venerdì 25 novembre 2016

Propagation News – 27 November 2016

This week NOAA, and us, got the HF propagation prediction wrong. The figures suggested that last weekend would be unsettled geomagnetically, but in fact conditions were actually much better. The K-index did eventually rise, but not until late Tuesday evening.

The prediction for this weekend, as you either read or hear this report, remains uncertain as the Earth succumbs again to the last of the, weaker than expected, plasma stream from a large recurrent coronal hole on the sun. NOAA has this weekend’s solar flux index predicted around 80 and the K-index at three or four, but geomagnetic conditions may improve slightly as we move into next week.

It is hard to say exactly how conditions will be for this weekend’s CW leg of the CQ Worldwide contest. If we continue to be hit with solar plasma maximum usable frequencies may be impacted adversely. However, as contesters tend to use larger than average antennas and power levels you should find the bands open to DX, at least up to 21MHz. Short occasional openings on 10 metres may also be possible, especially to the southern hemisphere, southern Europe and equatorial Africa. But we certainly won’t see the kind of conditions we have enjoyed during the last two or three CQ.

VHF and up propagation:

We have high pressure in attendance and conditions on the VHF and UHF bands are picking up well. This high should provide temperature inversions, both near the surface and aloft, due to descending air within the high pressure area. These temperature inversions are important in that they form discontinuities of the refractive index over short height spans and become the regions that can duct your VHF/UHF signals over long distances.

Tropo conditions provide DX on paths around the edges of large highs, so look for flat layers of low cloud or mist and fog, both visual signs of temperature inversions. Favoured paths are likely to be within the UK and across the North Sea into northern Europe. Things will probably change around midweek as low pressure moves down from Iceland. It is possible that the high may reassert itself late in the week, but this is a bit of a long shot.

The Moon is at apogee today so path losses are at their highest. Moon declination is still negative so with the Sun close to the Moon on Tuesday it is a poor week for EME.

We had a Leonid fireball meteor last week. While spectacular, a single rock doesn’t help meteor scatter conditions, so just hang in there for the major Geminids shower in a few weeks’ time.