A helix building is a small structure at the base of a longwave or mediumwave radio transmitter. Within, there are capacitors and coils for coupling the signal from the feed cable to the antenna. Active components such as amplifiers are usually not housed in a helix building. The interior of the helix building is typically covered with copper or aluminium, in order to control stray radiation.
For safety, helix buildings must be secured, due to the dangerously high voltages which may be present inside.
The helix building, which may be a simple switchbox for small transmitters, is sometimes also the base of a mast radiator.
References
Helix building Wikipedia(Text) CC BY-SA