One of the parameters of any conductive line is wave resistance. It acquires particular relevance in high-frequency radio transmission technology, where the slightest mismatch in the operation of the circuit leads to significant distortion at the output. On the other hand, every owner of a computer connected to others on a local network is confronted with the concept of “wave resistance” every day. It is worth noting that the advent of twisted pair Ethernet networks allowed the end user not to think about the connectors, grounding, terminators, and the quality of the connectors, as was the case with coaxial cable lines of 10 megabits (or less). However, even with respect to twisted pair, the term “wave impedance” is applicable. In general, we will dwell on the features of the operation of computer networks a little later.
So what is wave impedance? As already indicated, this is one of the characteristics of a conductive line based on metal conductors. The last reservation is necessary in order not to mix modern optical data lines and classic copper wires, where not charged particles act as carriers of energy, but light - other laws apply there. This value indicates what resistance the line provides to the generator (the source of modulated electrical oscillations). Do not confuse the active resistance, which can be measured with a conventional multimeter, and the wave resistance of the medium, since these are completely different things. The latter does not depend on the length of the conductor (this is already enough to draw conclusions about the "similarity" of the resistances). Physically, it equals the square root of the ratio of inductance (Henry) to capacitance (Farads). A small remark: despite the fact that the calculations use reactive component lines, the wave impedance of the circuit is always considered active in the calculations.
It is best to consider everything with an example. Imagine a simple circuit consisting of an energy source (generator, R1), conductors with wave impedance (R2), and a consumer (load, R3). If all three resistances are equal, all the transferred energy reaches the consumer and does useful work there. If on some site this equality is not observed, then an inconsistent mode of operation occurs. At the point where the correspondence is broken, a reflected wave appears, and part of the electromagnetic energy returns back to the generator. Accordingly, it is necessary to increase its power in order to compensate for the magnitude of the reflected energy. In other words, part of the energy is wasted, which means losses and suboptimal operation. In addition, in some cases, the mismatch generally disrupts the functioning of the entire line.
Now back to computer networks, where wave resistance plays an important role. For lines based on coaxial cable (50 Ohms), it is important to comply with the condition: the resistance of the network cards and the conductor between them must be equal. Only in this case does the system of terminators and groundings work. If any part of the cable line is physically slightly stretched (to hang a load on the conductor), then because of a change in the diameter of the conductors, the wave impedance will change in this place, a reflected wave will occur that disrupts the system. At the same time, the measured active resistance of the line may practically not change (budget devices will not register an increase in resistance at all). Attempts to restore the line by soldering conductors in the damaged area will further aggravate the situation, since not only a transition resistance will appear, but a mixture of different media (tin, copper), in which the waves propagate differently.
In the popular category 5 twisted pair cable, the impedance is 100 Ohms. Thanks to this, restoration by soldering and even twisting is allowed.