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Choosing the Right Coaxial Test Cables

Parent Category: 2019 HFE

By Peter McNeil

Of all the often-used components in an RF or millimeter-wave test facility, coaxial cables may be the most frequently overlooked. Coaxial cables can fail in many circumstances: when flexed to the point of shield degradation, when bent too tightly, when connectors are torqued too tightly (or not tightly enough) -- the list goes on. Many of these pain points are due to human error, but this can be somewhat mitigated when a lab is supplied with an adequate selection of test cables. This article attempts to provide an overview of the specifications and certifications to look for in a coaxial transmission line for a range of applicable test scenarios.

Critical Parameters

Impedance

There are many different metrics related to the performance of coaxial cables. Generally, coaxial cables come with either a 50Ω impedance or a 75Ω impedance. From a testing perspective, most test facilities come equipped with 50Ω equipment and, more often than not, it is not feasible to include 75Ω equipment into a lab. This is especially true for pricey Vector Network Analyzers (VNAs). When testing 75 Ω devices with a 50Ω VNA it is necessary to therefore attach Minimum Loss Pads (MLPs) or 50-to-75Ω adapters, 75Ω coaxial cables, and calibrate up to the 75Ω plane using either the often more accurate     Thru, Reflect, Line/Match (TRL/M) standard or Short, Open, Load, Thru (SOLT) calibration with a 75Ω calibration kit. More often than not, a 50Ω coaxial line is utilized beyond 1 GHz, as 75Ω coaxial cables are typically leveraged to relay audio/video signals in residential TV applications.

Measuring a 75Ω device with 50Ω instruments is most often accomplished with Minimum Loss Pads (MLPs, aka Impedance Matching Attenuators) which present a 50Ω impedance on one side and a 75Ω impedance on the other.  These pads often serve a dual purpose by adapting between the connector type used on the 75Ω side and that of the 50Ω side.  The pad can be used at the end of a 50Ω test cable, or between the instrument and a 75Ω test cable. The loss of the pad must be taken into account when making the measurement.

Attenuation & Insertion Loss

The insertion loss (S21) of a coaxial cable is a measure of the power lost from the input to the output of a coaxial assembly, while attenuation is a per-unit-length metric of loss that is frequency dependent. In terms of S-parameters, S21 should measure the same as S12 as a coax cable is a 2-port passive device. Attenuation is typically measured in decibels per unit length (per meter, feet, or 100 feet). On a benchtop test setup it is unusual to have cables longer than 24 or 36 inches.  However, this changes in some outdoor testing scenarios where massive runs of cable may be leveraged. This could occur, for instance, from a control room to a transceiver mounted at a distant location or from the ground-based test equipment to the top of an antenna tower/base station. In cases where long cable runs are utilized, selecting low loss cables can become increasingly important [1]. These cables are generally thicker and more durable with a solid center conductor as opposed to a stranded center conductor. The highly conductive, conformal surface reduces any high frequency resistive losses as the bulk of the signal propagates closer to the surface of the conductor. Moreover, jacketing materials that are particularly resistant to UV, humidity, temperature extremes, or even salt fog can be necessary for long term testing apparatus.

VSWR

Standing waves will occur in imperfect real systems. Standing waves will inevitably be caused by the mismatch that occurs between the test cables and subsequently connected components (such as antennas). In a coaxial transmission line there can be structural irregularities that affect the geometry of the transmission line, and therefore its impedance. Solid center conductors are often covered with highly conductive metals such as silver to have the combined benefit of lessening attenuation and smoothing out the inner surface with a conformal coating. Many coax imperfections can be a product of the cable manufacturing or handling. No transition from the coaxial line to a connector head is seamless. Swelling, cracking, or melting of the jacketing material can occur from undesired ingress into the coaxial line or connector head. Excessive flexing or vibrational strain can also cause changes in the electrical length of the cable, causing phase instabilities.

