Stefan, below a copy of my posting on this list from 3.11.09 maybe it's helpful. -- it's the CCIR- RECOMMENDATION 368-7 . There are no free sources on the internet for these graphs at least I was not
Dear friends: Some in spanish, but here you have all the UIT recomendations, P-series (Propagation): http://webs.uvigo.es/servicios/biblioteca/uit/product/P.htm P.368-9: Surface wave (10 KHz-30 MHz).