Have you ever wondered why a 75-ohm cable is used in television and 50 in a radio? There is a close connection with the antennas. I want to unify the approach, because the TV cable costs 5 rubles per square meter, and you have to spend money to pull WiFi. Users of VashTehnik portal have found a cheap solution in the form of selecting a cable with good characteristics at an affordable price. Let's try to answer what a home antenna looks like assembled from scrap materials.
The characteristics of a home antenna from the point of view of radio amateurs
A homemade home antenna was not considered a rarity in Soviet times. In the store, the device was not available, it was necessary to invent. The antenna of the house was made by everyone. Let's discuss the theoretical foundations.
The antenna design is simple, but there are many varieties. Initially, it was a radiating vertical( horizontal) piece of wire. About the length did not think until the study of the electrodynamics of radio wave propagation. It soon became clear that the antenna emits waves in the air because there is a distribution of currents and voltages on the surface, repeating the laws of transmission.
Ideally, the wire length is a multiple of a quarter of the wavelength( it can be otherwise).This distribution is the most profitable, and information begins to be broadcast.
Designs appear a few. First of all, a half-wave dipole, known as Hertz's vibrator. Resistance is approximately equal to a 75 Ohm resonant wave. This is a standard television cable. Coaxial cable is not a symmetrical line, it is also endowed with its own characteristic impedance. If the antenna parameters are not adjusted to the feeder, some of the power will begin to reflect, which will adversely affect the efficiency of the system. As a result, a bare wire without an antenna takes better than the wrong antenna.
Purchased TV antennas have a radiation resistance of 75 Ohms. It is clear that at each wavelength it is impossible to observe, therefore the real SWR( standing wave ratio) varies over the range. The parameter is measured by a special device. It is considered a good indicator when the CWS over all wavelengths does not exceed 2.
Those who want to do everything themselves should be remembered: a symmetrical home-made antenna is connected to an unbalanced coaxial cable through a balancing device. Recall that two standards are released, it is difficult to make universal equipment:
- In television, devices are used with resistance at an operating frequency of 75 Ohms. The TV jack, cable and antenna work with each other.
- In broadcasting, devices with a characteristic impedance of 50 Ohms dominate. The reasons are described below.
. Causes of differences in home antennas.
There are many viable designs and sizes of antennas. Why are devices with a wave impedance of 50 Ohms used in broadcasting and 75 Ohms in television? Depends on tradition. No one would have thought that the machines would have televisions. Informbureau reported on victories through the receiver. Radio serves military purposes, therefore, is designed to have mobility. This is the reason for the choice of cables with a resistance of 50 Ohms and vertical linear polarization. What does this mean in practice:
- If the polarization is horizontal, for reception, you need to more accurately aim the antenna at the broadcast point. Otherwise, the received signal drops to zero. It is clear that the sapper on a minefield has no time to figure out where the transmitter is. If the polarization is vertical, the antenna does not receive in a parallel to ground arrangement. Mobile systems become relatively free due to the fact that in the azimuthal plane the antenna pattern is identical in all angles. As for television, equipment is being deployed slowly, so the direction to the tower is approximately known.
- Resistance also plays a significant role. Broadcasting, including military, initially went at relatively low frequencies. We calculate the wavelength for the upper boundary of the CV: 299792458/30000000 = 9.993 meters. Half is about five meters. High antenna! Try to expand the field. Radio truckers and now work on these frequencies. What to do? Quarter-wave vibrators are used, the properties( efficiency and communication range) are somewhat worse. Size plays a key role. The pin height of 2.5 meters under the force to put in seconds any. On HF waves and below the signal goes around obstacles and goes beyond the horizon, spreading far. FM waves move in a straight line, so far from civilization there are zones of complete radio silence. In a military setting, this is unacceptable.
For the above two reasons, the standards mentioned are chosen. It is worth noting that the information is already slightly out of date, even for the HF range, ferrite antennas with winding are used. There is a third reason not to mix television and radio interferences. Broadcasting would make it difficult to take the right signal. In the light of the emergence of multiplexes, the reason looks ridiculous, so the future is unification, as the government demonstrates by adding radio television programs( on the same wavelength, with time division).It's time to start and create a standard one. Not the fact that elementary linear antennas will become leaders in the race.
Readers should understand that the geometric dimensions of a home antenna are not everything. If we want to receive television, we make an asymmetrical quarter-wave vibrator from a 75-ohm cable section, stripping the braid to the desired distance. Remember to rotate horizontally for maximum reception.
So, home television antennas are calculated for horizontal polarization. Moreover, the design now does not always correspond to a half-wave vibrator. Consequently, if you wish to install the RK - 75, you will need to coordinate the product, because the resistance of a quarter-wave vibrator is approximately 50 ohms, as attentive readers could already guess. In practice, home-made designs do not reach the indicator, even in broadcasting the products need to be coordinated. And, finally, a home antenna for a TV with satellite channels is parabolic. The signal level is relatively small, the direction to the spacecraft is direct and unchanged. This makes it possible to use high gain home antennas.
In our country, amateur radio was given primary importance. The antenna assembled at home was in every family.
So, the cable resistance is related to the characteristic impedance of the vibrator, which is most often used when transmitting specific radio and television information. This allows you to do a minimum of matching devices while maintaining the performance of the complex. Today, the rule is no longer maintained, but the tradition of using specific cables remains. Products are optimized for the transmission of the required spectrum( broadcasting and television) and frequencies.