Power Handling

Unlike a waveguide, a coaxial cable has much smaller dimensions and metallic surface area to dissipate heat and is therefore able to handle far less power. Moreover, waveguides tend to exhibit less attenuation than coaxial cables. This is why waveguides are typically used for antenna feeds in radar applications while coaxial cables suffice for cellular communications systems. This problem is exacerbated by the fact that power dissipation is a function of frequency, so a cable that can handle 1800W at 1 GHz can only handle 400W at 18 GHz. The center conductor, with the smallest surface area, will get the hottest which can rapidly degrade cable performance.     Even though coaxial cables are known not to pass large amounts of power, power handling is a pertinent figure especially when doing power tests. For instance, when testing a high power amplifier, a waveguide may be used on the output as well as couplers and attenuators to reduce power. Calculating the level of padding needed is critical in order to protect sensitive lab equipment.

Shielding Effectiveness

A common parameter that is often used to gauge the integrity of the shield is shielding effectiveness (SE). This parameter is a good measure to understand the electromagnetic compatibility (EMC) of the cable, or resistance to electromagnetic interference (EMI). The SE gauges the ability of the shield to prevent EMI and, in theory, can be expressed as the sum of the losses of a signal due to reflection, absorption, and re-reflection through the shielding barrier [2]. The SE is also proportional to the materials surface resistivity (which increases with frequency), permeability, and the thickness of the material.

Coaxial cables will often use only one layer of braided shielding. Any lack of coverage in the shielding can allow for interference or just general signal loss from radiating electromagnetic fields. Moreover, the shielding can slowly unravel with constant flexure over time, worsening the situation. The thickness of the shield should generally be several skin depths deep. The higher the frequency of operation, the thinner the shielding can be. Foil-based shields with materials such as aluminum often suffice, although they can subsequently limit the amount of flexure a cable can withstand. Several layers of shielding can be utilized for overall higher shielding effectiveness.

1909 cables fg1

High-reliability coaxial test cable under test.

Mating/Unmating Cycles

Connector heads in test environments will be more frequently mated and unmated than coaxial cables in permanent fixtures. Most of the connector degradation from mating/unmating is from improper handling (e.g.: torquing connector too hard, unaligned threading causing scraping, etc.). Still, leveraging stronger materials such as passivated stainless steel can extend the lifetime of the connector head and therefore the entire cable assembly.

Flex Cycles

Along the same line of reasoning, test cables are typically handled far more than most coaxial assemblies. Flexibility is therefore key where strain relief is often utilized to mitigate the strain that occurs at the point between the connector head and cable. Thinner cables seem to be more flexible during handling but that can be tricky as their smaller dimensions make them more sensitive to damage due to excessive bending. The “bend radius” parameter can include a “once only” bend radius defined as the point at which the cable can be bent without any adverse consequences (e.g.: a kink in the shielding, inner conductor fatigue, etc). This bend radius is more pertinent for more long-term or permanent installations, and it is notably smaller than the “repeat-flex” bend radius that can be defined as the radius that a coax can endure repeatedly without seeing signal degradation.

The parameter of flex cycles can provide insight to the general lifetime of a cable assembly as well. Some datasheets will specify the number of flex cycles a cable can endure at various bend radii. Phase and amplitude stability over flexure is a particularly important for test scenarios, as it allows for longer amounts of time between calibrations and more accurate measurements. Test cables that do not exhibit high stability/repeatability during flexure can cause significant measurement errors when the cables are flexed during a test.

Additional Features

Phase Stability

From the benchtop evaluation of components like switches, amplifiers, couplers, and so on, to antenna characterizations, all the way to production level testing with automated test equipment (ATE), phase related measurements, including phase stability and phase repeatability, are common and essential measurements. Phase stable cables can be necessary in a wide range of applications. Phase stability becomes increasingly relevant up into the millimeter-waves, as a phase offset at a lower frequency will produce a proportionally larger error at higher frequencies. Phase stable cables are also particularly critical in systems that distribute multiple coaxial lines from a single source or gather multiple coaxial lines into a single signal. In the systems that align signals from multiple antenna elements (switched beam arrays, phased array), the bit error rate (BER) can degrade with phase inaccuracies. In cases with long cable runs, phase instabilities can occur from the temperature cycling, causing changes in permittivity of the dielectric and ultimately altering the electrical length of the coax.

