Ground-Wave Propagation..
Ground wave works only with vertical polarization. One side of the antenna is the vertical radiator and the other side of the antenna is the earth ground. The surface wave in the air travels faster than the part of the wave flowing through the ground. The surface of the earth is curved like the curved part of a racetrack. On the curved track, a car on the outside of the track has to travel faster than the car on the inside lane to stay even, and the two cars travel in a curved path. Although the wave in the air travels faster than the wave on the ground, the two parts of the wave cannot be separated. Because of this, the radio wave also travels in a curved path that follows the curvature of the earth. The AM broadcast stations use ground wave propagation during the day and skywave propagation at night. Since radio waves at lower frequencies conduct better through the ground, an AM broadcast station on 540 kHz will be many dB stronger than a station on 1600 kHz, if both run the same power. This fact is important in understanding why ground mounted verticals do not work as well at high frequencies as they do on the broadcast band.
Direct Wave Propagation..
Antennas located on high structures can "look" over the horizon and "see" the receiving antennas. Because refraction is involved, direct waves travel 20% farther than light waves due to scattering of radio waves by the environment. Trees and other foliage are invisible to HF radio waves. Direct wave propagation is possible at all frequencies, but this mode of propagation is seldom used on our HF bands, but it is the usual propagation mode used by repeaters and others on VHF and UHF. If you watch TV on an outside antenna , you are receiving the signal by direct wave propagation.
Propagation by Refraction..
Refraction occurs when the lower part of a wave travels slower than the top part of the wave because the wave is passing through two media. These media can be tw
o layers of air at different temperatures or they can be air and a solid. One form of refraction is caused by a radio wave passing over a hill or ridge being bent as it passes over the obstruction. This is known as "knife edge refraction”. Another form of refraction occurs when layers of air of different temperatures bend the radio waves around the horizon. This is called tropospheric ducting. This mode of propagation makes long distance contacts possible at VHF frequencies. Tropospheric ducting does occur on 10m and lower frequencies and is noticeable when other forms of propagation are absent. On HF bands, many hams mistakenly call tropospheric ducting and direct wave "ground wave".
Ground wave works only with vertical polarization. One side of the antenna is the vertical radiator and the other side of the antenna is the earth ground. The surface wave in the air travels faster than the part of the wave flowing through the ground. The surface of the earth is curved like the curved part of a racetrack. On the curved track, a car on the outside of the track has to travel faster than the car on the inside lane to stay even, and the two cars travel in a curved path. Although the wave in the air travels faster than the wave on the ground, the two parts of the wave cannot be separated. Because of this, the radio wave also travels in a curved path that follows the curvature of the earth. The AM broadcast stations use ground wave propagation during the day and skywave propagation at night. Since radio waves at lower frequencies conduct better through the ground, an AM broadcast station on 540 kHz will be many dB stronger than a station on 1600 kHz, if both run the same power. This fact is important in understanding why ground mounted verticals do not work as well at high frequencies as they do on the broadcast band.
Direct Wave Propagation..
Antennas located on high structures can "look" over the horizon and "see" the receiving antennas. Because refraction is involved, direct waves travel 20% farther than light waves due to scattering of radio waves by the environment. Trees and other foliage are invisible to HF radio waves. Direct wave propagation is possible at all frequencies, but this mode of propagation is seldom used on our HF bands, but it is the usual propagation mode used by repeaters and others on VHF and UHF. If you watch TV on an outside antenna , you are receiving the signal by direct wave propagation.
Propagation by Refraction..
Refraction occurs when the lower part of a wave travels slower than the top part of the wave because the wave is passing through two media. These media can be tw
o layers of air at different temperatures or they can be air and a solid. One form of refraction is caused by a radio wave passing over a hill or ridge being bent as it passes over the obstruction. This is known as "knife edge refraction”. Another form of refraction occurs when layers of air of different temperatures bend the radio waves around the horizon. This is called tropospheric ducting. This mode of propagation makes long distance contacts possible at VHF frequencies. Tropospheric ducting does occur on 10m and lower frequencies and is noticeable when other forms of propagation are absent. On HF bands, many hams mistakenly call tropospheric ducting and direct wave "ground wave".
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