A backfire helical feed

A novel backfire helical feed for a paraboloidal reflector is described. The antenna requirements were a 20 percent bandwidth at
L
- band, right-hand circular polarization, a maximum voltage standing-wave ratio (VSWR) of 2.0, and a peak power capability of 200 kW unpressurized. The feed consists basically of a 3.5-turn helix wound around a
1\frac{5}{8}
-in rigid coaxial transmission line. The
50 \Omega
transmission line is coupled to the helix through a two-stage transformer within the coax followed by a half-wave coax to two-wire (unequal diameters) balun.