Coaxial Cabling

Coaxial cabling is the oldest and most venerable of transmission mediums. It has been in use since at least 1898, when Hertz made his original investigations of wave propagation on transmission lines. Despite its age, the electrical performance of coaxial cable is as good as anything else.

Physically, coax is a mess (see Chapter 7, "Generic Building-Cabling Standards"). The problem with coax is topological (Figure 10.1). To access the inner conductor, you must first peel back the outer jacket, outer shield, and inner dielectric. This error-prone procedure takes a long time in the field and often results in an unreliable connection. Pre-fabricated jumper cables made with good tools, under ideal manufacturing conditions, work just fine. Field-applied connectors don't. Coax connectors (compared to UTP) are extremely difficult to apply.

Figure 10.1. A coaxial cable has four parts .

Coaxial cable suffers from an overabundance of standards. To begin with, the mechanical properties of coaxial cable are controlled by a series of RG specifications. These specifications were written in the early sixties by the American military. The electrical properties in the RG specifications are very loosely specified. These specifications leave open many questions having to do with conductor construction, stranding, surface roughness, surface plating , use of steel cores, and dielectric material selection, all of which affect the electrical properties. Most modern cables rated for a particular RG class of operation will far outstrip the capabilities of that class. [84] If you want to know how a particular cable will perform, you have to examine the vendor's specifications for that type of cable.

[84] By as much as a factor of two.

From among all the thousands of variants available, I've selected a particular set of cables manufactured by Belden ( www.belden.com ) for all the examples in this chapter. The cables are listed in Table 10.1.

Table 10.1. Selected Belden Coaxial Cable Types

Belden cable type	Class	Jacket O.D. (in.)	Conductor stranding and gauge (AWG)	Conductor composition	Dielectric
8216	RG-174/U	0.110	7 x 34	Bare copper -plated steel	Polyethylene
84316	RG-316/U	0.098	7 x 33 ½	Copper-plated steel with silver coating	TFE Teflon
8259	RG-58A/U	0.193	19 x 32	Tinned copper	Polyethylene
8240	RG-58/U	0.193	20 Solid	Bare copper	Polyethylene
84303	RG-303/U	0.170	18 Solid	Copper-plated steel with silver coating	TFE Teflon
8237	RG-8/U	0.405	7 x 21	Bare copper	Polyethylene

POINTS TO REMEMBER

• The electrical performance of coaxial cable is as good as anything else, but physically, coax is difficult to handle.
• Coaxial cable suffers from an overabundance of standards.