The influence of chordal paths on signals propagating to the near antipode of an HF radio transmitter

The results of an HF radio propagation experiment at 15.270 MHz over an antipodal path are presented. The observations show a large and sudden presunrise increase in signal strength that is not predicted by a computer propagation model (multihop ray tracing with some random scattering) previously used successfully for the same geometrical path on a frequency of 11.790 MHz. It is shown that the anomaly can be explained when chordal propagation supported by the longitudinal ionospheric tilts (determined from critical frequency predictions) occurring along the most probable propagation paths are included in the computer model.