Friday, 6 November 2020

A bit more moonbounce, at last.

I am not a fan of high power. I do dabble with it, but I do not routinely run full legal power.

I can see, however, that in certain situations on VHF there is no alternative. In particular this applies to scatter modes of propagation, such as aurora, aircraft and meteor scatter and Earth-Moon-Earth (EME or "moonbounce" as I still keep calling it).

Recently I have been complaining that my maximum 70cms power output (65 watts) is a bit low for moonbounce. I am astounded, of course, to have worked anybody on 70cms moonbounce with that sort of power. When you add to that the path loss over 200dB, plus the vagiaries of the moon and the almost total lack of activity at 70cms, then it is surprising that I have a set of results to point to at all.

To make EME work you need to reach a budget of power and sensitivity which is pretty marginal at 65 watts. If, by the use of suitable equipment at each end you can reach the budget you can have a QSO. If you fall 1dB below it nothing happens at all. If you are on the margin, strange factors like Faraday Rotation, polarisation incompatibility or the Moon's variable distance can leave you with days of listening to white noise. On any one day you can look at the data and try to work out what budget of power you need, only to find that some obscure issue is preventing anything working. It helps to have some power in reserve.

But when it does work --- it is amazing. I have done very few things in amateur radio that have been more satisfying.

To reach your budget you can run a huge amount of power, have superb receivers or have massive antennas. Ideally all three. Or, in my case, none of these. To manage I use the same strategy as hundreds of other amateurs, I run a modest station and leave it to the big guns at the other end to provide the huge power level, the super-quiet receiver and the massive antenna. They don't mind, they love to work people with simple systems - it proves to them that their money was well spent.

Nevertheless, 65 watts did wear me down a bit. On a couple of occasions I heard stations who could not hear me. Mind you, they were using a kilowatt. The issue here comes down to having a balanced station. Do I transmit a signal comparable to my receive performance? Can I have a reasonable chance of reaching everyone I can hear (or hearing everyone who can hear me)? Of course, some stations will be out of balance too, but relying on them all to have super receivers does not seem too sensible. These people will have designed their station on the assumption that anyone calling them will have a reasonable set-up (whatever that is).

I have had a great time on 65watts. I would not wish to put anyone off from trying that sort of power level and you can work several stations using that approach. But I have bought a new linear. With a lot of inner doubts, I moved on and bought a Tajfun 1000 2m and 70cms amplifier. I had doubts because this has cost me more than any item I have ever bought for amateur radio. 50% more than I have ever spent before. 

It is broadly the same price as the IC-9700 (which I didn't buy, but would otherwise have been my most expensive item). Ouch. I will say more about it once I have put it fully to the test. Let me say that it claims to be able to run about 300W of data on either 2m or 70cms, though on 2m you do not have a low pass filter - luckily I already have a 2m bandpass filter.

The reason why I went for a 2m and 70cms amplifier was that, for some reason, I do not trust my 2m Gemini amplifier. Look back in the blog if you want to know why. I now have a hefty backup.

Tajfun 1000 2m and 70cm linear amplifier

I will have a better photo and more explanation about the Tajfun later. Early signs were that it is very happy running the 200W I expect to run. 300W on JT65 looks to be about the limit, which is exactly what it claims to be able to do.

So my 65W was about 12dB below those superstations running 1kW. 200W would raise me to -7dB, an increase of 5dB. In many cases that could be enough to make me heard - frankly even 1dB can be enough but you just never know with all the variables. If it was borderline I can now even muster another 100W which would add another 1.6dB.

Only time will tell if adding 5dB by running 200W will make much difference. This was a big expense and I am selling off one or two nice things to be able to afford it.

In normal operation on tropo contacts I doubt if 200W is really necessary. Where it may help is when I need to attract the attention of a station who is beaming away from me. It should help with marginal aircraft scatter contacts too.

What I am trying to do here is to bring my power more into balance with my receiver. I do not think that I need to go beyond 200W to do that on any band.

So I went on near moonrise and moonset to see what I could hear. 


I spent a very frustrating week thinking that I had lost my touch. I tinkered with the settings on WSJT-X, I footered with coax, I meddled with the sequencer, but of course nothing changed. A linear is no use if you cannot hear people. I thought I had wasted my money.

Moonbounce is very variable in the sense that you see crowd behaviour. Stations flock to it when conditions are right, when the moon is nearest, and at weekends. Of course, I was trying at the wrong time.

On 4 November I looked on 70cms at moonrise, nothing, then watched the highlights of the previous day's Vuelta cycle race from Spain on the television. Then once that was over I checked again, still nothing. The only small chink was that I could hear Anatoly RK3FG on 2m. I did not call him because I had already worked him, and anyway it was 70cms I was interested in. That was the first EME station I had heard in weeks.

After watching "The Great British Bake Off" (isn't that EXCITING??) I wandered back to the shack intending to shut things down. Anatoly was still coming in on 2m, 2 hours after I first heard him, during which time the moon elevation had gone from 1.5 degrees to 15 degrees. Not likely to be much doing now as 15 degrees is surely far too high for me to reach with an antenna fixed on the horizon. It would need an elevating antenna to do that. Or would it?

Even more suprising was to hear Dima, UA3PTW, on 70cms. I immediately called him and we worked easily. So the linear works. Perhaps my money is not wasted. A new country on 70cms (Russia), and a new square of course, plus a new "unique". Moonbouncers claim uniques they work a new station. I had worked Dima before but on 2m, not on 70cms. Very nice.

It did not quite go according to plan because Andrew, G0JCC, was having a conversation I had not noticed on the N0UK moonbounce bulletin board with Dima. Basically Dima was encouraging Andrew to give him a call, during which I had waded in with my big muddy tackety boots on and called Dima. Ooops. Apologies to Andrew. Andrew runs a comparable station to me but has better antennas - which is perhaps a better way of achieving the same aim of putting a bigger signal on the moon.

So when I popped up on the board to apologise to Andrew I attracted the attention of Anatoly who asked for a contact. I do not know if he knew I was on 70cms, but I thought I should make the best of it and call him on 2m. That contact was also completed straight away with no problems.


144 (red) and 432MHz (blue) contacts at GM4FVM 4 November 2020

So for that day, 4 QSOs, 3 squares and 2 DXCC. Best DX, UA3PTW in KO93 at 2553km. Not bad for 70cms, when you consider that the round distance via the moon was a bit over 800000km, not actually the apparent 2553km. 

All this proves that the new linear works. I could hear both stations on the loudspeaker. I must remember that I can work people at 15 degrees of moon elevation, even with horizonally fixed antennas. It is worth checking in the shack during breaks between TV programmes, even if all you expected to do was to turn everything off. Dima came straight back to me after my first call, he was not expecting it, so I am being heard. G0JCC worked Dima too, so I am on a pretty good level of competence if I can keep up with Andrew (this is the first time I had such a direct comparison with another station).

This much I have learned.

Next two days of listening at moonrise and moonset produced ... nothing.

I am worried about that, which shows that I may have learned nothing.

But, hey, the propagation to the Moon is always better at the weekend. And this is Friday!




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