|
Coaxial cable is essentially a wave guide transmitting radio and television frequencies down the cable and is immune to electromechanical interference.————————————————————————————————— HARVEY M. DEITEL, BARBARA DEITEL, in An Introduction to Information Processing, 1986 Coaxial CablesCoaxial cable (Figure 7-10) has a single wire with a very high capacity (that is, a large bandwidth). The conductor is wrapped in insulation, which is, in turn, covered with a wire mesh that keeps out electrical “noise” (static). The great capacity of coaxial cable allows it to carry many channels simultaneously, eliminating the need for thousands of separate wires to be strung. Cable TV companies use coaxial cable to bring many channels of subscription television programming to residences. Coaxial cable is also popular in local networking, as we will see later in the chapter. 
Sign in to download full-size image Figure 7-10. Coaxial cable. View chapterPurchase book John Crisp, in Introduction to Copper Cabling, 2002 Coaxial cablesCoaxial cable has a central insulated conductor which may be a solid wire or stranded. It is then enclosed in a conducting layer which is usually a copper or aluminum mesh or sometimes with a solid metal sleeve. It is then covered by an outer insulator called a jacket. The earthed braid provides a barrier against EMI moving into and out of the coaxial cable. The central core and the outer sheath share the same axis, hence they are coaxial and the cable is referred to coaxial cable or more usually just ‘coax’ (see Figure 7.1). 
Sign in to download full-size image Figure 7.1. Coaxial cable It can carry more data than the twisted pair that we will look at in a moment but is generally more expensive – there are many different propriety standards – and is not easy for general use. Compared with other cables, it is rather bulky and is more difficult to install as it cannot bend so easily. Coax is widely used for video and television but is not recommended for carrying data. The new standards are likely to continue its decline and possible demise. View chapterPurchase book David Large, James Farmer, in Broadband Cable Access Networks, 2009 2.2.1 DefinitionCoaxial cable is constructed with a center conductor surrounded by a dielectric of circular cross-section and by an outer conductor (shield), also of circular cross-section. Signals within the normal operating bandwidth of coaxial cable have a field configuration known as transverse electric and magnetic (TEM). In the TEM mode, the electric field lines go radially between the center and outer conductor and are of uniform strength around a cross section of the cable, whereas the magnetic field lines are circular and perpendicular to the length of the cable (see Figure 2.1). In a cable with a continuous, perfectly conducting shield, no electric or magnetic fields extend beyond the outer conductor, preventing both signal leakage and ingress. 
Sign in to download full-size image Figure 2.1. Coaxial cable basics. View chapterPurchase book D.R. Kaufman, in Instrumentation Reference Book (Fourth Edition), 2010 20.3.10.3 Other Considerations for Cables and ConnectorsCoaxial cables have an inner conductor insulated from the surrounding screen or shroud conductor; the screen is grounded in operation to reduce external interference coupling into the inner conductor. The inner conductor carries the radio signal. Industrial wireless devices are generally designed to operate with a 50 ohm load—that is, the coaxial cable and antennas are designed to have a 50 ohm impedance to the radio at RF frequencies. At the high frequencies used in wireless, all insulation appears capacitive, and there is loss of RF signal between the inner conductor and the screen. The quality of the insulation, the frequency of the RF signal, and the length of the cable dictate the amount of loss. Generally, the smaller the outer diameter of the cable, the higher the loss; and loss increases as frequency increases. Cable loss is normally measured in dB per distance—for example, 3 dB per 10 meters, or 10 dB per 100 feet. Cables need special coaxial connectors fitted. Generally, connectors have a loss of 0.1 to 0.2 dB per connector. View chapterPurchase book Clive Poole, Izzat Darwazeh, in Microwave Active Circuit Analysis and Design, 2016 3.3 Co-axial cableCo-axial cable, or “co-ax” is a very common form of RF transmission line that consists of a solid inner conductor together with a tubular outer conductor, the space between them being filled with a dielectric material, as shown in Figure 3.5. The term co-axial comes from the inner conductor and the outer shield sharing a geometric axis. Co-axial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880 [8]. 
