1) our pre-conceived ideas are wrong so we should try this,or
2) my preconceived ideas are wrong and I am a chump myself for not trying sooner.
This posting is rather about the second type.
On 27 October I worked NC1I on 70cms moonbounce. A fluke?
On 28 October I worked NC1I on 70cms moonbounce for a second time.
Here is what I wrote about my plans for 70cms on this blog on 19 July
This is not an Earth-Moon-Earth capable 70cms set-up. Or at least, it isn't designed to be. It is designed to be vastly better than no setup at all. Adequate, not excessive.
Erm. I was wrong about that. Clearly, it is EME capable. Or perhaps "it isn't designed to be" was my wishful thinking/ get-out clause.
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| 432MHz DX Maps on 27 October 2018 |
OK, lets get this straight. I still think my 70cms set-up should not be EME capable. To review:-
Icom IC-7100, Microset RU432-95 95W linear, 17m of Hyperflex coax, SHF Mini-70 pre-amp, 2m of dodgy RG-213 and a 12 element yagi with a boom length of about 2.5m (part of a dual band antenna).
This is not world class in any way. The IC-7100, admirable as it is, is a mobile rig. I have cut corners - if I had wanted to go for EME I would have bought a 300W linear and a better pre-amp, but the extra cost (£900 !!!) put me off that entirely. I used existing coax which works on 70cm OK but it not earth shattering. I could have reduced the RG-213 to 3/4 or even half its length if I really wanted to - or replaced it entirely. And, lets face it, a 12 element compromise yagi is hardly up to 70cm EME muster.
It is talk like that (the last paragraph) which I criticise others for using as justification for not trying.
Guilty.
Just because it looks difficult is not reason not to try.
It wasn't a plan, this 70cms EME thing. At least, there was no plan until I heard NC1I. For the past couple of weeks I have been trying aimlessly to get back into 2m EME and failing. Conditions have been poor. Then on 6 October I noticed Bernd, DL7APV, mention on a chat room that he was working on 432. I went to the frequency and heard him so I reported that and he suggested that we try a QSO the next day. It didn't work. Back to square one.
Bernd has what I would call a "Super Station". 128 x 11 elements. That is pretty big, and it is able to elevate, which puts my 12 elements fixed horizontal pretty much into the shade. Why was I not able to work him on 7 October? After a bit of pondering it dawned on me that my antenna is best near to my moonset when the moon is close to my horizon, and at that point Bernd has passed moonset and cannot see the moon (at least for now). If I could elevate my antenna I would be in with a chance. Or I need to try with somebody who is still seeing the moon at the moment when my antenna is working at its best.
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| DL7APV's 432MHz antenna ... |
Come 27 October if I had any hope it was of, maybe, working Bernd. The same problem arose - as I do not have an elevating antenna he was weak and had dropped out by the time ground gain had started to make my signals to the moon stronger.
As it happened my usual EME chat room was off the air. I decided to call CQ. I was using Live CQ to keep an eye on what was happening and I picked what seemed to be a clear frequency. By another chance I was 1kHz away from NC1I who was clear as a bell. This is a bit of a dream of mine on EME - stumbling across someone I can immediately hear. It isn't so unexpected; on looking up NC1I on QRZ.com it turns out that he has a Super Station and runs 1.5kW, or at least he can run that much.
So I thought it out. If he has 1.5kW and I have 95W, that is going to give him say 13dB advantage over me. I can hear stations down to, say, -26dB so he might hear me if I heard him at -13dB. At the start he was -16dB. At one stage I went out and raised my antenna height as I feared that the moon would disappear behind the roof ridge, but it turned out that raising it took about 6dB off the signal - it was better lower down. Ground gain and height advantages produce funny effects and my antenna height is not that important when it comes to the moon. Of course, I still thought it might be. So the plan was to keep the antenna as low as I could and stick it out until the moon fell low enough towards the horizon for ground gain to give me a chance of working NC1I.
