The measurement of free‐electron densities and collision frequencies in ionized‐gas flows by a probing transverse microwave beam is described. Interpretation of the observed reflection and transmission coefficients and their phases is developed on the basis of an idealized plane wave‐plane slab model, for which typical results are displayed. Application of the device and technique to a specific experiment—the buildup of ionization behind a strong shock in argon—is outlined. Effects of various departures of the microwave radiation patterns and flow configurations from the slab assumptions are discussed and examples evaluated.

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