Tuesday, 5 May 2015

Sporadic E weather and Flex AGC-T

I am a sceptic.

That does not mean that I do not believe things. It means that I need proof to convince me.

This is my third attempt to write on this topic. The other two have been deleted. You see, it is something I am confused about and cannot decide how to approach it.

There a theory that Sporadic E patterns are directly related to the weather.

There are a couple of snags with this. First of all the weather here has not got much to do with the weather 1000km away, which is where I am reflecting and refracting my signals off the ionised part of the E layer. And, that ionised layer is 100km up, whereas the weather as we know it is tropospheric, i.e. 14km up max.

The theory goes (and I saw this reported as "news" two days ago) about a "correlation" between the "Jet Stream" and Es propagation. Well, a "correlation" does not establish cause and effect. OK, the weather patterns and Es patterns might be related, but maybe because they share similar root causes, or happen at the same time of year.

The "Jet Stream" is a high (7 to 12km up around Europe), long distance (up to 4000km long), high speed (up to 400kmh) wind system which has a major influence on European and other weather. The Jet Stream certainly affects the weather here, but does it affect the pattern of Es? What is the mechanism for this? And what do they mean when they say there is a good "correlation" between weather systems and Es patterns.

There is a good correlation between putting fuel in my car's tank and it working, but there are a lot of other factors at work, like the engine, every single moving component, me, the road surface and a host of other factors including the weather.

There have been all sorts of theories about weather effects and Sporadic E. Like lightening, which has been put forward but never proved. I used to read about storms over the Alps "causing" Sporadic E. As somebody pointed out, the very good Es path between me and the Canary Islands, Madeira and the Azores does not really pass over the Alps. There are plenty of good Es paths which do not involve mountains or storms.

Then the inconvenient fact that lightening is limited to under 20kms in height and Es is at 100km is also generally ignored. Then you see people talking about red sprites and blue jets, not to mention elves and TIPPS, all of which sound like something out of a Harry Potter book. They are counterparts to lightening, which rise up, but sadly not as far as 100km.

This is the best comment I have seen, which is from a very useful paper (worth a read) by Michael Hawk, which you download here is:-
============
No studies intending to link thunderstorms to sporadic-E have resulted in conclusive data. For
years, the scientific community doubted any possible relation, given the simple fact that
thunderstorms occur in the troposphere (0-14 km altitude), and Es occurs at 100 km. In between
the troposphere and the ionosphere’s E region exist invisible barriers where the medium of the
atmosphere radically changes. These transition zones, named the tropopause and stratopause,
prevent certain interactions between layers from occurring due to the change atomic content,
wind, temperature, and other attributes.

=============
This would apply to most weather effects.

However, "weather" must be a factor of some sort. The widely understood explanation of Es is that the ionised particles are held by the Earth's magnetic field. Below this layer are particles of non-ionised air, which is subject to "weather" and therefore moves about, unlike the ions which are more or less fixed by the magnetic field. However, the weather at 100km is not like the weather down here on Earth. For a start the air is incredibly rare, so thin that it can stay ionised as a plasma, and thus create Es.

The clash of the moving air and the more rigid ionised plasma above it causes a "shear" effect which not only organises the layer, but concentrates the ions in the layer. Fair enough. But to link this to the Jet Stream acting 80km lower is a big step.

Yes, years ago I spent a long time trying to prove a correlation between weather patterns and Es myself, and it is hard to prove. The random nature of amateur activity makes comparisons hard to do. But there does seem to be something in it. I want to believe it but I cannot find any convincing proof. So, to make statement as if it was proved to be a major factor is going too far for me.

Maybe the correlation is for other reasons. Maybe storms over the Alps and Es are caused by the same cause (a link has recently been found between solar wind speeds/ auroras and thunderstorms, for example, though this is a different process). Maybe these are just coincidences - because weather in Spring is variable and so are Sporadic E formations.

I believe in Father Christmas, but I cannot prove he exists. Sometimes, it is acceptable just to believe in something because you see the signs and it would be nice to accept it. More likely in this case there is some common shared factor, or multi-factorial system, linking all this. But in the absence of proof of either, I find Father Christmas more likely to exist than the weather theory of Es.

It would be nice to look out the window, see the rain storm which we are now having, and say "Well, that is why I have not worked any Es on 6m this season". But that goes too far for me.

Perhaps weather is a bigger factor than we know. But let us not start claiming it is proved that it is cause and effect. Weather theorists, we love you, but lets see the proof rather than just patterns of similarity.

In the meantime, I believe that some opposite of Father Christmas, some ghostly sprite, is holding up Sporadic E this year.

Now, what about Flex AGC-T? I said recently that I have been having trouble with high speed WSJT modes not decoding using the Flex and Power SDR. It seemed to be related to the AGC-T setting.

AGC-T is a very enigmatic control.

Gianfranco, IU1DZZ, said something on the WSPR chat site which got me thinking. He had his set to 120, whereas I was having problems at 90. Gianfranco is using a slow speed WSJT program (WSPR) most of the time, though he has said he uses JT6M and others sometimes. Anyway, he said that the setting he uses is good for weak signals but not so good for strong ones. Or at least, that was what I think he said.

If AGC-T does work like that then it is sensible to treat it like a normal AGC. A normal IF AGC can work non-linearly if noise is high and/or signals strong. That is why we can adjust it. But AGC-T seems to work in some other way. I have tried setting it as low as 50, and I could copy a weak meteor scatter signal well, while at the same time it seemed to wipe out a strong signal entirely. With normal AGC, they would both be reduced by similar amounts, though the noise floor would tend to make the sound appear more equal. So this seems to work in a similar but different way.

With an IF AGC you are trying to prevent the IF stages being overdriven, but an SSDR like the Flex does not work that way. What I was seeing before was noise overdriving the VAC, apparently at (digital) audio stages. Also, it does not respond to changes in the level as I would expect - in other word it "maxes out" the VACs long before it reaches the default value of 90.

I do not understand it, and in most modes it works just fine. But the sudden loud signals on meteor scatter, and the long periods of white noise around them, seem to fox it in some way.

More testing needed...

...as always.

73.

Jim
GM4FVM

No comments:

Post a Comment