Optical DXing and refraction

Although my own experiments at 481THz have so far been limited to around 9km non line-of-sight, the amateur experts around the world have been achieving remarkable ranges both line-of-sight (LOS) and over the horizon (NLOS). In the UK several groups are looking for longer line-of-sight paths up to around 147km.

Some interesting information is coming to light (no pun intended) about the refractive index of the lower atmosphere at optical frequencies. At radio frequencies the refractive index (K) is around 1.33 and I had always assumed that for lightwaves it was just over 1. But tests suggest this is NOT the case at lower levels such as over the near line-of-sight paths now being attempted. A recent paper by Barry Chambers G8ACN on the UKNanowaves Yahoo group suggests a much higher figure may be more appropriate for a lot of the time meaning that what may appear to be a non line-of-sight path actually is one. He cites some observations by G8CYW and others of distant factory chimneys and cooling towers that should not be visible over the distant horizon but frequently are clearly visible.

Many are familiar with optical mirages, but the fact that the optical refractive index is frequently in the 1.1-1.33 range surprises me. It suggests many more distant optical paths may be workable with suitable equipment than was previously thought possible without having to resort to clear air scattering or cloudbounce. With 481THz cloudbounce and clear air scattering the additional path loss (compared with a LOS path) can be in the 30-50dB range, so equipment for NLOS paths is much more demanding than for a quasi line-of-sight path.