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Understanding Low Loss Coaxial Cables and Their Applications

Parent Category: 2019 HFE

By Dan Birch

Abstract

Coaxial assemblies form the wired backbone necessary to accomplish wireless systems. They are therefore utilized in a broad range of environments and circumstances. The military-style RG coaxial cables have been the go-to standard when first understanding what cables to leverage in a particular installation. However, there are times where these cables will not suffice and a low loss alternative is necessary. This article attempts to cover the general construction of a low loss cable as compared to standard RG assemblies and some application-specific considerations for these coax.

RG and Low Loss Coaxial Cables

The RG coaxial cable has been around for decades and was originally coined by the military as “Radio Guide” or “RF Government”. Similar to the MIL-STD-348 for connectors interfaces, the military had pre-specified dimensions for different coaxial transmission lines to generate a predictable system of interconnects to utilize in various military applications. As time passed, commercial variants of these military-born cables were introduced with similar construction and performance. Figure 1 shows some RG cables dimensions and their respective attenuations.

1904 cabl3s fg1

Figure 1 • Sample of different RG cables and their attenuation.

The range of impedances for RG cables can vary between 50 ohms, 75 ohms, and even 92 ohms. More often than not, the 50 ohm impedance coax are utilized for data applications (e.g.: WLAN, GPS, cellular, etc.) while the 75 ohm cables can be used for audio/video applications (e.g.: security system, CATV). Traditional RG cables, are quite lossy and this is especially apparent at long distances for wireless applications. For this reason, datasheets for cables used in wireless applications do not specify average insertion loss, or attenuation, in its decibel (dB) value, but attenuation over a specified distance such as: dB/100ft, dB/100m. Furthermore, the nominal attenuation value is listed at a number of frequency points within the cable’s operational bandwidth.

There is a significant need for various wireless applications (e.g.: WLAN, SCADA, PCS, ISM, etc.) to have an alternative to typical RG cable constructions--particularly for medium to long distance runs. Low loss cables offer an alternative to this type of cabling while vaguely holding up to the dimensions of the RG cables.

1904 cabl3s fg2

Figure 2 • Low loss cables will often employ materials that allow for low attenuations, including foamed dielectric materials and a solid layer of shielding.

What is a “Low Loss” Coaxial Cable?

As the name implies, Low loss coax will deliver a lower attenuation when applied as a replacement for similar diameter RG cables used in very same wireless applications. They offer a lower overall attenuation due to several key factors:

  • Solid inner conductor
  • Superior dielectric material
  • Superior shielding and more shielding
  • Application-specific jacketing materials

 

Inner Conductor

There are several factors that make a solid inner conductor less lossy than the stranded conductors that are often used in RG cables. The main contributor is the losses due to the proximity effect--the tendency for EM energy in a conductor to gather farthest away from nearby conductors carrying current in the same direction (Figure 3). This multi-conductor version of the skin effect, or the tendency for current to concentrate at the periphery of a conductor at high frequencies. And, while carefully stranded cables have been shown to reduce the losses due to skin effect [2], it does not offset the effects of proximity. This lack of uniform current distribution causes the AC resistance of a conductor to increase rapidly thereby increasing transmission loss with frequency.

1904 cabl3s fg3

Figure 3 • The proximity effect plays an important role in the degradation of signal performance at high frequencies. [1]

Another factor to consider is the conformity in the cross-sectional area of the coax. The characteristic impedance of a coaxial cable is directly correlated to the uniformity of its inner dimensions. Stranded center conductors are inherently less uniform than a solid inner conductor and can therefore cause more reflections and ultimately loss of signal. Stranded cables, however, are known to be much more flexible than solid inner conductors. Depending upon the application, this may or not may not be pertinent as there are many variations of coax with solid inner conductors that offer higher flexibility. Furthermore, the center conductor is not the only variable that limit the flexibility of a coax; the dielectric and shielding are also considerations for high flex cabling.

Dielectric

The main purpose of the dielectric material in a coaxial cable is to separate the inner conductor from the outer conductor while holding uniform cross-sectional dimensions across the transmission medium. One major consideration is the fact that a signal traveling in a dielectric is significantly slower than one traveling in free-space. This is the reason why a lower dielectric constant is preferred to minimize the delay in the line. Introducing air into the dielectric lowers the dielectric constant and can be done a number of ways including helically wrapping dielectric around the inner conductor, using dielectric spacers, or by foaming the dielectric material. The foaming is accomplished during the extrusion process by introducing bubbles to the perfluoropolymers such as Polyethylene (PE), Fluorinated ethylene propylene (FPE), and PTFE. For instance, the dielectric constant of solid Polytetrafluoroethylene (PTFE) is 2, while the dielectric constant of foamed PTFE is 1.6 (free-space is 1). The foaming minimizes attenuation in the coaxial assembly for two main reasons:

• Smaller loss tangent

• Larger center conductor

The introduction of air reduces the loss tangent, or the dielectrics inherent dissipation of electromagnetic energy. The lower dielectric constant allows for the dimensions of the foaming material to be smaller, this in turn, allows for a larger center conductor for a given diameter cable. Much of the resistive loss in a coax is due to the surface area of the inner conductor which is much smaller than that of the outer conductor. The larger surface area mitigates the losses due to the skin-effect and therefore minimizes resistive losses. As an additional note, foamed dielectrics also exhibit higher temperature stability, where less changes in electrical length occur due to temperature variations. This allows for a higher phase stability which is relevant for phase stable applications such as long cable runs in both indoor and outdoor environments. For plenum related applications such as vertical risers in building installations flame resistance dielectric materials are very important to minimize the spread of flames. In these cases the foamed FEP (FFEP) material allows for optimized electrical and environmental performance while maintaining cost-effectiveness due to the manufacturability of FFEP dielectric with the extrusion process.

