Checking my DGPS (circa 300 kHz) logs the other day, I was thinking again about propagation, and how the traditional multi-hop model is most likely not the only mechanism for reception over long distances. I believe ducting explains many of the "amazing" DX catches we get from time to time.
I came across this article from 2005, and I think it illustrates several points worth considering:
Ducting and Spotlight Propagation on 160m
http://myplace.frontier.com/~k9la/160m_Ducting_and_Spotlight_Propagation.pdfThe article opens with some details about German ham station ST0RY from 2003. The author noted that traditional multi-hop propagation modeling said the signals from this station should be 55 dB
below the noise floor at his location, 11,000 km away. Yet he was able to hear their signal several times.
He then modeled the path via ducting, which produced results on par with his actual reception conditions. I strongly suggest reading the entire article, I won't summarize the rest of it here but will quote several sections and my own comments about them.
Once the electromagnetic wave gets into the duct, it has to stay in it sometimes for very
long distances. That requires the nighttime ionosphere to be stable so that the electron
density valley retains its necessary characteristics.
This matches my observations about the DGPS band. I typically get the best reception from the most distant DX targets (Europe, Alaska when those station were on the air and now British Columbia) just before the band closes due to sunrise at the transmitter site, or here. This is not always the case, there are exceptions, but it is most common. Also conditions are best when the ionosphere is rather stable, not only no current geomagnetic storms, but when there have not been any for several days. When the ionosphere does get disrupted, say due to solar wind, it can take days or even a week or so to settle back down, and the very long distance DGPS to return. Note there's one exception (well, not quite an exception, more a corollary) to this: the onset of a geomagnetic storm / solar wind can sometimes produce a short period of excellent propagation, mostly on HF, as the MUF/foF2 value gets a bump. This only lasts for a few hours, then it all goes to hell of course.
...the range of elevation angles for getting into the duct in Figure 2
is quite small. This makes sense, since two conditions with respect to elevation angle
have to be met to get into a duct. The elevation angle must be high enough to get through
the E region peak. But it can't be too high, or it will also go through the F region. What
makes this complicated is the index of refraction it determines how much the
electromagnetic wave is refracted. The amount of refraction depends on two critical
factors: how close the signal frequency is to the electron gyro-frequency, and the angle
between the Earth's magnetic field and the direction of travel of the electromagnetic
wave. Even though the ducting mechanism (the electron density valley) may be present
worldwide in the nighttime ionosphere, getting into the duct may be easier on certain
paths compared to other paths even from the same QTH due to these considerations.
This goes a long way to explaining why we have exceptional openings to small areas of the world, rather than large regions. Again using DGPS/NDBs/longwave as an example I am familiar with, it's common to have excellent reception from a small DX area, which may last only a few minutes. Then the signals vanish, sometimes to be replaced with reception of stations from another area, perhaps as the entry/exit points of the duct move?
It's also interesting to do a ray trace at the time of Figure 1 at 0200 UTC, which puts
ST0RY well away from sunrise. Proplab Pro also shows ducting at this time. This is why
I earlier commented that sunrise might be more of a helper than an instigator in getting
into a duct. The help could be the tilt in the ionosphere that develops as sunrise
approaches (there's also a tilt in the ionosphere at sunset). This could favorably impact
the critical mechanics of refraction to get into the duct.
Does this partly explain the enhanced greyline propagation we're all familiar with? Signals are more easily able to enter and leave the duct (if there is one!) when both ends are in the greyline zone. Of course even if just one end is in a greyline zone, there is still a chance for ducting to occur, just a smaller chance.
The most plausible explanation for bringing the ray down in the dark ionosphere is an
irregularity in the ionosphere. These irregularities are the result of the day-to-day
variability of the ionosphere. We know that these irregularities exist, but we don't have a
good handle on them
Spotlight propagation is defined as a small geographic area that is favored with good
propagation at any given time. Oler and Cohen suggested that spotlight propagation is
simply the unpredictable result of coming out of a duct. I agree wholeheartedly with this, and further I believe that irregularities in the ionosphere are generally the cause.
This is an interesting concept to ponder. It very well could be that there's lots of RF
rattling around up there in the duct (based on many Proplab Pro ray traces that indicate
getting into a duct and staying in the duct is easier than getting out of the duct), but the
luck of the draw in terms of an irregularity to bring the wave down at your QTH in the
dark ionosphere determines if you have a QSO or not.
All this explains the apparently random nature of ducting and the amazing DX catches. It does boil down to physics in the end, but there's so many unknowns / things we cannot measure, it appears as pure luck to us mere DXers.
Earlier I mentioned that irregularities in the ionosphere are the result of the day-to-day
variability of the ionosphere. It's interesting to dig into this deeper to understand what
causes these day-to-day variations.
Two scientists with the Center for Space Physics at Boston University did just this. They
analyzed 34 years (1957 - 1990) of F2 region critical frequency data [reference 4]. Although this was a study about the F2 region, the results are very relevant to
propagation on 160m in the lower ionospheric regions
The result was they determined the
contribution of each of the three broad categories to the total variability: solar ionizing
radiation came in at around 3%, solar wind/geomagnetic activity/electrodynamics came
in at around 13%, and neutral atmosphere came in at around 15%
This suggests that while we want a relatively stable ionosphere overall (see much earlier) we also want some localized irregularities to provide openings into and out of the ducts, so we can take advantage of them. If we don't know exactly what we're looking for, or lack the ability to measure it, an alternative is to use real time DX catch reporting (WSPR and other networks) to try and identify ducting possibilities. Since the geographic areas for each end of a duct may be quite small, using online data from other reporters may not always work, although I think it is worth pursuing.
Even better would be to set up your own monitoring station so you can identify the actual ducting openings for your QTH, SDRs have become so affordable this is much easier to do today. A KiwiSDR for example can monitor 8 audio only channels at a time (more with the BeagleBone AI board, I think 14?) which would allow you to monitor lots of WSRP (or other) frequencies. In addition to submitting the data online, custom software could monitor it for any unexpected openings. Of course this requires that there be a WSPR or other station transmitting in the region where an opening to a duct exists.
Another alternative is post processing of SDR recorded I/Q data. This is the philosophy of my Amalgamated DGPS software. The entire 40 kHz band is analyzed and any decoded messages are counted/displayed. This means you don't miss those brief openings from a given region that might only last a few minutes. (Amalgamated DGPS can also run in a real time mode, but only on the Mac, and only with RFSpace/AFEDRI SDRs)
This is also what I do with my nightly SDR recordings of the 43 meter band, and how I catch those "bumps in the night". The same thing is practiced by MW DXers, and the NDB guys as well.
Since these ducting openings are brief and unexpected / apparently random, this suggests that the traditional DXing method of spinning the knob and hoping for a good catch is probably the least efficient method, and likely to result in most such openings being missed. Sorry Al Fansome, it's an SDR world now