Optimized Performance from Coaxial Feeder Cables and Connectors for Wireless Systems

Optimized Performance from Coaxial Feeder Cables and Connectors for Wireless Systems

Hugh R. Nudd

Andrew Corp.
Orland Park, IL

Modern multichannel wireless communication systems, such as cellular telephone systems, require high grade feeder cables for the connections between tower-mounted antennas and the radio equipment, which is typically located in equipment shelters at the base of the tower. Similarly, the terminating connectors that provide standard RF interfaces for the cables must meet exacting mechanical and electrical requirements for these systems. This article discusses the various critical characteristics for cables and connectors, how these characteristics are optimized in design and manufacture, and how the resulting system performance is maximized.

RF FEEDER CABLES

The explosive growth in services such as paging and mobile telephones has increased demand for coaxial cable. The growing complexity and sophistication of these systems have demanded increasingly stringent electrical and mechanical requirements that depend on appropriate material selection, product design and process technology.

Impedance

Coaxial cable impedance is determined by the dimensions of the inner and outer conductors and by the density of the foam dielectric, which also determines cable velocity. The standard 50 ohm RF characteristic impedance is a compromise between optimum attenuation (approximately 75 ohm) and optimum power handling at a lower impedance (approximately 35 ohm). Typically, RF cable impedance is required to be within 1 W from the 50 ohm nominal, with no single discontinuity producing a voltage pulse reflection of > 0.5 percent (a reflection of –46 dB or an impedance variation of 0.5 ohm). Impedance discontinuities can produce SWR and group delay discontinuities, which degrade system performance. Today’s process equipment allows cables to be manufactured with impedance variation controlled to within 0.5 ohm from nominal.

Shielding

Good RF shielding is a characteristic of RF cable with a continuous outer conductor. The construction is almost perfectly nonradiating and allows many feeders to be used in close proximity with no coupling or cross talk between cables. High shielding (measured as a low transfer impedance) depends on the ratio of the outer conductor thickness to the skin depth at the operating frequency. Foam cable outer conductor thickness is a few hundred skin depths in copper at wireless frequencies, and inherent shielding of a seam-welded, corrugated cable is several hundred decibels and improves with increasing frequency. In practice, shielding is limited by the connectors but is at least 120 dB. Braided or foil-braid cable may have a shielding effectiveness of approximately 90 to 100 dB at typical wireless frequencies; this effectiveness decreases at higher frequencies.

Attenuation

For maximum system efficiency, RF signals must be carried between the antenna and the radio equipment with minimum loss. Typically, system loss budgets allow 1 or 1.5 dB feeder loss. Attenuation varies approximately inversely with the cable diameter. For a given cable size, the best balance of electrical and mechanical properties is attained by optimum conductor corrugation design and selection of the best conductor and dielectric materials. Copper strip of at least 98 percent International Annealed Copper Standard conductivity is recommended for the conductors and physically blown, low density foam dielectric is recommended to provide the lowest available loss factor consistent with satisfactory mechanical and processing properties. Using low loss dielectric material is particularly important in the larger cables where a higher proportion of the total attenuation is determined by the dielectric. Table 1 lists conductor and dielectric contributions to cable attenuation for various cable sizes.

SWR

Reflections from three sources affect the measured SWR of a cable and its terminating connectors: reflection from the connector closest to the measuring equipment; reflection from the connector at the far end of the cable, reduced by cable attenuation; and additional reflections produced in the cable itself. Cable-produced reflections arise chiefly from small, periodic dimensional variations introduced by manufacturing processes. A single variation produces a small, undetectable reflection, but the effect of periodicity is that many of these small reflections are in phase and combine at particular frequencies (where the periodic spacing is an integral number of half-wavelengths). At other frequencies, individual reflections, essentially distributed among all phases, effectively cancel.

The result is that the cable contributes a number of narrowband spikes of higher SWR. Minimizing cable SWR requires a high degree of process control to eliminate or reduce sources of periodic dimensional or other variations. In practice, this degree of control requires high precision in all manufacturing tooling with continual monitoring and corrective actions should any SWR spike start to grow unacceptably.

Power Ratings

Peak power ratings (limitation by voltage breakdown) of foam-dielectric cables are determined by the cable dimensions. Average power ratings (limitation by heat transfer) are determined by the maximum allowable temperature of the foam dielectric. Continuous inner conductor temperatures of 100°C are the practical limit for polyethylene materials for satisfactory life.

