Friday, April 22, 2016

Antipodes DX

While discussing recent DXpeditions with a friend we noted the interesting propagation we have from here to the south Indian Ocean and nearby Antarctic islands. This is due to the antipodes of VE3 being located in that wide expanse of ocean skirting Antarctica to the southwest of Australia.

The antipodes is the opposite point on a sphere. On Earth you can easily locate it by inverting your latitude (north to south, or south to north) and adding or subtracting 180° to your longitude. There is a web site that makes it easy to find the antipodes of any point on Earth. Doing it for Heard Island nets the following humourous picture:

As you can see the antipodes of Heard Island is near Prince Albert, Saskatchewan (VE5). This is approximately 3,000 km west of my location of Ottawa. The antipodes of Ottawa is between Kerguelen (FT5X) and Perth, Australia (VK6).

The Wikipedia article I linked to above provides the following helpful map that shows at a glance the antipodes of any location on Earth.

Original can be found in Wikimedia Commons


It is worth understanding why antipodes propagation is so interesting. It concerns geometry on a curved surface.

Every ham who has ever worked DX knows there is a compass point to which you ought to point your directional antenna for best results. The line from you to the DX is a geodesic, although we more often call it a great circle path when referring to the Earth. A geodesic follows the surface of a geometrical object without veering left or right. It is the shortest distance between two points on a surface, curved or flat.

Radio waves most often follow a geodesic from the transmitter to the receiver. Scatter and skew paths make this less than a universal rule, however a geodesic is dominant on 40 meters and up. You can follow a geodesic in two directions, which in radio we call short path and long path (for a reason that should be obvious). Long path is less common but occurs more than you might realize. During solar maximums it is even possible to hear your own signal going right around the Earth back to you 135 milliseconds later. That is long path in the extreme (40,000 km).

Antipodes propagation and geometry

I would have liked to experience the recent VK0EK DXpedition from VE5 since antipodes propagation is quite fascinating. It is a relatively rare experience that I get from working VK6 and the nearby French territorial islands. It was very much in play during the Amsterdam Island (FT5ZM) DXpedition, which I worked without too much difficulty with only 10 watts, even on 40 meters. They could be heard on at least one band most of the time they were active.

Great circle is a circumference (from Wikimedia Commons)
The reason for the unusual propagation is geometry. It begins with a few basic facts that, I think, everyone is familiar with:
  • Radio waves follow a great circle. Skew path and scatter can also be exploited, though they are less common and are most often weaker than the direct path.
  • There is a unique great circle connecting two points on a sphere. The shorter distance between those points is the short path, which is the most common one we experience.
The second point has one important exception, for points that are at or near their antipodes. There are an infinite number of great circles between your location and your antipodes. That is, every compass direction carries the potential for direct path propagation.

This is both an opportunity and a dilemma, since there is no unique answer to the question: where do I point my yagi? The answer is to point it in the direction that works best! The direction that works best will often be the same as short or long path to distant locales that are concurrently heard. This is most notable on 20 meters and above. Indeed, the higher the frequency the fewer the choice of directions since on such long paths you are increasingly likely to encounter a patch of ionosphere with a lower MUF in many directions.

On 30 meters, which is less sensitive to the traversal of parts of the globe that are in darkness or daylight, the direction is less predictable since the MUF is more amenable and absorption is lower in the D and E layers of the ionosphere than on lower bands. On 40 meters and below the constraint is on paths in darkness or at least not far into daylight beyond the sunrise/sunset terminator. Unless you have a directional low-bands antenna you likely can't determine the actual path, only that the path is or is not propagation.

Some experimentation may be required on the high bands to know what direction works best. If you have more than one antenna, even a fixed wire antenna, the task is easier since signal strengths can be compared. For example, if the signal is stronger on an inverted vee than a yagi you know the yagi is not pointing in the best direction. On 40 meters where I have two inverted vees I can often resolve an approximate path.

I have noted signals from VK6 ranging over many compass points at different times of day and year. That is the curiosity that is antipodes propagation.

Antipodes propagation can be compared to lottery tickets. When you buy one ticket you get one chance to win. With antipodes propagation you get many tickets for the same price, and so many more chances to win.

Geographical dependence

The antipodes of North America falls across the Indian Ocean and a sliver of Antarctica. The lack of amateur radio operations in that area results in antipodes propagation being fairly rare for us. Perhaps that's why I find it so intriguing, due do its rarity, often only making a regular appearance during the occasional DXpedition.

It is less rare in other locales. For example, between New Zealand and the Iberian peninsula. But with 71% of the Earth's surface covered by oceans the odds are that your antipodes is not populated, just like mine. Except, that is, when a DXpedition to an uninhabited island takes place.

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