng shui detail.

The DCM antenna is mounted on a vertical, usually wooden, base rail and consists of several metal parts.

In the direction of the expected passage of the waves, the UHF antenna extends a metal supporting plate, which is called a traverse.

Across it, that is, parallel to the wave front, several resonator plates are installed on it. One is usually active, the antenna wire is taken from it, and placed in the middle. The other two are placed one in front of it (in the direction of the emitter), the other after it. Which in front of him is called the director, his role is to create an obstacle to the wave, forcing it to bend around him, forcing the wave to create a diffraction pattern, that is, the wave to resonate with itself (see the figure at the beginning).

The plate that is placed after the active resonator is called a reflector, that is, a reflector. It reflects the wave back onto the active plate, also amplifying the signal. It is clear that such effects on the wave are possible with strict adherence to the dimensions of the plates, so that they correspond to the lengths of the received waves. The lengths of the plates are made to the size of a half-wave - 0.5 λ. The active element is equal to half a wave, the reflector is a little larger, the director is a little smaller. The distance between the resonators is a quarter wavelength, 0.25 λ.

Often you can see not three plates, but many. This suggests that waves can be received not of one length, but of several lengths. Such antennas are called “multi-wave” or even “all-wave”. But we know that waves are meant only in our decimeter range.

Such antennas can be designed and installed at your own pleasure, taking advantage of the fact that radio waves invisible to us create very intricate patterns of reflections, diffractions and interferences in space. And if you place vibrator plates at the peak points of the waves, you can achieve a good resonance, which will significantly enhance the signal. Using this principle, a log-periodic antenna is built, in which resonators on both sides - right and left - are alternately connected to two buses in a checkerboard pattern.

Two cable buses are connected to two rows of resonators in a staggered pattern

Homemade option

Using scrap materials, you can easily make an indoor antenna - UHF antenna T2. For example, from two computer floppy disks, if you remove the actual magnetic surfaces of the disks from the envelope, you can easily get a Cheburashka antenna - a sort of big-eyed creature, if you have a little imagination.

An external version of the Cheburashka is also possible, then it is worth thinking about a more durable fastening of all parts and cable.

In addition to floppy disks, you also need a stick-stand, a piece of cable and a few nails or screws.

Waves shorter than 10m are called VHF. Due to the straightness of VHF propagation for their use. requirements direct visibility of the transmitter and receiver antennas. VHF has almost no diffraction, they cannot bend. convexity of the earth's surface, and ionization of the ionosphere is insufficient. for their reflection. For communication over long distances. m/y communications stations install intermediate stations (repeaters) or raise antennas to great heights. Communication within the line of sight is characterized by the possibility of simultaneous arrival at the reception point of not only a direct wave, but also a wave reflected from the ground above. Interference leads to ↓ field strength at the reception point, but it can be reduced to min correct. selection of antenna heights, distances from them and wavelength. VHF is the most Radio band used by the site. The large frequency capacity of this range and the operating radius limited by the line of sight are allowed. place a large number of simultaneous stations and transmit information over a wide frequency band. VHF allowed simultaneously transmitting a large number of TV programs, organizing thousands of telephone channels and digital communications. VHF is used for radar, radio navigation, communication with artificial satellites, airwaves, TV and radio astronomy. Meter and decimeter waves are used for TV, radio and PC with moving objects. Centimeter waves are used for multichannel communication. Sometimes meter waves are used for communication outside the line of sight, because they are a way.to go around.small.obstacles on the ground. on top The range of such communication is calculated. km, less often tens of km. The most difficult communication is on meter waves in large cities, where relaying through the center is used. a station whose antenna is installed on a high-rise building.

There are cases of long-range propagation meters and more HF. This is explained by the possibility of the state. atmosphere in which change. refractive index as it goes up to a greater extent, than in standard conditions. The curvature of the radio beam trajectory due to refraction increases, becomes. possible propagation of radio waves ||-but the earth's surface. or hitting them after refraction on top of the ground (superfraction). The waves falling on the ground are reflected, spread upward, refracted again, etc. In space near the surface of the earth and refractive upper layers, along which waves propagate at a distance tens of times greater than the line of sight distance. This makes it possible to receive TV programs from other cities and countries. For the appearance of waveguide channels in the atmosphere, it is required. an increase in t 0 of air as it rises upward and a strong decrease in humidity with height.

