The correlation function of log amplitude fluctuations of a wave reflected from a plane target in turbulent atmosphere is derived for a weak fluctuation region and is used to evaluate the aperture-averaging effect. The results show that the correlation length of the collimated beam propagating in a folded path is smaller than that of the wave propagating the same total lengnth in the forward direction, they both agree in order of magnitude with the radius of the first Fresnel zone. From this, the aperture averaging factor in a straight path is greater than that in a folded path,which means that the turbulence induced ingtensity fluctuation can be reduced by means of a reflector.The agreement of our experimental results with our theoretical prediction is excellent.