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Wednesday, December 10, 2008

Asian Propagation Mode intensifies, 7, 9 MHz

As anticipated, the annual summer occurrence of the daytime specialized Asian Mode has materialized.

This was apparent on December 9, here in Melbourne, with very strong signals from Malaysia, Vietnam, Myanmar, Japan and India audible in the period 0400-0500 in the 7 and 9 MHz bands.

This is 2pm-3pm local time, with this shortpath propagation following all-daylight transmission paths.

This phenomenon occurs each summer, and is more pronounced during years oflow or declining sunspot activity.

The propagation mechanism is not yet clearly understood, and has been studied by myself, with a few others, since it was "discovered" in the late1960s.

It is believed to be caused by multi-mode activity, caused by reflection off the top surface of Sporadic E clouds passing over the Australian continent.Under such circumstances, signals enter the ionosphere, enter the E and F layers, and are refracted back towards the earth's surface, striking the top surface of Sporadic-E clouds. Signals then continue upwards, and are refracted by the normal E and F layers back to earth.

This means that there is no intermediate ground reflection, resulting in greatly reduced absorption/attenuation on the total transmission path.

Propagation up to distances of 10,000 km, and greater, have been observed for this specialized daytime mode, on frequencies which would not normally propagate via conventional ionospheric refraction.

In past years, signals on frequencies as low as 4 MHz have been observed,from transmitters in the Indochina and Indonesia regions, at local noon Melbourne time.

Careful study of this mode will reveal that signals do not "fade-in" or"fade-out", but appear and vanish very abruptly, similar to long-distance Tropo propagation of VHF or UHF.

Signals will appear quickly, remain audible for variable periods, rangingf rom a few seconds to an hour (or more), with no fading. Signal strengths can be extremely high, hitting S9+20 regularly! Peculiarly, transmissions from the same geographical area, on different frequencies, generally are not audible simultaneously. Unfortunately, the abandonment of many HF services from Asia in recent years has significantly reduced the "pool" of frequencies which may be observed.

The mode, due to its unpredictability and variability, offers excellent opportunities for study and research of HF propagation into South Eastern Australia, and for extending the growing body of comparative data about the phenomenon in the present period of very low sunspot activity, global warming and climate change.

Documentation of this propagation event at a professional engineering level is expanding, and the phenomenon is now included in the formal training curriculum offered by the Australian Government's Ionospheric Prediction Service, Department of Radio and Space Services.

These are the entries of December 9, 2008, relevant to the HF Broadcasting Service, from a field site near Melbourne,in the Dandenong Ranges National Park, using the Eton E5 and a couple of metres of random wire for an antenna!

It would be interesting to explore this propagation mechanism in greater depth with a narrow band spectrum computer controlled specrum analyser, witha recordable digital screen display, measuring frequency versus amplitude over defined time spans, linked to a suitable professional receiver, rather than toys such as the Eton!

7130 MALAYSIA RTM Sarawak, dialects 0405-0430
7160 INDIA AIR Chennai, domestic 0425 to sign-off 0430
7210 VIETNAM VOV nat net 0415-0430 peaking 04207
270 MALAYSIA RTM Sarawak, dialects, 0405-0500
7285 MALAYSIA RTM Kajang, English 0400-0500
7290 INDIA AIR Thirunanathapuran, domestic, 0426-0450
7295 MALAYSIA RTM Kajang, English domestic, 0400-0500
9595 JAPAN R. Nikkei, Japanese 0405-0430
9730 MYANMAR Yangon, 0420-0445 domestic
9750 MALAYSIA V. of Islam, Kajang, super strength, English, 0400-0500
9760 JAPAN R. Nikkei, Japanese 0410-0500
9850 VIETNAM VOV dialects *0400-0500
9875 VIETNAM VOV dialects *0400-0500


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