We seek to understand and document all radio transmissions, legal and otherwise, as part of the radio listening hobby. We do not encourage any radio operations contrary to regulations. Always consult with the appropriate authorities if you have questions concerning what is permissable in your locale.

Author Topic: Ducting on LW/MW/SW and Spotlight Propagation  (Read 349 times)

Offline ChrisSmolinski

  • Administrator
  • Marconi Class DXer
  • *****
  • Posts: 26377
  • Karma: +62/-45
  • Westminster, MD USA
    • View Profile
    • Black Cat Systems
Ducting on LW/MW/SW and Spotlight Propagation
« on: September 22, 2020, 1743 UTC »
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.pdf

The 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.

Quote
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.


Quote
...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?


Quote
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.


Quote
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


Quote
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.


Quote
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  ;D
 
« Last Edit: September 22, 2020, 1749 UTC by ChrisSmolinski »
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 300 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline ChrisSmolinski

  • Administrator
  • Marconi Class DXer
  • *****
  • Posts: 26377
  • Karma: +62/-45
  • Westminster, MD USA
    • View Profile
    • Black Cat Systems
Re: Ducting on LW/MW/SW and Spotlight Propagation
« Reply #1 on: September 22, 2020, 1856 UTC »
I also came across this article from 1979: On High — Frequency Ionospheric Ducting — A Review
https://apps.dtic.mil/dtic/tr/fulltext/u2/a063873.pdf

Several interesting points:

Quote
The conventional multi-hop propagation , involving many bounces
between the earth and ionosphere , appeared less likely on account of the claim that
relatively large elevation angles at reception had been observed(about 20 degrees above the
horizontal)...

This has some very important considerations for antennas. Vertical antennas are favored by some due to low radiation angles which for multi-hop propagation is favored (even though vertical antennas inherently have a host of problems such as the need for elaborate ground radial systems, which few hobbyists are able to implement, resulting in high loss).  But if ducting is the primary mechanism for extreme DX, low radiation angles are only unnecessary, but actually could be unwanted. Instead the traditional dipole might prove superior, as would other easy to build/install antennas. Food for thought, anyway.

Quote
... differences in RTW signals propagating over land- or sea-paths had not
been detected

When thinking of multi-hop propagation, we want paths over the sea, as bounces over the ocean induce less loss than over land. With ducting, this becomes a non issue.


Quote
Further confirmation was obtained that RTW propagation is not confined to the
“twilight zone ” when it was observed that propagation orthogonally to this zone is also possible.

By twilight zone I assume they refer to greyline propagation.


Quote
RTW propagation modes were also found to be qualitatively predictable by use of world maps of foF2 and absorption , and by proper consideration
of the effects of ionospheric tilts. 16 , 17 The “ tilt mode ’ required the transmission
point to be in the daylight hemisphere . The ionosphere-ionosphere mode appeared
prevalent in the dark hemisphere while the earth-ionosphere-earth hop mode prevailed in the sunlit hemisphere ,

Predictable? Obviously I need to read up on ionospheric tilts next  :)

It was also noted that electromagnetic energy in
the tilt-supported mode could pass overhead , being undetectable on the ground at
sites located in the dark hemisphere , and yet detectable at a site in the day light
hemisphere[/quote]

Another example of the spotlight DX, were paths exist only between discrete regions of the Earth.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 300 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline ChrisSmolinski

  • Administrator
  • Marconi Class DXer
  • *****
  • Posts: 26377
  • Karma: +62/-45
  • Westminster, MD USA
    • View Profile
    • Black Cat Systems
Re: Ducting on LW/MW/SW and Spotlight Propagation
« Reply #2 on: September 22, 2020, 1913 UTC »
Further down the rabbit hole... 

The investigation of long-distance HF propagation on the basis of a chirp sounder
https://www.sparks.it/download/hfpropagation.pdf

Quote
Experiments on waveguide modes escaping from the ionospheric channel due to field-aligned scattering by artificial ionospheric turbulence are carried out. The conditions for trapping of radio waves in the ionospheric waveguide are investigated. It is shown that if the gradient of the critical frequency F0F2 is less than minus 2 × 10−2 MHz/100 km radio wave trapping takes place in the ionospheric waveguide at frequencies exceeding by 1–2 MHz the maximum observed frequency of the hop mode.

