08-11-2012, 05:58 PM
HF Propagation
1 HF Propagation.ppt (Size: 2.59 MB / Downloads: 28)
Properties Of Radio Waves
Waves travel in straight lines.
Waves will be “attenuated” or weakened if they pass through anything that conducts electricity.
The amount of attenuation (and therefore range) depends on frequency and what the wave must pass through.
For HF Systems increasing power does not increase range in proportion.
Direct Wave
Sometimes known as Line Of Sight (LOS) waves
The radio wave travels directly from source to destination.
Point to point as in space
e.g. from ground to an aircraft
Range is short at ground level because the radio wave travels in a straight line and the earth is curved. VHF propagates in a similar manner.
Sky Wave
Skywave uses a layer in space known as the ionosphere to refract its signal back to earth
Using the ionosphere, a radio wave can travel long distances
The distance traveled is known as the ‘skip’ or ‘hop’ distance
A Sky Wave reflected by the ionosphere is the most common method of HF communication.
Ionospheric VariationSolar Activity
Solar Cycle
The sun goes through a periodic rise and fall in solar activity due to Sunspot Activity varying in length from 9 to 14 years.
This cycle has a direct effect on the Ionosphere and consequently an effect on the propagation of HF radio waves
Antenna Systems
Not only are antennas very important, they are also very complex.
Unlike what has been shown so far, antennas don’t propagate a single wave ‘line’ – they propagate in all three dimensions, in different shapes and intensities
Antenna propagation varies with design, frequency, and height above ground