The Radio Frequency Coaxial Resistor Using a Tractorial Jacket

Radio frequency coaxial resistors are usually fitted with an exponential jacket, whose design is based on an approximation by a cylindrical transmission line. This approach is not consistent with the actual field distribution in the termination. An approximation which is more appropriate is a conical line, leading to a tractorial jacket. An analysis of this profile is given. It is shown that in the tractorial termination the electric field fulfills the boundary conditions at both the surface of the jacket and the resistor, and that the remaining parts of the field lines are represented fairly well. This ensures a reliable prediction of the properties of the termination. The residual difference between the actual waveform and the assumed TEM wave is expressed by means of a distortion factor. In the design, the length/diameter ratio of the resistor is the most important parameter. The factors influencing its choice are discussed in detail. For terminations below 80 ohms a length/diameter ratio between 8 and 20 is satisfactory, higher impedances requiring greater ratios.