Cabling technicians, in contrast to electricians, have occasion to work with somewhat more specialised materials. And the underlying theoretical basis also differs, writes David Herres.

Communication and data cabling usually involves lower power levels but higher frequencies, resulting in significant differences in how wiring is done and how circuits are to be understood.

An important concept in communications and data cabling technology is characteristic impedance. A complete mathematical and theoretical understanding is not necessary to properly install, troubleshoot and maintain equipment that involves characteristic impedance, but a fundamental knowledge is necessary in order to do this important work.

To get started, we must define some of the terms that we are going to encounter.

Impedance refers to the opposition to the flow of current in any electrical circuit, be it power (as in household AC), or the lower energy encountered in communications and data cabling.

Impedance is the vector sum of resistance and capacitive or inductive reactance. These quantities cannot be added in an ordinary way. They must be thought of as pulling in somewhat different directions. It is like two oxen hitched to a log, pulling at different angles. To figure the result, you have to calculate a vector sum that accounts for the angular difference and the amount of force exerted in each direction.

Impedance resembles resistance in a DC circuit, except that the reactive component is frequency dependent. Impedance, like resistance, is measured in ohms and conforms to Ohm’s Law. The symbol for impedance is Z.

Characteristic impedance is applicable for any type of cable, if the frequency is sufficiently high and/or if the cable is sufficiently long. Generally, the notion of characteristic impedance kicks in as the length of the conductor approaches one-tenth of the wavelength of the signal that the conductor is carrying. When this combination of parameters is reached, conventional circuit analysis rules no longer apply. Characteristic impedance and transmission line theory surface.

This shift in perspective is necessary for very long 50 or 60Hz power lines, as well as long runs of twisted cable used for telephone circuits. It is also necessary for shorter runs at higher frequencies.

A familiar medium for these applications is co-axial cable, so named because the two conductors are concentric and share the same axis. Coax has an inner conductor, frequently copper-clad steel, surrounded by a relatively thick insulating material. Outside the insulating material is a layer of metal foil, then a braided metal return conductor, usually grounded. Finally, the entire assembly is contained within an insulating jacket.

The coax cable is labelled with its characteristic impedance, such as 50O. This can be puzzling. Does it have to do with resistance between the two conductors, or resistance of the overall cable per some unit of length?

As an experiment, we may connect an ohmmeter to the inner pin and outer shield of a length of coax, say 10m long. If the two conductors at the far end are connected, we read a very low resistance. If they are not connected, we read a very high resistance. In either case, it is nothing like 50O. What’s going on?

The fact is that characteristic impedance cannot be measured by an ohmmeter. It can only be measured by an expensive instrument known as a time-domain reflectometer (TDR). This instrument determines characteristic impedance and measures the distance to faults in coax and twisted wire pairs by observing reflected waveforms. Characteristic impedance can also be determined by calculating, using properties of the conductor including dielectric constant of the insulating material, the distance between conductors and conductor size.

The good news is that most of us will never have occasion to measure or calculate characteristic impedance of coax or other communication and data cable. It is an intrinsic property of the medium and the correct value is supplied in the labelling of the product by the manufacturer.

So what is characteristic impedance, and why is it important to the working cabling technician?

To answer the first part of the question, we begin by taking a look at the anatomy of a coax segment (see Figure 1). At the high frequencies seen in data transmission, capacitive reactance becomes quite pronounced if the cable has significant length. The inner pin and outer shield become two plates of a capacitor. The unusual thickness of the insulating medium is not intended to insulate for a high voltage, but rather to provide uniform spacing between the conductors in order to limit and control capacitance.

Similarly, at high frequencies, the inductive reactance of the two conductors, inner pin and outer shield, becomes quite pronounced, even though these conductors are not formed into coils. The high frequency in conjunction with the length of the cable makes for significant values of capacitive and inductive reactance.

So how does the 50O characteristic impedance come about? To answer this question, it is necessary to perform a thought experiment. Imagine an infinitely long transmission line, coax or some other type of cable. Imagine that the DC resistance of the conductors is very low, so that it can be ignored, and imagine that the DC resistance of the dielectric material is very high, so that it can be considered a non-conductor.

This infinite transmission line can be represented as shown in the schematic. The cable consists of an infinite number of series-connected inductances and parallel-connected capacitances. When an electrical pulse is applied at the input, it will begin to charge the capacitance of each segment. And the current flow will be limited by the inductance of each segment. Since the length is infinite, there are an infinite number of capacitances and inductances, so the process will continue indefinitely. If the voltage and amperage are measured at the input, the impedance can be calculated using Ohm’s Law (i.e. O = V ÷ A). The impedance will be found to be (for this cable) 50O. This is a consequence of the size and spacing of the conductors and the dielectric coefficient of the insulating material. These variables are built into the design of the cable by the manufacturer and are intentionally chosen so that the desired value, 50O, is achieved.

But how, you may ask, do we get the characteristic impedance value of 50O when in the real world the cable is nowhere near infinite in length? The answer is the key to this whole discussion. Any length of uniform transmission line will act like an infinite length if the output is terminated in a load whose impedance is identical to the characteristic impedance of the transmission line.

A constant 50O value results regardless of length because the series inductances add to make the value higher even as the parallel (shunt) capacitances add to make the value lower. These two effects balance to result in a constant impedance value, calculated on the basis of input current and voltage.

That’s characteristic impedance of a transmission line. But why is this value important to the cabling technician?

It is very important, crucial in fact. For efficient (maximum) power transfer to occur, the characteristic impedance of the transmission line must match the impedance of the load. If output impedance is higher than input impedance, current flow (hence power transfer) is limited in accordance with Ohm’s Law. If the output impedance is lower than the input impedance, there will be greater voltage drop, also limiting power transfer.

If these values are not the same, some of the energy will be reflected back to the source. Because of collisions with the incoming stream, this will cause data loss and signal corruption, and can also damage upstream equipment, which may not be designed to see a voltage at its output.

The transmission line must have uniform characteristic impedance throughout its entire length. Any kinking or damage in manufacture or installation can result in localised variations in the characteristic impedance, giving rise to power loss and harmful reflections. In running data cable, especially unshielded twisted pair (UTP), such as Cat 5e and beyond, care must be taken to prevent workers from stepping on cable that is laid out on the floor. Coax is somewhat tougher, but still subject to damage.

Impedance matching is also important at the input end of the transmission line. A mismatch between the source equipment and transmission line characteristic impedance would limit power transfer and give rise to rereflections of any reflections coming from the output end or resulting from imperfections along the length of the transmission line.

We have seen how, above a certain frequency and line length, a conductor becomes no longer merely a piece of wire, but a transmission line, and some of the consequences of that change.