The paper discusses theoretical and numerical aspects of signal intensity calculations in the geometrical optics approximation for ionospheric HF wave propagation. We present an efficient method for calculating a flux tube of rays for a very general ionosphere. The method is new in the sense that it allows for the effects of electron collisions and the anisotropy produced by the Earth's magnetic field. The basic idea is to calculate the focusing of infinitesimally separated rays by integrating the differential equations for the deviated rays along a reference ray. Some of the first results obtained by this technique are presented.