Abstract: A quadrifilar helical antenna including four conductive helices having a common central axis, a common direction of turn about said axis, a common pitch, and a common length between opposite ends, the helices being uniformly spaced from each other by 90.degree., and a single dielectric helix concentric with the common axis, lying within and supporting the conductive helices at a nominal diameter. The dielectric helix has opposite ends, a plurality of turns having said common direction of turn, and a second pitch substantially greater than said common pitch. A casing contains the helices and is rotatably fixed to one end of the dielectric helix. A tuning device is fixed to the other end of the dielectric helix and rotatable relative to said casing, so that rotation of the tuning device twists the dielectric helix to alter the common pitch of the conductive helices without substantial variation from said nominal diameter.

Abstract: A lower die assembly used in a bending machine to form pre-punched three-dimensional structures has a plurality of plungers located within the V-shaped bending slot of the lower die. As the upper die presses against the pre-punched sheet, the sheet is pinched between an upper end surface of the plungers and the rounded end of the upper die so as to maintain the prescribed position of the sheet during bending. The lower die may include a pin stop which cooperates with positioning holes pre-punched in the sheet in order to establish the desired position of the sheet prior to bending.