Improvements in Precision Coaxial Resistor Design

The increasing demand for higher accuracy in coaxial line measurements and standards, particularly for the VHF and UHF bands, has led to the development of cylindrical metal film resistors of uniform surface resistivity in which the main characteristics have been considerably improved when compared to the cracked-carbon film resistors which have hitherto been used in precision coaxial resistor mounts employing a tractorial outer conductor. A description is given of an alternative design employing a conical resistor element with a cylindrical outer conductor which results in a much simpler mechanical design with fewer discontinuities than the tractorial configuration. It is estimated that with the new resistor element it should be possible to achieve a resistive termination of VSWR 0.996 in the range 0-4 Gc with a ¾ inch OD system. The principle of operation is explained in terms of electromagnetic wave propagation over a resistive film surface in which the tilt angle θ of the electric field from the normal to the film surface is given by sin θ= ρ/Z
_m
where ρ is the surface resistivity and Z
_m
the wave impedance of the medium. A simple explanation is given of the generation of the tractorial and conical systems by transformations from the simple parallel-plate line terminated in a resistive film sheet. Second-order effects caused by field penetration of the resistive film are described and formulas given for their evaluation and compensation.