Meteor Scatter and Tropo-scatter Extensions of Tropo-Ducting
On 27 January Bob, 2006, ZL3TY, worked, Dave, VK2AWD, Dave VK3HZ, Rhett, VK3VHF and Rex VK7MO on JT65 on 2 meters. There were two interesting things:
Firstly, there was a strong tropo-duct opening to Sydney that was allowing Bob to work this VK2 (around Sydney) on SSB but the Hepburn chart showed this did not extend to VK3 (around Melbourne) and VK7 (around Hobart) and no SSB contacts were made more than a 100 km south of Sydney. The extra sensitivity of JT65 allowed an extension well beyond that indicated by Hepburn but still took advantage of the tropo-duct at ZL3TY’s end. From the Hepburn chart it would appear that the first 500 km or so from VK3/7 would have been achieved by tropo-scatter and the last 1500 km by tropo-duct. From this one can conclude that it is worth attempting JT65 contacts when ducts do not extend over the full path – up to a further 500 km or so to take advantage of possible multi-mode propagation.
Secondly, the JT65 contacts were affected by meteors and both Bob, ZL3TY, and Dave, VK3HZ, Melbourne, noted many meteor pings on a path of 2287 km. In the past attempts at meteor scatter over this path have not shown any pings at all. The fact that meteors were being received suggested that tropo-ducting might be extending the range of meteor scatter. A little later Garry VK5ZK (at Goolwa, 2892 Km from ZL3TY) reported he could see some pings from Bob’s JT65 transmission and a sked was set up using FSK441. Bob decoded a number of pings from Garry. Peter, VK5ZLX at Angaston (2919 km) then reported pings from Bob and started transmitting. Bob also saw pings from Peter. The best was a short burn of 1.1 seconds reported by Peter.
The following day ZL3TY made numerous SSB and JT65 contacts into VK3/7, as far as Melbourne, indicating that the duct had extended further South and further towards VK. A further attempt was made between ZL3TY and VK5ZK/VK5ZLX but only one decodable ping was received by ZL3TY. A few hours later, when the duct was no longer reaching Melbourne a second attempt was made and VK5ZK received 5 decodable pings but nothing was received by ZL3TY or VK5ZLX. During the same day Ian, VK3AXH, near Ballarat, reported pings when listening to ZL3TYs JT65 signal over a path of 2387 km at the time when Bob was working JT65 into Melbourne – but JT65 was not detectable on tropo as Ian’s QTH.
While a VK5 contact was not completed, this exercise was sufficient to show that the normal limit of around 2300 km for meteor scatter can be extended if there is a good tropo-duct at one end.
It is worth some conjecture about what conditions allow a tropo-duct extension of meteor scatter. It would be expected that for efficient entry to a duct the signal would need to arrive at a very low angle and thus the meteor scatter component of the path would need to be reasonably long say 1500 km or more. Inspection of the Hepburn chart indicates this was the case between ZL3TY and the VK5s. However, the pings received by VK3AXH and VK3HZ are unlikely to have entered at the start of the duct which was within a few hundred km as the entry angle would be several degrees and also meteor scatter is unlikely over such a short path. This in turn suggests that some of the meteor extensions were entering the duct somewhere in the middle of region which is shown on the Hepburn chart to suggest a duct. There is some evidence on the Hepburn chart that of variability in the strength of the duct nearer to ZL. Thus while the best situation might be is a strong duct at one end combined with a non-ducted region of around 1500 to 1800 km it seems one should not overlook the possibility of entry somewhere in the middle of a duct.
On an international level this work suggests it is well worth looking at meteor scatter extensions using FSK441 at the end of long tropo-ducting paths. The work also suggests that station who are well inland and normally cannot participate in ducts over water can reach these ducts by meteor scatter where the terrain is not a factor
Decodes received by ZL3TY from VK5ZK and VK5ZLX at a distance of around 2900 km on the first day of testing.
235800 23.2 80 2 16 -150 K ZK VK5ZL ZM
233700 20.0 260 2 26 -64 5ZLX ZL3TY VK5ZK VK5ZLX ZL3TY VK5ZK VK5Z
233700 17.7 120 2 26 -64 X ZL3TY VK5ZK VKUZN
233200 7.0 80 2 16 -64 K5ZK VK5ZLX ZO
232600 4.6 80 2 16 -107 ZL3TY KQ ZL0TY
Decodes received by VK5ZK on the second day of testing. While Garry heard around 15 pings on the first day of testing these did not decode due to a set-up problem on WSJT with clip set full on at 99 and thus eliminating meteors (Murphy’s Law). Note that while hard clipping can be useful for eliminating meteors from tropo signals on JT65, clip should not be used on FSK441. Later versions of WSJT avoid this problem by automatically changing to the default values on change of mode.