Peak and average power ratings rarely are a limitation for feeder cables in typical wireless communication systems, though average power can be a concern for higher power (tens of watts per channel) multichannel systems or high power services such as paging. Peak and average power ratings for foam-dielectric feeder cables are listed in Table 2 .

Intermodulation

Intermodulation (IM) is caused by signals mixing in a nonlinear circuit (where current is not exactly proportional to voltage or output power is not exactly proportional to input power) that produces spurious signals at similar frequencies. These spurious signals can interfere with and degrade system performance. IM has been identified as a potential problem in multichannel communication systems where IM products generated by high power transmit signals can fall into receive bands.

A major cause of IM generation is ferromagnetic material in the current path. Ferromagnetic materials (stainless steel or nickel) are inherently nonlinear because their magnetic hysteresis effects produce permeabilities that depend on magnetic field strength.1 A second cause of IM is low contact pressures at conductor joints, which cause significant fractions of RF current to flow through surface oxide layers that are partially rectifying and, hence, nonlinear.2 Therefore, IM generation is minimized or eliminated by eliminating ferromagnetic materials in current paths, minimizing the number of conductor junctions and ensuring good metal-to-metal contact at junctions either by soldering (welding or brazing) or by designs that develop high contact pressures.

Multiconductor cable constructions (braid or foil-braid outer conductors or with stranded inner conductors) contain many conductor junctions for current to pass across. Since high contact pressures cannot be provided at these junctions, the cables contain a large number of IM-generating points. Seam-welded/corrugated cables have a single inner conductor for better IM performance. Comparative IM performance of a foil-braid cable and a seam-welded/corrugated cable is shown in Figure 1 .

Crush Strength

Crush strength measures the ruggedness of a cable and its ability to withstand the stresses of installation. Crush strength of a corrugated cable is determined by the outer conductor material and thickness and by the corrugation design, particularly the depth-to-pitch ratio. Deeper corrugations increase crush strength but also increase attenuation because of the longer current path. A thicker outer conductor increases crush strength but degrades flexibility (increases bend moment and decreases bend life).

Flexibility

For ease of installation, a foam cable must have appropriate bend moment, minimum bend radius and bend life. Super-flexible cables, used primarily in short jumper assemblies, are more flexible than main feeder cables. Superflexible cables have deeper corrugations, but the trade-off is higher attenuation. Newer, extra-flexible cables have a corrugation design midway between the designs of super-flexible and standard feeder cables, as well as intermediate attenuation and flexibility characteristics.

RF CONNECTORS

Generally, modern RF connector designs are smaller, lighter, quicker to attach and demonstrate improved performance. A connector designed for rapid attachment is shown in Figure 2 . The RingFlare™ connector features an expandable clamping ring that flares the cable automatically when the two-piece connector is tightened.

New interfaces such as the 4.1-9.5 DIN, similar in form to the 7-16 DIN only smaller, are becoming popular. Various RF connectors with type-N and 7-16 DIN interfaces, both used commonly,3 are shown in Figure 3 .

SWR

Connector SWR sets a minimum level for the SWR of a cable/connector combination and is the dominant factor in the SWR of short cable assemblies. Typical performance is a 30 dB return loss for two connectors on a short cable assembly (a single connector has a reflection that is 6 dB better than this). Coaxial connectors for wireless applications should have good SWR up to 2 GHz to cover extended specialized mobile radio, cellular and personal communications service frequency bands. Modern electromagnetic modeling software, wideband design and precision machining of the electrically critical internal dimensions help achieve low SWR.

Shielding

Shielding performance is set by the connectors and is best determined by measuring the signal radiated into a surrounding cavity. Connectors with reliable high pressure clamp contacts at the cable attachment display shielding performance of –120 dB or better. The leakage signal is so low that it is below the measurement threshold of typical test equipment, and the sensitivity of the equipment (–120 dB, for example) sets the lower limit of the performance that can be claimed.

Intermodulation

A connector designed for low IM generation will have no ferromagnetic materials in the current path, the minimum number of internal components (for the minimum number of conductor junctions in the current path) and good contact (by soldering or high contact pressure) at all conductor junctions. Table 3 lists comparative measured IM levels for various coaxial connectors resulting from two 20 W carriers.

