Egelsbach Transmitter Facility, Radio transmission facility in Egelsbach, Germany.

The Egelsbach Transmitter Facility is a US Army communication and SIGINT station on a roughly 30-hectare clearing in woodland near Egelsbach, south of Frankfurt. The fenced, barbed-wire site holds 20 horizontally polarised logarithmic-periodic shortwave antennas from TCI International, four radomes, and microwave dishes on a mast, two aimed at the former Rhein-Main Air Base.

The US Army has used the forest clearing near Egelsbach since the end of the Second World War. In the 2000s, while other US facilities in Germany were closed or reduced, this site was expanded, and a further radome about five metres across was added in 2011. Bunkers on the adjacent restricted area were later demolished, and their contents remain unknown.

The site's nickname in local reporting stems from a 2003 ZDF documentary that suspected programmes of the numbers station Cynthia were broadcast from here. The US military never confirmed this, yet the shortwave signal is strong enough to be received in the surrounding area with a receiver and no antenna. In late 2013 mayor Jürgen Sieling wrote to the US Army Garrison Wiesbaden asking who uses the facility and for what purpose; Colonel David H. Carstens answered in two sentences that it belongs to the US Army inventory and is run by the Department of Defense.

The site is a fenced restricted area guarded by US military police and local security services, with barbed wire along the perimeter; there is no visitor access. The antennas and radomes can only be seen from public paths outside the fence in the forest near Egelsbach.

A 2003 ZDF documentary suggested the site broadcast the numbers station Cynthia, which the US military never confirmed. The programmes can reportedly be received near the grounds with a receiver lacking an antenna, pointing to a strong shortwave signal.