Fluctuations in t 0 and humidity are constantly present in the troposphere. The refractive index of the air depended on them, and radio waves were scattered by the inhomogeneities of the ionosphere. This is a scattered field of observation far beyond the horizon. Small The field strength beyond the horizon is constant. Scattering of tropospheric waves by inhomogeneities is called far tropospheric propagation of radio waves. Creating troposphere communication lines is difficult, because The strength of the wave reflection field from the troposphere decreases very quickly with distance. Requires very powerful transmitters (1-50 kW), high directional antennas and highly sensitive receivers. Multi-channel communications are organized along troposphere communication lines. This connection does not require changing the wavelength during the day. Tropospheric communication lines compete with cable lines in hard-to-reach areas. Tropospheric stations form radio relay transmissions with an interval between stations of 300-500 km. The long-range propagation of VHF occurs due to their scattering on irregularities in the ionosphere. Scattering of events in the village. D or at the bottom of the s. E due to the heterogeneity of the electron concentration. Ionospheric communication lines are characterized by fading, seasonal and daily level changes. Signal distortion limits the spectrum width of transmitted signals to a band of several kHz, poet. TV and group signals multichannel.s-m cannot be transmitted over them.

Communication on meter waves due to ionospheric scattering allows operation around the clock at one frequency. Ionospheric scattering can be used to communicate with hard-to-reach areas. During periods of ionospheres. disturbances of heterogeneity in the lower regions of the ionosphere and ionosphere. Communication is improved.

Feeders and waveguides.

Electric chain and auxiliary devices using radio frequency energy. channel is supplied from the radioPRD to the antenna or from the antenna to the radioPR, called. feeder.

Feeders– these are power lines that transmit energy from the generator to the antenna (in transmitting mode) or from the antenna to the PR (in receiving mode). Basics The requirements for the feeder come down to its electrical tightness (no energy radiation from the feeder) and low heat losses. In the transmitting mode, the characteristic impedance of the feeder must be matched with the input impedance of the antenna (which ensures a traveling wave mode in the feeder) and with the output of the feeder (for maximum power output). In the receiving mode, matching the PR input with the characteristic impedance of the feeder is ensured in the last mode of the traveling wave, while matching wave resistance feeder with load resistance - the condition for the maximum transfer of power to the load PR. Depends from range radio waves used various types feeders: two or multi-wire air feeders; waveguides of rectangular, circular or elliptical sections; surface wave lines, etc. The design of the feeder depends on the range of frequencies transmitted through it. When transmitting el.mag. energy along the line tends to decrease. radiation from the line itself. For this wire, the lines are located. //-but also if possible. closer to each other. In this case, the fields of 2 are the same. in value, but oppositely directed currents are mutually compensated and energy radiation into the surrounding space does not occur. When creating an antenna, the opposite task is set: obtaining as much radiation as possible. For this use. the same long lines, eliminating one of the reasons depriving the feeder of radiating light. It is possible, for example, to move the line wires apart by some ے, as a result of which their fields will not compensate each other. This is what the slave is based on. V-shaped and rhombic antennas emitting cat wires. location under sharp ے one to the other, and a symmetrical vibrator, obtained by separating the wires by 180°. The compensating effect of one of the feeder wires can be eliminated by excluding it from the system. This leads to gain. asymmetrical vibrator. All antennas used This operating principle belongs to the asymmetrical class. antennas Also belonged to them. L-shaped and T-shaped antennas. A feeder radiates if adjacent sections of its two wires are flown by currents that are in phase, the fields of which reinforce each other. To do this, it is necessary to create a phase shift of half the wavelength, for example, due to a non-radiating loop. Common-mode antennas are based on this principle. The feeder will radiate if the distribution of m/u wires in some directions acquires meaning. stroke difference. You can choose the distance between the wires in such a way that in some directions the waves from both wires will be added. This is the use. in antiphase antennas.

