The Bifilar helix is constructed using two volutes with an equal number of turns, and their starting points positioned 180° apart. The ends of the volutes are connected with a shorting wire which adds to the structural integrity of the antenna.
Bifilar helix antennas are often constructed using thick metal rods or pipes, making them mechanically robust and able to withstand strong winds and harsh environmental conditions. These constructions make them ideal for shoreline installations. These antennas can be purchased off-the-shelf for many marine communication frequency bands and are well suited for rapid installation.
The 180° phase shift between the two volutes allows for circular polarization with an end-fire beam in the direction of the helix axis. The radiation pattern is stable with a well-defined end-fire lobe and low side and back lobes across the operation band. A typical radiation pattern is shown in the following image. ...Typical circularly polarized 3D gain pattern at the center frequency.
The input impedance exhibits oscillatory behavior at the high-frequency end of the band as the input resistance increases from 200 Ω to 300 Ω as illustrated in the second figure below. The -10 dB S11 bandwidth in a 200 Ω system is roughly 1.85:1.Typical circularly polarized radiation pattern cut. Typical impedance vs. normalized frequency.