Category Archives: Essays

Moonbounce – A Science for Everyone

Posted by on May 14, 2024 No comments
4 yagi antennas pointing at the sky

By John Berry GM8JBJ

On 12th May I bounced signals off the Moon.

OK, I’d been bouncing signals off the Moon ever since I put up the antenna system a fortnight before. Just that I didn’t know there was anyone on the other end to decode my 13-character message. Until, that is, on the 12th Mr Potts, callsign NC1I, replied from Massachusetts, USA.

Successful QSO between GM8JBJ and NC1I shown on WSJT-X screen.

Moonbounce (or Earth-Moon-Earth, EME) communication is a very cool thing to do. Just imagine the cred you’d get when next in a bar and you throw that out there when things go quiet.

And importantly, anyone can do it.

Background

The Moon is about 400,000km away (less at perigee, more at apogee). Hence, the flight time for a signal from Earth to Moon and back is about 2.7 seconds. The corresponding round-trip free-space path loss for a signal is a huge 400dBi – no way anyone can bridge that gap. But whilst the Moon is a poor point reflector with a reflection coefficient of 6.5%, it’s also a 3,500km diameter billboard that reflects from all points across its area. The result is a real total path loss of around 250dBi and that’s (just) low enough for radio amateurs to exploit our natural satellite to communicate almost globally.

Basic Kit

Today, almost everyone uses Joe Taylor (K1JT)’s ultra-narrow-band algorithms to code and send a very short message. The message is pared to the bone – it’s even skinnier than FT8. And using his software, WSJT-X, this is sent repeatedly. So, you need a computer, a sound card, and the application. The setup is the same as for FT8, but you’ll configure to use Q65B with a 60-second period.

Video of a transmission via the Moon using Q65B (specks on waterfall and tones in noise) on 432.093MHz at https://hamradio.engineering/moonbounce-eme/

The transmission modulates the audio of a rig and uses upper sideband (USB). The simplest bands for moonbounce are 144MHz and 432MHz. 1296MHz is another option, but above 432MHz you’ll need a high stability reference to hold the frequency steady and 23cm kit gets expensive. Of course, you could do it at any GHz frequency amateur band, but I’m assuming no-one starts at 10GHz or the like. So, you need a VHF/UHF SSB rig.

And then there’s the antenna.

Antenna

There are three options when it comes to antennas: huge, big, and modest. Huge VHF/UHF antennas are for Americans and others with acres of land. They are typically 48 or more stacked and bayed Yagis. Since you must point at the Moon, you can imagine the mechanics and electronics to automatically rotate and elevate 48 Yagis. Radio amateurs with huge antennas are the heavy lifters of the moonbounce world. Without them, novices wouldn’t get off the ground.

The 432MHz array – Frank Potts, NC1I. At https://www.qrz.com/db/NC1I

Then there’s big. ‘Big’ describes folk like me who put up four or more Yagis. That’s easily done. It’s all standard kit. My antennas are on a five-metre scaffold pole. Yaesu and others make az/el rotators, and at VHF/UHF the feeder doesn’t cost a mortgage. Automatic Moon tracking can be done on an Arduino or the like. And many folk in this bracket run 500W.

4-Yagi array at GM8JBJ at https://www.qrz.com/db/GM8JBJ

Finally, there’s modest. There’s a whole movement of folk who use single Yagis and a barefoot rig. And many of those use a lightweight long Yagi mounted on a homebrew tripod. The advantage of this is that you can also avoid spending on a rotator, or on a low noise pre-amp (since you’ll be close to the Yagi feed point). These simple systems are often used /P from some piece of open high ground.

Single antenna portable EME operation at https://www.hamradio.me/operating-tips/eme-on-a-budget-part-2.html

Personal Qualities

There are two. Belief (that you can do it). And patience.

It also helps if you do a lot of research to optimise your kit.

Results

The software does everything for you. But you must do the research to know when is best to try.

Conclusion

The truth is that Moonbounce is not that easy. There are several propagation mechanisms that will thwart your attempts.

First, there’s Faraday rotation. The polarisation of the transmitted wave is sometimes distorted along the path. And there’s nothing you can do about it – save use rotatable antennas, and that’s hugely complex.

Then there’s libration fading. The Moon wobbles. And sometimes the reflected waves received back on Earth create an interference pattern in time with deep fades. There’s nothing you can do but wait till the effect stops.

And of course, it’s stupid to try when the Moon and Sun are both within your antenna’s beam. The Sun adds noise to a sensitive system. But you can just wait till the Moon is on its own.

And then there’s the little issue of few folk listening at any point in time. But you can always arrange a sked on a chat room.

None of these issues diminish its street cred. They even enhance it because they make it complicated.

But here’s the secret. Given the simplicity of the kit, anyone can do it.

Links

More detail from John Berry GM8JBJ https://hamradio.engineering/moonbounce-eme/
John Berry QRZ page https://www.qrz.com/db/GM8JBJ
NC1I QRZ page https://www.qrz.com/db/NC1I
HamRadio.me EME on a Budget https://www.hamradio.me/operating-tips/eme-on-a-budget-part-2.html
WSJT-X Digimode Software https://wsjt.sourceforge.io/

50MHz Sporadic E by John Berry GM8JBJ

Posted by on June 21, 2022 No comments

A first (for me)

The Sporadic E (Es) season has been pretty muted this year. Maybe it will improve. It’s followed a typical trajectory with openings allowing paths from central/southern UK to central and southern Europe. But for us at 55˚N, it’s been more meagre.