Waveguide– artificial. or natural a channel capable of supporting waves propagating along it, the fields of which are concentrated inside the channel or in the area adjacent to it. Waveguide types:

1) Shielded. There are screens. waveguides with highly reflective walls, to the cat. include metal waveguides, electrical guides. waves, as well as coaxial and multi-core shielded. cables, although the latter are usually classified as transmission lines (long lines). To screened Waveguides also include acoustic waveguides with fairly rigid walls.

2) Unshielded. In open (unshielded) waveguides, field localization is usually due to the phenomenon of total internal. reflections from the interfaces of two media (in dielectric waveguides and simple light guides) or from areas with smoothly changing environmental parameters (ionospheric waveguide, atmospheric waveguide, underwater sound channel). Belongs to open waveguides. and s-we are on the surface. waves directed by the interfaces between media.

Basics sacred waveguide - beings. it contains a discrete (with not very strong absorption) set of normal waves (modes), propagating with their own phase and group velocities. Almost all mods are available. dispersion, i.e. their phase velocities depend on frequency and differ. on group velocities. In the screen waveguide phase velocities usually exceed. the speed of propagation of a plane homogeneous wave in the filling medium (speed of light, speed of sound), these waves are called. fast. If shielding is incomplete, they can leak through the walls of the waveguide, re-radiating into the surrounding space. These waves are called. leaking. In open waveguides the distribution slow waves, amplitudes cat. quickly decrease with distance from the guide channel.

Despite the rapid development of satellite and cable television, reception of terrestrial television broadcasting is still relevant, for example, for places of seasonal residence. It is not at all necessary to buy a finished product for this purpose; a home UHF antenna can be assembled with your own hands. Before moving on to considering the designs, we will briefly explain why this particular range of the television signal was chosen.

Why DMV?

There are two good reasons to choose designs of this type:

1. The thing is that most channels are broadcast in this range, since the design of repeaters is simplified, and this makes it possible to install larger number unattended low-power transmitters and thereby expand the coverage area.
2. This range is selected for digital broadcasting.

Indoor TV antenna “Rhombus”

This simple, but at the same time, reliable design was one of the most common in the heyday of on-air television broadcasting.

As can be seen from the sketch (B Fig. 1), the device is a simplified version of the classic zigzag (Z-design). To increase sensitivity, it is recommended to equip it with capacitive inserts (“1” and “2”), as well as a reflector (“A” in Fig. 1). If the signal level is quite acceptable, this is not necessary.

The material you can use is aluminum, copper, and brass tubes or strips 10-15 mm wide. If you plan to install the structure outdoors, it is better to abandon aluminum, since it is susceptible to corrosion. Capacitive inserts are made of foil, tin or metal mesh. After installation, they are soldered along the circuit.

The cable is laid as shown in the figure, namely: it did not have sharp bends and did not leave the side insert.

UHF antenna with amplifier

In places where a powerful relay tower is not located in relative proximity, you can raise the signal level to an acceptable value using an amplifier. Below is circuit diagram device that can be used with almost any antenna.

List of elements:

• Resistors: R1 – 150 kOhm; R2 – 1 kOhm; R3 – 680 Ohm; R4 – 75 kOhm.
• Capacitors: C1 – 3.3 pF; C2 – 15 pF; C3 – 6800 pF; C4, C5, C6 – 100 pF.
• Transistors: VT1, VT2 – GT311D (can be replaced with: KT3101, KT3115 and KT3132).

Inductance: L1 – is a frameless coil with a diameter of 4 mm, wound with copper wire Ø 0.8 mm (2.5 turns must be made); L2 and L3 are high-frequency chokes 25 µH and 100 µH, respectively.

If the circuit is assembled correctly, we will get an amplifier with the following characteristics:

• bandwidth from 470 to 790 MHz;
• gain and noise factors – 30 and 3 dB, respectively;
• the value of the output and input resistance of the device corresponds to the RG6 cable – 75 Ohm;
• the device consumes about 12-14 mA.

Let us pay attention to the method of power supply; it is carried out directly through the cable.

This amplifier can work with the simplest designs made from improvised means.

Indoor antenna made from beer cans

Despite the unusual design, it is quite functional, since it is a classic dipole, especially since the dimensions of a standard can are perfect for the arms of a UHF vibrator. If the device is installed in a room, then in this case it is not even necessary to coordinate with the cable, provided that it is not longer than two meters.

