A 50-ohm input impedance for helical beam antennas

A helical beam antenna with uniform conductor size has a nominal impedance of 140 ohms. This may not be convenient for some applications. The purpose of this note is to call attention to the fact that the impedance can be adjusted to 50 ohms by increasing the conductor size close to the feed point at the ground plane. This lowers the characteristic impedance of the conductor-ground plane combination (acting as a transmission line) and transforms the helix impedance to a lower value over a substantial bandwidth