Unusual HF Propagation Phenomena

This page has a collection of audio samples from unusual HF propagation phenomena, from round-the-world propagation (delay 138 ms) to moonbounced signals (delay 2.39 sec), and to ducted transmission (delay ~140-300 ms).

Three paths from Japan to Norway along the grayline

The Japanese station JA3YBK was received in Oslo (press here for audio), 30. November 2003, 08:20 UTC on 21.004 MHz during the CQ WorldWide contest. The sound is reverberant due to the multiple copies that are received. This can be seen as extra signals, and it is in particularly easy to see after the first short dot (‘e’ in ‘test’). The right-hand figure zooms in on the ‘e’.

ja3ybk2ja3ybk3

The main signal must have arrived directly via the short-path. The second signal which starts where the red line ends, has most likely propagated via the long-path. The distance from Kobe to Oslo is about 8350 km or 8350/300 000 ≈ 28 ms. The long path is 31650 km, or about 31650/300 000 ≈ 106 ms. The difference between short-path and long-path is therefore about 106-28 ≈ 78 ms.

The third signal which starts at the end of the green line, is probably the short-path signal with an extra round around the earth. As the earth’s circumference is about 40 000 km, it is delayed by about 40000/300 000 ≈ 133 ms (or rather 138 ms which is the established round-trip time, taking into account an additional 1400 km path length due to reflections between the ground and the ionosphere). As the AGC was on (in Fast mode on my K2), one cannot make amplitude comparisons of the echoes.

On 30 November 2003 the sunspot number reached 116, much higher than the smoothed average for 2003 which was 63.7. But as propagation is only weakly correlated with daily sunspot numbers (see Correlation Between MUF and Solar Flux on C. Luetzelschwab K9LA's excellent propagation site), this only partially explains the unusual propagation. A much more important factor is that Southern Norway and Central Japan were connected by the grayline at the time, that the propagation paths did not pass too close to any of the polar regions, and that the K-index was only 1 so the there wasn't much disturbance near the polar regions anyway. See figure below for grayline at the time (generated by GeoClock).

Grayline

(A Norwegian language version of this report appeared in Amatørradio in september 2007: Trippelekko fra Japan)

Propagation via aurora reflection

Here is a recording of the Swedish station SM6CNN in contact with me on 29. October 2003, 22:42 UTC on 28.024 MHz. The direct distance is about 275 km. Note the noise-like signal which is typical for signals that have been reflected off the aurora on the higher HF and on VHF frequencies. There was a strong geomagnetic disturbance that day with the K-index at Dombås, Norway reaching a value of 9 which explains the occurrence of aurora reflection.

Polar flutter

Here is a recording of JW/DJ3KR on Spitzbergen from 24. August 2007, at about 21:45 UTC. This was only a few minutes after I had contact with him on 7.002 MHz. Notice the fluttery character of his signal. On this day it wasn’t too bad and it wasn’t too hard to copy his signal, but on other occasions, signals that travel over the polar zone may be impossible to copy correctly. The K-index at Tromsø, which lies between Oslo (60 deg. N) and Spitzbergen (78 deg. N), at the time was 0.

Noise bursts from Jupiter and the sun

Noise emitted from both Jupiter and the sun can be heard around 20 MHz when conditions are right. Consult the pages of The European Radio Astronomy Club.

Moonbounced echoes on 6792.5 and 7407.5 kHz

Recently, a facility with big enough antenna (300 by 365 m) and high enough power (3.6 MW) was used to set a new record for how low in frequency on HF one can get echoes from the moon. This was also done during the HAARP moonbounce experiment which encouraged radio amateurs to listen on 19 and 20 January 2008. At my location in Oslo, I only heard the direct signal from Alaska, but many in the US heard good moon echoes.

Below is the result of analysis of a such a recording. It is shown here with kind permission from T. Hammond, N0SS, in Missouri. The data was received on 7407.5 kHz on 20 January 2008 at 07:29:30 UTC + the time given in the figure (16 min 25 sec). Frequency analysis was done with a resolution of 8k bands and a Hamming window, and the sampling frequency was 16 kHz. The marked region in the left-hand figure shows that the delay was about 2.39 seconds and the right-hand figure shows a Doppler shift of about 7 Hz, which actually is negative due to reception on the lower sideband.