Power Ratings

Peak and average power ratings for coaxial connectors are determined by voltage breakdown and thermal transfer considerations. Essentially, a connector is a transition from the line size of the coaxial cable to that of the connector interface, and the peak power rating is determined by the smallest line size, which is usually the connector interface. When the interface size is larger than the cable size (a 7-16 DIN connector on a 0.375" cable, for example), the cable rating sets the value for the combination. Average power rating, which is more difficult to specify, is determined by the maximum safe, long-term inner conductor temperature for the cable-connector combination. The RF power that produces this temperature depends on a number of variables, some of which are unknown to the connector manufacturer. For example, average power ratings may increase or decrease according to the degree of heat sinking provided by the connector body and its mounting arrangement. Power ratings may be affected by contact pressure applied by spring fingers at the interface and the associated contact resistance (often the connector manufacturer supplies only one of a pair of mated connectors). For these reasons, some manufacturers do not provide average power ratings for connectors. The better manufacturers supply conservative ratings derived from testing under standardized conditions. Peak and average power ratings for connector interfaces are listed in Table 4 .

Sealing and Weatherproofing

Outdoor RF connections, such as antenna-to-cable and jumper-to-feeder connections, must block moisture ingress reliably to prevent failure over the life of the system (typically 20 years). Connectors for outdoor use should incorporate sealing O-rings and meet the water exclusion requirements of IEC529, IP68. A typical test requirement is no evidence of moisture ingress after total immersion under a three-foot (1 m) head of water for 24 hours. Generally, extra sealing protection is added with butyl vinyl tapes or with a sealing elastomeric tube that shrinks down on the connection when an internal support is removed.

SYSTEM PERFORMANCE

The electrical characteristics of the cable and connector products directly influence the important system parameters of attenuation, SWR and IM generation.

Attenuation

Minimal attenuation of a feeder cable will maximize output power, coverage and efficiency. However, in practice, because the lowest attenuation cables are the largest in size, a trade-off is required between attenuation and mechanical considerations (ease of installation, space occupied on the tower and windloading). System loss budgets may allow a maximum of 1 or 1.5 dB for a single feeder, which determines the smallest cable size that can be used for a given antenna height.

SWR

Low SWR in the RF transmission path minimizes distortion from echoes and group delay variation from reflections in the system. System SWR is determined by combining the effects of the individual components. To determine the input SWR of an antenna-feeder system, the components at the top of the run (antenna and top jumper) are combined, the resulting reflection is reduced by the line attenuation to produce the effect at the bottom of the line and this effect is then combined with the components at the bottom of the line (the feeder, bottom jumper and surge arrestor).

Adding all reflections in phase (adding the reflection coefficients) produces the unlikely case of maximum possible SWR. Reflections almost always combine in random phase and a technique such as determining the root of the sum of the squares of the individual reflections is used to find a typical, or probable, system SWR.

IM Generation

A modern multichannel communication system IM requirement may be –110 dBm with 20 W (+43 dBm) carriers. Individual components can require IM generation levels better than this because combined IM signals produce a higher overall result. Typical IM levels required from individual cables and connectors may be –115 dBm, or –158 dBc at the 20 W carrier level.

CONCLUSION

To maximize wireless system performance, RF feeder cables and connectors are precision engineered, use specialized materials and are manufactured using exacting process technologies to attain the required electrical and mechanical characteristics.

ACKNOWLEDGEMENT

The foam-dielectric feeder cables are HELIAX® coaxial cables.

References
1. G.H. Stauss (Ed.), “Studies on the Reduction of IM Generation in Communication Systems,” NRL Memorandum Report 4233 , Naval Research Laboratory, Washington, DC, July 1980.
2. B. Carlson, “RF/Microwave Connector Design for Low IM Generation,” IICIT Proceedings , 25th Annual Symposium, 1992.
3. J. Paynter and R. Smith, “Coaxial Connectors: 7/16 DIN and Type N,” Mobile Radio Technology , April 1995 and May 1995. Reprinted as Andrew Corp. Bulletin No. 3652, May 1995.

Hugh R. Nudd joined Andrew Corp., Lochgelly, Scotland, in 1978 and transferred to the company’s Orland Park, IL headquarters in 1982, where he has handled design engineering for coaxial cable, waveguide, connector and accessory products. Currently, Nudd is cable product development manager at Andrew Corp.