Propagation News – 3 November 2024

Last weekend’s CQWW SSB contest showed just how good autumnal HF conditions can be. Many contesters worked more than 100 countries or DX entities on each and every HF band.

With a solar flux index of 238 on Saturday and 246 on Sunday, plus low Kp indices all weekend, it was about as good as it could get. There were two three-hour periods where the Kp index hit four and 4.33, but it was otherwise in the ones and twos.

An X-class solar flare occurred on Saturday 26 October at around 07:15hrs UTC, but this had little effect and the MUF over a 3,000km path was back above 28MHz quite quickly.

The rest of the week saw numerous M-class and C-class solar flares occur and a Kp index that did get up to four at times, but once again the MUF remained above 28MHz during daylight for most of the time.

Interestingly, there have been times when the critical frequency over the UK has hit 14MHz, making 20m almost a local chat band via Near Vertical Incidence Sky Wave or NVIS communications! These openings may be short-lived so make the most of them should they occur.

Night time critical frequencies have generally been around 4.5-6MHz, meaning a MUF over a 3,000km path in excess of 14MHz in the first half of the night and around 10-12MHz in the second half.

The solar proton flux has been above the 10MeV warning threshold but is now falling as of Thursday 31 October. This may improve signals that pass through the polar regions, such as the UK to the far west of Canada and Alaska, and the UK to Japan.

Next week NOAA predicts that the SFI will start at around 240, but perhaps fall to around 195-200 as the week progresses. ESA predicts that the solar wind speed could increase around November 4th and could cause disruption.

As always, we are at the mercy of solar flares and CMEs, which could disrupt HF propagation, so we recommend keeping an eye on solarham.com for daily updates.

 VHF and up

The Autumn Equinox period continued to bring F2-layer propagation to 50MHz with almost daily morning openings to VK and JA and afternoon openings to the Americas for some.

Stations in the far South and West of the island were favoured. Side scatter in the Indian Ocean brought the JAs on a 90-degree beam heading, an effect noted at previous sunspot maxima.

The autumn is typically a time for quiet settled weather, a season of mist, and this is where we are going for the period through to the end of next week. It’s worth a couple of reminders of how it all works since, although it’s likely to be pretty much the same ‘big picture’ of dominant high pressure, there will be subtle differences.

Firstly, there are two types of high pressure; one that builds in warm air and the other in cold air. The warm air version is the current offering, and this means quite cloudy conditions at times with moist air below the temperature inversion and very dry air above the inversion.

Second, the quality of the ducting is determined by the change of the refractive index across the inversion and a large part of the refractive index change comes from a contrast of moisture, so signs of cloud and/or fog moisture are good omens for a lift. It is possible that we may swap over to a cold air high later next week, which could lessen the Tropo quality.

Meteor scatter prospects are on hold pending the Leonids meteor shower, which peaks mid-November. A dominant high pressure means that rain scatter is not likely to be a big player for the GHz bands next week.

There is a possible heads-up for aurora since the solar conditions still offer chances for the Kp index to rise – anything over Kp=4 is worth a longer look – and don’t forget that ‘watery’ signals on the lower HF bands such as 40m and 80m are also a good sign.

For EME operators, Moon declination reaches a minimum (that is, maximum negative declination) next Tuesday when the Moon only reaches nine degrees peak elevation and is only above the horizon for six hours between 1200 and 1800hrs. Path losses, on the other hand, are falling all week.

144MHz sky noise is moderate to high, peaking at more than 3,000 Kelvin on Tuesday 5 November before dropping back to moderate levels.

Category: GB2RS Propagation News