Helical antenna having adjustable beam angle

A multifilar helical antenna comprising:

• a plurality of 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;
• a single dielectric helix concentric with the common axis, lying within and supporting the conductive helices at a nominal diameter, the dielectric helix having opposite ends, a plurality of turns having said common direction of turn, and a second pitch substantially greater than said common pitch;
• means for fixing opposite ends of the conductive helices to said dielectric helix;
• a casing containing the helices and connected to one end of the dielectric helix;
• a tuning device connected to the other end of the dielectric helix and rotatable relative to said casing;
• whereby rotation of the tuning device twists the dielectric helix to change the common pitch of the conductive helices without substantial variation from said nominal diameter.