I started calling NC1I at 08:24. He finally heard me at 09:25. I was calling for an hour and during that time I had all sorts of doubts, in addition to the antenna height issue above. Doubts about antenna direction and whether this contact was possible at all or not. I had many doubts about Doppler effect and whether I had factored it in properly. Then, as the moon fell below 4 degrees (and all the other European stations had lost the moon below their horizon) NC1I's signal started to climb up each segment -13, -12, -08, -04 ... so it was easy to see that ground gain was coming to my aid. Also. my antenna was beginning to function better as the moon started to get closer to the horizontal angle at which the boom is fixed.
When I called at 09:24 I knew immediately as soon as NC1I replied that he had heard me. Even though it took a minute longer to receive the signal and decode it, he must have clicked on my callsign. His frequency moved about 150hz, the amount I had miscalculated the Doppler effect. He must have seen me. And then ... well it just went like a normal QSO. Nothing special about my first trans-Atlantic EME contact.
After the contact he was calling CQ and within fifteen minutes he had disappeared entirely. Moonset had arrived.
Just to prove that it was not a fluke I tried again the next day
I know that logic comes in here - if I hear a station at -15dB and he has a 13dB advantage over me on transmit, then he is likely to hear me at -28dB. Add to that the fact that his receive station might have a better pre-amp and receiver than I have (almost a certainty). Thus he WILL hear me. The EME path is usually steady and I bet he lives somewhere which is electrically quiet (like on a ranch!). Still, I never really expected him to reply both times.
As for the distance (5133km on six figure locators) ??? Well, logic also tells me that if I can reach the moon it hardly matters where on the world my contact is. With an indicative path distance of 752,034km on 27th (a mere 747,302km the next day) it matters little which of us is where on the Earth. Still, working round Europe has seemed a lot easier than reaching the US on 2m. On 70cms my lower power is more or less balanced by more antenna gain so my ERP is much the same, so why am I surprised to do it on 70cms first?
I suppose these 70cm figures give a clue to my pessimism built on years of ignorance
2012 NIL
2013 1 QSO dx 92km (GM)
2014 NIL
2015 1 QSO dx 30km (GM)
2016 1 QSO dx 5km (GM)
2017 1 QSO dx 82km (G !!!)
2018 47 QSOs, 8 DXCC, 2 continents, dx 5133km
Lessons to learn? I am often wrong. I can even believe my own negative script. 95W and 12 elements did do it and I suppose if I had done a careful budget I could have worked that out in advance. Still, my job was to make my tiny signal reach him, which involved sticking at it until the ground gain kicked in, not raising the antenna too high, paying close heed to the moon's movements and not allowing my doubts to overwhelm me.
Here I go again. "I am surprised by those two contacts"
I should have tried this ages ago.
73
Jim
GM4FVM
Congratulations Jim!
ReplyDeleteYes 70cm is a very good EME band. It has a lot less noise both cosmic and man-made than 2m. If you can elevate your antennas even more so of course.
You are right that the total transmitted ERP relative to an isotopic antenna is increased by more antenna gain per unit length at 70cm, but remember that if you calculate things that way you will find that the "path loss" is greater at 70 cm than at 2m, so you are no better off at all.
I partially quote from K1JT, Joe Taylor's chapter on EME in the 2010 ARRL Handbook.
http://physics.princeton.edu/pulsar/K1JT/EME_2010_Hbk.pdf essential reading!
K1JT writes:
The dependence of path loss on lambda squared suggests that EME should be nearly 20 dB more difficult at 1296 MHz than at 144 MHz. This conclusion is misleading, however, because of the assumption of isotropic antennas.
Thus if one assumes a fixed size of antenna, such as a parabolic dish or Yagi array of effective frontal area A, the frequency dependence is reversed: for a given transmitted power, lunar echoes would be 20 dB stronger for every decade increase in frequency, rather than 20 dB weaker. Most practical situations fall somewhere between these two extremes of frequency dependence. For reasons explained in detail below, amateur EME communication is feasible with roughly comparable degrees of difficulty over nearly two decades of frequency, from 144 MHz to 10 GHz.
