Convenient formulas are deduced, which relate electron densities and collision frequencies of uniformdense plasmas to two standing wave signals of a probing microwave reflected at a plane boundary of theplasma. In an experiment with a uniform afterglow plasma produced in a waveguide, the electron densityfrom2×1013 cm−3to3×1011 cm−3is measured with reasonable value of the collision frequency by using aX-band microwave. The technique provides a mean for detecting an electron density as high as1×1016 cm−3with the X-band microwave.