One of their options is any conductive line impedance.It becomes especially true in the high-frequency radio transmission technology, where the slightest misalignment of the work loop leads to significant distortion in the output.On the other hand, every owner of a computer connected to others in the local area network, faced daily with the notion of "characteristic impedance".It should be noted that the emergence of Ethernet twisted-pair allowed the end user is not particularly think about the connectors, grounding and terminators as connectors, as was the case with the coaxial cable lines at 10 megabits (or less).However, even with respect to the twisted pair apply the term "impedance".Generally, on the specifics of operation of computer networks discuss later.
So, what is the characteristic impedance?As already mentioned, this is one of the characteristics of the conductive lines on the basis of metallic conductors.The latter clause is necessary in order not to mix modern optical data transmission lines and classic copper wires, which are the energy carriers are not charged particles, and the light - there are other laws.This value indicates how important resistance line has a generator (power modulated electrical vibrations).Do not confuse the resistance that can be measured by conventional multimeter, and the wave resistance of the medium, as it is completely different things.The latter does not depend on the length of the conductor (already enough to draw conclusions about the "similarity" of resistance).Physically, it is equal to the square root of the ratio of inductance (Henry) to capacity (Farad).A small remark: despite the fact that the calculations used reactive components line characteristic impedance calculation circuit is always considered active.
best to consider all of the example.Imagine a simple circuit consisting of a power source (generator, R1), conductors having characteristic impedance (R2), and consumers (load, R3).In case of equality of all three resistances entire transmitted energy reaches the consumer and it performs useful work.If at any part of this equality is not respected, there is a mismatched mode.At the point where the line is broken, there is a reflected wave, and the electromagnetic energy is returned back - to the generator.Accordingly, it is necessary to raise power to compensate for the magnitude of the reflected energy.In other words, part of the energy spent "wasted", which means losses and non-optimal operation.Furthermore, in some cases, even mismatch disrupts the functioning of the line.
Now back to computer networks where the characteristic impedance is important.For the lines based on the coaxial cable (50 Ohm) is important to respect the conditions: resistance network card and the conductor between them must be equal.Only in this way the system works and termination grounds.If any portion of the cable line is physically a little stretch (hang on conductor load), then the change in the diameter of the conductors in this place will change characteristic impedance, the reflected wave will arise, breaking the system.This measured resistance line can hardly change (budget instruments did not register an increase in resistance).Attempts to restore the line by soldering wires on the damaged area will further exacerbate the situation, as it will not simply transfer resistance, and a mixture of different media (tin, copper) in which waves propagate differently.In popular
same twisted pair Category 5 characteristic impedance is 100 ohms.Because of this recovery may be soldered, and even twisting.