From noise voltage frequency spectral data Ss（ω） taken from carrier channels along many transmission lines in China

relative power spectra are calculated. Then by step-by-step integration method

the equations for corresponding correlation coefficients

or normalized autocorrelation ρ ate derived in terms of Ss（ω） and difference time τ. The shapes of the calculated ρ-τ curves are in general irregular

but in several cases where maximum Ss（ω） occurs at lower measuring frequency

p-r curves resemble the shape of damped cosinusoid e-aτ cos 2 πf τ. Finally the following parameters are calculated: thedifference time τ1

τ2 and τ3 at which | ρ|≤

| ρ|≤1/e2 and | ρ|≤ 0.05

exponentialattenuation constant a

average frequency and no. of zero crossings N

in intervals τ1

τ2-τ1

τ3-τ2 and τ3. It is found N/τ3f= 2

τ3 determines the upper limit of frequency ofsignal envelope for optimum reception. When higher carrier frequency （>40000 Hertz） is used

τ3 is usually smaller

this upper frequency limit 1/τ3 becomes higher.