We have calculated the intensity of RASS (radio acoustic sounding system) echoes scattered by refractive index fluctuations produced by acoustic waves. Because of temperature changes in the atmosphere, the sound speed decreases in the troposphere and the shape of the acoustic wave front becomes elliptical. When the shape of acoustic wave fronts is significantly distorted from a sphere, the backscattered radio wave does not focus on the radar. Consequently, the spot size of the RASS echo becomes large, and the peak intensity of the RASS echoes decreases. We numerically estimated the echo power loss due to defocusing effects for an atmosphere whose sound speed decreased linearly with altitude. The loss was found to be a function of the range, radar beam width, radar wavelength, and degree of distortion of the acoustic wave front. When the temperature gradient is 5° C/km, the echo power loss at a range of 10 km for 50- and 500-MHz radars becomes as large as 0.5 and 16 dB for radar beam width of 2.0°, and 7.0 dB and 19.0 dB for a 5.0° width, respectively.