David
DeleteThanks. I was not aware of that relationship - maybe just as well really. My 70cm antenna is quite a bit shorter than the 2m one even though they share the same boom. I do not have the figures with me right now but from memory the 2m one is about 2.9m and the 7ocms is about 2.4m, so I am on to a loser from the start!
73 Jim GM4FVM
Now another thing to consider is the polarisation. Unless one station has variable polarisation then you can run into important one way EME effects.
ReplyDeleteBy definition your horizontal isn't necessarily his horizontal if he is a quarter of the way around the world from you. This is called Spacial Offset.
Now add in the effect of Faraday rotation of the signal and you find that one station (you) can hear the other when the other can't hear you and vice verse. This is more likely to happen when Spacial Offset is about 45 degrees. In sophisticated EME software you may see a figure called MNR (Maximum Non Reciprocity). This figure represents what the chances are of having one way reception on EME between fixed polarity stations.
As the moon appears at different declinations over the month, the Spacial offset between stations varies so giving some hope that a suitable time may be found, though we have other variables such as earth moon distance, sky noise to contend with.
That is why programs like MoonSked were written.(Blatant plug).
I refer you to the appendix in MoonSked that reprints an article by Paul N1BUG on MNR which was his original thought experiment on the subject. He produced software for DOS that calculated MNR between stations and I adapted it for MoonSked. You will find that Joe Taylor has now even included MNR in WSJT. It is one of the least well understood figures and many, many amateurs have entirely the wrong idea of its purpose and usefulness.
Having at least one station with an adjustable Polarisation antenna allows the optimum receive polarisation to be chosen for any circumstance, however note because of the combined effects of Spacial and Faraday rotation, the optimum receive and transmit polarisation may not coincide. MoonSked will calculate the optimum transmit pol for a pair of stations if you know the best receive polarisation at any time.
Most of these one way effects occur on DX, or rather not on local same continent EME paths, because Spacial offset is usually very similar for stations with a small continent such as Europe.
In the early days when everyone had fixed horizontal antennas, you just had to wait for Faraday rotation to come right for you even for local stations. At 70cms the rate of change of Faraday rotation is much slower than at 2m, so you could find no echoes for hours if you were unlucky. Or you could be "locked out" for hours between continents. Leading to the false conclusion that those "over there" were all deaf or running illegally high powers or both.
On 6m where Faraday is changing at an even higher rate than 2m it is hardly worthwhile having the huge variable polarity antennas, as wait a few minutes and it will be OK. But at 70cm it is a very big deal. One of the reasons why dishes with their easy rotatable pol feeds are a benefit at 432 and were popular.
So you can see each band has its own problems (and solutions) on EME.
It's what makes it so interesting.
Note, I haven't worked anyone on datamodes on 70cm EME yet, though I have worked a huge 70cm Canadian radio astronomy dish (150 foot diameter from memory) on CW EME in the olden days with 90 Watts out from a microwave modules amplifier.
I also heard the activation of the huge Arecibo 1000 foot antenna by Joe Taylor's group on CW SSB and Data modes and was copied by them on CW after the event from their wide band recording of all of the band.
Keep up the good work!
David GM4JJJ
Jim,
ReplyDeleteSometimes it is good to be wrong, isn't it ?
At the moment I do not have any 70cm antennas up, but I do have an old 9 element Tonna (N-connector version) that I would try out at some stage. The advantage of EME or other space based QSOs is that it is *possible*, though not ideal, to have a low antenna position.
I should certainly try the German station out with "QRP EME". Power on 70cm is limited to about 50W out of my IC-910.
On the other hand this huge German station has worked EME with a station running **a dipole** on 70cm, so your experience does not really surprise me too much.
Now you know what is possible, so I am looking forward to hearing more about 70cm EME from you.