What Are Helical Antennas?

Helical antennas are radiating structures formed by winding a conductor into a helix, typically mounted over a ground plane and fed at one end. They are among the few antenna designs that naturally produce circularly polarized radiation, a property that makes them particularly valuable in satellite and space communication links where the orientation of the transmitter and receiver may not be fixed. The helix geometry couples electromagnetic wave propagation along the conductor to radiation into free space through a mechanism that depends sensitively on the ratio of the helix circumference to the operating wavelength.

The concept was developed systematically by John D. Kraus at Ohio State University beginning in the late 1940s. Kraus identified two distinct radiation regimes determined by the size of the helix relative to the wavelength: the normal mode and the axial mode. These two regimes have different radiation patterns, polarization characteristics, and practical applications, and they are treated as separate design problems in antenna engineering. The helix interacts with transmission line structures and waveguide components in feed systems, and its impedance characteristics are closely related to those of other traveling-wave structures.

Normal Mode Operation

In the normal mode, the overall dimensions of the helix are small compared to the operating wavelength. The radiation pattern is broadside, directed perpendicular to the helix axis, and the radiated field is circularly or elliptically polarized depending on the pitch angle. Normal mode helical antennas behave similarly to short dipoles or small loops in terms of radiation resistance and efficiency, which tends to be low unless special matching networks are used. This mode finds use in compact embedded antennas for handheld devices and body-worn sensors, where physical size is the primary constraint and gain requirements are modest. Adjusting the helix diameter and spacing between turns allows the designer to trade off radiation resistance against polarization purity.

Axial Mode Operation

The axial mode, also called the end-fire mode, is achieved when the helix circumference is approximately equal to one wavelength and the turn spacing is near one quarter wavelength. Under these conditions, a wave travels along the helix and radiates predominantly in the direction of the helix axis, producing a directional beam with circular polarization. The IEEE Xplore paper on helical antennas for circular polarization by Kraus established the foundational design equations relating helix geometry to gain, beamwidth, and axial ratio. Gains of 10 to 15 dBi are achievable with helices of ten to twenty turns, and multiple helices can be arrayed to increase gain further. The axial ratio, which measures the purity of circular polarization, is close to unity across a bandwidth of roughly 1.7:1 in frequency, giving axial mode helices a notably wide usable bandwidth compared to resonant antennas. This combination of gain, circular polarization, and bandwidth is why axial mode helices are standard on ground stations communicating with satellites and spacecraft.

Design Parameters and Performance

Key parameters governing helical antenna performance include the pitch angle (typically 12 to 14 degrees for axial mode), the number of turns, and the diameter of the ground plane. The antenna-theory.com overview of helix antennas provides design formulas and measured performance data across these variables. Input impedance in axial mode is approximately 140 ohms, which is often matched to 50-ohm coaxial transmission lines through a tapered section or a quarter-wave transformer. VHF and UHF circuits operating in the 100 MHz to 3 GHz range represent the most common deployment frequencies, though the design scales well into the microwave bands. The IntechOpen chapter on helical antennas in satellite radio channels covers propagation effects and link budget considerations specific to satellite applications.

Applications

Helical antennas have applications in a wide range of fields, including:

• Satellite ground stations requiring circularly polarized uplink and downlink beams
• Spacecraft and launch vehicle telemetry antennas
• GPS receivers and navigation systems
• Amateur radio moonbounce (Earth-Moon-Earth) communications
• Weather satellite data reception