Poldhu () is a small area in south Cornwall, England, UK, situated on the Lizard Peninsula; it comprises Poldhu Point and Poldhu Cove. Poldhu means "black pool" in Cornish. Poldhu lies on the coast of Mount's Bay and is in the northern part of the parish of Mullion; the churchtown is to the south-east. On the north side of Poldhu Cove is the parish of Gunwalloe and the village of Porthleven is a further to the north.

Poldhu Point became the site of one of the main technological advances of the early twentieth century when, on 12 December 1901, a wireless signal was sent by Thomas Barron in Poldhu to St. John's, Newfoundland, and received by Guglielmo Marconi. The technology was a precursor to radio, television, satellites and the internet, with the earth station at Goonhilly Downs a nearby example.

The beach at Poldhu was heavily mined during World War II to prevent any prospect of a German force landing there. As a result, on 24 April 1943, Royal Air Force Volunteer Reserve members Mair Myfannwy Richards and Reginald Thomas Smith both died instantly when Mair trod on an unmarked mine.

In January 2016, Poldhu Cove was inundated with thousands of pink plastic bottles, brought onto the beach with successive tides. The National Trust, which organised the clean-up, thought they had likely come from a container ship, and had been washed overboard in recent storms.

Marconi's Poldhu Wireless Station

thumb|left|The station, with four masts.

The site is famous as the location of Poldhu Wireless Station, Guglielmo Marconi's transmitter for the first transatlantic radio message on 12 December 1901. Marconi received the transmission on Signal Hill, St. John's, Newfoundland. The station was built partly on cliff top pastures that had been enclosed in 1871 and partly on medieval fields belonging to a nearby settlement, Angrouse. During the work two Bronze Age barrows were flattened and a bronze dagger and urn were recovered. The original twenty mast circular aerial was destroyed in a storm on 17 September 1901. Marconi hastily built a temporary aerial of 50 wires suspended in a fan shape from a cable between two 200 foot (61 m) masts. Fleming estimated the transmitter's radiated power was around 10–12 kW. The frequency used is not known precisely, as Marconi did not measure wavelength or frequency, but it was between 166 and 984 kHz, probably around 500 kHz.

In the first episode of the 2018 female-led adaptation Miss Sherlock, "Poldhu" is the name of a brand of wireless medical telemetry devices in the form of a capsule that is swallowed by the user, which the murderer exploits as triggers for liquid bombs that destroy the abdominal cavities of her victims.

References

<!-- Openstreetmap coordinates (50.033757, -5.260624) linked to description:

hamlet Poldhu Cove (which is about south of town Helston in Cornwall and about 29km south-west of city Truro) found about 2 km north-west of middle of village Mullion (which is about 9 km south of town Helston, ditto and about south-west of city Truro ) -->