Helical Antennas
A helical antenna consists of a conducting wire wound into the shape of a helix, usually mounted above a ground plane and fed at one end. Its behaviour depends strongly on the relationship between the size of the helix and the wavelength of the signal. Two main operating modes exist: the normal mode, in which the helix is small compared with the wavelength and radiates broadside like a short dipole, and the axial mode, which is the form most often meant when helical antennas are discussed.
In the axial mode the circumference of each turn is roughly one wavelength, and the antenna radiates a focused beam along its axis, in the direction it points. This gives useful gain that increases with the number of turns and the overall length of the helix. A distinctive feature of the axial-mode helix is that it produces circular polarisation: the radiated field rotates as the wave travels, with the sense of rotation set by whether the helix is wound clockwise or anticlockwise.
Circular polarisation makes helical antennas valuable for links where the orientation of the antennas is not fixed or where signals pass through changing media. Communication with satellites is a common example, because a satellite's orientation relative to the ground station varies, and a circularly polarised antenna maintains a connection regardless of rotation. Helical antennas are also used in some space, telemetry and radio applications. To work well, the helix's diameter, the spacing between turns and the number of turns must be chosen for the intended frequency and the desired beam.
Frequently asked questions
- What is the axial mode of a helical antenna?
- It is the mode in which each turn's circumference is about one wavelength, causing the antenna to radiate a directional beam along its axis with circular polarisation.
- Why are helical antennas useful for satellite links?
- They radiate circularly polarised waves, so the link is maintained even as the orientation between the satellite and the ground antenna changes.
- How is the gain of an axial-mode helix increased?
- Adding more turns and lengthening the helix narrows the beam and raises the gain along its axis.
