The modern market offers a huge range of antennas for receiving terrestrial television. There are two main types of these products that allow you to receive the meter and decimeter range of radio. 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 the size and preservation of the necessary parameters under the influence of weather conditions. In this article, we will discuss the existing types of these products, consider what their parameters are, how to conduct testing. And for those who like to craft, we will tell you how a decimeter antenna is made with your own hands.
What's the difference?
Let's try to explain in a nutshell how to determine what kind of product is in front of you. The UHF antenna looks like a ladder. Install them parallel to the ground. Meter TV antennas are crossed aluminum tubes. The appearance of both types is shown in the photo below. There are also combined antennas, when both the "ladder" andcross tubes.
The problem of choice
It would seem that everything is simple. However, at the same time, the buyer faces the question of how to choose the right device, what parameters to pay attention to. In general, it is best to test TV antennas directly in the conditions in which they will work. The passage of a radio signal is often individual for a particular area. So, the product in the laboratory shows some results, and in the "field" - completely different. There is a certain tactic that allows you to test both meter and decimeter TV antennas. However, choosing such a product in the store, we are not able to conduct a full test. Not a single seller will agree to give us several different antennas for testing. In this case, you have to trust the characteristics of these products. And hope that the selected antenna will perform its functions according to passport data, and not real conditions.
Main parameters
The decimeter antenna is characterized primarily by the 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 chart is determined in the horizontal and vertical planes at the level of 0.707 from the largest value. So, according to this parameter (the width of the main lobe), the 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 importantparameter 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 allocated by the antenna under the condition of a matched load at the moment of receiving a signal from the main direction to the power (with the same load) when receiving from the lateral 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 efficient directional radio receivers. They are widely used in areas of clearly weak television air. Antenna (decimeter) type "wave channel" has a high gain and has good directivity. In addition, these products have relatively small dimensions, which (along with a high level of amplification) 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).
Working principle
A decimeter antenna of the “wave channel” type is a set of elements: passive (reflector) and active (vibrator), as well as several directors that are installed on a common boom. The principle of its operation is as follows. The vibrator has a certain length, it is in the electromagnetic field of the radio signal and resonates at the frequency of the received signal. An electromotive force (EMF) is induced in it. On theeach passive element is affected by an electromagnetic field, which also leads to the emergence of an EMF. As a result, they re-emit secondary electromagnetic fields. In turn, these fields induce an additional EMF on the vibrator. Therefore, the dimensions of the passive elements, as well as their distances to 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 provides an increase in the efficiency of the design compared to a single vibrator. Thus, even an ordinary indoor UHF antenna can provide stable signal reception.
Reflector (passive element) is installed behind the vibrator 0, 15-0, 2 λ0. 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 rear of the antenna is significantly reduced. If it is necessary to receive a television signal over long distances, as well as in difficult conditions, in the presence of a large amount of 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 on the wave receiver (Tele-Sputnik No. 11, 1998), it was noted that inIn the case when the signal source is not a standard (that is, not a laboratory) generator and a radiating antenna, but the signal is broadcast by a television tower, weather conditions and the location of the receiver play a significant role. This especially affects the operation of products in the UHF range. This is explained by the fact that the wavelength in the decimeter range is smaller, respectively, the obstruction of obstacles is much worse, and any signal reflections play an important role in the quality of the received picture. In particular, even the wall of a house can be a wave reflector. So, in the absence of 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 direct line of sight, then you can use the reflected wave. In fact, an indoor decimeter antenna works on this principle. After all, it is difficult to catch a direct wave in a room if the windows face the opposite direction. 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.
Method to compare antenna parameters
In order to test receiving devices, they need to create the same conditions:
1. Choose the installation location where your antenna will work. You can use the balcony, roof or mast. The main thing is that both the height and the place are the same for all products.
2. The direction to the source of the broadcast signal should behold to within three degrees. To do this, you can make a special mark on the mounting pipe.
3. Measurements should be taken 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 receivers only.
Testing should only be done on products of the same kind. For example, an indoor UHF antenna should not be compared with outdoor or meter receivers. It should be understood that field trials may produce results that differ materially from those in the lab.
Decimeter antenna for digital television
Recently, the media are increasingly talking about the need to switch to digital television. Many have already done this, and someone else is thinking. So far, the signal is being broadcast in both modes. However, the quality of analog TV leaves much to be desired. In this regard, people are interested in what decimeter antennas can be used for T2. Let's deal with this issue. In fact, digital television broadcasts on a UHF channel. So a standard UHF antenna may be suitable for its reception. In stores, you can often see receiving devices that indicate that they are designed for digital television. However, this is a marketing ploy that allows you to sell a standard decimeter antenna for more than it costs. By purchasing such a product, you cannot be guaranteed that it will provide the bestreception than what is already 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 much more powerful generators are used for digital television transmission than for analog television. This is done in order to speed up the transition to the new standard. After all, viewers want to see a clear image, not "snow" on the screens. Therefore, if a receiver is displayed in the window, which says “Decimeter antenna for DVB T2”, you should know: this does not mean at all that you have some special product in front of you. It's just that a not entirely honest seller wants to cash in on an ignorant buyer. You should also be aware that the program for the transition 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 given free of charge. In some cities, this equipment is in test mode, so the signal may be unstable or weakened. Do not worry, the center's employees will always tell you how to solve the problem with the quality of signal reception.
Do-it-yourself decimeter antenna
The length of the UHF waves falls within the range from 10 cm to 1 m. From this feature, their name comes from. Electromagnetic oscillations at this frequency propagate mainly in a straight line. They practically do not go around obstacles, they are only partially reflected by the troposphere. In connection withthis makes long-distance communication in the decimeter range very difficult. Its radius does not exceed one hundred kilometers. Consider a couple of examples of how to make a decimeter antenna at home.
The first version of a home-made television broadcast receiver will, so to speak, be assembled on the knee from improvised materials. UHF channels are located on the segment from 300 MHz to 3 GHz. Our task is to make an antenna that will operate at these frequencies. To do this, we need two beer cans with a volume of 0.5 liters. If you use a larger capacitance, then 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 an ordinary 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 pair of self-tapping screws, tools, a coaxial cable, a connector, terminals, and an insulating tape. We put on a television connector at one end of the cable and solder it. We put the second end into the terminal block. Next, we attach the terminals to the necks of the cans with screws. The wires must fit snugly against the metal. Now let's start assembling the antenna itself. To do this, on the horizontal bar we fix the cans with their necks towards. The distance between them should be 75 mm. Insulating tape can be used to secure the cans. Everything, the antenna is ready! Now we should find a place of stable TV signal reception and hang our “hanger” in this place.
Digital TV receiver
This section is for people who don't want to use a regular (analogue) product, but want a special UHF antenna for the new format. With your own hands, such a receiving device is also assembled elementarily. To do this, we need a square wooden (or plexiglass) frame with a diagonal of 200 mm and a regular RK-75 cable. The option presented to your attention is a zigzag antenna. She has proven herself well when working in the range of digital television reception. Moreover, it can be used in places where there is no direct line of sight to the signal source. If you have a weak broadcast, you can connect an amplifier to it. So, let's get to work. We clean the end of the cable by 20 mm. Next, we bend the wire in the shape of a square with a diagonal of 175 mm. We bend the end outward at an angle of 45 degrees, the second stripped end is bent to it. We connect the screens tightly. The cleaned central vein hangs freely in the air. At the opposite corner of the square, carefully remove the insulation and the screen in a 200 mm section. 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 of thick wire should be used. This will provide the best 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.
