Contents
Is helical antenna directional?
The radiation is linearly polarised parallel to the helix axis. In the axial mode or end-fire helix, the dimensions of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna’s axis.
What is the geometry of helical antenna?
It consists of a helix of thick copper wire or tubing wound in the shape of a screw thread used as an antenna in conjunction with a flat metal plate called a ground plate. One end of the helix is connected to the center conductor of the cable and the outer conductor is connected to the ground plate.
What is the radiation of a helical antenna?
The radiation of helical antenna depends on the diameter of helix, the turn spacing and the pitch angle. Pitch angle is the angle between a line tangent to the helix wire and plane normal to the helix axis. D is the diameter of helix. S is the turn spacing (centre to centre). α is the pitch angle.
When does the helical antenna operate in axial mode?
When the helix circumference is near the wavelength of operation, the antenna operates in axial mode. This is a nonresonant traveling wave mode, in which instead of standing waves, the waves of current and voltage travel in one direction, up the helix.
How is impedance matching done in a helical antenna?
Impedance matching (when C=λ) to standard 50 or 75 ohm coaxial cable is often done by a quarter wave stripline section acting as an impedance transformer between the helix and the ground plate. The maximum directive gain is approximately: where N is the number of turns and S is the spacing between turns.
What’s the best way to build a helical antenna?
There are at least two good methods for constructing a Helical antenna the first of which is documented in VHF-UHF Manual D.S. Evans (G3RPE) and G.R. Jessop (G6JP) Third Edition printed by RSGP in 1977. Although this is a old book, its design ideas are very quick, easy and strong.