Sunday, 28 June 2015

Doing the VHF treble

My late Uncle Ernie was a great gambler. He did "trebles". When he gambled, he bet not on one thing happening, but three things happening. Although he would not get regular small winnings, he reckoned his fewer big winning would be more.

He never did win a treble.

Footballers also want to win the "treble", league cup, plus domestic and European championship, or whatever, and score a hat-trick. Trebles are big things.

I though of this when reading "Fouritalia" a news sheet produced by IW0FFK for Four Metre Band (70MHz) enthusiasts in Italy. He included a letter from IZ5ILX describing his VHF activities as "doing the treble".

You can find Fouritalia (in Italian of course) here.

Thanks for that idea for a name. "Doing the Treble" is exactly what I do.

I know that some of you have been sceptical about me running several rigs. Well, that is how I do the treble, and I often do quadruples too.

This is not odd. HF DXers do something similar, moving up and down bands with MUF. But it is different with Sporadic E. With F layer propagation you can get gradual shifts, and just use one rig up and down the HF bands. With the VHF treble you have to jump around, taking your chance as it comes.

Here are some diagrams. You have to take some things for granted. As the radius of the Earth is about 6371km, and the Sporadic E layer is about 100km above that, any scale diagram would either have too little detail or be too wide. So I have tried to get a lot closer to true scale here, but my diagrams are still not flat enough. This makes the angle of radiation looking too high - for real good DX I suggest you use the lowest angle of radiation you can.

You also have to take into account that radio signals do not go a straight lines with parallel sides, but spread out in cone shapes into open space. Hence the inverse square law. I will show them as straight lines for now and say something else  about this later.

Finally, the E layer is shown roughly parallel to the Earth's surface (it almost is, but not always) and the angles by which the signals are sent back to the Earth are symmetrical. This is true as far as it goes, but if you go into more detail you find other variations. For here we stick to "the angle of reflection is equal to the angle of incidence". Relative to the line X-Y on the following diagrams, the signal appears to be bent at the same angle on each side (that is only an illustration too, as the signal appears to be reflected at an imaginary point above the E-layer, but that is not for discussion now).

The actual mechanism is not quite like a mirror would reflect light, as I show it. There are in fact many refractions plus many minor and one major shallow reflection, and there can be a lot of variation. Let us just take the simple analysis for now. Making it more complicated does not help us in a general discussion. There is quite a good explanation in the ARRL handbook and I hope to say more about this later.

This diagram should be pretty familiar (click to enlarge if needs be) ;-
As the level of ionisation increases, radio signals which otherwise pass out into space are able to be bent back down and reach a DX station.

With lower ionisation they pass straight through into space. When increasing ionisation allows propagation to reach the "maximum useable frequency" (MUF) the radio signal will be bent down to the point where it just grazes the earth. This is the maximum distance able to be reached.

The effect of even higher ionisation is best shown in a later diagram. In this diagram it looks as if the ionisation rises until the signal bends down and disappears somewhere in space (actually it stops being refracted). However, you do not usually stop hearing stations. As the ionisation continues to increase you start getting your signal bounced from a point "P" and you get shorter path lengths (but the angle in incidence at P still equals the angle of reflection at P, just the possible angles are different at different levels of ionisation). If you do not understand this for now, just move on and it should become clearer.

So there is a level of ionisation which gives you the best DX you can get, if you fire your signal off at a low angle of radiation. You can exploit this, but as ionisation increases, you get short skip. Any good DX-er then goes up to higher frequencies to get more DX. Not so many stay with short skip (unless they are hunting for squares or nearby countries).
 At this point things begin to get difficult for the diagram as the signals are shown as straight lines. They are not straight lines in real life. They are cones with a point at the tx antenna which expand out and hit the E-layer at slightly different angles. So as the ionisation increases further, you still get signals bounced back over a fairly wide area. But in general, as the ionisation increases the "skip shortens".

