In the Age of Fiber, Long Copper Runs are Still Necessary—How Low Loss Coaxial Cables Meet the Need for Large Coax Installations
by Amar Ganwani – Senior Product Line Manager, L-com
There has been a significant transition from copper-based transmission to fiber. This trend is readily apparent in the cellular industry. The traditional long coax installed from the base of a cell tower to its tower-mounted amplifiers (TMAs) has shifted to a long fiber run that utilizes the CPRI (or eCPRI) protocols in 4G and 5G fronthaul. Even for 5G backhaul, deep fiber installations are viewed as critical components to support the traffic loads of dense urban areas alongside microwave backhaul, millimeterWave backhaul and a non-terrestrial infrastructure. However, this does not entirely sidestep the continuing need for short to medium runs of copper-based coax. These cables are paramount in the growing cellular infrastructure and virtually every other wireless communication system, including SATCOM, IoT, WLAN, GPS, wireless telemetry, and defense systems.
Coaxial cables find utility in practically every high frequency application—from the various types of test and measurement applications to outdoor commercial installations, where long lengths of coax are still deployed in cell towers and distributed antenna systems (DAS). They’re found in everything from temperature- and humidity-controlled labs to harsh marine and space applications and thus, vary in structure and construction to best suit the use case. The process of installing a coaxial cable in the field is markedly different from the comfortable confines of a test lab. Tight spaces limit the amount of torque that can be put on a connector. Vibrations can loosen these connections, worsen passive intermodulation distortion (PIM), and cause unforeseen attenuation in longer cable runs. The construction of most commercial bulk coaxial cables has not been optimized to mitigate the damaging effects of environmental and mechanical stresses. Low loss coaxial cables aim to fill that gap that traditional RG coaxial cables leave with a level of cable and connector ruggedization that ensures consistent performance over its lifetime. This article dives into the various use cases of the low loss coaxial cable and exactly how these cables are suited to outdoor installation applications.
The Difference Between a Low Loss Coaxial Assembly and a Bulk “RG” Cable
Coaxial cables are the go-to interconnect for interfacing with high frequency circuits. The non-dispersive nature of this media allows these cables to accomplish signal transmission from one point to another without much concern around introducing undesirable modes. Another advantage of coaxial cables is their broadband performance from DC up to their upper frequency limit; this characteristic allows a singular coaxial assembly to remain usable across several frequency bands. The major electrical specifications for coaxial cables are return loss, or its variation in voltage across the length, referred to as Voltage Standing Wave Ratio (VSWR), and the attenuation, or insertion loss per unit length. The unit length can vary between insertion loss (in decibels) per 10 feet and go all the way up to 100 feet.
Coaxial cable vendors have generally leveraged the old military RG specification for coax. This standard was originally released in the 1930s and defined the inner conductor and outer conductor diameters and material in order to control the impedance (50Ω, 75Ω or 92Ω) and electrical performance of the transmission line. The term “RG” originally referred to either “Radio Guide” or “RF Government” by the military. Now, this military standard has long since retired and is replaced with the MIL-DTL-17 specification that provides a much more detailed explanation of the construction of a mil-spec coax and a list of maximum attenuations over a number of frequencies. The term “RG” has become more of a generic industry term that might vaguely refer to the coax dimensions of a part. However, the variations between these assemblies are significant. The choice of conductor and dielectric materials are not consistent. There are also differences in center conductors—stranded or solid inner, as well as in shielding materials—braided or foil-. All of these factors significantly impact the attenuation and VSWR performance of the cable over its lifetime.
The low loss coaxial cables offer the same cost-effectiveness as bulk RG coaxial cables by being manufactured in reels with long lengths of coax. However, these cables are custom tailored to outdoor commercial installations with either a high level of flexibility, a fire retardant, low smoke, non-halogenated jacketing material, or waterproofing for longer cable runs and direct burial cables.