1909 cables fg2

Hi-Rel Coaxial Test Cables.

In the millimeter-wave portion of the spectrum, cables are dimensionally much smaller which makes them far more susceptible to bending and flexing. Minor amplitude and phase changes in the RF range are therefore exacerbated for some millimeter-wave cable constructions. One potential solution for this is the armoring of millimeter-wave test cables to prevent overextension while bending the cabling for test. This way, semi-rigid cables are not entirely necessary as the flexible alternative will also not be over-flexed. In general, phase instabilities can destroy the integrity of these test systems by degrading the accuracy and precision of test measurements. Naturally, cables that experience phase instability will require more frequent recalibrations. Phase stable cables undergo extensive temperature cycling to ensure phase stability over temperature. Part of this procedure will often involve the use of a specialized dielectric such as microporous, foamed, or air-spaced materials that experience less dielectric losses and more mechanical stability over temperature. Similarly, skew matched cable assemblies involve two or more phase stable cables that are matched in time delay, allowing for signals to propagate within picoseconds of each other.

Severe Test Environments (Mechanical, Electrical, and Chemical Testing)

High reliability test cables may be necessary in harsh test environments. Typically these types of test cables are more often leveraged for military, industrial, or various outdoor applications where cables are subject to harsh environments. Many military cables for instance require a life test or a burn-in test where a coax is subject to stress-acceleration with temperature, humidity cycling in an oven chamber over a predetermined period of time. Many industrial microelectronic circuits and interconnect must meet Mechanical, Ingress, Climatic/Chemical, and Electromagnetic compatibility (MICE) parameters before being leveraged in harsh industrial environments. This is not dissimilar to military standards for circuits and cabling where a cable is subject to mechanical tests such as shock, vibration, crushing, and bending as well as chemical tests and non-standard electrical tests such as insulation resistance, and dielectric withstanding voltage. These are all pertinent parameters depending upon the applicational usage of the coaxial cable. A cable that may be run over will necessarily need some shock, impact, and crushing resistance. This could be accomplished through the use of armoring over the cable.

A cable subject to chemicals such as hydraulic fluid or even something as simple as sea water will need an inherent salt or chemical resistance. Otherwise, the cable jacketing will break down in some way causing contaminants to ingress further into the cable as stated earlier. In environments with harsh chemicals, jacketing material is likely the most critical part of the coaxial cable as it provides a shield from these contaminants. Thermoplastics with special plasticizers can be used to give inherent UV or chemical resistance. Thermosets such as neoprene can also be leveraged especially for mechanical stability as these materials can handle far more tensile and impact strain than the average thermoplastic. Combinational materials such as thermoplastic elastomers (TPE) are also employed to combine the simplicity of stripping a thermoplastic with the inherent strength of a thermoset. Materials with fire resistance might also be necessary in some test environments where fire retardant properties can be added to thermoplastics such as Teflon (FR-PTFE), Polyvinyl Chloride (FR-PVC) or Polyethylene (FR-PE). Dielectrics with inherent fire resistance can also be leveraged such as Fluorinated Ethylene Propylene (FEP).

Conclusion

Coaxial cables provide a common backbone to transmit RF and millimeter-wave signals. There are, however, a number of considerations involved in choosing the right cable for a specific test. This depends on parameters such as electromagnetic and chemical stressors in the environment as well as basic parameters such as impedance, attenuation, and VSWR. The selection of test cables can mitigate much of the stress of testing.

References

1. http://www.highfrequencyelectronics.com/index.php?option=com_content&view=article&id=2202:understanding-low-loss-coaxial-cables-and-their-applications&catid=183:2019-04-april-articles&Itemid=189

2. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970036055.pdf

About the Author

Peter McNeil serves as Technical Content Manager for Infinite Electronics.

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