It is time to tell important information: why was the cable winding torn off when designing the simplest asymmetric vibrator for receiving digital television, when they showed how to make the simplest antenna? Recall, for these purposes, a piece of coaxial cable of the selected length was stripped of insulation, supplied with a f-connector and stuck into the console. Positioned horizontally and, if possible, perpendicular to the direction of arrival of the signal. This simple design catches the signal in cities without difficulty. But, pay attention, it was recommended to use braid in walkie-talkies, closing it on the core at the base when docked to the board. Already told how to make the antenna with your own hands for any occasion, and now we come to the subtleties.
Frequency Range
For making the right antenna, find out the frequency range. In the future, the task is reduced to the search for finished structures. Probably, readers will not take in the library a textbook on the calculation of antennas and calculate the size. It is possible to use ready-made drawings, it is required to determine the channel frequency in Hz and the signal structure( type of polarization).
Information, concealed in manuals for the manufacture of homemade antennas
In the review of the manufacture of the antenna Kharchenko mentioned that for each frequency use a fixed wire diameter. The longer the wavelength, the thicker the material. It is clear that under certain conditions it is no longer possible to get the wire of the desired diameter, therefore, they are constructed from plates or create several contours. As for simple vibrators, it was mentioned that increasing the thickness of the copper conductor expands the frequency band of the received frequencies. What is the basis of the principle of operation of fractal biconous antennas, invented more than a century ago.
It turns out that the thickness of the wire makes adjustments. For long-wave antennas and it is recommended to use antenna cords. This material is similar to a single core vole. Strong and thin antenna cable does not distort the characteristics, unlike conventional wire, the thickness of which can be significant. As a similar point to consider when designing. The length of a half-wave vibrator is less than the ideal calculated one. Exclusively in this case, the exact frequency is provided in the resonance. The thicker the wire, the further the care. As for the cords and thin wire, in their case the length is 97% of the calculated. Now it becomes clear why the sheath was removed when fishing for digital multiplex.
For radio stations, the screen was left mainly for good performance. The external insulation of the cables is strong and the antenna for the radio is durable. True, the length should be taken less than the estimated. It is easier to determine the required dimensions empirically, cutting off a little from the cable and measuring the reception distance. And now we will give the table where lengths of half-wave vibrators and thickness of a wire are given. The numbers are tabulated. The columns are channel numbers, in rows of half-wave vibrators depending on the two thickness of copper wire, 1.5 mm and 4 mm.
It can be seen that the difference is 1 cm to several millimeters and decreases with increasing frequency. This is not surprising, rather the correction is associated with a decreasing wavelength. Let us clarify that the matter concerns television, let us indicate the frequencies for the reduced channels.
These are carrier images close to the left border of the channel strip. The sound is symmetrical above. Accordingly, it would not be an error to tune the half-wave vibrator to this value. Then for the middle frequency the antenna will be a bit short, which is what is required. Above is the exact table, we are guided by it to create a television reception system. What to do if the wire is different in diameter from the specified. Respectively slightly change the length of the homemade antenna. By the way, do not forget that for television a half-wave vibrator is horizontal, and for radio receivers it is vertical, this takes into account the polarization of the signal. In the first case, the antenna is perpendicular to the direction of the broadcast station.
Matching and balancing home-made antennas
Those who are used to using purchased antennas will need to learn two questions:
- Balancing home-made antennas.
- Antenna matching with cable( feeder).
Let's start with the second, since it is more obvious. In each line, the wave moves, reflecting from the walls. Specified angle of reflection. Actually, the wavelength in the line varies, which is not so important in this case. Copper resistance does not play a great role, because the wave runs in a dielectric, although the active component imposes restrictions. Energy is reflected by the screen and does not go beyond the line.
Of course, the higher the conductivity of the metal, the losses will be less. The perfect coaxial cable with gold plated. By the way, the other type of lines - waveguides - has a similar quality. Therefore, they are expensive( and in the passport is indicated the percentage of precious metals to be handed over to the state during disposal).
When a wave runs inside a line, it is reflected from obstacles. It is easier to illustrate on the example of equipment for the study of high-voltage lines. A special van with hardware in the cable sends an electrical audio signal. That is reflected from any coupling, but since the repairmen have a map, they are not interested in such heterogeneity. But the cliff is noticeable against the background of regular sources of reflection. From the example it is clear how homogeneous the line is. Coordination aims to eliminate interference with signal movement. As a rule, we are talking about such heterogeneity as unequal wave resistance of the line sections.
An illustrative but weak example: to present a canal with water flowing through it. The channel narrows, a cycle is formed. But the wave in the line still “narrows” or “expands” the channel, still partially turns back. Moreover, the process is repeated many times: a wave, like a pendulum, runs to and fro, gradually fading out. Such processes reduce the quality of reception. Not so easy to achieve agreement. Antennas are endowed with impedance, exactly as the line.
With radiation, the wave experiences resistance. Accordingly, part of the power is lost. This resistance is called wave.
The difference from the active one is that the characteristic impedance depends on the wavelength( signal frequency).Measured using a generator and high-frequency voltmeter, as usual. This resistance is equal to 50 Ohm for radio, and 75 Ohm for television. According to the cable. In practice, it is not always possible. For this purpose, matching devices are assembled, for example, quarter-wave transformers. Transmit energy almost without loss, the fine tuning of the antenna is conducted on the CWS meter. In the case of half-wave vibrators, it is permissible to shorten the shoulders and see if the parameters have changed for the better.
Antenna balancing is important. This plays a role on meter and decameter waves, with increasing frequency the effect decreases. But 12 channels( along with FM radio) fall into the range, which means that the question is not superfluous. The dipole with two shoulders is a symmetrical device, and the cable is an asymmetrical line. The result is a potential difference that distorts the radiation pattern, which increases the level of side lobes. Due to the currents induced on the braid of the line, spurious radiation and parasitic reception appear. If this is not a bad thing for radio, in the case of television every chance of getting a third-party part along with the useful signal. Similar effects interfere in large cities with a strong level of interference.
Balancing devices for half-wave vibrators
Television adopted the standard 75 Ohm cable. And the wave resistance of the half-wave vibrator is 75 Ohms. Reconciliation is not required in this case. But a balancing device must be assembled. And show how to do it. Below is a drawing of the simplest device for this purpose. He himself was taken from the site http: //lib.qrz.ru/node/ 1033, where he appeared, we believe, thanks to one of the many books on the design of antennas. Below is a table showing the lengths a and b, and the wire is a length of coaxial cable.
Of course, other types of antennas are also invented, some suitable as indoor ones. Half-wave vibrator is good simplicity. Such a homemade antenna adopts a digital multiplex, made of improvised material. Long vibrators put on stretch marks and hang between houses( TV reception).Some of the structures reach the height of a residential block of flats. It is not surprising that such radio amateurs put rarely.
In the era of the USSR, citizens managed to contact the United States. Long waves skirted the earth's surface and were hardly picked up by the special services direction finders. Not to say, similar methods were used by dissidents, but lovers indulged in, turning a blind eye to the ranges allowed for household communication.
Now readers know how to make an antenna on their own.
