Skywave Propagation..
Skywave propagation occurs when radio waves are reflected from the ionosphere. Practically all HF communication is done by skywave. In the ionosphere, the waves are really refracted twice, and they just appear to be reflected. The reflections are frequency sensitive, meaning each ham band reflects differently from the others. Low frequencies, such as 80m, reflect mainly from the lower levels of the ionosphere and the reflected signal comes nearly straight back down. This causes 80m to propagate to points from local out to more than a few hundred km in the daytime. At night, when the D-layer and E-layer are absent, signals striking the ionosphere at lower angles may propagate many thousands km on 80m. On the bands from 10m to 20m, high angle signals pass straight through the ionosphere and do not reflect back down to the nearby stations. The low angle signals on these higher bands reflect from the ionosphere near the horizon and return to the Earth some km away. The in-between region can’t hear the TX signals nor can you hear signals coming from this region. The in-between region is called the "skip zone". Only when the ionosphere is weakly ionized do you have a skip zone on 80m. Another interesting type of skywave propagation seen on the higher HF bands is called chordal hop propagation seen frequently in trans-equatorial propagation, which is propagation crossing the equator. When this occurs, signals entering the ionosphere are trapped inside
the F2-layer then they are finally refracted back to earth across the equator thousands km away. There is no propagation between the signal entry point and the exit point. This is skip in the extreme. One time when we working Europe and North America at the same time, we could not hear the European stations because our path to him was via chordal hop propagation. Another way of describing chordal hop propagation is to call it ionospheric ducting. Skywave propagation sometimes produces an effect called "backscatter." What happens is the radio waves that strike the ionosphere, instead of only reflecting father away from the transmitting station, part of the signal reflects backwards toward the TX station. Stations that are too close to hear each other by direct wave can communicate by the backward reflecting waves. Both stations that communicate by backscatter must point their directional beam antennas in the same direction although their direction toward each other may be at some other azimuth. Backscatter will confuse front-to-back measurements of directional beam antennas. This is because, when you turn the back of the antenna toward the station you are hearing, you may be able to hear him on backscatter from a direction opposite from him. You will be hearing him from the ionized atmospheric cloud in the opposite direction. During intense solar magnetic storms, when aurora occurs at high latitudes, stations are able to communicate by backscatter on VHF and UHF by both stations pointing their directional beams toward the aurora. This will be due north for stations in the Northern Hemisphere and due south for stations in the Southern Hemisphere. Audio from aurora backscatter will have a "wispy" sound.
Skywave propagation occurs when radio waves are reflected from the ionosphere. Practically all HF communication is done by skywave. In the ionosphere, the waves are really refracted twice, and they just appear to be reflected. The reflections are frequency sensitive, meaning each ham band reflects differently from the others. Low frequencies, such as 80m, reflect mainly from the lower levels of the ionosphere and the reflected signal comes nearly straight back down. This causes 80m to propagate to points from local out to more than a few hundred km in the daytime. At night, when the D-layer and E-layer are absent, signals striking the ionosphere at lower angles may propagate many thousands km on 80m. On the bands from 10m to 20m, high angle signals pass straight through the ionosphere and do not reflect back down to the nearby stations. The low angle signals on these higher bands reflect from the ionosphere near the horizon and return to the Earth some km away. The in-between region can’t hear the TX signals nor can you hear signals coming from this region. The in-between region is called the "skip zone". Only when the ionosphere is weakly ionized do you have a skip zone on 80m. Another interesting type of skywave propagation seen on the higher HF bands is called chordal hop propagation seen frequently in trans-equatorial propagation, which is propagation crossing the equator. When this occurs, signals entering the ionosphere are trapped inside
the F2-layer then they are finally refracted back to earth across the equator thousands km away. There is no propagation between the signal entry point and the exit point. This is skip in the extreme. One time when we working Europe and North America at the same time, we could not hear the European stations because our path to him was via chordal hop propagation. Another way of describing chordal hop propagation is to call it ionospheric ducting. Skywave propagation sometimes produces an effect called "backscatter." What happens is the radio waves that strike the ionosphere, instead of only reflecting father away from the transmitting station, part of the signal reflects backwards toward the TX station. Stations that are too close to hear each other by direct wave can communicate by the backward reflecting waves. Both stations that communicate by backscatter must point their directional beam antennas in the same direction although their direction toward each other may be at some other azimuth. Backscatter will confuse front-to-back measurements of directional beam antennas. This is because, when you turn the back of the antenna toward the station you are hearing, you may be able to hear him on backscatter from a direction opposite from him. You will be hearing him from the ionized atmospheric cloud in the opposite direction. During intense solar magnetic storms, when aurora occurs at high latitudes, stations are able to communicate by backscatter on VHF and UHF by both stations pointing their directional beams toward the aurora. This will be due north for stations in the Northern Hemisphere and due south for stations in the Southern Hemisphere. Audio from aurora backscatter will have a "wispy" sound.
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