Dear LF Group,
I Have just looked through the many e-mails that have arrived since the fun
& games at the beginning of the week. Derek G3GRO has taken some digital
pictures of the site and has more details which will no doubt be available
soon, but here is some brief info:
The QTH was the Porthcurno Museum of Submarine Telegraphy, which was/is the
terminal for many submarine cables going around the world. It is in a WW2
bunker cut into the granite cliffs - there is an amateur radio club
(GB2PK), with a shack just by the entrance to the tunnels, which was kindly
made available to us (see
http://www.porthcurno.org.uk/photoAlbum/tunnatwar.html - the "north tunnel
entrance" photo - the shack is in the flat-roofed building the guard is
standing in front of). The first problem was getting an antenna up - the
club had some wire antennas, but these were rather in the shadow of the
cliff, so we put up about 80m of sloping wire, running from a flagpole
outside the entrance to a short lattice tower on top of the cliffs about
40m above, part of the bunker complex. This wasn't easy because of the
brambles and gorse bushes covering everywhere! The antenna was about 10m
high at the ends, but only about 5 or 6m where it passed over the cliff
top. Earthing was the next problem - since everything was built into the
granite, ground rods of the diamond-tipped variety would have been needed.
Since the site was open to the public, covering the area in radial wires
would not have been popular either. So we just used the normal mains earth,
which fortunately worked quite well. However, the mains noise level was
hopelessly high.
In an attempt to improve reception, we ran out 100m of coax to the cliff
top, and set up a loop antenna. The noise level was still high, but reduced
somewhat after museum closing time, and we were able to have 136kHz QSOs
with G3KEV and GM0MRF on Monday evening. There were some RF feedback
problems, not surprising with the lack of RF ground - so we had to run
reduced power, giving about 2.5A antenna current. On Tuesday morning we set
up for 73k, and managed to work G3AQC, although the noise was so bad that
Laurie's very strong signal was only an M copy in DFCW. The TX antenna
seemed to work quite well, with a loss resistance down around 30 ohms, and
4.5A antenna current. However, an improvement on receive would be needed to
work GM0MRF. Running out a total of 240m of coax for the receive antenna
had little effect on noise, even though the loop was now quite a long walk,
and considerable distance from the museum or overhead power lines, or
anything else apart from brambles. Various experiments showed that the
feeder was not to blame, but while messing around on the cliff top with a
battery operated RX, I found that if the antenna was lifted up far enough
so that none of the wet grass or other vegetation could come into contact
with the loop element, the mains noise was significantly reduced. After
suitably propping up the antenna against the wind, the noise level in the
shack was dramatically improved, and we were immediately able to work
GM0MRF and MI0AYZ. So Tuesday's total on 73kHz was G3AQC, G3LDO, G3YXM,
GM0MRF and MI0AYZ. Unfortunately, we did not see Mike or John's sigs,
probably because it was fairly late in the evening before we had decent
reception available.
While dismantling the station on Wednesday morning, noise levels on both 73
and 136 were really quite good, so it looked as if we had finally cracked
the problem - we shall know what to do next time! Thanks to all those
involved for making the expedition a success, Including the Porthcurno
Club, who have at least gained a new antenna for their HF station. I only
had time for a quick look round the museum, but it was very interesting,
and definitely worth a visit if you are in the area. The amateur radio club
shack forms part of the exhibition, and contains quite a range of WW2 era
radio gear in working order.
Cheers, Jim Moritz
73 de M0BMU
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