Friday, 12 April 2019

"Almost Sporadic E" and 6 metre band readiness

At the start of April we find ourselves back in the "Almost Sporadic E Season".

For the past ten days I have been decoding one-way FT8 signals on 50MHz which look like Sporadic E, and so have others. Sometimes I have called CQ and been heard all over the place ...
Stations reporting hearing GM4FVM on 50MHz, 7 April 2019, on PSK Reporter
All of this has produced just one QSO for me - SP4MPB on 4 April. The rest of the time I just see parts of signals being received. Even more frustratingly, I see myself being reported as heard be other stations, but I hear no replies. Collections of dots and lines which are almost enough to decode but not quite there, then maybe at a different time several decodes. Although I can receive them, none of the stations answer when I call. Lots of activity, even quite a few decodes at time, but only once a QSO.

Almost Sporadic E.

There could be lots of explanations for this, but I think it is all the more noticeable thanks to WSJT-X and that darned mode FT8. I have speculated before about the ability of the "weak signal" modes to receive signals which are otherwise inaudible, and this may exploit the effect of partial reflection in the ionosphere.

"Above MUF propagation" is a fairly silly expression, but what it means is "above normal MUF propagation", in other words by using very sensitive protocols like FT8 we can decode signals we never noticed before.
Plenty of 6m activity on PSK Reporter, 9 April 2019, no QSOs though.

Partial reflection occurs when there is refraction, but usually we do not notice it because the refraction produces a much stronger signal which swamps the partial reflection. It may cause phase differential QSB, which many of us note on radio signals without giving much thought to the causes. However, just now the E layer is only strong enough to produce some refraction but not enough to deflect the signal back to Earth, so the weaker reflected one is heard.

Partial reflection is really only capable of being understood as a quantum effect. I do not think that I am alone when I say that at school I was taught the wave theory of radio propagation in my physics classes. It is very comforting to think of radio progressing as a wave, especially as many observed radio effects can be explained by wave theory. But wave theory does not explain partial reflection. Like many more complex aspects of elecro-magnetic theory, we need to consider quantum effects to understand it.

When considered in quantum terms, radio energy travels in straight lines carried by photons, and these are only deflected by:-
a) gravity (but the Earth's gravity is too weak to have any significant effect)
b) collision (producing what we know in radio as scattering)
c) refraction (the main way signals are deflected back to Earth)
d) reflection (quite rare at common radio frequencies).
It is in the process of refraction that partial reflection occurs, and it is often dismissed as a byproduct as it is very weak.

You could add to that list e) absorption. Radio signals encountering a very dense object will be effectively absorbed. The object gets hotter, and the photons involved cease to exist. Not really deflection, but worth mentioning. Although there is some absorption (and scattering) in the atmosphere, the radio signals we notice getting weaker do so mostly due to the Inverse Square Law

However, you will note that neither electric nor magnetic fields deflect photons, so neither the ionisation itself, nor the Earth's magnetic field, will deflect a photon in the ionosphere. What we hear back on Earth is almost returned to us due to refraction (plus a tiny element of reflection).

I had written 10 long paragraphs on the subject of the analysis of radio propagation from a quantum perspective. It is all so complex it hardly seems worth it. I have just deleted them. Instead I will show my home drawn diagram from the earlier posting again, and it applies equally to the E-layer as it does to the F-layer:-

The partial reflections, shown by the white lines, would normally be swamped by the main signal, but just now the ionisation is not quite strong enough to refract the main signal back to Earth. So we have some weak reflections, and we would never notice them were it not for using FT8 and other very sensitive modes.

As partial reflection is a quantum effect it happens in the world of probabilities. We can see through glass yet from the correct angle it can reflect light too. Up to 4% of a light beam falling on a single layer of glass is probably reflected by this process. "Up to" and "probably" being key here. The effect we see with glass is faint and ghostly. You have probably seen reflections from glass, when you expect to see through a shop window but also see a faint impression of yourself as a reflection.

In the dire 1960s UK comedy TV series "Harry Worth", Harry used to open the show with a recording of himself reflected in a shop window .... no Jim, don't go back there again. Harry Worth's TV show should be forgotten but, yes, it did feature partial reflection in the titles. Yes, Harry was a bit of an amateur scientist. Anyway, partial reflection is a faint, shadowy event, and that seems to explain what I am hearing.
Harry Worth (the things we used to find funny) Photo Wikipedia (Fair Use).
Sure you can also explain these strange signals in other ways. For example, maybe the ionisation is rapidly variable and just reaching levels able to support refraction. I do not think so, as these signals are very weak (Es is very strong), and there is a one-way element. I can copy one or two or even three signals from station, but they do not hear me at all. At other times different stations hear me and I do not hear them.

If you look at my diagram and imagine another signal coming the other way you can see that the angles do not match up - you could not reverse the angles symmetrically as the path of the signal passing the other way would need to be pass through a different patch of ionisation - and Es clouds are notoriously localised.

Whilst it is possible a second patch of ionisation would occur, or the first one is wide enough, it is unlikely to happen until there is much more ionisation - not until into the Es season proper. If this sounds odd it is due to the asymmetry of the path - the partially refracted signal is not straight, whereas the reflected one is more likely to arrive at a different angle as it passes through different paths of ionised space. While the path my exist, the signal will be a different strength. This effect becomes less as more ionisation occurs.

Certainly there is marked one way propagation in the present period. Soon we will have more ionisation, stronger refraction, and we can forget these odd lumpy traces on our waterfalls.
In order to be ready, and as predicted in my last posting, I have readied my 6m linear amplifier. I have taken the TE Systems 0552G out of its box, cleaned all the corroded cables, tuned it up, and Hey Presto!, it works. I suspect that the trouble was simply power starvation after the 50 amp drawn through the PSU output terminals melted everything that would melt and scorched and blackened everything else.

I have imposed a current limit of 40 amps, but I can still get 200W output for that so it is no hardship.

As I suggested in the last posting, the three element 50MHz antenna seems to be fine for now. Perhaps by late May I will feel the need for something bigger.

Also mentioned last time, I am investigating more power on 70cms. Not a lot more, but maybe up to 3dB more would help the Earth Moon Earth strike rate a lot.
At last, thing are beginning to look better for the new season. Well, I did work SP4MPB, so technically the Es season has started and the Lyrids meteor shower should be going soon.


P.S. I have to post this now, as I have spent so much time drawing diagrams of one-way paths, small patches of ionisation, asymmetry ... grrr ... I am going MAD. Quantum theory, who needs it?

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