Multiline TRL (mTRL) is a distinct calibration method from the LRL/LRM family discussed in the previous chapter, but many aspects are shared. All are based on fundamental ideality assumptions about transmission lines (difference between S-parameters of two lines are based solely on the complex propagation constant and line length difference, all lines in the calibration have the same characteristic impedance and propagation constant, etc.). If a match component is used in either family, its measurement data together with any model information will be used explicitly to set directivity.

The LRL family will only use two lines at any given frequency and the user selects which ones through the band definitions (see LRL/LRM Calibration of this guide). mTRL uses all lines (up to 10 are allowed) at all frequencies with their relative contributions determined in a least-squares sense which will de-emphasize those lines with non-optimal lengths (for each frequency) and those that have anomalous measurement data (to help reduce connection repeatability effects). The principles behind the method have been covered extensively in the literature. See, for example,