RF & Microwave

Unusual amateur radio propagation modes

17 May 2024
Source: mLu.fotos/CC BY 2.0

Propagation is loosely defined as the ability for a radio signal to travel from one place to another. While a radio signal may be emitted in all directions (isotropically), part of that signal may be reflected and refracted numerous times before reaching its destination at the receiving station.

“Normal” propagation consists of either line-of-sight communication or “skip.” In line-of-site communication, remember the walkie-talkies of childhood, where one child stands within sight of another and talks into the walkie-talkie. Some combinations of power output and frequency limit communication to line-of-sight.

Sporadic-E

One of the most unpredictable modes of propagation is sporadic-E. Sporadic-E is where a random parcel of air becomes ionized due to the sun’s energy. This ionized cloud becomes a reflector of radio waves, just like a floating mirror. The ionized cloud may linger for a few seconds or a few minutes, and can theoretically occur at any height, but it is much more common at high altitudes (95 km-150 km above the surface). At this height, radio signals can be bounced long distances.

Sporadic-E can cause signals of many different frequencies to bounce. However, amateur radio operators normally talk about sporadic E as a 6-meter (50 MHz) phenomenon. Typically, 6-meter signals only travel locally. With sporadic E, suddenly the distance is increased significantly for the duration of the event. For example, a station in New Mexico may make lots of contacts to bordering states (Texas, Colorado, Arizona), and then will make a contact in Kentucky at seemingly random times. Furthermore, propagation between these places may be limited; the station may have a perfect copy on Kentucky one minute and have no copy at all in the next.

name="_s7lepvov16bj">Meteor scatter

When a meteor enters Earth’s atmosphere, the friction between the meteor and the air, especially at high speed, causes the meteor to heat up and break apart. In this process, the meteor leaves behind a small, ionized cloud of material for a few seconds. Just like sporadic-E, the ionized trail can be used as a reflector for radio signals.

The ionized trail behind a meteor may only last for a few seconds or a minute. Therefore, long voice conversations are not favorable for these conditions. Instead, amateur radio operators tend to use digital modes, where short messages are sent through the radio using computers. One mode is a very fast, machine copyable morse code, and others include JT-65, FT-8 and FT-4, which use timed, pre-programmed messages. This makes the most of the time where the meteor trail is ionized.

Some, including the author of this article, originally assumed the operator had to see a meteor and point a directional antenna at it. This is not true. There are millions of meteors that burn up in the atmosphere without the big spectacle referred to as a “shooting star.” These smaller meteors make up the bulk of meteor scatter communication.

name="_yc03oyj1405m">Grayline

Normal modes of propagation focus on the fact that the ionosphere separates into different zones during the daytime, but then recombines at night. Each layer reflects different frequencies and is located at different heights. The end result of normal propagation is that signals below 14 MHz generally reflect better during the nighttime and above 14 MHz during the daytime. This is why the 10-meter band (28 MHz) “dies” sometimes around sunset, as there ceases to be a reflective surface to return radio waves back to Earth.

Slice of the ionosphere during the daytime and the nighttime. Image source: IonosphereLayers-NPS.gif, CC BY-SA 3.0, .Slice of the ionosphere during the daytime and the nighttime. Image source: IonosphereLayers-NPS.gif, CC BY-SA 3.0, .

Ask any kid who has been told to “be home by dark” and learn that there is a big “gray” area between light and dark, literally. In that area of twilight, the D layer is beginning to disappear and the F2 and F1 layers are combining back into the single F layer. As a result, the nighttime amateur radio bands are starting to open up, but the daytime bands have not yet surrendered.

Grayline propagation occurs daily during this time. Daytime bands, such as 10 meters (28 MHz), 12 meters (24 MHz) and 15 meters (21 MHz) begin to lose their normal propagation patterns. However, there is a sudden strengthening in propagation along north-south paths, along the terminus between daylight and nighttime. Amateur radio stations in the United States will have a much easier time talking to Brazil and Argentina during grayline propagation than they might have all day. It actually enhances propagation along north-south paths.

Of all of the unusual propagation modes, grayline is the most predictable. Daytime bands benefit most from grayline propagation.

name="_ulidiyu4m05w">Tropospheric ducting

VHF and UHF radio waves often travel in a line-of-sight fashion. It makes them great for local communication, as well as communication with spacecraft. Tropospheric ducting is the exception, occasionally extending the range of VHF/UHF signals, but not really affecting other signals significantly.

Tropospheric ducting occurs where density differences in the atmosphere cause the radio signal to refract. Instead of simply bending direction, some weather anomaly (such as a strong cold front) causes that reflection to be contained along a meteorological boundary. It’s like shooting a BB gun into a culvert- it bounces around, but really only comes out at the other end.

A tropospheric ducting event can look like this: a station in DC normally talks to a few folks around the DC area on a VHF frequency. One day, he talks to a few of his friends in DC, but then he makes contacts along a stripe between DC and southern Vermont. Typically, this would be impossible with VHF or UHF, but tropospheric ducting makes it possible.

name="_rrwfj2z3kx4w">Conclusion

The mysteries of radio started being unlocked in the late 1800s and early 1900s. Since then, so many advances in technology have led to discoveries on how radio waves travel, and thus more about the physics behind them. For example, early radar displays during World War II were plagued with “noise” that turned out to be showers and thunderstorms. Many of the propagation methods discussed in this article have been due to these recent discoveries. Moving forward, new radio technology will bring about even more discoveries about how information can be passed between people.



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