The design of a radio-frequency coaxial resistor

Coaxial resistors used as reflection-free terminations of concentric transmission lines are usually enclosed in a jacket, the characteristic impedance of which is ¿3 times lower than the resistance. It is shown here that a much better performance can be obtained if this factor is about¿4.5 provided additional means for compensating the reactive component of the resistor are used. The performance improvement due to an undercutting in the line and to a special compensating line is discussed in detail, taking also into account the discontinuity reactances in the resistor holder. Design methods and formulae are given, and their use is illustrated by an example. Voltage standing-wave ratios of 1.01 can be obtained with resistor lengths of, ¿/8. The frequency range is several times greater than with a non-compensated ¿¿3¿ jacket.