PROPAGATION

Radio wave propagation on earth takes place between two boundary layers, the earth’s surface and the ionosphere. In attempting to explain how such propagation mechanisms work, ionospheric physicists and other scientists have created mathematical models that allow us to predict, or simulate, what we see in the real world.

But the real-world propagation mechanism is much more complex. Consider radio waves emitted from an antenna somewhere above the earth’s surface: The antenna radiates energy in an infinite number of directions that excite a nearly unlimited number of propagation modes that then travel, or propagate, through the area between the boundaries. Such modes do not travel in isolation but interact whenever they come together to create other modes that are different from the original excitation energies. This process is called mode combining.

When the radio waves travel through uniform boundaries, such as over uninterrupted lengths of seawater under an all-daytime ionosphere, the combined modes propagate with little change. But when an abrupt boundary change is encountered, such as the coast or a day-to-night change in the ionosphere, previously stable modes interact, mode combining again takes place, and a new set of modes is launched, different in all directions.

Consider the real-world environment, where there is a complex combination of ground and water. In addition to the varying complexities introduced by the different ground and water influences, consider the always changing ionosphere, where the day-night terminator sweeps through the area, continually perturbing the already unstable ionosphere. It is easy to see that the propagation medium is indeed so complex that any mathematical models conceived to simulate it must be simplifications.

Scientists have studied and measured radio wave propagation for many years, but the resulting knowledge still doesn’t permit us to exactly simulate the natural process. Nevertheless, for distances greater than a wavelength where we can neglect near-field distortions, emerging theory considers two mechanisms: Groundwave propagation and skywave propagation. The total field can be considered to consist of groundwave plus skywave energy, and that energy is best explained as being a number of interacting modes in which the total electromagnetic energy propagates.

Where expected signal quality is:

A--Excellent opening, exceptionally strong, steady signals greater than S9

B--Good opening, moderately strong signals varying between S6 and S9, with little fading or noise.

C--Fair opening, signals between moderately strong and weak, varying between S3 and S6, with some fading and noise.

D--Poor opening, with weak