Identifying 14 MHz Propagation Modes Using FST4W SNR and Spectral Spread

The FST4W protocol within the WSJT-X family of weak signal communications programs has an advantage over the widely used WSPR protocol in that it estimates spectral spreading. With modern equipment of modest cost, readily available to the radio amateur, spectral spread at the transmitter and receiver can be less than 30 mHz. This is much lower than spectral spread imposed on signals by ionospheric refraction or ground or sea scatter. Simple two-dimensional scatter plots of spectral spread and signal to noise ratio, alongside time series plots, show clear clustering attributable to different propagation modes. Using a single FST4W transmitter in Northern California and reports from eleven receivers from 2.4 km to over 3000 km to the west, north and east spectral spreading/signal to noise ratio clusters for surface wave and ionospheric 1F and 2F paths were easily identifiable. Other clusters were not so obvious. In particular, the prevalence of 2F ground side-scatter, or skew off great circle propagation, also termed 'above the basic maximum usable frequency' propagation, at ranges of 40 to 1000 km was unexpected. This mode was also seen after dusk at more distant receivers, following on from 1F propagation as the maximum usable frequency fell. This mode was easily tracked across different receivers by its high spectral spread, 500 mHz to 650 mHz, some eight times that of 1F propagation. Instances of 'above the basic maximum usable frequency' nighttime propagation due to, we hypothesize, refraction from patches in the ionosphere with much higher electron density than the background plasma were identified by their low spectral spreading at 1000 km and 1525 km. Identifying the particular propagation mode over a path may be of interest to the radio amateur, for example, if the current mode is 2F ground side-scatter, antenna headings along the great circle path may not give best results. Propagation mode identification using FST4W could be a radio amateur contribution to the ionospheric science programs of the 2023 and 2024 Festivals of Eclipse Science, charting changes in propagation modes as changes in solar flux affected ionospheric dynamics and structure.