What is wave impedance

One of the parameters of any conductive line is the wave impedance. It becomes particularly relevant in high-frequency radio transmission technology, where the slightest misalignment of the circuit's operation leads to significant distortions at the output. On the other hand, every owner of a computer connected to others on a local network faces the notion of "wave resistance" on a daily basis. It should be noted that the emergence of Ethernet networks based on twisted pair allowed the end user not to think much about connectors, grounding, terminators and connector quality, as was the case with coaxial cable lines of 10 megabits (and less). However, even with respect to a twisted pair, the term "wave impedance" is applicable. In general, the features of the operation of computer networks will be discussed later.

So, what is wave impedance? As already mentioned, 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 transmission lines and classical copper wires, where energy carriers are not charged particles, but light - other laws act there. This value indicates how much resistance the line provides to the generator (the source of the modulated electric 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 resistances). Physically, it equals the square root of the ratio of inductance (Henry) to capacitance (Farada). A small remark: in spite of the fact that the reactive components of the line are used in calculations, the loop impedance is always considered active in calculations.

It is best to consider everything by example. Imagine a simple circuit consisting of an energy source (generator, R1), conductors having a wave resistance (R2), and a consumer (load, R3). When all three resistances are equal, all transferred energy reaches the consumer and performs useful work there. If on the other hand this equality is not observed, then an uncoordinated mode of operation arises. At the point where the correspondence is violated, a reflected wave appears, and some of the electromagnetic energy returns back to the generator. Accordingly, it is necessary to increase its power in order to compensate the value of the reflected energy. In other words, some of the energy is wasted, which means losses and a suboptimal mode of operation. In addition, in some cases, the mismatch generally disrupts the functioning of the entire line.

Now back to the computer networks, where wave impedance plays an important role. For lines based on a coaxial cable (50 ohms) it is important to observe the condition that the resistance of the network cards and the conductor between them should be equal. Only in this case the terminator and grounding system works. If any part of the cable line is physically stretched a little (hang the load on the conductor), then due to a change in the diameter of the conductors, the wave impedance will change at this point, a reflected wave will appear, which disrupts the operation of the system. In this case, the measured active resistance of the line can practically not change (budget devices do not register an increase in resistance at all). Attempts to restore the line by soldering conductors in the damaged area will further exacerbate the situation, since not only a transient resistance will appear, but a mixture of different media (tin, copper) in which the waves propagate in different ways.

In the popular twisted pair of category 5, the impedance is 100 ohms. Due to this, it is allowed to restore by soldering and even twisting.