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View Full Version : Propagation Primer Part 2: The geometry of propagation

KL7AJ
08-05-2012, 10:03 PM
The surface of the Earth is much much closer to the upper reaches of the ionosphere than it is to the center of the Earth. Although this may seem obvious, it's easy to forget when calculating how many skips it takes to get from point A to point B. To get a realistic perspective of this, imagine a large orange. If the inside of the peel is the surface of the earth, the outside of the peel is the outer limits of the usable ionosphere.

This means that the maximum distance you can get with a single skip, assuming you launch a truly horizontal (zero angle ) signal is about 3000 kilometers. This is the maximum skip distance presented on any Digisonde.

If we ignore magnetic effects, ionospheric propagation is like bouncing a ball down a hall with a very low ceiling.; For all practical purposes, we can consider the "ceiling" as being horizontal, with little consideration for the curvature of the earth.

With this grossly over-simplified model, skip distance is a direct function of the height of the ionosphere at the midpoint and the launch angle. Of course, launch angle is a bit difficult to define, since we aren't launching laser beams, we're launching beams with very wide lobes. As an expedient, we can consider the launch angle as being the MAXIMUM power angle of any particular radiation pattern.

The "height" of the ionosphere is equally "rubbery" since the reflection does not take place suddenly at one altitude, but is a slow process, taking place over many wavelengths of propagation. A highly derived figure known as "virtual height" is an attempt to arrive at a usable reflection height.