A compressible and deployable helical antenna (2) includes a supporting structure (7) having a helical shape. The supporting structure (7) includes a core (18), especially of foamed material, and a carrier layer (19) of composite fiber material, the carrier layer (19) being located on the core (18). An electrically conducting layer (8) is located on the carrier layer (19). An antenna unit (1) includes the compressible and deployable helical antenna (2) and a reflector plate (3). The reflector plate (3) is connected to the helical antenna (2).