Chasing Meteor Scatter on FT8, a fruitless business.

My last posting (finalised yesterday) was about the length and peak of the Sporadic E season. You can read it here, though it is just below this one on the blog.

This time I am mentioning a factor which makes 50MHz and 70MHz contacts at this time of the year difficult to classify. 

Difficult to work too.

There are often a lot of single decodes cropping up on my software just now. Some of them do carry on weakly to a second decode and the odd one makes it to a full QSO. Then there are sudden decodes which appear to come from nowhere, sometimes linking into distant propagation modes like TEP which seem like strange mini-openings. But after one strong decode they are gone. What is this thing which is teasing me?

The clue lies with meteor scatter. As I write we are in a meteor shower. When I look at dxMaps I see similar decodes on 144MHz, often over pretty well the same paths, on the 2m band. There are two differences - 1) the 2m ones don't often link into anything because those dx propagation modes are rare on 2m, and 2) on 2m they are listed as meteor scatter rather than Sporadic E.

Ah, I have at last found a reason why I watch 5 or 6 bands simultaneously. To find things like this, of course.

The maps of 6m and 2m propagation look very similar, but on 6m there are postings showing Sporadic E on FT8, whereas on 2m there are decodes as meteor scatter on MSK144. Look at the listings and you see that the 6m ones are just single decodes but the 2m ones are meteor scatter QSOs.

Meteor scatter propagation happens in the E layer when signals are scattered from ion tails left by meteors. At 2m these are fleeting short pings, but the same event produces longer periods of propagation on 6m. Thus it is possible to have good strength contacts on 6m using MSK144 and these show up as meteor scatter contacts.

Ionisation caused by meteors is short lived on higher frequencies but the effects last longer as we use lower frequencies. This longer ionisation period on 50MHz persists and can allow what looks like "standard" Sporadic E because FT8 can decode signals during bursts (at least briefly). This slightly longer period for 6m can permit a decode which is little affected by Doppler shift. So it is possible to have a 6m decode using FT8, and that tends to be posted as Sporadic E. If it happens to last long enough, a QSO can result using modes which normally do not respond well to meteor scatter.

Of course, it is ionisation on the E layer, it is short lived, so it is Sporadic E. But as it is directly caused by meteors it follows the general principals of the meteor scatter mode. Thus is tends to happen principally at the same time as meteor showers. Yesterday was the peak of the Lyrids shower, and these short decodes peaked then with me. It tends to peak at about 06:00 local time and decline during the day, as did this effect yesterday. You can get meteors at any time, but during the morning meteor shower peaks I see this type of propagation peak too.

Sometimes, during an intense shower you can get enough direct ionisation to form a small area in the E layer and get some prolonged propagation, but this is not so common. In a weaker form it can occur at any time too, but mostly I only notice it during shower peaks.

I first found out about this through a posting by another amateur who goes meteor scatter hunting on 28MHz during every meteor shower. On 10m the effect is more long lived than on 6m, and he can have quite a lot of success. I wish I could find that posting again. Anyway, once I knew what to look for it was easy to see. Here are the two maps for yesterday, 22 April at around 08:00:-

dxMaps of 144MHz on 22 April 2024

 

 

 

dxMaps of 50MHz on 22 April 2024 

 

At this time of year, and during meteor showers generally, you can turn on PSKReporter or DXMaps and see lots of 50MHz decodes reported, while there are almost no QSOs possible (try selecting "Only DX-cluster" from the DXMaps options during one of these events to see the difference).

So what difference does it make that this form of Es is created directly but briefly during a shower, when "standard" Es is just the same ionisation which has been organised into a layer (for details of that complex process see here)? The immediate effect is that it is very difficult to have a QSO on FT8 like this directly after a meteor burst. FT8 is not designed to handle it. MSK144 is a far better mode for this purpose. Also, the station at the other end may not be expecting to deal with single decodes in the way that most meteor scatter operators do. MSK operators will have long watchdog periods and keep transmitting for ages. Plus the key difference is that the MSK144 algorithm is far more sensitive to the weak signals as the level of ionisation tails off during the period of the ping.

It is Sporadic E, Jim, but not as we know it (with thanks to Gene Roddenberry, Bones, and the crew of the Starship Enterprise for that misquote).

To make full use of this effect, I suggest you switch to MSK144 and work meteor scatter on 6m - if you can find anyone else with the same idea. Or go down to 28MHz where the ionisation hangs around long enough to have an FT8 QSO (possibly). However, it only lasts for a day or two around a meteor shower peak.

Me, I just sit and watch as operators chase individual decodes. It must be frustrating. It was for me until I worked out why this happened at the meteor shower peaks.

73

Jim GM4FVM