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Working HF DX stations located near Antipodes
Working HF DX at the longest possible physical distance on earth is often referred to as working stations near Antipodes. These situations present several unique challenges that may never be observed while working regular DX at shorter distances that are not as far as the Antipode. These challenges manifest themselves while using directional beam antennas and may not be observed or appreciated by those of us who have omnidirectional antennas. The challenges of working stations at or near the antipodes are not only experienced by the station holding the frequency and calling CQ but also by those who are trying to work him.

We cover the concept of Antipodes in an article under the section Geodesic for Terrestrial HF Radio. However, to recap for those who might be new to the term, an Antipode is a location that is at the opposite side of the globe from where a station might be located. Hence, every operator has a unique antipode based on his location. For the sake of simplicity, if we assume the earth to be a perfect sphere (though it is actually not so and is Oblate in shape), then the antipode from any QTH could be found by passing a straight line from the QTH on the earth’s surface through the center of the earth’s core till the line emerges on the other side of the sphere. This location on the other side of the earth’s surface is the Antipode. For spherical earth, the perimeter of the earth would be around 40000 Km. Hence the distance from a station to its antipode will be half of that and be 20000 Km. This would be the longest possible physical distance from any location.



The uniqueness of the Antipode and the Geodesic Path
Radio signals unless acted upon by extraneous phenomena, always travel (propagate) in a straight line at a constant velocity. This propagation path is known as the Geodesic path. Due to the near-spherical nature of the earth’s surface, even though the propagating radio wave follows the Geodesic path, the apparent effect is to find that the radio signals tend to go around the earth to return to the originating point after completing a full circle. In practice, on account of propagation losses, the actual radio signals may or may not go very far or be readable along the entire Geodesic path. The coverage range may be limited. However, that’s a story for another day.

The effect of the Geodesic property of signal propagation along the non-planer (spherical) surface of the earth, is to make all signal paths originating from any point, and radiating in any direction to travel across the earth via different surface paths to eventually converge at the Antipode. After converging at the Antipode, the signal would continue on its onward journey. The extended path beyond the Antipode is what we call the Long-Path (LP). The bottom line is that irrespective of whichever direction around the 360° azimuth that one might beam the radio signal, it will eventually have to converge and pass through the Antipode. Another interesting fact about the Antipode is that since all path distances to it are equal irrespective of which direction it is coming from. Hence, antipode is a unique location which does not have a distinct Short-path (SP) or a Long-path (LP)

Unlike normal DX, where one might have to beam the antenna in a specific direction towards the DX, the unique location at the Antipode can theoretically be reached by beaming in any direction. However, one must keep in mind the fact that adequate propagation may or may not be available along every direction leading to the Antipode. The inability for the signal to reach from most directions is due to the multiple ionospheric skips that are required along those paths, and that the MUF prevailing along these paths that may not support conditions for needed propagation. However, from the geographic perspective, the azimuth heading of the beam for Geodesic travel to the Antipode is inconsequential. All paths converge at the Antipode.




Challenges of establishing contact with stations near antipodes
Normally, the radiation from a directional antenna flares out to cover broad areas across the earth’s surface as the distance to the DX increases. We have discussed this in another post and also suggested that swinging of an antenna beam too frequently to precisely point at the DX is usually never required because of the large coverage area at long distances. Read the post Need We Fuss About Exact Antenna Bearing for HF DX? to check it out.

However, the above suggestions do not hold for antipode or near Antipode operations. Due to the Geodesic path followed by the signal along the spherical surface of the earth, the beam flare-out of an antenna maximizes its bulge at approximately 10000 Km away from the transmitter and thereafter starts converging. The beam converges into a very narrow shape as it approaches the antipode. The area around the antipode covered by the exceptionally narrow-end of the converging beam ensures that very small regions around its path are illuminated. Even at a couple of hundred kilometers away from the path of the main beam of the arriving radio signal, the places located near the antipode may not receive any copyable signal. To reach such station locations, the transmitting operator may need to swing his beam around in an entirely different direction. Even then, a contact may not be possible due to a lack of propagation opening along that alternate path.