Shielding

The shielding also plays a role in the attenuation, especially for high frequency signals if there is a lack of coverage, signal degradation will occur. The outer conductor acts as the return path for the inner conductor and carries current in the opposite direction. This, in turn, functions as a shield because it carries equal and opposite signals to those in the center conductor. The skin effect plays an important role in the shielding as well where the braided shields perform well at low frequencies (hundreds of kHz) but as the frequency increases and the signal is pushed towards the surface of the shielding, fields can emerge through the holes in coverage that can cause EMI. Much of this is mitigated through the addition of a bonded or non-bonded aluminum foil shielding preventing losses in the transmission line. The tradeoff for the addition of aluminum foil is less flexibility, this is mitigated through bonding the aluminum foil to the dielectric. Shorter connections make require a higher flexibility or tighter bend radius while long cable runs would likely require far less attenuation without the need for flexibility.

Jacketing

Jacketing materials can make or break a coaxial cable based upon its environmental conditions. Outdoor applications will require resistance to moisture from rain and humidity, resistance to vibrational strain from wind, resistance to UV and, in some cases, resistance to chemicals/oils. Typical PE or PVC cable jackets may not be able to hold up these conditions causing the jacketing crack, swell, melt or otherwise degrade. Plasticizers can be inserted into the jacketing material that generate UV resistant properties. For underground burial applications, a flooded cable, or a cable covered in a water resistant gel, may be necessary to prevent moisture ingress through minor flaws in the cable jacket. Plenum applications can use FEP, FR-PVC, or kynar to meet the UL-1581 standard that includes tests for flammability and suitability for plenum applications. Aside from the use of typical thermoplastics such as PE, PTFE, PVC, and FEP, Thermoplastic Elastomers (TPE) can be used. These materials benefit from both the manufacturability and flexibility of a thermoplastic and the inherent mechanical strength (e.g.: elongation, tensile strength, resistance to abrasions, etc.) of thermosets such as neoprene, epoxy resin, and polyurethane.

Practical Considerations in Wireless Network Installation

Indoor and outdoor wireless installations will invariably require coaxial cables of varying dimensions and operational frequencies to route to amplifiers, antenna feeds, or access points. Generally speaking, the thinner cables are highly flexible and can therefore be used for short pigtail connections to access points, surge protectors, and amplifiers. The thicker cables can be used for medium/long antenna runs such as base station tower installations that can be over 100 feet off the ground. Thicker cables will have a larger inner conductor surface area and therefore less overall resistive losses and lower attenuation. This is necessary for long cable runs where attenuations can become unmanageable beyond 100 feet. Smaller cables are not subject to this and can therefore afford smaller dimensions with a higher level of flexibility.

Application-Specific Considerations

Direct burial cables are relevant in a number of applications including WLL, GPS, WLAN, WISP, WiMax, and SCADA. These are often governed by long cable runs that must be moisture resistant. As stated earlier, thicker cables that with flooded cable jackets offer the benefits of a reduced attenuation as well as resistance to moisture infiltration. Indoor building installations must meet National Electrical Code (NEC) and have cables tested against standards such as UL-1581 to be viable. As shown in Figure 4, Cables routed in risers and plenum spaces such as in HVAC vents are necessary flame resistant as these cables can rapidly spread flames. This is especially true for ventilation systems that push air through an entire building. For this reason CMP-rated or plenum-rated cabling go through more stringent testing than CMR, or riser-rated cabling. The jacketing material in these cables must be flame resistance and also mitigate flame propagation. Dielectric materials such as FEP can be used as well to minimize flammability.

1904 cabl3s fg4

Figure 4 • Amplifiers, access points, and antennas all require coaxial routing with varying lengths and requirements in building installations.

Base stations can have long feeder cables that extend beyond 150 feet. These coaxes will be exposed to UV, vibrations, moisture in the atmosphere, and in some cases, Passive Intermodulation Distortion (PIM). Both connector heads and cable are serious considerations in these environments so that vibrational strain does not cause the connectors to cause temporary lapses in signal transmission. Moreover, low PIM connector heads can be employed as they are specifically built to eliminate common sources of PIM such as contact between dissimilar metals and ferromagnetic materials. Industrial Supervisory control and data acquisition (SCADA) applications can be exposed to a fairly broad range of environmental stressors depending upon the industrial application. If the cable jacketing material may be exposed to chemicals such as oil, it is important that it does not degrade.

Conclusion

Wireless applications cover a very large swath of applications across many industry verticals from commercial to industrial. These cable installations can be small jumper cables for routing to radio equipment to very long runs in base stations and vertical risers in tall buildings. Each of these applications come with material, construction, and installation considerations that ensure signal integrity through its operational lifetime. While there are RG variations of cable that are considered sufficient for all of these applications, employing low loss cable assemblies can allow for much more design flexibility and improved overall system performance.

1. Kazimierczuk, Marian K. High-Frequency Magnetic Components. Wiley, 2014.

2. https://engineering.dartmouth.edu/inductor/papers/stranded.pdf

3. Johnson, Howard W., and Martin Graham. High-Speed Signal Propagation: Advanced Black Magic. Prentice Hall, 2011.

About the Author

Dan Birch serves as a product manager at L-com.

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