A New Coaxial Flow Calorimeter for Accurate RF Power Measurements up to 100 Watts and 1 GHz

Abstract

Establishing traceability of RF power measurements at power levels in excess of a few watts has historically used methods that we will categorize into two major branches. One class of methods uses low-power sensors traceable through microcalorimeters operating in the milliWatt range. This class is exemplified by the power measurement technique described by Bramall [4], in which the low power sensors are used to measure the insertion loss of attenuators or couplers, which are then cascaded to provide the required attenuation or coupling factor to enable measurement of high power using low power sensors. The other major division is the direct measurement of the higher power using high power calorimeters. The basic theory and history of flow calorimeters is described well in chapter 5 of Fantom [1], and recently available commercial flow calorimeters are described in their respective user manuals [2][7].The cascaded coupler method has been refined to the point at which, for 100 Watt measurements below 1 GHz, NIST reports the ability to calibrate transfer standards with an uncertainty of 0.67% [3][8]. This uncertainty seems adequate to provide traceability for typical power sensors giving an overall uncertainty of 3% to 4% [5], but is higher than the 0.6% required to calibrate the most accurate of high-power RF sensors [6]. The method is also reported to be, “cumbersome and lengthy”[8].Commercially available calorimeters [2][7] represent that the user will obtain measurement uncertainty in the neighborhood of 1.25%. One of the referenced models claims 0.5%, but “not including load error”, which apparently does not include offset due to a leakage path. In real calibrations performed by the authors, that unit’s total error exceeded 2% of full scale. However much better results have been shown to be possible, such as by Bird[6] showing that their lab can calibrate to 0.6% when required. In the project being reported on, the authors addressed the challenge of finding as many of the sources of error in a flow calorimeter as possible, and followed up on the findings by developing new instrumentation, process automation, and heat flow to minimize error as much as possible.