Spiral antennas consist of a thin metal foil spiral pattern etched on a substrate, usually fed from the center, and located over a cavity. The etching contains a symmetrical pattern of at least two arms, but we build spiral antennas with more arms to radiate in multiple modes or to suppress unwanted modes. The two-arm version can be fed using a simple balanced line requiring a balun. With more arms we need a feed network called a beamformer, which contains an output port for each spiral arm and a separate

input for each spiral mode. This network ideally divides the power into equal-amplitude outputs with a linear phase progression between them. The phase progression of each spiral mode cycles one or more times through all phases, and the complex summation of any set of mode voltages equals zero. The number of complete rotations through all phases equals the mode number; mode 1 has one cycle, 2π radians; mode 2 has 2 cycles, 4π radians; and so on.

Spiral antennas Antennas

• Introduction to Spiral Antenna

• Archimedean Spiral

• Equiangular Spiral

• Spiral Construction

• Balun Feed in Spiral Antenna

• Infinite Balun in Spiral Antenna

• Modulated Arm Width Spiral Antenna

• Conical Log Spiral Antenna

• Fedding Conical Log Sprirals

Helical antennas are formed by winding a helix or helices from a single or multiple conductor. This type of antenna is a natural choice for producing radiation that is circularly polarised. The antenna operates by setting up travelling waves on the conductors forming the helix. This class of antenna exhibits other useful operational features, such as nearly real input impedance and wide bandwidth.

Helix Antenna

• Introduction

• Helix Parameters

• Feed of a Helical Antenna

• Long Helical Antenna

• Short Helical Antenna