A classic tropospheric duct

I really enjoy working tropo enhancements on VHF and UHF. Usually though they move around with the weather and can be quite wide, which makes them hard to identify against a complex background of other contacts.

However, on 3 April I stumbled across a long lasting and quite narrow duct on 70cm. The background is that a high pressure system had developed to the north of Scotland and was destined to affect our weather for ten days or more. Seeing the opportunity of the good weather, I had scheduled some antenna work. As barometric pressure was moderately high but not declining I though nothing much would emerge on the tropo front. Usually good conditions emerge only on the trailing edge of high pressure systems, when pressure is falling. However, even during long lasting highs it is possible to find good conditions.

With all this in mind, and while working on my 2m antenna, I passed the shack door on the way to collect some tools. I noticed a very strong station calling CQ. This was SM6VTZ in JO58.  I have worked Chris a dozen times on 23cm, 70cm and 2 metres over the past four years, but only during good conditions. The striking thing about seeing him on the waterfall this time was not only how strong he was, but that nobody else was to be seen. Apart from one station, conditions were normal.

Although he had finished calling CQ, I called Chris at 12:49 and gave him a +20 report, and he replied with +23dB. The path length is 879km. These are not unusual reports, the previous time we worked on 70cm was in September 2024 when the reports were +14/+29dB. It is worth bearing in mind that during normal conditions I cannot hear Chris at all and on a flat band I can usually work only about 300km with much lower reports.

What was different about this contact was the almost complete absence of any other station, not just on 70cm, but also on 23cm and 2m. Chris came on to KST chat room where other SM stations reported hearing nothing from me.

On KST Chris and I exchanged reports as the duct built over time, later reaching +24/+34dB. Having started before 12:49 this went on until after 16:50. I heard SM6CEN in JO57 at 15:19. I guess that Hakan is on the southern edge of the duct as signals to him were much lower at  -19/-16dB at a time when VTZ was reporting my signal via KST at 50dB higher than Hakan.

I also worked MM0INH who I think worked VTZ too, but at lower signal strengths than I did. Dale is 43km to the west of me. I was not aware of any other stations working Chris though he may have worked another station in North East England. It is clear that this duct was narrow.

Extract from station log at GM4FVM covering 3 April 2025

Click to enlarge images if you need to.

So how do I know that this was a narrow duct? Because it follows the general pattern for ducts:-

1) I did not work anybody else on 2m, 70cm or 23cm that day, and I would have expected to if this had been a general enhancement or a moving or wide duct.

2) It was extremely stable over a long time - several hours

3) Stations outside the duct did not get into it 

4) Signals were extremely strong as ducts pass signals very efficiently 

More usually ducts move with the weather. On this occasion, with a very stable weather pattern, it stayed in one place. I have experienced this before, but it is unusual.

Here is the best map I can do at the moment. In the absence of Log Analyser I have used K2DSL's "ADIF to Map" site. Once I had the map by this method I went back to the way I made maps years ago - passing the image through two pieces of software to add text and crop to size. I am still seeking a better solution.

Contacts at GM4FVM during 3 April 2025

Ducts are common. What was uncommon this time was finding such a narrow long lasting duct, with no other DX possible for the whole day. I like ducts, and after all it was a duct which allowed me to work EA8TJ on 5 August 2018, a distance of 3260km. But others in various parts of Scotland could access that duct, whereas this time it was very much narrower.

I shall look more carefully for such things in future. And next time I will try to remember to ask Chris to try 1296MHz too.

[EDIT. I should have mentioned that in contrast to this narrow duct, the next day I also worked SM6VTZ, this time on 144MHz. Unlike 3 April, contacts on 4 April were all round the compass and nothing like as strong. I worked 15 on 144MHz and 9 on 70cm, spread almost evenly around the North Sea. 5 x G, 4 x OZ, 5 x LA, 3 x GM, 6 x PA and one SM. That could hardly have been more different]

73 Jim

GM4FVM