A 4 mm microwave interferometer has been developed for measurements on electron concentrations in low‐density plasmas (ne≥1010cm−3). The microwaves are transmitted via Lecher wires, inserted across the discharge tube, to obtain high spatial resolution (≲one wavelength). The sensitivity of the system is improved by eliminating the influence of possible variations in the klystron frequency. A microwave phase shift as low as 2&pgr;×10−3rad can be detected in this way. Density oscillations can be scanned by means of a sampling technique. An example of applications on low‐pressure arc discharges is given.