This gives a distance to the moon of 0.5*2.39*3e5 km ≈ 358,000 km and a velocity component of the moon away from the earth of 0.5*3e8*7/7.407.5e6 ≈ 140 m/s (moon-earth movement + rotation of both the transmitter and the receiver relative to the moon). Press images for larger figures.

N0SS-HAARP-2400ms-MoonEchoN0SS-HAARP-8Hz-Doppler

See also YouTube video from K7AGE in California on 19 January 2008. The best echoes start at 4:20 in the video, at 9:55 the display changes to amplitude display, and at 13:15 the frequency changes to 7407.5 kHz.

Magnetospherically ducted echo

Signals in the 1.8 – 4 MHz range may pass the ionosphere and be ducted in the magnetosphere out to a distance of several earth radii over to the opposite hemisphere where they will be reflected on top of the ionosphere. The round-trip time varies with the latitude of the transmitter, or to be more accurate with the position relative to the magnetic Equator. Typical delay times are 140 - 300 ms. At my location near Oslo, Norway, the expected delay is about 308 ms, but unfortunately I have yet to hear such an echo.

1.      The first echo is from a signal which has followed the path in the left-hand figure below out to a distance of about 1.5 times the radius of the earth over to the Southern hemisphere where it has been reflected back. The recording was done by G3PLX on 26. November 2006, at about 21 UTC (also local time), on 1998 kHz. It has a delay of 210-220 ms, consistent with his location in Northern England. The transmitted signal was a chirp using a 100 Watt SSB transmitter with a 50 m longwire antenna. Note the stereo effect as the transmitter and receiver are in the right and left channels respectively. The chirps are exactly 5 sec apart, timed against GPS, so this will enable one to calculate the exact samplerate, which is about 8100Hz. Listen to the signal here (with kind permission from P. Martinez, G3PLX).

2.      On 17. February 2006, at 0345 UTC (2245 local time) K4MOG heard his own 3.5 MHz signal coming back to him after 165-168 ms.  He explained this as the travel time around the earth (138 ms) plus delay in the transmitter/receiver switching. I would like to offer an alternative explanation as it is very seldom that signals at this low frequency travel around the world. At his location in Georgia, US, the magnetospherically ducted delay was 143 ms at the time, only 5 ms more than the round-the-world travel time, so his observed delay can just as well have been a magnetospherically ducted echo. See the right-hand picture below which shows the path it could have taken, which in this case only goes out to about 1 earth radius. Listen to the signal here (with kind permission from G. Greneker, K4MOG).
 

Long-delayed echo (LDE)

Here is an echo recorded by CB station 20CB110 in Norway (Radøy 50 km North of Bergen on the West coast) on frequency 27.025 MHz on 11 May 12.21 UTC. The main echo is delayed by as much as 7.5 seconds. The FM-signal was emitted from a 5/8 wavelength vertical antenna with a radial system. Notice the double echo (easy to hear initially on the word ‘test’). The second delay comes about 0.45 seconds after the first one. At this time there was sporadic E propagation to central Europe, but sun spot activity was at its minimum. This is one of 8-9 echoes heard at this time, selected because it is double. The delays were in the 7-7.5 second range except for one echo which was delayed by almost13 seconds. The utterance which is spoken is: “Test–signal rundt hele verden (2x)”=“Test–signal around the whole world(2x)”. The cause of the long delayed echo here is not known, but it could just be a simplex repeater which can be found in the CB band in Europe. There is a remote possibility that it could be due to e.g. mode-conversion in the ionosphere.

Mode conversion is one among the list of the Five Most Likely Explanations for Long Delayed Echoes. The magnetospheric duct echo above corresponds to explanation no 1, and is the mechanism which is best understood.

Norwegian language versions:

©LA3ZA, Sverre Holm

13 Apr 2009.