Helical Antenna
Understanding Helical Antennas
The axial-mode helical antenna is one of the simplest ways to generate circular polarization with moderate gain. A wire wound in a helix over a ground plane naturally produces CP radiation along the helix axis. More turns increase gain and narrow the beamwidth.
Axial Mode Design
- Circumference: C = lambda (one wavelength at the operating frequency).
- Number of turns: More turns = higher gain. N = 5 gives ~12 dBi; N = 10 gives ~14 dBi.
- Pitch angle: 12-14 degrees. Controls the helix height per turn.
- Ground plane: At least 3 lambda/4 diameter for good performance.
Helical Antenna Advantages
- Naturally produces circular polarization without additional feed network.
- Wide bandwidth (1.7:1 typical).
- Simple construction (single wire on a support tube).
- Gain increases simply by adding more turns.
C = lambda (circumference ~ 1 wavelength)
Gain = 15 N (C/lambda)^2 (S/lambda) (linear)
G (dBi) = 10 log(15 N (C/lambda)^2 (S/lambda))
Example: 5 turns, C = lambda, S = 0.25 lambda:
G = 15 x 5 x 1 x 0.25 = 18.75 = 12.7 dBi
Beamwidth: 52 / (C/lambda x sqrt(N x S/lambda)) degrees
3 dB axial ratio bandwidth: ~2:1
Frequently Asked Questions
What is a helical antenna?
A helical antenna is a wire wound in a coil shape. In axial mode (circumference = lambda), it produces circular polarization and endfire radiation with 10-15 dBi gain. It is one of the simplest CP antennas, widely used for satellite and GPS.
How does a helix produce circular polarization?
When the circumference equals one wavelength, current travels around the helix and produces a rotating electric field at the axis. The rotation direction follows the helix sense: right-hand wound = RHCP, left-hand wound = LHCP.
What is the bandwidth of a helical antenna?
Axial-mode helical antennas are inherently wideband, with usable bandwidth ratios of 1.5:1 to 2:1. The circular polarization is maintained over most of this range. This is significantly wider than patch antennas, which typically cover 3-5%.