Courtesy of Pasternack : Coaxial Cable Power Handling Part 3
In the previous sections of the “Coaxial Cable Power Handling” blog series, a brief overview of coax power handling, and how cable size, cable type, signal frequency, and electron velocity through a cable, influenced the power handling capability of a coaxial cable. In this section, attenuation/loss, environmental factors, VSWR, and cable capacitance, will be touched on in respects to coaxial cable power handling.
Attenuation or Loss
One of the important constraints that limit the power handling capability of a line is attenuation. Attenuation is the loss generated by the inner and outer conductor of the coaxial cable and its dielectric medium. As most of the heat is generated in the center conductor, any heat generated as a result of dielectric losses will be dissipated within the dielectric. The construction of the dielectric is key in determining the power handling capability of the coaxial cable.
Thus, lower loss RF coaxial cables will have a higher power rating than higher loss RF coaxial cables. Additionally, there is a degree of loss as a signal travels along a coaxial cable which is proportional to the length of the cable and is specified in terms of a loss over a given length or decibels over a given length, e.g. 0.5dB / 10 meters.
Environmental Factors and VSWR
Non-optimal operating conditions may require a coaxial cable to be derated. Environmental temperature can induce derating. For example, if the coaxial cable is operating in a high temperature environment, it cannot dissipate as much heat, operating temperature will increase. Hence, a de-rating factor is applied if the coaxial cable is used at high temperatures. Altitude plays a role but only at significant heights.
Thus, if the cable is operating at a high altitude and under reduced pressure, cooling will be less effective and temperature rise within the cable will be greater. Additionally, if the coax cable is operating under conditions where the VSWR is high, de-rating may be necessary due to the variations of high and low current along the coaxial cable such that the power dissipation may cause higher levels of power to be dissipated locally.
The capacitance of the coaxial cable is the ability to hold a charge. Capacitance exists between the inner and outer conductors of the coaxial cable which is proportional to the length of cable as well as the dielectric constant and the inner and outer conductor diameters. The larger the capacitance, the longer it takes a signal to reach full amplitude inside the cable. Thus, a high capacitance is an undesired characteristic when determining the performance of a coaxial cable.
For a detailed look at the data used to develop recommended maximum power ratings for RF coaxial cable power handling, see Power Handling Capabilities of RF Coaxial Cables, 1971, http://www.dtic.mil/dtic/tr/fulltext/u2/912072.pdf. in which “representative coaxial cable types were subjected to high power CW and pulse tests” which were “conducted over a range of frequencies and ambient temperature and pressure attitudes.”