19-06-2012, 03:13 PM
RADIO WAVE PROPAGATION
[attachment=24814]
PROPAGATION
Propagation: How radio waves travel from point A to point B;
and the events occurring in the transmission path that affect the communications between the points, stations, or operators.
When the electrons in a conductor, (antenna wire) are made to oscillate back and forth, Electromagnetic Waves (EM waves) are produced.
These waves radiate outwards from the source at the speed of light, 300 million meters per second.
Light waves (waves we see) and radio waves (waves we hear)are both EM waves, differing only in frequency and wavelength.
EM waves travel in straight lines, unless acted upon by some outside force. They travel faster through a vacuum than through any other medium.
As EM waves spread out from the point of origin, they decrease in strength in what is described as an "inverse square relationship".
For example: a signal 2 km from its starting point will be only 1/4 as strong as that 1 km from the source. A signal 3 km from the source will be only 1/9 that at the 1 km point.
HOWEVER…..
Modern receivers are very sensitive and extremely
small power provides usable signals. Waves can
be received many thousands of kilometers from the
transmitting station. For Example, Voyager 2 transmitted signals over many billions of kilometers from outer space with only 25 W of power!
POLARIZATION
The polarization of an antenna is the orientation of the electric field with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation
Radio waves from a vertical antenna will usually be vertically polarized.
Radio waves from a horizontal antenna are usually horizontally polarized.
LINE OF SIGHT, GROUND WAVE, SKY WAVE
Ground Wave is a Surface Wave that propagates or travels close to the surface of the Earth.
Line of Sight (Ground Wave or Direct Wave) is propagation of waves travelling in a straight line. These waves are deviated (reflected) by obstructions and cannot travel over the horizon or behind obstacles. Most common direct wave occurs with VHF modes and higher frequencies. At higher frequencies and in lower levels of the atmosphere, any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye senses.
Space Waves: travel directly from an antenna to another without reflection on the ground. Occurs when both antennas are within line of sight of each another, distance is longer that line of sight because most space waves bend near the ground and follow practically a curved path. Antennas must display a very low angle of emission in order that all the power is radiated in direction of the horizon instead of escaping in the sky. A high gain and horizontally polarized antenna is thus highly recommended.
Sky Wave (Skip/ Hop/ Ionospheric Wave) is the propagation of radio waves bent (refracted) back to the Earth's surface by the ionosphere. HF radio communication (3 and 30 MHz) is a result of sky wave propagation.
[attachment=24814]
PROPAGATION
Propagation: How radio waves travel from point A to point B;
and the events occurring in the transmission path that affect the communications between the points, stations, or operators.
When the electrons in a conductor, (antenna wire) are made to oscillate back and forth, Electromagnetic Waves (EM waves) are produced.
These waves radiate outwards from the source at the speed of light, 300 million meters per second.
Light waves (waves we see) and radio waves (waves we hear)are both EM waves, differing only in frequency and wavelength.
EM waves travel in straight lines, unless acted upon by some outside force. They travel faster through a vacuum than through any other medium.
As EM waves spread out from the point of origin, they decrease in strength in what is described as an "inverse square relationship".
For example: a signal 2 km from its starting point will be only 1/4 as strong as that 1 km from the source. A signal 3 km from the source will be only 1/9 that at the 1 km point.
HOWEVER…..
Modern receivers are very sensitive and extremely
small power provides usable signals. Waves can
be received many thousands of kilometers from the
transmitting station. For Example, Voyager 2 transmitted signals over many billions of kilometers from outer space with only 25 W of power!
POLARIZATION
The polarization of an antenna is the orientation of the electric field with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation
Radio waves from a vertical antenna will usually be vertically polarized.
Radio waves from a horizontal antenna are usually horizontally polarized.
LINE OF SIGHT, GROUND WAVE, SKY WAVE
Ground Wave is a Surface Wave that propagates or travels close to the surface of the Earth.
Line of Sight (Ground Wave or Direct Wave) is propagation of waves travelling in a straight line. These waves are deviated (reflected) by obstructions and cannot travel over the horizon or behind obstacles. Most common direct wave occurs with VHF modes and higher frequencies. At higher frequencies and in lower levels of the atmosphere, any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye senses.
Space Waves: travel directly from an antenna to another without reflection on the ground. Occurs when both antennas are within line of sight of each another, distance is longer that line of sight because most space waves bend near the ground and follow practically a curved path. Antennas must display a very low angle of emission in order that all the power is radiated in direction of the horizon instead of escaping in the sky. A high gain and horizontally polarized antenna is thus highly recommended.
Sky Wave (Skip/ Hop/ Ionospheric Wave) is the propagation of radio waves bent (refracted) back to the Earth's surface by the ionosphere. HF radio communication (3 and 30 MHz) is a result of sky wave propagation.