Cables for Short Coax Runs
Applications that Leverage Short Lengths of Coax
The RG-316 and RG-142 cables are often used as pigtails to connect to a desired external antenna, wireless equipment, or a larger diameter cable. In other words, pigtails are essentially impromptu adapters, allowing a user to switch between a specialized board, Access Point, or Wi-Fi card connector (e.g., MMCX, MCX, SSMB, FME, QMA, S/E Type 237, etc.) to a more standard RF connector interface such as RP-SMA, N-type, RP-TNC, and SMA (Figure 1). The very same technique of using extremely flexible cable runs in order to rapidly connect with another standard RF interface can be employed in short antenna feeds as well as jumper cables. These short coaxial installations are often done in tight spaces where, for instance, the coaxial feed to an antenna must be woven and bent through interconnect/equipment to reach the antenna port.
Understanding the Low Loss Cables for Short Cable Runs
As shown in Table 1, the outer diameter of the RG-316 is a thin 2.59 mm. The inner conductors of these cables are stranded in order to increase their number of flex cycles and bend radius. This stranding significantly increases the attenuation of the cable—hence the exclusive use of short coax lengths. The low loss 100-series alternative offers nearly the same outer diameter with a lower attenuation and an installation bend radius of 6.4 mm, much lower than the conventional 1-inch (25.4 mm) bend radius for most RG-316 cables. The RG-58 and RG-142 coax cables leverage the same outer diameter of 4.95 mm and offer very similar attenuation performance; both are often used for short cable runs and jumper cables. The low loss alternatives for these cables—the 195-series, 200-series, and 240-series—each outperform the standard RG coax’s attenuation performance and installation bend radius.
Within the low loss series of coax, the 195- and 200-series offer the same outer diameter and installation bend radius of 0.5 inches (12.7 mm). However, the 200-series exhibits a more optimal attenuation performance, largely due to its thicker solid bare copper inner diameter of 1.12 mm over 0.94 mm diameter of the 195-series. The 240-series cable offers the lowest attenuation with the thickest inner conductor diameter of 1.42 mm and minimum installation bend radius of 0.75 inches (19.05 mm). These electrical and mechanical characteristics directly translate to practical considerations.
One key factor in pigtails and jumper cables is their need to be flexible. A tight bend radius is enormously helpful in mitigating any failures that could occur due to coax kinking, or accidently bending the cable so tightly to generate a permanent deformation in its cross-sectional area. This causes a change in impedance at the non-uniformity, leading to reflections at the point of the kink, and a jump in attenuation rendering the cable unusable. While the cable lengths used for pigtails and jumper cables are generally short, it is still important to minimize the losses in the system—optimizing the losses in an antenna feed will directly extend the link budget of the communications system.
Cables for Outdoor Medium Distance Runs
A Look at Applications that Require Long Runs of Coax
Longer distance runs that go well above 50 feet are often found in base station and cell tower applications. Older generations of base stations send a signal through a large feeder cable to the tower-mounted amplifier (TMA) in order to amplify the highly attenuated signal, then through jumper coax and finally to the final multi-port passive antenna for uplink and downlink communications. More modern cell towers still leverage multiple jumper coaxial cables ranging from 6 to 12 feet in length to connect from the remote radio head (RRH) to the multi-port antenna. Newer generations of base stations tend to integrate the RRH into the antenna structure to eliminate the use of jumper cables while the nascent massive MIMO base station will have an active antenna structure with an integrated Radio Unit (RU) and Distributed Unit (DU).
Similar to a traditional base station, a DAS will often utilize a long coax run. A passive DAS includes a number of remote radio heads (RRH) that are often called head-end units (HUs). These HUs interface with remote antenna units (RAUs) that are distributed throughout an area of coverage (e.g., building, stadium, subway, etc.) to provide uniform coverage. The general passive DAS architecture connects these geographically disparate multi-band antennas, or RAUs, to a more centralized HU via copper connection to process the incoming RF signals.
Wireless Internet service providers (WISP) can set up towers with point-to-point (PtP) line of sight (LoS) connections or point-to-multipoint (PtMP) non-line of sight links (NLoS) to provide broadband wireless Internet access where traditional access methods are not available (e.g., cable, satellite, etc). These towers provide connectivity for the end user as well as backhaul for the network to connect to a point where there is more equipment to support the Internet connection. Internet access can also be provided through WiMAX towers connected to WiMAX-enabled routers for broadband access. There are a number of WLAN applications that require an easily routed, low loss RF coax to connect from radio equipment to an external antenna. This can also be seen in Supervisory Control and Data Acquisition (SCADA) and telemetry applications where high gain directional antennas are used to provide medium throughput synchronous transmission to monitor and track industrial equipment. The integrity of any of these wireless connections is highly dependent on its wired backbone, much of which is coax-based. For longer runs, a thicker coaxial cable is generally leveraged because of its lower attenuation performance. The resistive losses from the conductors lessen when the diameters of the inner and outer conductors are larger, also when the conductivity of the metallic material used is higher.