It looks as though you want a large enough variation (decrease, or increase if you're looking at it in the opposite direction) in foF2 for this ducting mode to occur. This change in foF2 is likely around the terminator, so another explanation of why greyline propagation works.  It also suggests high resolution foF2 maps could be used to predict this mode, at least to some degree?

Also... they were apparently using the Russian version of HAARP for their experiments.  So if a DX were to have the funds to set up their own HAARP facility... just thinking out loud here  ;D
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 300 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Σ

  • Blog author
  • Sr. Member
  • *
  • Posts: 351
  • Karma: +9/-5
    • View Profile
    • ΣSDR KiwiSDR online
Re: Ducting on LW/MW/SW and Spotlight Propagation
« Reply #3 on: October 28, 2020, 1631 UTC »
Just now responding since I didn't see it when originally posted.

After reading this I am reminded how I always wondered how multi-hop propagation maintained enough signal strength to overcome the multiple land based hops. Over the ocean, maybe, but over land, too many variables - reflectivity, or lack of reflectivity of the earth at that point, scatter from the terrain or urban development, etc. It never made a lot of sense to me that multi-hop was realistic for as often as it is attributed.

With ducting I am very familiar with it on VHF/UHF as the eastern seaboard often gets temperature inversion propagation up and down the coast. DXing NOAA weather radio broadcasts can be very interesting and it is helps to determine potential 2 meter openings. I always assumed that you didn't get any form of that on HF due to the larger wavelength but I suppose there is enough elbow room in the atmosphere to allow for an HF waveguide to form. 
- Rob

CT/MA border
Afredri SDR-Net with multiband dipole at 65 ft.
Email: commsigma@gmail.com
KiwiSDR online - http://sigmasdr.ddns.net:8073/
ΣSDR Blog - https://n1nte.blogspot.com/

Offline pinto vortando

  • Hero Member
  • *****
  • Posts: 706
  • Karma: +2/-0
    • View Profile
Re: Ducting on LW/MW/SW and Spotlight Propagation
« Reply #4 on: October 31, 2020, 2311 UTC »
Interesting stuff although some of it is beyond my level.

Radio textbooks discuss LW propagation in terms of groundwave and mention ducting only in the VHF/UHF realm.
Based on my years of LW listening, there are 15 or so beacons (the number was easily twice that as little as three years ago but decommissioning has taken its toll)
that can be heard at my location with rather rudimentary receiving equipment most any time regardless the weather, atmospherics, or time of day.
The practical limit seems to be about 300 miles.  So, OK, these are probably groundwave signals.  However, what is the explanation for the nighttime DX ?
Most likely skywave would be my guess.  However, there has been a noticeable feature of the nighttime DX that has me curious.  Oftentimes, the nighttime
propagation is only so-so in every direction except for signals booming in in droves from a particular direction.  One night it could be northern Quebec and the
next night Manitoba, or Florida, or wherever.  This seems to fit the ducting model. 

Anyway, just goes to show that maybe we have not yet got this propagation thing all figured out.

edit: 
For example, this morning LW propagation was unremarkable.  Only the regular stronger beacons scattered about ON and QC
were heard here in MI except for the following:

212  YGX  Gillam  MB
244  TH    Thompson  MB
248  WG   Winnipeg  MB
250  FO    Flin Flon  MB
269  UDE  Delta  MB

All with very good signals and all in the same direction and nothing heard in between.
So is it skip or a duct  ???

The only maverick was 254  5B  Summerside on PEI with the loudest signal ever in here from that beacon.
« Last Edit: November 01, 2020, 2239 UTC by pinto vortando »
Das Radiobunker somewhere in Michigan

Offline pinto vortando

  • Hero Member
  • *****
  • Posts: 706
  • Karma: +2/-0
    • View Profile
Re: Ducting on LW/MW/SW and Spotlight Propagation
« Reply #5 on: November 14, 2020, 1925 UTC »
OK, so this morning the LW propagation was rather bland, mostly the usual stronger not too distant Canadians.
However, three Florida beacons: PKZ, FIS, and JA all had rather good signals in here to Michigan.  Nothing else heard in between other than one Ohio beacon.
The only odd ones were 329 YEK Eskimo Point NU and 334 YER Fort Severn ON at 1000+ miles in the almost exact opposite direction both with a big signal.
Das Radiobunker somewhere in Michigan