News - Helical Antenna: The Ideal Choice for Broadband Circular Polarization

A Spiral Antenna is a typical example of a wire antenna, characterized by its spiral-shaped structure. It is a broadband antenna suitable for VHF and UHF bands.

The helical antenna operates over a frequency range of approximately 30 MHz to 3 GHz, primarily covering the VHF and UHF bands.

The Construction and Working Principles of Spiral Antenna

A Spiral Antenna is formed by winding a conductor into a spiral shape and connecting it to a ground plane via a feedline. With a simple structure, it naturally generates circularly polarized waves and is widely used in extraterrestrial communications such as satellite relay systems.

The figure above shows a Spiral Antenna system used for satellite communications. Such antennas typically require ample outdoor installation space.

The Spiral Antenna consists of a coil of thick copper wire or tubing shaped into a helix, working in conjunction with a flat metal ground plane. One end of the helix is connected to the center conductor of the coaxial cable, while the outer conductor is attached to the ground plane.

The image above illustrates the structure of a Spiral Antenna, with a detailed view of its components.

The radiation characteristics of a Spiral Antenna are primarily determined by the helix diameter, the spacing between turns (pitch), and the pitch angle.

The pitch angle is defined as the angle between the tangent to the helix and the plane normal to the helix axis, and is given by:

Where:

•D is the diameter of the helix

•S is the pitch (center-to-center spacing between adjacent turns)

•α is the pitch angle

Operation Mode

Spiral Antennas operate in two primary modes:

•Normal Mode (also known as the perpendicular radiation mode)

•Axial Mode (also known as the end-fire mode or beam radiation mode)

Each mode is described in detail below.

In the normal radiation mode, the radiated field is perpendicular to the helix axis, and the radiated wave is circularly polarized. This mode is achieved when the helix dimensions are small relative to the wavelength. In this case, the radiation characteristics of the helical antenna can be considered as a combination of a short dipole antenna and a loop antenna.

The figure above illustrates the radiation pattern of a Spiral Antenna operating in normal mode.

This mode is determined by the helix diameter D and the spacing between turns S. The disadvantages of this operating mode include low radiation efficiency and narrow bandwidth; therefore, it is rarely used in practical applications.

Axial Mode

In the axial radiation mode, the radiated field exhibits end-fire characteristics along the helix axis, and the radiated wave is circularly or nearly circularly polarized. This mode is achieved when the helix circumference is increased to the order of one wavelength (λ) and the spacing between turns is approximately λ/4. Under these conditions, the radiation pattern is broad along the axis with directional characteristics, and side lobes appear at angles offset from the axis.

The figure above illustrates the radiation pattern of a Spiral Antenna operating in axial mode.

When the antenna is designed for right-hand circularly polarized (RHCP) waves, it will not receive left-hand circularly polarized (LHCP) waves, and vice versa. This operating mode is easy to implement and is more commonly used in practical applications.

The main advantages of the Spiral Antenna are as follows:

•Simple structure and easy to design

•High directivity

•Wide bandwidth

•Capable of circular polarization

•Suitable for HF and VHF bands

The main applications of the Spiral Antenna are as follows:

•Single helical antennas or their arrays are used for VHF signal transmission and reception

•Widely used in satellite and deep-space probe communication systems

•Applied in telemetry links between ballistic missiles, satellites, and earth stations

•Used to establish communication between the Moon and Earth

•Also plays an important role in radio astronomy applications