Null Forming in Circularly Polarized Antenna Patterns Using Reactive Loading of Multi-Arm Archimedean Spiral Antenna

A multi-arm spiral antenna, of N arms, can have N modes of operation. The integer n represents the operating mode which is determined by the signal phase increment between adjacent arms as fed from the antenna terminals. For the antenna to radiate effectively, the active region must be enclosed by a circumference of mode number (n) times the operating wavelength. Each mode generates a different radiation pattern. Modes, with n between 2 and N-2, are capable of forming radiation pattern nulls. This paper shows a way to excite noninteger modes along with integer modes of radiation, which combined, can create a null which can be steered away from the antenna vertical bore sight. Furthermore, variable reactive loading of spiral arm ends offers additional steering control and positioning of nulls in a 2D angular hemispherical space.