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coaxial, coaxial cable, central core, braid, conductor, screen, dielectric, radiation, oscillatory circuit, cylindrical capacitor, bifilar coil, quality factor, capacitance, coaxial transformer, Faraday transformers, Rutrof transformers, isolation choke, harmonic filter, common mode current, filter plug , cutoff angle, power addition, long line, ballast, wavelength, quarter-wave transformer, standing wave, idle, short circuit, matching, exponential transformer, wave resistance
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1 ___ COAXIAL
Greetings friends. In touch Timur Garanin.
Today’s video, I dedicate it to the majesty of coaxial cable.
First, we discuss, of course, its direct purpose. It is called coaxial because its central core and braid are aligned. Due to this structure, the entire electric field, as well as magnetic, are located in the space of the dielectric between the two conductors. And do not go outside, as in the case of twisted pair.
Due to the fact that the field does not leave the cable space, the radiation loss tends to zero. While in a twisted pair cable, radiation losses are noticeable, especially at large distances.
However, the coaxial cable, like any asymmetrical line, has a daunting problem - the susceptibility to interference from the outer braid, and vice versa, the ability to radiate from the outer braid in case of current distortion. That is why coaxial cable is always used together with balancing devices when working on a symmetrical load.
Due to its design, which actually consists of two inductively coupled conductors and a long cylindrical capacitor, the coaxial has found many alternative applications.
Let's start with the consideration of the oscillatory circuits from the coaxial, which I mentioned in the previous video about TRAP antennas. For a better understanding, here are a few simpler examples.
Imagine an ordinary single-layer cylindrical coil. Obviously, such a coil has a significant inductance. But besides inductance, such a coil will also have a capacitance. This capacity occurs between adjacent turns of the coil. But since the turns of the coil are wound sequentially, the resulting capacity will tend to zero. Theoretically, such a coil can be considered as an oscillatory circuit, but in practice due to the vanishingly small capacitance, high resonant frequency and low quality factor, i.e. it is extremely difficult to obtain stable oscillations in such a coil.
Another thing is a bifilar coil. Such a coil is wound with a double wire, and the end of the first wire is connected to the beginning of the second. The number of turns of a bifilar coil with the same dimensions and wire coincides with the number of turns of a conventional coil, their inductances differ slightly.
But the capacity of the bifilar coil is much higher. This is directly related to the fact that each turn of the first half of the inductance lies next to the turn of the second half of the inductance. Therefore, the turns of such a coil are simultaneously plates of a full capacitor.
Due to the increased capacity, as well as better flux linkage between the halves of the winding, the quality factor of the formed oscillatory circuit is very high. And the oscillation frequency lies already in the radio range. Therefore, bifilar coils are really put into practice. However, only at relatively high frequencies. Because to create a lower frequency circuit, you have to wind a huge bifilar.
And now let’s remember that we have a coaxial with an excellent linear capacity. We wind a coaxial cable in the form of a coil and connect its conductors as in a bifilar: we solder the end of the central core to the beginning of the braid. And the oscillating circuit is ready.
Due to the large capacity of a long coaxial capacitor, we can operate in a wide frequency range from tens of kilohertz to Gigahertz units. Moreover, thanks to constant linear parameters, we obtain the exact reproducibility of the parameters of the oscillatory circuits. For coils of cable of the same length, the capacitance and inductance of the oscillating circuit will always be the same. When changing the length of the coaxil, the inductance and capacitance will change simultaneously and in proportion.