Multipath Propagation

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Multi-Path Propagation

One feature of this chart is that by changing the angles at Point A and Point B, it is possible to explore the many different propagation paths that may occur as a signal travels from A to B, or conversely from B to A. When two or more of these paths exist simultaneously within the same communication circuit, the result is known as multipath propagation.

In the image below, we see one example of a possible multipath circuit. One path travels from Point A to Point C1 and then on to Point B. The reflection point, C1, has an elevation of approximately 113 miles. A second path travels from Point A to Point C2 and then on to Point B, where the reflection point is approximately 97 miles above the Earth.

Because the path through C2 is shorter, that signal arrives at the receiver slightly before the signal traveling through C1. When the two signals recombine at the receiving location, they may reinforce or cancel each other depending on their relative phase. This can produce effects such as fading, distortion, or a noticeable echo-like quality in the received signal. The severity of these effects depends on the difference in path lengths, signal strengths, and propagation conditions.

As the refractive layer lowers and rises during the day, the distance and time distance of arrival (TDOA)  also changes.  Some amateur radio operators are able to view the Doppler shift that occurs as the layer changes.  As with multi-path propagation, the changes in height can cause fading, and distortion.  

While this tool looks at primarily the angles and distance, some of the multipath possibilities can be related to the ionospheric region through which the signal travels.  It might also be that instead of simply going up and down in a direct path from transmitter to receiver, a signal might go off to the left or right and still end up at the receiver.  In some conditions and frequencies, the multipath could also be the result of interactions with mountains, buildings or other large objects.  

Some amateur radio operators have assembled specialized equipment to measure and study multipath propagation. Time and frequency standard stations such as WWV and WWVH in the United States, and the former CHU station in Canada, have often been used as reference signals for such experiments.

Although this tool was not specifically designed to model multipath propagation, it can be used to visualize how multiple propagation paths might exist between two locations. Images generated by the tool can be saved and combined in applications such as PowerPoint to illustrate multipath scenarios. A future version of the tool may include features specifically intended for multipath analysis.

In the graph, the dashed line appears as if it is below the Earth circle between point A and point C2.  That is an artifact of an imperfect redrawing of the path.