Admittance of waveguide radiating into plasma environment

Several important effects on radio transmission occur during the reentry of a space vehicle into the earth´s atmosphere. Of these effects, signal variation due to changes in antenna admittance may be significant. These changes result from the formation of an ionized sheath about the vehicle as it reenters the atmosphere. The sheath may have a profound effect on the propagation of electromagnetic waves and may thereby modify the antenna admittance characteristics. Knowledge of the input admittance of the antenna enables one to compute the field amplitudes at the antenna input terminals when fed by a specified power source and, from this, to determine the change in radiated signal level that results from antenna mismatch. This change in radiated signal level is in addition to that due to pattern distortion. Besides the change in radiated signal level, the reflected wave may adversely affect operation of the transmitter and may even result in transmitter burnout. This report investigates the changes in input admittance of a rectangular waveguide terminated in a groundplane covered by a uniform plasma slab contiguous with the groundplane. Such radiators provide wide angular pattern coverage and find application as telemetry and communication antennas in reentry vehicles. Numerical calculations have been performed for the case of lossless plasmas in which the electron densities are all below the critical density, i.e., the effective relative permittivity of the slab ranges between zero and unity. Curves of input conductance and susceptance for this configuration are given as a function of relative permittivity for several slab thicknesses. These admittance values are used to compute reflection coefficients and signal loss resulting from mismatch. These are also presented graphically. The calculations show that, except for thin slabs, the aperture admittance is very close to that for a slab of infinite thickness. A qualitative explanation of this effect is also given.