As a newcomer to modern data modes, one event did stand out, during one 50MHz Es opening.

Background

Es propagation (via the E region at around 110km up) is caused ultimately by the Sun. Isn’t everything? In our summer, the Earth tilts such that the Tropic of Cancer (about 23˚N) is facing the Sun. The ionosphere at 23˚N gets cooked and electrons are produced in sufficient density. If incident at shallow angles, VHF signals may get returned to the Earth from reflection points at that latitude.

Up at 40˚N, above Greece for example, the cooking is a bit less efficient, and the electron density is lower. It does however densify from time to time because of electron movement. The cause of this sporadic densification is both complex and unclear, but two of several likely variables are atmospheric tides and waves, themselves caused by interaction between the Earth’s and the Sun’s geomagnetic fields.

The resulting Es propagation gives rise to a feeding frenzy on 50MHz data modes, with southern operators making the most of intra-regional single (1E) and two-hop (2E) paths. Great if you’re in the thick of it and want to clock up numbers.

Up at 51˚N (London), operators exploit reflection points about 600km away for a 1,200km path to central and Eastern Europe. And less frequently they exploit more distant reflection points, down to central Africa, the Middle East, and the like for a 2,000 to 5,000km twin hop. There’s often plenty to be had in early afternoon on a good summer’s day.

For us at 55˚N, it a different and more frugal picture. The sun’s cooking is less intense at reflection points over the south of England, and the incidence of usable reflection points is lower. We can participate a bit in the feeding frenzy, but any contact east-west and north is normally DX indeed.

7th June 2022

The day started normally enough with FT8 contacts in southern and central Europe with southern UK stations participating.

The efficiency of the reflections grew to a point at about 2:00pm where reports (of received signal power above channel noise) passed +10dB. For reference, FT8 is 23dB more effective than SSB phone and that’s why it’s so popular – distant contacts are possible on FT8 when they’re just a dream on SSB.

With such a good positive signal power I guessed a voice call might be possible. So, I tuned to around 50.150MHz, the SSB calling frequency. To cut a long story short I was working station after station across Europe on phone with signal reports of 5/9+20dB.

It occurred to me that if this was the intensity of the ionisation around 45-50˚N, it might have improved too at more northerly latitudes. As the afternoon bore on and the Earth turned, reflection points in mid-Atlantic might support northerly trans-Atlantic QSOs. So, I excused myself from the phone melee, fired up WSJT-X and turned the beam WNW towards Canada.

PSKReporter showed that I was lighting up stations on the east coast of the US and Canada. The PSKReporter screen is shown adjacent.

For those not familiar with PSKReporter, stations using WSJT-X can select a reporting mode. They will, on receipt of a data message from another station, telegram a central server via the Internet, reporting the received signal level. The time of receipt will be flagged on the PSKReporter web site. Those flags are shown in the adjacent image with the time since signal receipt.

Now, a couple of things to note. First, just because my signals are flagged as received does not mean the station doing the receiving is manned. Many amateurs leave their kit on to provide this reporting as a service. And, observing the FT8 signals, it was clear that many data messages were only being partially received, let alone decoded. This was sporadic E at its most sporadic.

As the minutes passed, the received signals to the US and Canada improved and reporting moved westward, starting at Nova Scotia, and ending around Ohio.

At 3:17pm I managed to exchange with WA1NLG in Massachusetts at received signals both ways of +4dB. Then the opening faded and received signal levels diminished. A first for me, then it was all over.

Those in the southern lands between 35 and 45˚N were still managing QSOs with the US at parallel latitudes. But for Scots at 55˚N, it was over.

At +4dB, I could dream about a phone contact to the US on 50MHz! Just a few more dB needed. Maybe next time.

8th June 2022

So, what happened the next day?

The day started in a similar way. Legacy ionisation was allowing some Es contacts, and even receipt of signals from the US and Canada. But few operators were completing any contacts. The distant stations’ CQs could be decoded, but then they faded. Incomplete QSOs were rife.

Again, Sporadic E at its most sporadic! In short, it was a dead duck.

So, what was different between the days?

People speculated on chats. Whatever caused the densification of electrons on the 7th had changed or was absent the next day. The electron clouds had dispersed. One thing different was that we were experiencing huge occurrence of lightning storms in northern hemisphere mid latitudes after a period of extensive high pressure over southern Europe.

Now, weather occurs in the troposphere up to around 12km. The E region giving Es is around 110km – above the weather and in the thermosphere. But lightning storms do cause upward turbulence. And they might be just the sort of occurrence to disperse clouds of densified electrons. Maybe that’s an explanation. More reading needed!

Post Note

On a QSO with GM0OQV he reported that on the weekend of the 11th and 12th June it had been the ARRL VHF contest. During that, he had several FT8 contacts with USA and Canada – and several phone contacts too – all on 6-metres.

Just my luck to have been doing something else that weekend!

John GM8JBJ