Also at this point I start trying to "do the treble". As the ionisation rises, the skip on 6m shortens. But when it reaches a certain level, that means that the ever-increasing MUF reaches 4m. The MUF has risen from 50MHz to 70MHz. Eventually, when the MUF rises very high, the distance I can work on 6m gets even shorter, it gets shorter on 4m and suddenly I can work DX on 2m. The MUF has reached 144MHz.

So I band hop. I watch all three bands. I try to "do the treble".
I watch 6m on DX Maps and see, perhaps, station working between Southern England and Northern Spain. That is quite short skip for 6m, maybe 1000km.  Perhaps I can see MUF prediction on DX Maps showing an Es cloud over the Bay of Biscay. I cannot get into this contact along the line A-B on the diagram above. The 6m skip is too short. I am too far away to reach the end of the path. But I can try changing up to 4m. If the ionisation over the Bay of Biscay will support an MUF of 70MHz I can work into Portugal along the line C-D. And very often, it works. It is actually further than A-B, which I cannot reach anyway.

That is just "doing the double". I still enjoy it. If 2m opens too, I can get along path E-F.

The point is that sitting and waiting on any one band rules out the possibility of watching the MUF rise and the skip length changing on the others. If you listen to silence on 4m, you do not realise that there is an opening which you cannot work on 6m, and you can get into that on the higher band if you swing your beam and just try calling CQ. Yet, despite this being pretty simple well-known stuff, I know VHF operators who sit in hope on 2m watching the world chatting on KST chat line, not realising that they might as well turn their rig off. If they were listening on 6m or 4m (even if their country does not have 4m to transmit on) they might know how the bands are behaving.

So, I am not aimlessly moving about the bands. I can be looking at the MUFs and thinking about the levels of ionisation. Often I can see that the 6m paths are too short for me, and too far away. So I think in terms of higher frequencies, trying to cross the same ionisation spot which I see on the map. Or sometimes I can hear 4m open to strange commercial data stations, but no amateurs. Then I starting wondering if 2m is about to open too.

On 22 June I did the treble. I had worked Slovenia, Slovakia, Romania and Serbia on 6m at about 10:00. However, the skip shortened, and the 6m Es paths were all in Europe and too short for me. Later on 4m I knew that I was being heard in Italy (best not to say how) before 16:00. But the skip on 4m shortened again, and I lost that path. Then I thought and moved to 144MHz and worked IW1CHX, IZ1ESM and I2FAK. When that faded I thought that the 4m skip length had probably lengthened again and dropped back to 4m and worked Slovenia, Southern Germany, Greece and Croatia.

The DX distance of those contacts were all much the same. They involved jumping up and down the three bands. Doing the treble.

The point about this is that while I was working good DX on 6m, I was not going to get anywhere on 4m. Once I knew that I could get DX on 4m then the good DX disappeared on 6m. Eventually when the 4m DX went I could move up to 2m for a short while, and after than 4m was good again. These are the steps and stairs I go up and down regularly.

I am not going to say that this is easy. Unlike F layer DX, you cannot do it easily on one rig. Es can appear very quickly. You might think that Es can just appear on 2m and vanish immediately. I have shown that 2m Es is more easy to find if you follow it up from 6m via 4m and then wait for it. Most of the time it does not come. Sometimes the process is very quick. There is a lot of guessing, but it is informed guesswork.

It is great fun though.

So there are three things I would say about this as some sort of advice.

Firstly, watching paths you cannot reach is a good pointer for finding ones you can. I try to learn the way the bands work that day and follow that.

Secondly, after a band closes, try the one below in frequency and that might now have paths you could not reach before. Never switch off after an event without trying the lower bands or you might miss something.

Finally, try doing "the quadruple". I leave 10m on WSPR all day. I learn from that where the very early and late openings are.

What I am talking about here is "smart working". It will not make a band open if the MUF is not high enough. But I think it gives me the best chance of following the best paths.



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