Comparison of propagation predictions and measurements for midlatitude HF near-vertical incidence sky wave links at 5 MHz

Signal power measurements for a UK-based network of three beacon transmitters and five receiving stations operating on 5.290 MHz were taken over a 23 month period between May 2009 and March 2011, when solar flux levels were low. The median signal levels have been compared with monthly median signal level predictions generated using VOACAP (Voice of America Coverage Analysis Program) and ASAPS (Advanced Stand Alone Prediction System) HF prediction software with the emphasis on the near-vertical incidence sky wave (NVIS) links. Low RMS differences between measurements and predictions for September, October, November, and also March were observed. However, during the spring and summer months April to August), greater RMS differences were observed that were not well predicted by VOACAP and ASAPS and are attributed to sporadic E and, possibly, deviative absorption influences. Similarly, the measurements showed greater attenuation than was predicted for December, January, and February, consistent with the anomalously high absorption associated with the "winter anomaly." The summer RMS differences were generally lower for VOACAP than for ASAPS. Conversely, those for ASAPS were lower during the winter for the NVIS links considered in this analysis at the recent low point of the solar cycle. It remains to be seen whether or not these trends in predicted and measured signal levels on 5.290 MHz continue to be observed through the complete solar cycle.