This work focuses on the planar helix and its variations, particularly for application in travelling-wave tubes (TWTs). In addition, the book also briefly describes the application of the planar helix in circularly polarized antennas. As mentioned earlier, among the different types of vacuum electron devices (VEDs), travelling wave tubes (TWTs) stand out due to their large bandwidth and high efficiency. Also, the circularly polarized antennas are very useful for satellite communication and for reducing multipath effects in communication systems.