"After several discussions of the subject on various Internet reflectors, I am quite sure we are dealing with troposcatter. The word 'Tropo' is not in any of the books or research articles on propagation I have read. It seems 'Tropo' is used by amateurs for various propagation modes taking place in the troposphere because they do not know better. The word 'Tropo' should never be used alone. Let me try to describe the relevant propagation modes (ranges in brackets) in the troposphere:
"Always available are: Line of sight (0-30 km, depending on height). The antennas see each other and have more or less clearance of 1. Fresnel zone. Generally S9 signals with QRP. Diffraction (30-100 km?). Waves follow the curved earth a little, thus extending line of sight somewhat. Knife edge diffraction in mountain areas is a better known but special case of diffraction. Refraction. (30-100 km?). Waves bent towards the earth due to the fact that the atmosphere gets thinner at higher altitude. Actually humidity plays a greater role than atmospheric temperature and pressure. Diffraction and refraction combined extends line of sight range, but signals will have QSB.
"Troposcatter (100-700 km). Scattering in the troposphere in the common volume of the beams. Range is limited by antenna gain and power. 10w and 10 dB at each end give about 300 km range anytime on 144 MHz, most SSB stations work up to 700 km anytime, two eme-stations at 900-1000 km can work each other anytime with very reliable signal strength. Most QSB on troposcatter takes place on the shorter range, 2-300 km where lifts of 20 dB are not uncommon. The lifts on troposcatter signal strength are often caused by increased refraction, which effectively decreases the scatter angle. Note: each degree of scatter angle costs 9-12 dB in signal strength, so set your self up with low angle radiation if you want daily DX using troposcatter.
"Forms of tropospheric propagation only available occasionally include Inversion (200+km ?). Temperature/humidity sudden increases at higher heights, 100-1.000 m up and radio waves are reflected back to earth. Ducting (1.000+km, e.g. Hawai to California at 4000 km). Radio waves are trapped like in a wave guide between an inversion layer and the ground or between two inversion layers. Little attenuation, thus good signal strengths. Often signals are only heard at each end of the wave guide, and conditions are only good between relatively small geographical areas.
"All the above mentioned propagation modes are of tropospheric origin, where the cause of propagation is some kind of change in the refractive index of the atmosphere - that is change in temperature, density and humidity.
"Scatter is composed of simultaneous reflections from many small objects. If all the resulting small signals arrive in phase at the RX the scatter is coherent and the signal quality is Q5. If all the small signals arrive more or less out of phase at the RX the scatter process is incoherent and the signal sounds distorted, much like aurora. Tropospheric forward scatter along a great circle bearing is almost coherent, so signal quality is OK. However we can experience tropospheric side or back scatter when our beams do not point at each other, so the common scatter volume is not along the great circle bearing. This is mostly heard on stations with big (narrow) beams and QRO linears. The backscatter signal sounds distorted, much like aurora (which actually is just 'backscatter from field aligned patches of auroral ionization'). So if you get reports on bad audio it could be caused by tropospheric backscatter, so make sure both beams point at each other before you investigate your audio.
"Note that troposcatter works from 50 MHz to 10 GHz (28 MHz also. G0AEV) and in principle the propagation mechanism does not change. However we often find troposcatter to be less on 50 MHz, but this is mostly due to the average amateur tower being smaller in wavelengths on 50 MHz than on the higher bands. (The size of the scattering volumes relative to the frequency of the radio waves being propagated also has a major impact on the relative efficiencies of troposcatter. G0AEV).
On 10 GHz few amateurs have the power on to do troposcatter, but when enough stations run 10 w and a dish a few m in diameter on 10 GHz they will realize that 300 km+ can be worked anytime. Just like 300 km can be worked any time on 50 MHz with a modest setup. A serious contest station on 50 MHz will be able to work 700 km any time and this requires no eme setup, just some power and low angle of radiation at both ends. The ranges given in Six and Ten Report July 97 neither fit theory, nor my practical experience."