The interaction between the ionized wake of a capsule enteringthe Martian atmosphere and the circularly polarized radiation emittedby an antenna located on the aft part of the capsule is theoreticallyinvestigated in this study. A simplified mathematical model of theatmosphere of Mars, the entry-trajectory of the capsule, and the flowfield surrounding the capsule are used in the analysis. The near wakeof the capsule is approximated by a cylindrically stratified plasmashell consisting of n plasma regions. The plasma in each region isassumed to be homogeneous, anisotropic, and conducting, and moving ata uniform velocity relative to the antenna. The antenna is representedby a turnstile antenna located off-axis λo/4 above an infinite groundplane and operates at the signal frequencies of 400 MHz and 2.295 GHz.
Integral expressions for the cylindrical components of thefield vectors are obtained by a rigorous relativistic formulation ofthe problem, and are evaluated using the techniques of asymptoticexpansions to yield the radiation patterns of the antenna. Radiationpatterns for the special case of an on-axis antenna radiating througha uniform, lossless, and isotropic plasma shell are presented and arecompared with the free space patterns.
The analysis shows that blackout occurs during the entry ofa capsule into the Martian atmosphere, Before and after blackout,the radiation patterns of the antenna exhibit an on-axis null regionwhose angular extent is proportional to the electron concentration ofthe plasma. Also, sharp peaks which are attributed to leaky wave radiation,are present in the null region of the patterns. For the non-nullregion of the patterns, the values of the gain function of the antennaoscillate about the free space values. As the electron concentrationof the plasma increases, the peaks in the radiation patterns become morenumerous and more sharply defined. The effects of the motion of theplasma on the radiation emitted by the antenna are to shift the peaksof the radiation patterns to smaller cone angles and to introduce morepeaks into the patterns.
For the low velocity case corresponding to an entry into theMartian atmosphere, no serious motional or depolarization effectsoccur, and communications with the capsule can be satisfactorily carriedout when the condition of blackout does not exist.
