Y-stations were British Signals Intelligence collection sites initially established during World War I and later used during World War II. These sites were operated by a range of agencies including the Army, Navy and RAF plus the Foreign Office (MI6 and MI5), General Post Office and Marconi Company receiving stations ashore and afloat.
The "Y" stations tended to be of two types, Interception and Direction Finding. Sometimes both functions were operated at the same site with the direction finding (D/F) hut being a few hundred metres away from the main interception building because of the need to minimise interference. These sites collected traffic which was then either analysed locally or if encrypted passed for processing initially to Admiralty Room 40 in London and during World War II to the Government Code and Cypher School established at Bletchley Park in Buckinghamshire.
In World War II a large house called "Arkley View" on the outskirts of Barnet acted as a data collection centre at which traffic was collated and passed to Bletchley Park, it also acted as a "Y" station. Many amateur ("ham") radio operators supported the work of the "Y" stations, being enrolled as "Voluntary Interceptors". Much of the traffic intercepted by the "Y" stations was recorded by hand and sent to Bletchley on paper by motorcycle couriers or, later, by teleprinter over post office land lines.
Direction finding Y stations
In addition to wireless interception, specially constructed Y stations also undertook direction finding on enemy wireless transmissions. This became particularly important in the Battle of the Atlantic (1939–1945) where locating U-boats became a critical issue. Admiral Dönitz told his commanders that they could not be located if they limited their wireless transmissions to under 30 seconds, but skilled D/F operators were able to locate the origin of their signals in as little as 6 seconds.
The design of land based D/F stations preferred by the allies in World War II was the U-Adcock system, which consisted of a small, central operators' hut surrounded by four 10 m high vertical aerial poles usually placed at the four compass points. Aerial feeders ran underground and came up in the centre of the hut and were connected to a direction finding goniometer and a wireless receiver that allowed the bearing of the signal source to be measured. In the UK some operators were located in an underground metal tank. These stations were usually located in remote places, often in the middle of farmers' fields. Traces of World War II D/F stations can be seen as circles in the fields surrounding the village of Goonhavern in Cornwall.
Y-station sites in the United Kingdom
- Beachy Head, Sussex
- Beaumanor Hall, near Loughborough, Leicestershire
- Beeston Hill, Beeston Regis, Norfolk
- Bishop's Waltham, Hampshire 
- RAF Canterbury, Kent
- RAF Cheadle, Cheadle, Staffordshire
- RAF Chicksands, Bedfordshire
- RAF Clophill, Bedfordshire
- Cromer, Norfolk
- G.P.O. Transatlantic Radiophone Station Kemback, near Cupar Fife
- Foreign Office Denmark Hill, Camberwell (Metropolitan Police)
- Met Office Dunstable, Bedfordshire
- Felixstowe, Suffolk
- Gilnahirk, Belfast
- Gorleston, Norfolk
- Harpenden, Hertfordshire (Army, No. 1 Special Wireless Group)
- HMS Flowerdown, Winchester, Hampshire For more information see RAF Flowerdown
- HMS Forest Moor, Harrogate, Yorkshire
- Kedleston Hall, Derbyshire
- RAF Kingsdown, Hollywood Manor, West Kingsdown, Kent
- RAF Monks Risborough, Monks Risborough,Buckinghamshire
- Foreign Office Knockholt, Kent
- Army Markyate, Hertfordshire
- North Walsham, Norfolk
- Foreign Office Sandridge, Hertfordshire
- Saxmundham, Suffolk
- Army Shenley Brook End Milton Keynes
- South Walsham, Norfolk
- Southwold, Suffolk
- Stockland Bristol Nr Bridgwater, Somerset
- Stockton-on-Tees, Cleveland
- RAF Waddington, Lincolnshire
- High frequency direction finding (Huff-Duff)
- Y-stations in World War I
- Pidgeon, Geoffrey (2003). "15. Box 25 - The RSS and Hanslope". The Secret Wireless War: The Story of MI6 Communications 1939–1945. UPSO Ltd. pp. 103–118. ISBN 1-84375-252-2. OCLC 56715513.
- R.B. Sturtevant, AD7IL (December 2013). "The Secret Listeners of 'Box 25, Barnet'". Popular Communications (CQ Communications, Inc) 32 (4): 22–26. ISSN 0733-3315.
- Nicholls, J., (2000) England Needs You: The Story of Beaumanor Y Station World War II Cheam, published by Joan Nicholls
- The operators huts can still be seen in the centre of the circles.
- "The National Archives - Piece details HW 50/82". Retrieved 2008-05-10.
- "HMS Forest Moor is Decommissioned". Navy News. 17 November 2003. Archived from the original on 11 June 2011. Retrieved 12 May 2014.