The modern market offers a huge range of antennas for receiving terrestrial television. There are two main types of these products, allowing to receive the meter and decimeter ranges of radio. They can also be divided at the place of use into outdoor and indoor. Fundamentally, they are not much different. Here, first of all, emphasis is placed 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 parameters they have, how to conduct testing. And for fans of tinkering, we will tell you how to make a 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 decimeter range antenna looks like a ladder. Set them parallel to the ground. TV meter 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 and cross tubes are combined.
Problem of choice
It would seem that everything is simple. However, the question arises before the buyer about how to choose the right device, what parameters to pay attention to. In general, it is best to test the TV antennas directly in the conditions in which they have to work. The passage of a radio signal is often individual for a given 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 full-fledged testing. No seller agrees 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 the passport data, and not the actual 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 of the highest value. So, according to this parameter (the width of the main lobe), it is customary to divide the diagrams 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 chart. It is determined by the ratio of the power released by the antenna under the condition of a coordinated load at the time of receiving a signal from the main direction to 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 antenna design.
What does the term “wave channel” mean?
TV antennas of this type are highly effective directional receivers of radio signals. They are widely used in areas of clearly weak television broadcast. The antenna (decimetric) of the “wave channel” type is highly amplified and has a 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 (after the name of Japanese inventors, who patented this device).
Principle of operation
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 action 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. It induces an electromotive force (EMF). Each passive element is affected by an electromagnetic field, which also leads to the emergence 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 distance to the active vibrator, are selected so that the induced EMF due to the secondary fields is in phase with the main EMF, which is induced in it by the primary electromagnetic field. In this case, all the EMFs are summed, which provides an increase in the design efficiency compared to a single vibrator. Thus, even a conventional UHF indoor antenna 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 diagram 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 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.
Forward 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 a laboratory) generator and a radiating antenna, but the signal is broadcast by a television tower, a significant weather conditions as well as the installation location of the receiver play a role. This especially affects the operation of products in the DMV range. This is explained by the fact that the wavelength in the decimeter range is shorter, respectively, the envelope of obstacles is much worse, and any signal reflections play an important role as the received picture. In particular, even the wall of a house can be a reflector of waves. So, in the absence of direct visibility, this property can be used - to receive a reflected signal. However, its quality will be lower than that of direct. If the level of the broadcast signal is high, but there is no line of sight, then you can use the reflected wave. In fact, the indoor decimeter antenna works on this principle. Indeed, it is difficult to catch a direct wave in the 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.
A technique for comparing antenna parameters
In order to test the 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 place 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 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 a single wire, changing only the receivers.
Testing should be carried out only for products of one kind. For example, the indoor UHF antenna should not be compared with outdoor or meter receivers. It should be understood that field trials can give results that will differ significantly from laboratory ones.
Digital Television Antenna
Recently, the media have increasingly insisted on the need to switch to digital television. Many have already done this, while others are still pondering. So far, the signal is being broadcast in both modes. However, the quality of analog TV is poor. In this regard, people are interested in what decimeter antennas can be used for T2. Let's deal with this issue. In essence, digital television broadcasts on the DMV channel. So a standard UHF antenna may be suitable for its reception. In stores, you can often see the receiving devices, which indicate that they are designed for digital television. However, this is a marketing move that allows you to sell a standard UHF antenna more expensive than it costs. Buying such a product, you will not have a guarantee that it will provide a better reception than what is already at your place and has been working for more than one year. As we said earlier, quality depends mainly on the level of the broadcast signal and the conditions of direct visibility. However, it should be borne in mind that in most cities much more powerful generators are used to transmit digital television than for analog. This is done in order to accelerate the transition to the new standard. After all, viewers want to see a clear image, and not “snow” on the screens. Therefore, if a receiver is displayed in the window that reads “Decimeter Antenna for DVB T2”, you should know that this does not mean that you have a special product in front of you. Just not quite an honest seller wants to cash in on an ignorant buyer. You should also be aware that the program of transition to the new standard provides for the establishment of advisory centers. In them you can get comprehensive information on any issue related to digital television. All consultations are free of charge. In some cities, this equipment is in test mode, so the signal may be unstable or attenuated. Do not worry, the employees of the center will always tell you how to solve the problem with the quality of signal reception.
DIY decimeter antenna
The length of the UHF waves lies in the range from 10 cm to 1 m. Their name came from this feature. Electromagnetic vibrations 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 this regard, long-range communication in the decimeter 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.
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 in the interval from 300 MHz to 3 GHz. Our task is to produce an antenna that will work precisely at these frequencies. To do this, we need two beer cans with a volume of 0.5 liters. If you use a larger capacity, then the received frequency will decrease. For installation, you need some kind of frame, you can use a 10 cm wide board. 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, it is necessary to prepare a pair of self-tapping screws, tools, coaxial cable, connector, terminals, insulation tape. We put a television connector on one end of the cable and solder it. The second end is brought into the terminal block. Next, we attach with screws to the necks of the jars of the terminal. The wires should fit snugly against the metal. Now let's start assembling the antenna itself. To do this, on the horizontal crossbar we fasten the cans with the neck facing. The distance between them should be 75 mm. An insulating tape can be used to fix the cans. Everything, the antenna is ready! Now we should find a place for the stable reception of a television signal and hang our “hanger" in this place.
Digital TV Receiver
This section is intended for people who do not want to use a conventional (analog) product, but want a special decimeter 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 (possible from plexiglass) frame with a diagonal of 200 mm and an ordinary cable RK-75. 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 visibility 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 form of a square with a diagonal of 175 mm. We bend the end outward at an angle of 45 degrees, the second stripped end bends to it. We tightly connect the screens. The stripped central core hangs freely in the air. On 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. In the lower part, where the two ends are connected, copper brackets made of thick wire should be used. This will provide 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 rain.