Two potential failure modes must be considered when determining the power handling capability of an RF coaxial cable:

1. Peak Power (Voltage Breakdown)
2. Average or CW Power

The Peak Power (Voltage Breakdown) occurs when the voltage gradient between the cable center conductor exceeds a limiting value causing the signal to arc across the path of the least resistance. Generally, the path of the least resistance is located at the cable/connection junction. Catastrophic breakdown is not the only problem: the existence of corona, usually around the center conductor, produces other deleterious effects. Corona cutting is a concern with PTFE insulators whereupon the PTFE is eroded causing the formation of cavities (usually without carbonization).

Coaxial cable assemblies are typically rated with the peak power handling much lower than what the interface can handle. To maximize peak power of the cable assembly, a high voltage (HV) connector should be used. A higher voltage potential is achieved by overlapping the dielectric thereby increasing the airline arc path. A drawback to this design is that connectors which are modified in this way generally have greater VSWR at higher frequencies.

If the transmission line has reflections, the voltage and the current along the line will have maximums and minimums. The cause of this nonuniform distribution is superposition of the incident and reflected waves. Breakdown is a function of the maximum voltage. Higher reflection results in lower voltage handling. Even high-performance assemblies with low VSWR can have poor peak power handling if they are connected to an unmatched load. Peak power handling is dependent on frequency since the typical value of VSWR is proportional to the increase in frequency. The most common breakdown at high altitudes (usually greater than 70,000 ft.) is ionization breakdown in the air path. For vacuum and space applications, the main type of breakdown is multipaction breakdown. Click here for more information with regard to multipaction.

For average power rating of a cable with a pulsed signal, multiply the peak power rating by the duty cycle.

Frequency range, ambient temperature, altitude, physical size, and the thermal properties of each layer of construction are the primary factors which determine the average power handling capability of an RF coaxial cable. The Average Power failure occurs when the level of power transmitted results in resistive and dielectric heating at a rate higher than the rate at which the heat can be conducted away through the different layers of cable and dissipated from the outermost cable layer to the environment. A buildup of heat energy causes the internal cable temperature to exceed the maximum rated dielectric temperature. Convection, conduction and radiation are methods to remove heat from the cable assembly. Conduction transfer of heat through the outer and inner conductors of a cable is particularly effective for short assemblies. For very high altitudes and space applications, the air is too thin or nonexistent and convection cooling is ineffective. Heat from the cable assembly can only be removed by radiant heat and conduction.

Astrolab, Inc. has developed a unique computer-modeling program that accurately predicts the power rating for coaxial cables of varying designs and materials. This program was used to generate the charts found in the coaxial cable sections of the Astrolab catalog (see Section C). Power Handling is calculated for convection cooling only. Conduction and radiation are included in the safety margins. These charts provide the CW or Average Power rating for all Astrolab cables versus frequency. The following calculation shows how to use the CW power charts for non-standard temperature conditions:

Where:
P = power at temp, t1 and altitude a1
Ft = temperature derating factor at temp. t1
Fa = altitude derating factor at altitude a1
Pf = power level at frequency f1

Example:
What is the Average Power rating for Astrolab 32051 cable at 12 GHz at an ambient temperature of 100°C and an altitude of 30,000 ft.?

Pf = 580 Watts (see data – page C:11 Astrolab Catalog)
Ft = .58 (see chart)
Fa = .68 (see chart)

Therefore:
P = 580 (.58)(.68) = 229 Watts

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