Power Rating of Solid-Polyethylene-Insulated Coaxial CablesAuthor: R.J.Edwards G4FGQ © 2nd November 1999
This program assists with selection of dimensions of RF coaxial cables when input power is high enough to cause concern about the temperatures to which plastic materials may be subjected. It will also assist with estimating the power rating, Zo and attenuation of a cable when nothing is known except its radial dimensions. Results are valid from 0.5 to 1000 MHz at temperatures up 150 degrees Celsius.
The softening temperature of low-density polyethylene is 85 degrees C and the melting point is 120 degrees C. Break-down voltage stress is >2,500 volts per millimetre which is high enough not to be a power input limitation at HF.
The physical properties of a flexible PVC jacket may be permanently degraded if operated above 60 degrees C for prolonged periods.
Cable users should allow an operating safety margin due to environmental uncertainty. A margin of 25 degrees C is reasonable. In any case it may be considered wasteful of RF power to run cables hot even when safe. An increase in resistance of copper with temperature and the consequential further increase in RF power dissipation in the cable is taken into account by the program.
Conditions and Assumptions
A current loop is a position of maximum current on a line with standing waves. Computed temperatures and loss apply to the current loop nearest to the generator end of the line. Highest temperatures occur at this loop. It is assumed the line is long enough to contain it. Without standing waves the hottest part of the line is immediately at the generator end.
Heat conducts radially outwards from inner to outer conductor to environment. Inner conductor is copper with skin effect fully operative at frequencies> 0.5 MHz. Outer conductor is copper or aluminum, either solid or braided. To simplify, outer conductor is 0.35 mm = 0.014" thick for all cable types. The PVC jacket is dark coloured with a matt surface. A moderate outdoors breeze reduces the rise above air temperature of surface of the PVC jacket by 50 percent.
When enclosed in a brick duct the current loop is well inside the duct. The duct (drilled hole) diameter is 5mm greater than jacket diameter. There is no longitudinal cooling air flow through the duct.
Ambient temperature is the shaded air temperature in the cable's vicinity. Always use worst-case climatic and seasonal conditions to estimate highest ambient temperature before including a safety margin.
Four cable types similar to RG-58, RG-217, RG-11 and RG-59B can be entered in the program by hitting keys H, J, K and L respectively. This can be done only after one complete set of input data has been entered in the program.
Use keys N and M to vary the diameter of the inner conductor, and so also Zo, over a range of values while observing attenuation and the surface temperature of the jacket. Note that lowest temperature occurs in the vicinity of Zo = 50 ohms at frequencies below 50 MHz. At UHF optimum Zo has lower values.
Use keys Z and X to vary cable input power while observing temperatures, etc.
Maximum possible SWR is that at the input of 1/4-wavelength of s/c or o/c line in which conditions all input power is dissipated in the line itself. To enter a greater SWR into the program is meaningless. Maximum SWR varies with temp.
Reminder: Power handling capacity in watts is inversely proportional to SWR.
It appears USA cable manufacturers' stated power ratings are absolute maximum values at an ambient temperature of 25 degrees, SWR=1:1 with cables suspended in still air. At greater power levels a cable may suffer permanent damage. Users are at liberty, of course, to apply their own safe operating margins.
After entering inner and outer conductor diameters and jacket thickness, make sure the computed jacket diameter is equal to the measured value. If not, re-adjust outer diameter and/or jacket thickness until it is.
If the cable impedance Zo is known, after readjusting the outer diameter the inner diameter can be varied until computed and actual Zo's are equal.
Zo and attenuation may deviate from manufacturers' nominal values by 5% or more due to dimensional drift during and between production runs.
If inner conductor is stranded enter 90 percent of diameter over the strands or, if Zo is known, enter an inner diameter which results in correct Zo.
Guesstimating temperature: 55C is much too hot to hold in the hand. 45C is too hot to be held indefinitely. 37C is blood temperature and is very warm to the hand. 25C is a cozy living room temperature.
In these notes "attenuation" is an intrinsic property of a cable independent of its terminations. It is sometimes referred to as its loss in decibels per unit length when terminated in its own characteristic impedance Zo.
Voltage stress is not uniform between the inner and outer conductors. It concentrates most strongly in the dielectric in contact with the inner conductor. The computed maximum voltage stress per mm is always greater than cable volts divided by the dielectric thickness. The discrepancy increases with Zo.
Important: Computed data applies to the location of the current loop nearest to the generator end of the cable. Cable attenuation stated in terms of dB per 100 feet applies only to a very small fraction of a wavelength at that loop. The remainder of the cable is at a lower temperature and therefore has a lower attenuation. When there are high standing waves on the cable its temperature varies with distance from the generator as a series of alternating hot and cool spots. The distance between hot spots is 1/2-wavelength.
To obtain the attenuation in dB per metre when the cable is at a uniform temperature it is necessary to set input power to zero and ambient temperature to the required value. At HF the attenuation (dB/metre) temperature coefficient is 0.2 percent per degree - half of the TC of the DC resistance of copper.
PTFE, Teflon-insulated cables have Zo 4.5% greater than polyethylene for same dimensions. DB attenuation is 4.5% smaller. Temperature rise above ambient is approximately 9% less for the same power input. Softening temperature is 260 C.
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