Transequatorial VHF-UHF Propagation

Transcription

1 Transequatorial VHF-UHF Propagation the next challenges for VK Roger Harrison VK2ZRH GippsTech Conference 2007 Churchill, Victoria Australia 1

2 A Rayleigh-Taylor production There are holes in the sky Where the ions don t run Those holes are long and large That s why T-E-P is fun! - with due acknowledgement to Spike Milligan Copyright Roger Harrison VK2ZRH 2007, st Australian serial rights granted to the Eastern Zone Amateur Radio Club, Victoria, for publication in Proceedings of the GippsTech Conference

3 TEP defined Transequatorial propagation (TEP) involves the reliable reception of signals or the making of two-way contacts over very long paths that cross the geomagnetic equator on frequencies well beyond the usual MUFs often at unexpectedly high signal strengths. 3

4 Two modes of TEP Afternoon-type experienced predominantly between 1400 and 1900 LMT (relative to the path mid-point) Evening-type predominantly occurs between 2000 and 2300 LMT (relative to the path mid-point) Each is now referred to by its mode of propagation 4

5 Two modes of TEP Afternoon-type is called super mode TEP propagation involves two F-layer reflections north and south of the geomagnetic equator without an intermediate ground reflection ( chordal hop ) MOFs typically MHz, up to MHz sometimes path lengths range typically from 4000 km to 10,000+ km often strong, steady signals only small Doppler shift peak seasonal occurrence is equinoctial occurrence rate follows the solar cycle 5

6 Two modes of TEP Evening-type is called ducted mode TEP propagation is via ducting or guiding through equatorial plasma bubbles field-aligned bubbles or tubes of depleted ionisation which thread the nighttime equatorial ionosphere, extending symmetrically north and south of the geomagnetic equator MOFs often extend to 144 MHz some 432 MHz This is where the next challenges are for VK 6

7 Ducted mode TEP in detail From how we experience it, to the mechanics of how it happens 7

8 Times and durations of openings predominant occurrence between LMT may start earlier, occasionally extends past 2400 openings typically last minutes, sometimes less a series of openings often occurs, from seemingly continuous (overlapping), or up to 30 mins or so apart duration of openings decreases at the higher frequencies 8

9 Times and durations of openings 9

10 Seasonal characteristics peak occurrence during the equinox months: Mar-April / Sept-Oct the higher frequencies have shorter seasons, fewer openings on low VHF, openings still occur thru the solstice months 10

11 Solar cycle dependency occurrence rate of openings maximises during solar maximum years openings never disappear during solar minimum years MOFs are greater during solar maximum years however,144 MHz openings have been reported during the 2006 and 2007 equinoctial seasons as we pass across solar minimum 11

12 Paths are bisected by the geomagnetic equator cross the geomagnetic equator within a small range of angles close to 90 degrees the path terminals commonly lie in a zone between about 10 and 30 degrees geomagnetic latitude (ie. near magnetic conjugate) paths having an obliquity of 15 degrees or more to the orthogonal experience far fewer openings The higher the frequency, the closer to orthognal with the magnetic equator the path needs to be 12

13 Paths: global sectors 13

14 Paths: VK-Asia the paths predominantly worked on 50 MHz and 144 MHz via ducted mode TEP note how the path terminals are near conjugate areas curiously, off-path signals have been recorded: note the ---- line between Singapore and the Darwin-JA path 14

15 Signal characteristics Distinctive, rapid flutter fading over long-term peaks and troughs; occasional drop-outs. Signal strengths can range from S1 to S9+; S3 to S7 are more usual; decline with increasing frequency. The strongest signals can exceed free-space levels for the distances involved. Doppler shifts ranging from +/ 20 Hz at low VHF (50 MHz) to excursions of 400 Hz on 144 MHz. Doppler smearing of 100s of Hz up to 1-2 khz. Voice modulation (SSB, FM, AM) can suffer severe distortion. 15

16 Signal characteristics Doppler shift of 144 MHz signals on the Africa-Greece path, measured by SV1DH Fimerelis.and Uzunoglu 1981/1 16

17 About those bubbles As promised we come to the mechanics when the sun sets on the equatorial ionosphere (about 1900 LMT), the base rises in altitude the more viscous F-layer flows over the less viscous neutral atmosphere this sets up a Rayleigh-Taylor instability ripples form in the base of the F-layer the top of a ripple, having little ionisation, rises into the F-layer this forms a bubble that swiftly elongates north-south along the geomagnetic field lines 17

18 Equatorial plasma bubbles Simulation of the dramatic development of an equatorial plasma bubble (Huba and Joyce 2007) 18

19 Equatorial plasma bubbles rise at speeds of typically m/s drift eastwards at speeds of m/s some bubbles have been recording rising at >2 km/s! reach peak heights of km at the geomag. equator the feet can extend to >25 0 geomag. latitude km in diameter, spaced km apart may be bifurcated (think elephant s trunk!) 19

20 Equatorial plasma bubbles seasonal occur most often around the equinoxes (it s geometry!) greater occurrence during solar maximum years still occur through years of solar minimum first 144 MHz contacts 1977 year after Cycle 21 mimimum! manifest as spread-f on ionograms from equat. ionosonde stations cause off-great circle/multi-path fading and distortion on HF signals bubble walls are known to be leaky 20

21 Ducted mode TEP The path geometry in detail 21

22 A real life path South Africa to Athens 1980s (After Uzunoglu and Fimerelis 1988) 22

23 Path geometry Entering and exiting a bubble ray path tangent to field line at height of entering bubble elevation angles range from ~2 0 to ~20 0 a cone of acceptance a communications zone on the ground radius of this zone given by: r o = 1.42 x /f 144 = 987 km; 432 = 329 km - after Heron and McNamara

24 Path terminals VK-JA dashed ellipse call areas most often worked from Darwin on 2m reported 2m contacts from VK4-JA cover call areas 0,1,2,3 & 7 areas are close to conjugacy paths cross geomagnetic equator at or very close to 90 0 THE NEXT CHALLENGES? 24

25 The next challenges for VK 1/ bridge VK-JA on 432 MHz 2/ extend the world VHF-UHF TEP DX record ~8000 km * 3/ extend VHF ducted mode TEP via meteor scatter * VK4BFO holds the 144 MHz VK record 6763 km to JI7DMB, 15/04/91 25

26 Planning for the challenges location: optimise the conjugate communications zones plan around the path loss arrange skeds choose times around when the sunset line is 90 O to the geomag. equator know when bubbles appear HF multipath, Vanimo ionograms 26

27 Planning for the challenges plan to hit those bubbles tangent to a field line - after McNamara

28 Planning for the challenges follow that bubble! from west to east, low to high elevation - after Heron

29 Extend TEP via meteor scatter Consider this: 29

30 Ducted mode TEP Between the idea And the reality Between the motion And the act Falls the shadow - T.S. Eliot, "The Hollow Man" 30