Paper gives the description of a two-frequency, two-time coherence function of HF field, propagating in the fluctuating ionosphere, in the framework of a complex phase method for a smoothly inhomogeneous dispersive medium with local inhomogeneities embedded. The coherence function is calculated numerically on the basis of the analytical expression, derived by the complex phase method. The results of numerical simulation of the coherence function are presented for a concrete one-hop HF path of propagation and an inverse power law spatial spectrum of the ionospheric electron density fluctuations. General properties of the coherence function are discussed.