Short-wave radio signals are sometimes reflected from layers in the atmosphere back down to the planetary surface. These reflections can travel much further than the direct ground-wave, which is limited by the curvature of the planet (the flat-earthers generally miss these simple points!). Just as ships disappear over the horizon, so direct radio signals also become invisible.

Online analysis sites such as pskreporter.info provide maps of where radio signals are being heard, and at what strength. Sometimes, the annulus, or ring, is clearly visible, where the ground-wave direct signal cannot be heard, and equally, the reflection is too far away. This image shows a segment of an annulus.

The gap, or the difference between the direct ground-wave and the reflected wave is called the skip distance. In this case, it's around 500km from the G8PHM main station location.