Measurements of admittance of a slot antenna exposed to a nonuniform plasma layer

When antennas are used on vehicles in hypersonic flight through earth and other planetary atmospheres, the thermally generated plasma layer is usually close enough to the antenna near fields that the layer has a strong effect upon the antenna admittance. Earlier a program of measurements was reported showing that the reflection coefficient of a slot antenna exposed to a shock-tube-generated plasma is a function not only of the electron density and collision frequency in the free stream, but also of the thin nonuniform boundary layer plasma parameters next to the groundplane. Limited admittance measurements with the nonuniform layer of a plasma jet indicated that it is preferable to measure the complex admittance in order to discern the quite different dependence of the real and imaginary parts upon the various parameters. In the new work, the antenna used is the Teflon-plugged open-waveguide slot antenna. The leading edge of the slot ground-plane was made sharp to minimize the effects of the flow, and the electrostatic probes were used to monitor the ion density just downstream of the aperture. Prior studies showed that a boundary layer of approximately 2-mm thickness would exist over the aperture in the low-pressure shots, compared with an 0.6-mm thickness in the high-pressure shots.