HF Long Path (LP) – Getting acquainted
The Great Circle round-the-world path is always unique between any two radio stations on earth. In one direction along the circular path (Great Circle), the distance is shorter than the distance along the opposite direction. The direction along the shorter of the two distances is called the Short Path heading, while that in the opposite direction is the Long Path heading. There is only one exception. It is when the DX station is located at a point that is equidistant along both the SP and the LP. This point on earth is called the Antipode. Communication with another station located at the Antipode throws up a very unique propagation effect… Let us ignore the Antipode case for the time being and see what happens when we try to reach a DX station via the usual SP or via LP by turning our antenna beams towards LP by swinging the beam 180° around.
We cover the concepts of both SP and LP along with a lot of other information in the article Geodesic for Terrestrial HF Radio. Therefore in this post, let me provide a few brief pointers related to the exciting world of working DX via the Long Path.
Long Path (LP) – A few brief tips & pointers
Although LP may not be as simple, prominent, or as recurrent as SP, it opens up new vistas and interesting possibilities of DX. The usual pitfall of distinguishing an LP opening from that of inadvertently working a station on the SP at the back of a beam does exist and happens quite often with inexperienced operators. The experienced HF operators, however, know how to distinguish between the two.
Please remember, that all directional antennas like the Yagi, etc that we use, only have a finite Front-to-Back (F/B) ratio. Hence, a strong signal arriving at the back of a beam from a nearby station may be easily copyable even though both the stations may have turned their beams 180° around to attempt an LP QSO. Do not get mislead by the back of the beam radio signal reception between the two of you. That’s not LP but SP with reduced signal strengths due to lower gain at the back of the beams.
A typical example of an SP and a robust LP scenario
I am presenting below a set of scenarios for both SP and LP as it manifested from my QTH at New Delhi, India to the rest of the world on the 12m band at 14:00+ UTC on 19th December 2019. The graphics should provide a fairly clear understanding of the manifestation of LP. The antennas used to arrive at the propagation scenarios projected in the illustrations use 5-element Yagi installed at 15m (45 ft.) above ground at both ends of the circuit. QSO would have, of course, been possible even with a 3-element Yagi.
The illustration below depicts the SP scenario prevailing for destinations around the world at the time and band cited above.. You may notice that SP allowed me to primarily work southern hemisphere at this time on 12m band.
On the other hand, LP allowed me robust openings into North and Central America and the EU. You can also clearly see that the territory covered by LP runs above the north of the SP coverage region and also borders of coverage areas of SP and LP run alongside one another and they do not overlap.
Keep a watch for the LP openings that might occur on the right bands at certain times. Quite often when a band might appear to be substantially closed for DX on SP, the LP option may spring a few very pleasant surprises. Try to look for these openings on the DX bands from 20-10m. Do not bother to look for them on the top bands below 40m.
Those of us who are fortunate to possess a decent directional antenna will find the phenomenon of HF long path rather exciting and pretty rewarding. These openings may be elusive but when they occur, it is sheer joy. The rest of us who use either omnidirectional or bidirectional antennas need not bother because in both cases, the LP will automatically be available whenever it exists.
Happy DXing everybody… 73, de Basu VU2NSB.