This paper presents calculations of ionospheric electron temperature and density perturbations and ground-level signal changes produced by oblique high-frequency (HF) radio waves. Our analysis takes into account focusing at caustics, the consequent Joule heating of the surrounding plasma, heat conduction, diffusion, and recombination processes, these being the effects of a powerful oblique ''modifying'' wave. It neglects whatever plasma instabilities might occur. We then seek effects on a secondary ''test'' wave that is propagated along the same path as the first. Our calculations predict ground-level field strength reductions of several decibels in the test wave for modifying waves having effective radiated power (ERP) in the 85- to 90-dBW range. These field strength changes are similar in sign, magnitude, and location to ones measured in Soviet experiments. The location of the signal change is sensitive to the frequency and the model ionosphere assumed; so future experiments should employ the widest possible range of frequencies and propagation conditions. An ERP of 90 dBW seems to be a sort of threshold that, if exceeded, might result in substantial rather than small signal changes. Our conclusions are bases solely on Joule heating and subsequent refraction of waves passing through caustic regions. ¿ American Geophysical Union 1990