Modification of a 1811A Hewlett–Packard time domain reflectometry (TDR) system is described for long‐time drift reduction. Triggering methods for single‐channel as well as dual‐channel TDR systems are discussed. It is shown that the long‐time drift of the 1024‐point incident signal is reduced to less than one discrete point over a period of several hours. This modification extends the upper frequency limit of a TDR system to about 10 GHz for precision permittivity measurements.