In this age of misinformation, you can add coaxial cable to the list of items that are easy to go wrong based on an internet search. Many of the false online comments about coaxial cable relate to cable loss, cable impedance, and standing waves. Here are some basics that can help sort out the truth from cable mythology.
One misconception concerns cable impedance and cable loss. The characteristic impedance (Z0) on the transmission line is the ratio of the amplitudes of the voltage and current of a wave propagating along the line. Another way to say is that this is the ratio of the voltage and current of a wave moving in one direction in the absence of reflections in the other direction. In addition, the characteristic impedance is by definition the input impedance of a transmission line, ideally when the transmission line is infinitely long.
All homogeneous transmission lines have a characteristic impedance. There are two main standards for coaxial cable. Fifty-ohm coaxial cable is used in professional and commercial installations, while seventy-five-ohm coaxial cable is found mainly in home TVs and high-frequency FM installations. Demanding computer applications require higher values. Other available characteristic impedances include 25, 95 and 125 ohms.
The physical specifications that determine the characteristic impedance are the distance between the conductors, which is determined by the thickness of the dielectric layer, the dielectric index and the thickness of the conductors, which determines their resistance. You can find it online coaxial cable impedance calculators which receive input data on the width of the dielectric of the outer diameter, the width of the inner diameter of the conductor and the values of the dielectric constant or propagation rate (VoP, the speed at which RF propagates through cable conductors) to calculate the inherent coaxial impedance cable.
However, there is some confusion about the internal impedance and cable loss. Different coaxial cables with the same internal impedance may exhibit different signal loss properties. Consider the coaxial RG-59 / U, often used for low power video and radio signal connections. The cable has a characteristic impedance of 75 Ω and a capacity of about 20 pF / ft (60 pF / m). The suffix / U means for general use. The RG-59 is often used at baseline video frequencies, such as composite video. But its high-frequency losses are too great to allow its use over long distances. These applications use RG-6 or RG-11 instead. Both RG-59 and RG-6 coaxials have an internal impedance of 75 Ω, although their loss properties are quite different.
A video blogger even made a YouTube video concerning the replacement of the RG-59U by the RG-6 in a normal home television installation. In short, it recorded a signal about 15% higher, replacing RG-59 with Rg-6. In the residence he served, television reception switched from 14 channels to 33 channels with better coaxial cable.
The reason for the different performance becomes clear when studying the cable structure. Conventional coaxial cable RG-59 has 20 AWG copper central conductor. Some brands use copper-coated steel for their central conductor material. The standard RG6 coaxial cable has a larger 18 AWG copper core conductor, which contributes to its lower losses. There may be differences in shielding. RG-59 cables typically have less shielding than RG-6 versions. The standard coaxial RG-6 has two shielding layers compared to one for the RG-59. The low-loss versions of the RG-6 have four shield layers and are called quad-shields. For super long cable lines there are special cables with low losses. One such type is the LMR 600, although it is 50 Ω and is designed primarily for power and wireless communication infrastructure.
Example of a low loss cable LMR-600 used for long cable lines. Pay attention to the knitted shield and the layers of foil shield.
When installed in a building, residential, commercial or industrial, or outside it, the design and installation must comply with the current version of the National Electrical Code (NEC), unless the property is owned and operated by the electricity company, in which case is covered by the National Electrical Safety Code. NEC covers construction specifications, permitted uses, unauthorized uses and related aspects of coaxial installation, as well as other permitted wires, devices and operating procedures).
Coaxial applications are:
CMP – Plenum, suitable for use in ducts, chambers and other spaces used for ambient air.
CMR – riser, suitable for use for vertical passages in the shaft.
CMG – General, suitable for communications, if not in the riser or plenum.
CMX – Suitable for use in homes, if not in the plenum.
These classifications refer to the flammability of the outer layer and its potential for hazardous smoke distribution.
There are other applicable NEC cable type items, including item 725, Class 1, Class 2 and Class 3 for remote control, signaling and limited power circuits.
A signal transmitted in a transmission line is reflected in whole or in part back to the source when it encounters an interruption in the characteristic impedance in the line or if the line is not completed in its characteristic impedance. This may be due to incorrect choice of hardware for termination or cutting, mismatch of the receiver or physical damage to the line that occurs after installation.
This leads to another misconception about coaxial cables: radiation. There are some comments that suggest that a mismatch at the end of the cable may cause the coaxial cable itself to emit some of the signal. In fact, the only condition that can cause a coaxial cable to emit is when the current flowing in the central cable is different from the current flowing in the screen. In other words, the sum of the central wire and the currents on the screen is not equal to zero, which leads to what is called total current. Note that this can happen even when the impedances at each end of the cable match the impedance of the cable.
The classic example of when an unbalanced current can flow in a coaxial cable is when a transmitting antenna is attached to one end and the element attached to the central conductor of the cable emits more than the element attached to the cable shield. There may be other situations where unbalanced currents may occur, but these do not usually occur when cables connect ordinary electronic equipment.
Finally, we mention a misconception about standing waves. Standing waves are a consequence of signal reflections. If a step voltage is applied to the end of a cable with a characteristic impedance corresponding to that of the source, the step will propagate down the line. Assuming the signal meets an open circuit at the end of the cable, the current will abruptly go to zero. As the charge continues to arrive at the end of the line until no current leaves the line because it is open, maintaining the charge requires that there be equal and opposite current flowing at the end of the line. This equal and opposite current is the reflected current. There is a reflected tension that behaves in a similar way.
As the reflected wave moves back up the cable, it adds to the incident wave, where both are in phase, and subtracts from the incident wave, where both are out of phase, forming a standing wave. Where the points of minimum energy and maximum energy appear in the cable depends on the frequency.
The idea of a standing wave has given rise to a misconception that is still found in some corners of the Internet. The misconception is that a coaxial cable must have a length corresponding to half the wavelength at the frequency of the signal it carries. Websites that serve CB radio sometimes still contain statements of this kind. The rationale for this statement is that the point of the half-wavelength corresponds to the minimum voltage for the frequency in question. An interesting point about such advice is that it often ignores the VoP of coaxial cable when offering cable length instead of basing RF wavelength advice in free space. In this way, a cable length equal to half the wavelength in free space will not lead to a minimum voltage at its end for the frequency in question.
In any case, the standing wave ratio measured in a coaxial cable depends on the measuring point. This really has nothing to do with the energy supplied to the load. The way to minimize reflections in a coaxial cable is simply to make sure that the impedance of the source and the load is equal to that of the cable.