Some theories content that 160 meter propagation changes little if any when the SSN is between 0 and 50. The D-layer leve remains the same because the ionizing source is not related to the SSN.
Popular Myth- We don't understand medium frequency (300-529 kc long wave aviation and marine navigation beacon band, 530-1700 kc AM broadcast band, 160 meter amateur band and 120 meter shortwave tropical broadcast band) radio wave propagation conditions and therefore it can't be forecasted.
Fact- Yes it can and is on a regular basis at KN4LF Daily LF/MF/HF/6M Frequency Radiowave Propagation Forecast
http://www.kn4lf.com/kn4lf6.htm .
a.) Medium frequencies encompass 300 to 3000 kc. The simplest way to look at medium frequencies with respect to propagation issues from a layman's point of view, is to accept the fact that propagation is poor the majority of the time (See definition #6. Electron Gyro Frequency Absorption), especially past approximately 1250 miles (one refraction off of the E layer), with occasional short-lived good periods as far as 3200 miles.
Medium frequency radio waves possess elliptical polarization, with the signal splitting into ordinary and extra-ordinary rays. These rays can propagate in or out of phase, more often out of phase. The out of phase extra-ordinary ray represents a 50% power loss on the receive end of a path.
b.) Why is medium frequency propagation poor the majority of the time? D layer absorption! At daytime the D layer which is at an approximate height of 30-60 miles in the mesosphere, totally absorbs medium frequency RF signals the majority of the time. I say the majority of the time because at higher latitudes, during the winter season and especially at the low part of a sunspot cycle, daytime penetration of RF signals through the weakened D layer and then refraction via the E layer and/or Sporadic E (Es) clouds does occur. Another issue is the fact that the D layer does not totally disappear at night. Many books that deal with wave propagation erroneously state that the D and E layer's disappear after sunset, totally incorrect thanks to Galactic X-Rays, Cosmic Rays and Lightning.
c.) Background electromagnetic radiation in the 1 to 10 Angstrom range (Hard X-Rays) is a major source of ionization of the day time D layer, with our Sun as the source of Cosmic Rays, also playing a role.
The following paragraph was contributed by Carl Luetzelschwab K9LA, a scientist with a very good understanding of radiowave propagation.
.....A couple years ago I was playing with Proplab Pro on a one-hop 936km path on 160m during daylight. I plotted absorption versus sunspot number. I expected a nice monotonic increase as the sunspot number increased. But the plot showed that absorption started at about 60dB at zero sunspots and was constant out to a sunspot number of about 50. Then it started climbing, reaching 100dB at a sunspot number of 150. This suggested that there was something other than hard X-rays and cosmic rays as the source of daytime D region absorption. So I dug into Davies 1990 (page 61), Hunsucker and Hargreaves (page 31), and Brekke (page 233). They all seem to point to the Lyman-alpha line of the solar spectrum at 1215 Angstroms ionizing NO as the main source of the quiet daytime D region. So in terms of my absorption versus sunspot number plot, the flat portion up to a sunspot number of 50 is probably due to the Lyman-alpha line ionizing NO. Then above a sunspot number of 50 the hard X-rays start contributing as the Sun becomes more active.....
Carl has produced two really good .pdf files on 160 meter propagation in 2003 and 2004. Read them here: 160 Meter Propagation & Disturbances To Propagation . He also has a propagation website with allot of good information on it at K9LA's Amateur Radio Propagation.