Understanding the Low Loss Cables for Medium to Long Cable Runs
The low loss 400-series and 600-series cables offer an alternative to the RG-8 cable and have quickly become the standard for outdoor Wireless Local Area Network (WLAN). The 900-series is the thickest of the three with double the bend radius of the 600-series, but exhibits significantly less attenuation, allowing for longer lengths of coax. All three of these cables offer a far lower attenuation than the standard RG-8 cable. For the RG-8 drop-in, or the 600-series of coax, the attenuation is generally much lower due to the use of a foamed Polyethylene (PE) dielectric. This material exhibits a much lower dielectric constant and loss tangent than standard PE, two parameters which directly contribute to transmission line losses from the dielectric material. The 400-. 600-, and 900-series cables all utilize this very same dielectric material and are ruggedized for outdoor environments with a UV-resistant PE jacketing material.
In outdoor applications, jacketing material becomes particularly important with environmental stressors such as UV, wind, and rain, as well as moisture- and salt-laden atmospheres. Plastic materials experience photooxidative degradation when exposed to the ultraviolet rays in sunlight. This breaks down polymer chains and causes the material to become increasingly brittle and eventually exposes the internal shielding and dielectric to the elements. UV-polymer stabilizers are often inserted into common plastic materials such as PVC or Polyethylene (PE) in order to imbue them with UV-resistant properties. Fire resistant plastic materials such as flame retardant Polyethylene (FRPE) and Polyvinyl Chloride (FR-PVC) are employed to best suit riser and plenum spaces.
Fire Retardant and Direct Burial Low Loss Cables
Most stock RG coaxes do not consider fire retardant, low smoke, non-halogenated jacketing materials. However, these are necessary in order to meet code requirements for building installations (Table 2). Riser rated cables go through a vertical burn test to ensure that the jacketing material is self-extinguishing, while the plenum rated cables must be self-extinguishing and must not re-ignite. The use of non-halogenated materials is required in closed spaces where the toxic acidic fumes from halogens such as fluorine, chlorine, bromine, and iodine could be lethal. The low smoke zero halogen (LSZH) cables can self-extinguish, emit limited smoke, and do not release any halogens for potential inhalation and harm.
Direct burial low loss cables include an inert polymeric waterproofing compounding applied to both the foil and the braid, making them entirely watertight. This differs from standard underground cabling that typically applies a water resistant gel exclusively to the jacketing material. However, this may not suffice in the event of an accidental digging up of the cable, when extensive shear forces are applied to the cable by work equipment, or when there are manufacturing defects in the cable jacketing. In the event that cable jacketing is damaged, the water resistant gel will not be able to protect the coax. However, if there are several layers of protection with a robust jacketing material and waterproofed shielding, the dielectric and inner conductors will also be buffered from any moisture ingress. This ultimately protects the coax installation from its main environmental stressor under ground: moisture.
The Merit Behind Employing Low Loss Cables
The traditional reels of bulk RG coaxial cables are not typically optimized for outdoor installations. Long runs of coax are likely to prematurely fail due to exposure to moisture, vibrations, and UV. These cables are also more likely to undergo tight bends during installation, introducing yet another potential avenue for failures. Outside of all these field-based considerations, the coax itself is generally not constructed to minimize losses due to the conductors or dielectric material, but rather to manufacture the coax in the cheapest possible way.
Low loss coaxial cables are specifically designed for these outdoor wireless communications applications, including WLAN, cellular, WLL, GPS, DAS, mobile antenna, and SCADA. These cables often leverage a bare copper center conductor, a foamed dielectric, and two layers of shielding to optimize the attenuation performance. Low loss cables are also specifically designed to operate in varying environments, with outdoor, direct burial, fire retardant, zero-halogen, and general purpose cable variants. This allows the end user to select the best cable for a particular installation application as opposed to being confined to a stock model of coax with standard jacketing material.