The effects of antenna location, cone angle, nose radius, signal frequency, vehicle velocity, and wall temperature on rf signal attenuation are examined. For the assumptions of a clean air equilibrium flow and a thin slab electromagnetic model, the results indicate that a great reduction in rf signal attenuation can be achieved by proper aerodynamic shaping of the vehicle. Sample calculations are made for a transmission frequency of 600 me for nose bluntness varying from J-in. to 1-ft radius, cone angles from 10° to 60°, wall enthalpies from 300 to 1200 Btu/lb, and velocities from 20,000 to 24,000 fps for a range of altitudes of 50,000 to 200,000 ft. Altitude-velocity maps for lines of constant attenuation are also presented. Results indicate that, for sharp slender bodies, rf attenuation can be limited to 10 db throughout the re-entry envelope. Nomenclature a = speed of sound C Q = speed of light in a vacuum db = attenuation dS/dn = entropy gradient normal to streamline e = charge E = electric field f t = transmission frequency, cps g = mass conversion factor, 32.2 Ibm/slug hi = enthalpy h w = wall enthalpy / = energy conversion factor, 778 ft-lb/Btu J = current density L = plasma thickness m = electron mass N = number density of electrons HO = £0/Co = 27T/X0 Ri = radius of curvature in the hodograph plane R n = nose radius S = specific entropy T = transmission coefficient u = velocity component in axial direction v = velocity component in radial direction v = electron drift velocity v ni = velocity normal to ray v ri -velocity along ray V = freestream velocity V e = re-entry velocity, kft/sec V\ira = limiting velocity W = F/F lim X = (cop/co) 2 y = ratio of specific heats A0 = angular increment from ray to ray rj e = electron density, electrons/cm 3 0 = flow angle B c -cone half angle, deg X 0 = free space wavelength fj. = Mach anglê = stream function co = transmission frequency, rad/sec co c = collision frequency, collision/sec cop = plasma frequency, rad/sec
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