The remains of the Marconi Wireless Telegraphy Company’s long wave transmitting station, universally then known by the company as the ‘Carnarvon’ station, are situated on the west-northwest slopes of Cefn Du, west of Llanberis. The transmitter site and antenna field area is 1km2 when the perimeter of guarding blockhouses is included.

Salvaged on closure in spring 1939, most of the substantial concrete mast bases, guy wire anchors and elevated counterpoise support mast bases, trackway beds and inclines remain in place and in good condition, though often heavily overgrown. Remains of ten WW1 guarding blockhouses also remain, all but two in an entirely ruinous state.

The site consisted initially, from 1914, of one inverted-L antenna of 32 radiating wires (callsign ‘MUU’), aligned on a great circle path to its receiving station at Belmar, New Jersey, USA. The antenna was then claimed repeatedly by Marconi to be directional, but by 1920, his engineers knew this was incorrect.

On September 22, 1918, in its original form and using a timed disc transmitter of 200kW, Carnarvon sent the first direct, non-relay messages from Wales to Australia, though there was then no transmitter to respond in Australia.

The antenna was extended 900’ to the east with two large pylons ca. 1921. It was then extended as one, much larger antenna, with the addition of a new array in 1923, but at an angle of 130 degrees to the 1914 array, north-east towards Llanberis. The extension, from 1925 onwards, operated as a separate antenna, this now under the callsign 'MUU' and from circa 1930, to bring it into line with the British standard 'G' prefix, 'GMU', whilst the earlier, lower array operated under the callsign ‘GLC’. A third antenna – ‘GLJ’ - was installed in 1925. From circa 1919 onwards, all antennas were tied to ground and were no longer of inverted-L types; they were now half-loop in form.

The station was active between mid-late 1914 and November 1938 and was the most important long wave transmitter station in Britain, handling high traffic volumes of imperial, financial and press communications. Early static photographic image transmissions were also sent from the station to America. All new transmitter designs were trialled at Carnarvon, an important station where Marconi engineers gained experience in their operation before deployment elsewhere. The receiving station (NPRN 421024) was at Tywyn (then known by the company as ‘Towyn’ station).

Source: Rowlands, J., Marconi's Carnarvon Station 1912-1939: a journey into early commercial wireless in north Wales Second Edition, 2023.

RCAHMW, 2023.