Similarly, the DX operators at the receiving end who might be located only several hundred or perhaps a thousand kilometers from one another may need to swing their beams in totally different directions to reach the same DX station who was calling CQ. In the animated illustration below, one can see that the direction of the approach of signal to the Antipode may be from any azimuth angle all around in 360°. The direction of beam headings of stations near and around the Antipode point would have to be in the direction of the approaching signal path.

Another unique feature of communication near the Antipode is that many stations around this point might report Long-path (LP) openings, much more often than what one might expect normally around the world. The reason is that those station locations that fall beyond the Antipode in the direction of the Geodesic path of the beam will essentially be receiving signals via path lengths greater than halfway around the earth. Hence, for them, the contact will be on an LP, even though they may just be a few hundred kilometers beyond the Antipode.

HF DX near Antipodes

Typical example of working an HF DX station located in a region close to the Antipode. The station at one end of the DX path in this example is at Chattam Ils. (ZL7) while at the other end are stations from Europe. The animation shows the direction of arrival of signal from various directions along various Geodesic Great Circle paths and converging at the Antipode.



In our example scenario above, we have a station located at the Chattam Islands (ZL7 call) in the South Pacific several hundred kilometers East of New Zealand mainland. ZL7 is a very rare DX and is a very sought after by DX hunters. The stations in Europe face the unique challenge produced due to the Antipode. The Antipode of ZL7 falls pat in the middle of the populated landmass of continental Europe which is somewhere in the south-eastern region of France.

In the animated illustration above, the ZL7 station is marked with a blue marker while his antipode location in Europe is marked with the green marker. The orange-colored circle around the antipode designates a 300 Km radius around the antipode. The antenna beam in the illustration is deliberately made narrower than what one might find in practice, to be able to distinctly highlight the phenomena under discussion.

As we see in the animation, the beam heading from the ZL station would need to change drastically if he were to contact stations all around in Europe. To reach, western France, Spain, Austria, Poland, etc which are all fairly close to each other, the ZL operator with have to swing his directional antennas all around in 360°.

Similarly, the stations in Europe wanting to contact our ZL7 friend cannot all point their antennas in a nearly similar direction as one might expect in the case of a usual DX. In regular DX scenarios, more often than not, most stations that are within a couple of hundred thousand square-kilometers from one another in a particular region may all expect to hear or establish contact with the DX at nearly around the same time of the day, by pointing their antennas in the same direction, thus creating a good on-band pileup. However, in this antipodal scenario, it won’t be possible if the DX (ZL7 station) and/or the responding stations (Europe in our example) are using a high gain, narrow beamwidth antenna.

Not only would the stations in Europe need to point their beams in different directions but they may need to wait and work the ZL7 station at entirely different times of the day or on different bands depending on the possibilities of propagation openings along their great-circle geodesic path to the ZL7 land. For the stations very close to the antipodal location in Europe, there may be a silver lining. Despite the narrow convergence effect near the antipode, several scatter mode propagation phenomena due to the non-homogeneity of the ionosphere may allow them to establish contacts with the ZL station even though they may not be exactly within the geodesic beam coverage area. Such signals may be considerably weaker yet quite workable at times. However, those stations that might be more than a couple of hundred kilometers away may find it rather difficult.

HF radio communication near the antipodes brings to fore these interesting and rather unique conditions based on Geodesics of the Earth’s geometry. However, most of us rarely have the Antipodes of our station locations falling at landmasses with substantial population or radio amateur presence. Try finding your Antipodes and you will most probably find that they fall somewhere in the ocean or near an island with no radio amateur presence. ZL7 region is amongst the very few practical exceptions.

Challenges of working HF DX near Antipodes 1

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