Designations:

• A - two cans with a volume of 500 mg (if you take tin and not aluminum, you can solder the cable instead of using self-tapping screws).
• B – places where the cable shielding is attached.
• C – central vein.
• D – place of attachment of the central core
• E – cable coming from the TV.

The arms of this exotic dipole must be mounted on a holder made of any insulating material. As such, you can use improvised things, for example, a plastic clothes hanger, a mop bar or a piece of wooden beam of the appropriate size. The distance between the shoulders is from 1 to 8 cm (selected empirically).

The main advantages of the design are fast production (10 - 20 minutes) and quite acceptable picture quality, provided there is sufficient signal power.

Making an antenna from copper wire

There is a design that is much simpler than the previous version, which only requires a piece of copper wire. We are talking about a framework loop antenna narrow range. This solution has undoubted advantages, since in addition to its main purpose, the device plays the role of a selective filter that reduces interference, which allows you to confidently receive a signal.

For this design, you need to calculate the length of the loop; to do this, you need to find out the frequency of the “digit” for your region. For example, in St. Petersburg it is broadcast on 586 and 666 MHz. The calculation formula will be as follows: L R = 300/f, where L R is the length of the loop (the result is presented in meters), and f is the average frequency range, for Peter this value will be 626 (the sum of 586 and 666 divided by 2). Now we calculate L R, 300/626 = 0.48, which means the length of the loop should be 48 centimeters.

If you take a thick RG-6 cable with braided foil, it can be used instead of copper wire to make a loop.

Now let's tell you how the structure is assembled:

• A piece of copper wire (or RG6 cable) with a length equal to L R is measured and cut.
• A loop of suitable diameter is folded, after which a cable leading to the receiver is soldered to its ends. If RG6 is used instead of copper wire, then the insulation from its ends is first removed, approximately 1-1.5 cm (the central core does not need to be cleaned, it is not involved in the process).
• The loop is installed on the stand.
• The F connector (plug) is screwed onto the cable to the receiver.

Note that despite the simplicity of the design, it is most effective for receiving “digits”, provided that the calculations are carried out correctly.

Do-it-yourself MV and UHF indoor antenna

If, in addition to UHF, there is a desire to receive MF, you can assemble a simple multiwave oven, its drawing with dimensions is presented below.

To amplify the signal in this design, a ready-made SWA 9 unit is used; if you have problems purchasing it, you can use a home-made device, the diagram of which was shown above (see Fig. 2).

It is important to maintain the angle between the petals; going beyond the specified range significantly affects the quality of the “picture”.

Despite the fact that such a device is much simpler than a log-periodic design with a wave channel, it nevertheless shows good results if the signal is of sufficient power.

DIY figure eight antenna for digital TV

Let's consider another common design option for receiving “digits”. It is based on the classic scheme for the UHF range, which, because of its shape, is called “Figure Eight” or “Zigzag”.

Design dimensions:

• outer sides of the diamond (A) – 140 mm;
• internal sides (B) – 130 mm;
• distance to the reflector (C) – from 110 to 130 mm;
• width (D) – 300 mm;
• the pitch between the rods (E) is from 8 to 25 mm.

The cable connection location is at points 1 and 2. The material requirements are the same as for the “Rhombus” design, which was described at the beginning of the article.

Homemade antenna for DBT T2

Actually, all of the examples listed above are capable of receiving DBT T2, but for variety we will present a sketch of another design, popularly called “Butterfly”.

The material can be used as plates made of copper, brass, aluminum or duralumin. If the structure is planned to be installed outdoors, then the last two options are not suitable.

Bottom line: which option to choose?

Oddly enough, the simplest option is the most effective, so the “loop” is best suited for receiving “digits” (Fig. 4). But, if you need to receive other channels in the UHF range, then it is better to stick with “Zigzag” (Fig. 6).

The antenna for the TV should be directed towards the nearest active repeater, in order to select the desired position, you should rotate the structure until the signal strength is satisfactory.

If, despite the presence of an amplifier and reflector, the quality of the “picture” leaves much to be desired, you can try installing the structure on a mast.

In this case, it is necessary to install lightning protection, but this is a topic for another article.