Longpath (LP) Experiment using ZL6B
This exercise started on the morning of 15th November 2006, I was looking for evidence of Sunrise/Sunset enhancements, commonly called greyline. I had actually been checking on the HF bands for a number of years for evidence of this phenomenon without success. This is not to say that it does not happen on the lower bands, just that the NCDXF beacon chain only covers 14MHz to 28MHz. Finding a suitable and reliable source of HF energy has proved difficult to say the least that is why I use the NCDXF beacon chain with the monitoring program, FAROS.
I should mention here at the start that my recording computer is synchronized to GPS via a HP Z3801A receiver. Upon commencement of recording the ZL6B beacon data on 14.100MHz, with the above program, the beacon was being received from about one hour following sunset at ZL for about 145 minutes each morning. These signals were confirmed by FAROS to be LP. As ZL is almost at our antipode, I was extremely suspicious that the program was getting it wrong. FAROS measures the time taken for the signal to arrive to make this decision. The time difference for signals arriving via SP and LP is only 9 milliseconds. To eradicate any confusion I asked Derek G3RAU, who has a beam, to check the arrival path for me. He was able to confirm that signals were actually arriving via both paths but at the times that I was recording LP the signal strength via that mode was higher. This concurs with FAROS, that software will choose the bigger signal when both paths are available.
Apart from operating (CW) my interest has always been propagation. Therefore I was intrigued as to which mode/s the signals were utilizing. From my QTH, LP to ZL is over the edge of N. Africa and the southern part of the Atlantic then clipping the Antarctic, Fig-1. I attempted to get ionosonde data from several stations in the Antarctic region but the only one that has been available is Casey, which is roughly 13 hours (daylight time) from my reflection point in the Antarctic. Port Stanley data is available but I considered this too far north of my reflection point for data extrapolation. One thing that Casey data was able to show me and verified to some extent by the Port Stanley data was that blanketing sporadic-E was prevalent each day at my reflection point if I extrapolated the data forward. This would then cause the circumstances described in Table-1 to be observed at or between my reflection point/s.
Fig-1. LP (black line) from ZL6B to G4FKH
A description of the two exotic propagation modes.
- Chordal Hop Mode – A very specific way of signal ducting is called chordal-hop propagation. In this ducting mode, the waves are guided along the concave bottom of the ionospheric layer, acting as a single-walled-duct. Two kinds of chordal-hop modes can be distinguished.The first one is more a daytime phenomenon and found where the F-region displays two tilted blobs of highly ionized plasma quite some distance apart. The first refraction within a chordal-hop is in fact partly a refraction that does not propagate fully downward, but propagates further along the rather concave ionosphere to meet another region that is ionized high enough to refract it downward to earth. Fig-2 below shows an excellent example of how a radio signal can experience a chordal hop. The second kind of chordal-hop propagation is established by successive chordal-hops. During nighttime the F-region characteristics change rather profoundly. After sunset the F1-layer migrates slowly to a higher height and recombines with the F2-layer. The single existing F2-layer is also migrating to a higher height. The difference between daytime and nighttime height of the F-region can often be more than several hundred kilometers. This process forms a rather concave shaped F2-layer bottom side, which might again act as a single-walled-duct.
- Trapped Mode or Inter Layer Ducting. – Fig-3 Inter-layer ducting is very similar to chordal hop propagation except that the signal does not travel along the base of the ionosphere. Instead, it propagates within the lower E-region and higher F-region. The ionosphere consists of a depletion of electron density between those two regions. This area of depleted electron density is known as the E-valley region. The E-valley is stronger and capable of ducting radio waves when the area is under sunlight and more strongly ionized. Ducting during night-time is also possible, but because the E-valley electron density is much weaker, lower frequencies must be used. When the angle of incidence is shallow enough, radio waves might travel within this E-valley, bordered by the top side of the E-layer and the bottom side of the F-layer and ducting may occur. When the angle of incidence is too high then the signal wave will simply pass through the E-layer when returning back toward the earth. When the angle is shallow, the signal will be alternately refracted from the top side of the E-layer to the base of the F-layer and back to the E-layer. This process may repeat numerous additional times until it encounters conditions that would cause the end of the ducting. The signal may encounter an area of irregularity of non-horizontally stratified electron density in the E valley region. This may cause the signal to change direction and increase the angle of incidence with respect to the E- or F-region so that penetration through the layer occurs. Another condition to end ducting might be that the critical E-layer frequency drops to a level that causes the signal to penetrate this E-layer and return to the earth.
Fig-2. Chordal hop propagation mode caused by two tilted highly ionized blobs forming a concave tilted F-region.
Fig-3. Multiple hops by a concave tilted ionosphere found at the dark side of the globe
Because as I’ve mentioned there is a lack of confirmatory data, there are two possible modes at the Antarctic that would account for my receiving enhanced signals here. Both are described in Table-1 and shown in Fig-2 / Fig-3. Either of these circumstances would enable the signal strength at the receiver to be enhanced enough for reception.
This experiment lasted from 15/11/2006 to 07/03/2007 when LP disappeared. The duration of received signals showed a sinusoidal curve, as I would expect because the time difference of sunset in NZ and sunrise here grew apart, then got closer and grew apart again. The co-incidence of sporadic-E at the required place in the Antarctic follows the same pattern. Received signal strength remained pretty stable, at about 1 S-point during the reception periods.
I believe that keen DX-ers would know that LP is available with ZL at the above time of year but I wonder how many are aware of the exact mechanisms involved. My experiment showed that LP to ZL is available between (or before) November and March. To take better advantage, (I use a multi-band dipole) a beam of some description is necessary. The signals would then be lifted up to about S-5 or S-6. The reception of ZL6B was on 14.100MHz; nothing was heard from this station on any of the higher bands during this period. Good luck and good DX-ing.
This experiment/research is continuing, results will be reported here when further conclusions are newsworthy.
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