Sign in to download full-size image Figure 3.5. Co-axial cable construction. The insulating material separating the inner and outer conductors may be a solid dielectric, such as polyethylene (PE), polypropylene (PP), fluorinated ethylene propylene (FEP), or polytetrafluoroethylene (PTFE). Alternatively, larger diameter co-axial cables may use air as a dielectric, in which case solid dielectric spacers need to be included to provide physical separation between the two conductors. The outer conductor may be of solid or mesh construction, the latter offering increased flexibility. RF co-axial cable should not be confused with the numerous types of shielded cable used to carry lower frequency signals, although these may be superficially similar in appearance. In the case of RF co-axial transmission line, the dimensions must be precisely controlled to achieve a specific conductor spacing, so as to maintain a constant characteristic impedance. By contrast, the conductor spacing in low-frequency shielded cable is of no significance. For the co-axial cable of Figure 3.5 the shunt capacitance per unit length, in farads per meter is given by [3]: (3.3.1)C=2πεln(D/d)=2πεoεrln(D/d) where D is the outer conductor internal diameter and d is the inner conductor diameter. Series inductance per unit length, in henrys per meter is given by [3]: (3.3.2)L=μ2πlnDd=μoμr2πlnDd At low frequencies, the resistance per unit length is the sum of the DC resistance of the inner conductor and the outer conductor. At higher frequencies, the skin effect increases the effective resistance by confining the conduction to a thin layer of each conductor. The shunt conductance is usually very small because the types of insulators in use today all have a very low loss tangent. Characteristic impedance of co-axial cableNeglecting resistance per unit length (a reasonable assumption for most practical cables), the characteristic impedance is determined from the capacitance per unit length, given by equation (3.3.1), and the inductance per unit length, given by equation (3.3.2), by applying expression equation (2.5.11) for a generic transmission line as follows (assuming μr = 1): (3.3.3)Z0=LC=12πμoμrεoεrlnDd≈138Ωεrlog10Dd The loss per unit length is a combination of the loss in the dielectric material filling the cable, and resistive losses in the center conductor and outer shield. These losses are frequency dependent; the losses becoming higher as the frequency increases. Skin effect losses in the conductors can be reduced by increasing the diameter of the cable. Consequently, very low loss co-axial cable tends to be of large diameter and often has air as a dielectric (with solid dielectric spacers). The velocity of propagation inside the cable depends on the dielectric constant and relative permeability (which is usually 1), that is: (3.3.4)v=1εμ=cεrμr The co-axial cable could be considered as a circular waveguide with the addition of a center conductor. This means that energy will propagate in the cable in distinct modes. The dominant mode (the mode with the lowest cut-off frequency) is the TEM mode, which propagates all the way down to DC (i.e., it has a cut-off frequency of zero). The mode with the next lowest cut-off is the TE11 mode. This mode has one “wave” (two reversals of polarity) in going around the circumference of the cable. To a good approximation, the condition for the TE11 mode to propagate is that the wavelength in the dielectric is no longer than the average circumference of the insulator; that is that the frequency is at least: (3.3.5)fc≈1πD+d2με=coπD+d2μrεr Hence, the cable is single-mode from DC up to this frequency, and might in practice be used up to 90% of this frequency [9]. Assuming the dielectric properties of the material inside the cable do not vary appreciably over the operating range of the cable, the characteristic impedance given in equation (3.3.3) is frequency independent above about five times the shield cut-off frequency. The shield cut-off frequency is the frequency above which the energy in the cable is partially propagating in the form of both waveguide modes and TEM modes, all traveling at different velocities. Below the shield cut-off frequency, the energy propagates through the cable as a TEM wave with no electric or magnetic field component in the direction of propagation. Co-axial cables are widely used to carry RF energy from A to B, sometimes over considerable distances. In the case of flexible co-axial cables, the inner conductor is usually made from multi-stranded wire and the outer conductor is made from wire braid. At microwave frequencies, we often come across “rigid” or “semi-rigid” co-axial cables where both inner and outer conductors are fabricated as solid metal cylinders, common materials being tinned or silver plated copper, copper clad steel, and copper clad aluminum. The advantage of rigid and semi-rigid cables is that they have lower losses than flexible co-ax cables and are generally used in applications where flexibility is not so important, such as fixed interconnections between subsystems inside pieces of equipment. Some sections of semi-rigid co-ax, with typical RF connectors, are shown in Figure 3.6. 
Sign in to download full-size image Figure 3.6. A selection of semi-rigid co-axial cables, with RF connectors. (reproduced by kind permission of L-com Global Connectivity, 45 Beechwood Dr. N. Andover, MA, USA (http://www.))Finally, it is worth noting that for low-power situations (e.g., cable TV), co-axial transmission lines are optimized for low loss, which works out to be a characteristic impedance of about 75 Ω (for co-axial transmission lines with air dielectric). For RF and microwave communication and radar applications, where higher powers are more often encountered, co-axial transmission lines are designed to have a characteristic impedance of 50 Ω, a compromise between maximum power handling (occurring at 30 Ω) and minimum loss [10].
|
