People in the street think it's enough to buy an expensive TV antenna to enjoy their favorite programs. With satellite TV the statement is true, but on Earth things are different. Obstacles, weather conditions, terrain features are included in the work. It is important to understand how to conduct reception. Otherwise, even the best TV antenna, personally made according to the drawings, will not catch anything. We will discuss the nuances of communication, let's see why they want digital radio( even television) to be transferred to the HF and HF frequencies, what are the pros and cons. Without an understanding of the obvious things, it’s difficult to make an antenna for TV by yourself: it’s impossible to set a technical task for yourself.
Features of the reception of a signal to a TV antenna
The principle of operation of a TV antenna is based on the conversion of aerial waves into electric current by the conductors of the structure. The shape and dimensions are clearly adjusted. For receiving the same channel, there are many designs of TV antennas. The former are cheap, the latter are endowed with a large gain, but a narrow radiation pattern.
During the 1980 Olympics, it turned out that TVs do not catch the signal. .. at the foot of the Ostankino Tower. Reception in a circle of 2-3 km uncertain. The vertical directional pattern for television broadcasting depends on the range, the width of the lobe does not exceed 10 degrees. This means that the signal will not reach the surface of the earth, therefore, no sense will emerge from any TV antenna designed independently. In the district of the tower, televisions see only side petals, which are not distinguished by large power, small width.
It turns out that the reception area resembles a concentric ring.
Extremely badly catches the TV at the foot, as the signal level rises from the epicenter, rises, reaches a peak, falls, disappears, then the so-called line of sight begins. This is the area where the reception is sure if the source of the broadcast is not obstructed by obstacles.
There are many known formulas for determining the specified radius, we give one with a few reservations:
- There is in nature diffraction, the ability of electromagnetic waves to go around obstacles. And with increasing frequency, the quality weakens. The sun's rays pass in a straight line regardless of the obstacles, the waves of SV and KV range go beyond the horizon at a sufficient distance. The broadcast area is extended by hundreds of square kilometers. The formula given below is given for normal conditions, valid for a range of television broadcasting. This is the decimeter and meter ranges. Long waves are reflected from the ionosphere. This ability is lost at 30 MHz. For these two reasons, the use of HF and CB for broadcasting is beneficial from the point of view of providing the audience with a quality signal at no extra cost. The disadvantage is that you cannot put a lot of information into broadcasting. The point is in the Kotelnikov theorem, without going into details, we mention that the carrier is provided at least 6 times higher than the maximum harmonic of the transmission spectrum. Translated into Russian, this means that there is a border in terms of the amount of information above which you cannot jump. Of course, the complexity can be solved intellectually by coding the broadcast, but the conversation does not apply to today's topic. At the moment, the Government’s desire to broadcast digitally to HF is being implemented.
- Refraction causes a fact: depending on the state of the atmosphere, the waves begin to refract in a bizarre manner. Seafarers know the phenomenon of the green ray, which occurs after sunset. The spectrum of the star is split according to the frequencies into components, the color of the foliage passes towards the observer. The phenomenon increases the reception range of the TV antenna, but the gain is highly dependent on the weather. There is an area of the penumbra, where the broadcast is caught, then disappears. This should be clearly understood by residents of remote areas. If the TV antenna device was conducted in clear weather, it is not a fact that you can watch your favorite shows in the rain. On the contrary, in cloudy weather the signal is better captured. It has already been said that, at the foot of the Ostankino television tower, there was an uncertain reception on a TV antenna, this is also the case of the petals of the broadcast station. Mentioned about diffraction and refraction, the interference remained in reserve. The meaning of the term: two waves come to a common point, if the frequency is the same( as in our case), summation occurs. The result on the TV antenna depends on the phase difference of the terms. If two strong signals come with a difference of 180 degrees, we get a subtraction. It means that just a zero will appear on the TV antenna, not giving an image. The farther from the tower, the weaker the phenomenon, the reflected rays become smaller. And the broadcasting zone, on the contrary, is a spotted field of light and shadow( if people could see the radiation).
So, there is a circle around the broadcasting tower, where the TV antenna catches the signal, and taking into account the phenomena described above, the radius is:
R = 4.15( √h1 + √h2),
where h1 and h2 are the transmitting and receiving heightsantennas. The area found by the formula makes it possible for anyone to describe the broadcast station by the specified zones:
- A line of sight within a radius of 0.8 R. An area where reception is certain, provided that the direction to the broadcast station from the TV antenna is not blocked by extraneous objects. The audience will notice that with the height of the Ostankino tower 400 meters square root is 20 meters. At any height of the installation of the receiving TV antenna, the maximum will come out to increase the range by 20%.For catching a signal from the ground, you need direct visibility to the tower, the TV antenna amplifier will not help. If it is impossible to find in azimuth a clear sky in the direction of broadcasting, search for the reflected beam coming from unimaginable directions.
- The zone of the penumbra is large, where the TV antenna will be received, with a reservation to the weather. The right comes refraction. Refractive conditions are highly dependent on temperature and humidity. Therefore, do not scold the TV antenna if there is no reception. It is useless to turn, and the amplifier is able to help. The amplitude of the signal varies. The region of the penumbra lies within 0.8 R - 1.2 R according to the formula given above. The attentive reader will notice that the difference will amount to those 20% promised by experts due to refraction. Due to changing conditions, sometimes the increase in the range of reliable reception of a TV antenna reaches 40%.
- What extends longer than 1.2 R is related to the shadow zone, where, without a reflection from the ionosphere, reception on a TV antenna will fail. It has already been said that the signal emitted by broadcast towers is not capable of this. Draw conclusions.
Conclusions on the design of TV antennas
From what has been said it is clear that the TV antenna should be placed wisely. Outside the city dominates the nearby obstacles in the direction of the tower. A forest, stretching away, can not interfere. This is due to the described phenomena of diffraction and refraction, in addition, the line of sight gradually goes up for the simple reason that the radiating TV antenna is higher than the receiving one. The angle is easy to calculate, if you know the distance to the broadcasting point, the coordinates of the location of the emitter. Based on the acquired information, simply calculate the installation height of your own TV antenna.
There is an opinion that the higher the better, but not every summer resident can erect a 400-meter tower. Consequently, on the ground, we take a compass, a map, determine the installation location of the future TV antenna, and begin to evaluate the obstacles. In the case of the forest follow the measure the height of the trees on the edge. This is simply done with a protractor and ruler. From the trunk we count the meters until the direction to the top is 45 degrees from the ground. The specified distance is considered the desired height. For advanced, we report that the ratio of height to range to the base of the trunk is equal to the tangent of the angle measured by the protractor in the direction of looking at the top. It is necessary to measure from the ground, or add to the resulting final value( in meters) the distance from the ground to the eyes. You can calculate the tangent on a personal computer and on any calculator.
Do not forget about polarization. In television, applied horizontal. If you tilt the TV antenna, the signal level will begin to fall. It is logical to conduct the installation by level, so that the directors of the wave channel or the eight of the frame structure are correctly oriented not in relation to the hill with the house, but in the real field of the Earth. In this case, a TV antenna assembled from improvised means will give the maximum gain.