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A circular-aperture flush-mounted antenna for which a theoretical description of the antenna-plasma interaction has been developed was used to obtain a comparison between theoretical and experimental antenna admittance in the presence of ionized boundary layers of low-collision frequency. The antenna was located in a flat-plate model which was in turn located in the expanding-flow environment of a reflected-shock tunnel. The electron-temperature and electron-density distributions in the plate boundary layer at the antenna location were independently measured using voltage-swept thin-wire Langmuir probes for one of the test conditions. The antenna admittance was measured using a four-probe microwave reflectometer and these measured values were found to be in good agreement with those predicted from the theory. The response of this antenna was found to be very sensitive to changes in the profile and magnitude of electron density for 0.1 < n e /n e ) CT < 1.2. For values of n e /n e ) CT greater than 1.2 the phase rotation continued to provide a measurement of electron density. Measurements were also performed with another type of circular-aperture antenna which did not satisfy all of the constraints of the theoretical model, yet good agreement was obtained between the calculations and the experimental results. Nomenclature b s = normalized shunt susceptance E AP = electric field at aperture H* = complex conjugate of the magnetic field n e = electron density n e)cT -critical electron density for antenna frequency ds = surface element V 0 = impressed voltage Y a = aperture admittance 7 IN = normalized input-admittance, defined by Eq. (1) pi = normalized length of transmission line for matching network of the RAM C-II Antenna F = reflection coefficient defined by Eq.(2) d = boundary-layer thickness A G = guide wavelength A = mean free path v = collision frequency co = electromagnetic frequency x,y,z = coordinates of ground plane

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