Chordal Hop Propagation
Amateur radio operators communicate around the globe on the HF bands thanks to signals propagating by ionspheric skip. High layers of the atmosphere become densely populated with ions - charged particles that result from solar radiation. HF signals, and sometimes VHF signals, are bent (refracted) back toward the earth by these layers of the ionosphere to allow long-distance, over-the-horizon communications. (Read more basics about ionospheric propagation in the suggested related articles below.)
The longest distance typical of a single F-layer ionsopheric skip is about 2500 miles. However, reflection of signals from the earth back into the ionosphere are common, allowing multi-skip propagation and much greater communications distances.
Multi-skip signals become very weak, in part due to earthly reflections in which signals get absorbed by the earth and scattered by irregular surface terrain and manmade features.
However, under some conditions ionospheric propagation can occur without surface reflections, thereby reducing signal losses and achieving very long distance communications. Signals traveling in multiple refractions within the ionosphere before returning to earth can support long-path propagation in which signals travel around the globe in the opposite direction of the shortest distance over the surface between stations. Under some ionospheric conditions, a station may even be able to receive its own signal from propagation around the world, and other stations may hear a slight echo effect from reception of both the short path and long path signals.
How is this possible? What's going on here?
The refractive strength of ionospheric layers changes with solar conditions, the time of day, and the signal frequency. The steepness of the angle of an ionospheric skip will change commensurately with these factors. When sunspot numbers are great, the ionosphere becomes very densely populated with electrons and positively charged ions, and it is very effective at bending HF signals steeply back to the surface of earth. However, higher frequencies, such as the 10-meter band's 28 MHz signals, will be refracted at shallower angles than lower frequencies, such as the 20-meter band's 14 MHz signals. The refractive effect reduces as frequency increases.
Additionally, on the night side of earth where the ionosphere weakens, signals of any frequency will not be refracted as steeply as on the daylight side of the planet where the ionosphere remains strong. Given a proper balance of ionospheric conditions and emitted frequency, the geometry of ionospheric refraction can resemble that of Figure 2 that presents potential ray traces of an HF signal propagating by chordal hops.
The name of this propagation phenomenon comes from a geometric term. A chord is a line within and across an arc of a circle or sphere. Signal ray traces over the night side of the earth refract shallowly to form multiple chords until reaching the daylight side where they are more steeply bent back to the earth to be received by stations. The daylight-side ionosphere maintains a stronger refractive effect with its dense population of ions, while the night-side effect is sufficient only to produce the shallow angles of the chordal hops.
From this geometry it is easy to see how long-path propagation can result with chordal hops. Since the signals are traveling great distances without the need to reflect from the earth, signal strength is better preserved.
-- Stu WØSTU