MW1CFN/u

About 20 years ago, I was quite keen on exploring the extensive underground slate mines of North Wales.  It's an interest that I've taken up again, this time with my son and daughter, in the past few months.

Before I go any further, if you go down any mine, it's a firm unwritten rule that you never - ever - damage or remove anything from there.  It's our collective heritage for all to see, enjoy and respect.  The best - and safest - way to explore our mines, is by booking a trip with Go Below.

As I refamiliarised myself with the sheer maze that is Cwmorthin slate mine, near Blaenau Ffestiniog, I came to thinking: I could try radio down here!

The whole of the inside of this mountain, above and below lake level, is quarried-out.

 

Quarry plan. Much of the blue bits, above lake level, collapsed in the 1880s.  It's very difficult to use such a map, not least at it represents multiple levels, and isn't very accurate.  Instead, you have to memorise it from walking it all!

Well, this morning, I took the chance to go underground on my own - my son and daughter were too busy - but with my trusty FT818, and a 80m dipole.

The selection of 80m was due to the expectation that longer wavelengths get through rock better than shorter ones.  But a 40m-long wire is about the limit of what is practical to deploy in a mine with wet, cold hands, and where the occasional paying groups travel past; I had the pleasure of explaining HF radio to two such groups, who were a bit bemused to find me down there!

About to go in to Cwmorthin slate quarry.  It was very cold and windy outside, much nicer inside!

 

I'd heard some people in Yorkshire had received amateur HF WSPR signals underground, but I can't recall reading any details of which band they used.  In any case, their hills are mainly limestone, which is a very different proposition to the multiple granite-slate sandwiched layers of North Wales.

Aerial view of Cwmorthin ('Cwm' means valley in Welsh). The radio position was directly under the ridge on the top left, where the snow runs out (you can see the slate spoil tips below that).

The only previous experience I'd had with slate was of the high attenuation my old house's roof caused at 14MHz - I could never get a decent signal to or from an attic dipole.  So my hopes of receiving through ~180m of rock horizontally, and ~225m vertically (estimated from maps), were not very high.  

Vertical section, showing how the miners chased the slate veins.

So, using some old iron pegs to support the insulators, I strung the dipole just above the floor within the connecting tunnels between chambers, which are often a hundred or more metres long.  One leg was within one tunnel, the other turned 90 degrees into another tunnel.  Somehow, I miraculously found a new piece of wood to support the other end of the dipole wire!

An amazing fungus spreads its way over an old timber pit prop (~50cm long)

 

I was restricted to RX-only, as I didn't have the space to carry a matching box, and the antenna was never going to be a naturally good match in the way it was mounted! 

The big problem with many digimodes is their need for fairly accurate time-keeping on the computer.  This is an essentially insurmountable problem underground!  I overcame it to a degree by switching on the Pi on the surface, where there was a mobile phone signal, and then leaving it on as I went underground.  Any reboot underground quickly saw the Pi lose good timing.  It would therefore be much easier to use PSK-31, OLIVIA or such modes, which have good sensitivity (very good, in the case of OLIVIA's slower rates), but without the need for any time synchronisation.

A piece of new wood props up one end of the dipole in a side tunnel.

Tuning to the 80m FT8 frequency - no hint of a signal, but quite a bit of RFI from the Raspberry Pi, not helped by being very close to the antenna.

Well, I wasn't expecting much, so I switched up to 60m.  Again, nothing.  But 60m is often a bit dead in the daytime, so I went up yet again, to 40m.

Bingo!

The 40m waterfall had loads of FT8 signals on it.  I couldn't quite believe it, but here's the proof (you may have to click on the 'view in YouTube' link if it doesn't play in Blogger):

 

At 30m, I could only detect a couple of weak FT8 signals, although they were at reasonable distances.  By 20m, there was no more to be detected, not even using WSPR.

I also had a listen to 40m SSB.  Yep, there were signals there, too!  Both of the ones I heard were from France, of which this was the best - no call sign given, sadly.  Quite appropriate, given that the lower levels of this mine were flooded for 30 years, and eventually drained by drilling a hole from below - an extremely skilled and dangerous task - by the Isler company, founded by Camillo Isler, born in France.  In characteristic Welsh fashion, the level was immediately named 'lefal Ffrench' (the French level)!

 

My listening location was not only deep inside the mountain, but also beyond five blind corners with long tunnels between each.  So there was no realistic prospect of leakage from the outside world via the adit opening.  I'll test that idea next time, by going down a level, maybe more. 

I'm intrigued that slate is so transparent to 40m signals at this depth. I'll have to do some research and think about what is going on.  

UPDATE: I visited again briefly 15/12/2021 to see if I would lose the signal deeper into the mountain by about 60m horizontally.  Other than one fairly weak 40m signal that I couldn't decode due to time sync issues (I await a real-time clock unit for the Pi to resolve this), everything had vanished.  It's not yet clear whether it's leakage from outside via the tunnels (unlikely), or a changing depth, or nature of rock, above me that allows signals to get in so deep into the mine. The addition of one more granite bed by beeing a level deeper, for example, could make all the difference between reasonable and no reception.  I'm also going to run tests in a different, nearby mine.  It will all take time, though.

As always, we must remember the men who lived exceptionally hard lives within these mines, and the many who died, for very little reward in the end, in order to roof the world.  Cwmorthin, indeed, was and remains known locally as 'y lladd-dy' - the slaughterhouse - due to the very high number of injuries and deaths here, which prompted some of the earliest UK workplace safety investigations.

You can find some lovely images of Cwmorthin here.