Coaxial cable for wireless communications
The most important lines of communication are the ones closest to home.
System coverage, spectrum efficiency and cost-effectiveness all drive wireless communications technologies. Yet, in the rush to improve communications systems, it is often the proverbial “nail in the shoe of the horse that missed the battle” that gets overlooked. Of all the elements in land mobile, cellular, paging and PCS systems, the most unassuming is also the most ubiquitous_coaxial cable.
Coax is a major part of a technician’s work: for transmission lines, for the numerous feeders and interconnects that link system components and, in recent years, “leaky” cable for in-building antenna systems. The essential idea behind coaxial cable (for the first two cases) is simple and elegant: two concentric conductors to provide a self-shielded line, minimizing radiation loss that might affect other components, with as little attenuation as possible. Because they have a range of impedances from 30V to 100V and no external fields (except intentionally, with leaky lines), other than at the terminations, coaxial lines are used everywhere in RF. In practice, a number of factors complicate the choice and use of coax. Size, insertion loss, flexibility, connectibililty, attenuation and cost are all variable when considering different applications and environments.
Reviewing the basics The basic elements of a coaxial line are a center conductor, a separating dielectric medium, an outer conductor and a jacket. The composition of these elements varies from manufacturer to manufacturer, and usually depends on the use.
* Center conductors _ Heat is one of the factors affecting the power capability of a coaxial line, and heat is mostly generated in the center conductor. For wireless applications, center conductors are usually solid copper wire. Some stranded center conductors have been introduced that trade slightly higher attenuation for the benefit of additional flexibility in applications that require tight turns or for rotating antennas. Temperature variations can also affect the length and connectivity of the center conductor over long runs, so some manufacturers stress the bonding between the conductor and the surrounding dielectric.
* Dielectrics _ Dielectrics perform two functions: they isolate (insulate) the two conductors and transfer heat from the inner conductor outward. The type of dielectric used limits the power handling of the line. (The peak power handling, or voltage breakdown, of the coax is also frequency-dependent.) Several dielectrics are in current use; primarily air, synthetic beads and air, solid synthetics (including polyethylene [PE], polypropylene [PP], tetrafluoroethylene [TFE] and Teflon), and synthetic foam. Foam increases the velocity factor of the line and simultaneously reduces line loss. Manufacturers have even produced varieties of foam, with different bubble, or cell, structures. Compression strength of the foam can be a factor in certain types of installations.
Air dielectrics are used primarily on transmission lines in exposed settings. Because condensation in the line can occur when the ambient air temperature reaches the dewpoint of the air within the line, dry-gas pressurization or dehydration equipment can be used. This serves to prevent corrosion, voltage breakdown and an unwanted increase in voltage standing wave ratio (VSWR) from degraded electrical performance of the conductors.
* Outer conductors _ Perhaps the widest variation, and basis for competition, among various coaxial cable types lies in the outer conductor. Outer conductors may be solid (including smooth-wall, corrugated or helical types), tape, braid or a combination. Proponents of solid outer conductors point to their moisture resistance and consistent electrical performance. Braiding proponents point to the comparative lower cost, simpler connectivity and less likelihood of permanent damage from kinking. Some technicians prefer to use corrugated conductors outdoors for their low-loss capability, such as in antennas feeders, and use braided conductors indoors, but overall desired performance and use should be considered for each system installed. One particular concern for inter-rack and intra-rack connections is the bending radius ability of the cable.
* Jacketing _ Most RF coaxial cables are jacketed with polyvinylchloride (PVC) plastic because it provides flexibility and a moisture barrier in a relatively thin skin. Elastomer jackets are also used in high-flexibility types of cable. Additional treatments are given some cables used outdoors to block deterioration of the plastic by UV radiation from the sun. Such deterioration can lead to moisture intrusion of the line. For installation purposes, the ease of stripping also depends on the jacket type.
As mentioned previously, bonding of these four basic cable elements to each other is receiving increased attention from manufacturers, particularly the bonding of the center conductor to the dielectric and the bonding of tape-and-braid outer conductors to the dielectric.
Radiating cables A radiating cable couples the RF energy propagating along its length to the surrounding environment, with a minimum of downline attenuation, and thereby functions as a continuous antenna. Used in bounded locations, such as tunnels or in buildings, radiating cable has shields or milled openings designed to radiate low levels of energy. Placement of these lines next to conductive or high-dielectric constant materials (the ground, a tunnel wall, in free-space, etc.) can increase or decrease desired performance.
Other factors Signal loss as a measure of transmission performance for coaxial cable is usually quoted in terms of decibels per 100 feet. Variations among manufacturers are about 0.3dB at 450MHz and 0.4dB at 900MHz. In most mobile communications applications, low loss, rather than high power capacity, determines cable size. Shielding from interference is another factor, particularly between transmit and receive cables. Shielding in semi-rigid cables with solid outer conductors is about 120dB. Tape and braid flexible cables provide about 90dB of shielding. With either type, the weak point in shielding is where the cable joins the connector.
Codes and standards for your cable installation should also be considered. Is a plenum-installation fire rating required? Do military-type performance standards have to be met? Time required for installation and maintenance, dependability of supply, material cost, crush or compression strength are all variables that are valued differently for each type of user, commercial, public or private, and each type of installation, transmission or interconnection.
Consult with several manufacturers or their distributors. Get flexibility, installation, cost and performance quotes that are consistent with the environment and system design for your specific application.
The following companies are among the manufacturers and suppliers of coaxial cables and connectors. For a complete list, refer to the Buyers’ Guide section of MRT, December 1997, page 22.
Andrew 10500 W. 153rd St. Orland Park, IL 60462-3071 708-349-3300; 800-255-1479; Fax 708-349-5444 www.andrew.com
Cablewave Systems Div. (RFS) 60 Dodge Ave. North Haven, CT 06473-1124 203-239-3311; Fax 203-234-7718
Commscope-General Instrument PO Box 1729 1375 Lenoir-Rhyne Blvd. Hickory, NC 28603-1729 704-459-5030; 800-982-1708 Fax 704-459-5099 www.commscope.com
Eupen Cable U.S.A. 284 Racebrook Road Orange, CT 06477 203-799-7900; Fax 203-799-3871 www.eupenusa.com
Times Microwave Systems 358 Hall Avenue Wallingford, CT 06492-3555 203-949-8432; 800-867-2629; Fax 203-949-8423 www.timesmicrowave.com
Trilogy Communications 2910 Highway 80 E Pearl, MS 39208-3409 601-932-4461; 800-874-5649 Fax 601-939-6637 www.trilogycoax.com