A Single Bead Broadband Coaxial Thermistor Mount

A new broadband coaxial thermistor mount is described which has as its major feature the virtual elimination of a dc to RF substitution error commonly encountered in the measurement of the RF power. Secondary features include the simplicity of the design and the potential ease of matching thermistors for temperature-compensated power measurements. The termistor mount utilizes a single center tapped thermistor in place of the two thermistor beads normally used in coaxial mounts. The data taken on the thermistor mount indicate the elimination of an error described in detail in a paper by Glenn Engen, "A dc-RF Substitution Error in Dual Element Bolometer Mounts." Typically, this error is in the order of 1 percent at 10 mW and 2 percent at 15 mW of applied RF power for conventional dual bead thermistor mounts, and is due to the two beads having different dissipation constants and a shift in resistance when RF power is applied. Measurement of resistance shifts and the dissipation constants of the respective sections of the thermistor for a number of tapped thermistor beads and mounts have been made. The results indicate that this substitution error is typically reduced to below 0.1 percent up to 15mW of applied RF power. The error calculation is based upon a derived expression for the substitution error. A coaxial thermistor mount is described which is usable at RF frequencies up to 12.4 Gc/s.