Between Dusk and Dawn

By   2 June 2021 06:05

Our radio friends in New Zealand informed me that the article on 60 metre propagation was published in the NZART magazine “Break-In”.

Below, a link to the article as I submitted it.


(taken from the longer version of the study)

The author, radio amateur, became intrigued by ionospheric propagation during the late 70’s of the previous century. Reports of exceptional propagation during periods of high solar activity inspired the author to build a receiver for the 50 MHz band. Stations from Africa and the Americas were heard during the maximum of solar cycle 21. Just before the next solar cycle peak, permits to transmit were issued. Many contacts followed to all continents. Two reports were written, analysing the observations during cycle 22.

Late 2015, a previously unallocated frequency band around 5.3 MHz was assigned to Dutch radio amateurs. Exploring this uncharted territory revealed unexpected propagation phenomena, such as reception by a station in Tasmania. Experiments followed which revealed that signal strengths peaked around the same time on consecutive days. The estimated path loss was considerably less than various mathematical models predict.

After the New Zealand administration issued experimental 5.3 MHz licences in 2018, it became clear that wave propagation between western Europe and New Zealand can be exceptionally good via the long great circle path across South America. Series of experiments demonstrated that signal strengths peak at specific times, with a diurnal pattern.

Over time, it became clear that contacts between Europe and New Zealand can be made with very modest power levels nearly every day. Observed path losses were surprisingly low.

Research was initiated, looking for explanations for the observed phenomena. Data was gathered from various sources. Much of it came from software, optimised for weak signal communications. The software estimates the signal to noise ratio which can be used to determine path loss between transmitter and receiver. This software also records and uploads reception reports to databases, from which data was retrieved. Data was also derived from observations by the author as well as various reports of other radio amateurs.

A number of possible explanations for the signal enhancements are discussed.

Tilts in the F2 layer, linked to changes in the ionosphere during sunrise and sunset, are believed to provide the condition for rays to get trapped in a duct between the E and F regions. Rays through this duct suffer minimal absorption and ground reflection losses, providing a valid explanation for the observed very low path losses.

Extensive ray tracing experiments confirmed this assumption and also showed the importance of horizontal E region gradients during twilight. The gradients reduce elevation angles towards the F region, widening the range of rays to enter or exit the duct. It is also believed that the gradients provide a focusing mechanism, converging rays and enhancing signal strength. The observed signal peaks coincide with the start of the E region ionisation. It is assumed that the E layer gradients reinforce the effects of the F2 layer tilts. Because sunrise in Europe coincides with sunset in New Zealand, signal enhancements associated with that path are most pronounced.