A Helical antenna is a antenna consisting of conducting wire wound in the form of a helix.in most cases,helical antennas are mounted over a ground plane.the feed line is connected between the bottom of the helix and the ground plane. The ground plane can take different forms.one is for the ground to be flat,however it can be cupped in the form of a cylindrical cavity or in the form of a frustum cavity. Typically the diameter of the ground plane should be at least 3. λ/4

The parameters of helical antenna are- D- diameter of a turn on the helix antenna. C- circumference of a turn on the helix antenna. S-vertical separation between turns for helical antenna. a-pitch angle,which is the angle formed by a line tangent to the helix wire and a plane perpendicular to the helix axis. a = pitch angle = tan-1 (S/pi D) N-numbers of turns on the helix antenna. H- total height of the helix antenna

Helical antenna can operate in one of the two Principal modes :normal mode and axial mode. NORMAL MODES For a helical antenna with dimensions much smaller than wavelength (NL << l), the current may be assumed to be of uniform magnitude and with a constant phase along the helix. The maximum radiation occurs in the plane perpendicular to the helix axis, This mode of operation is referred to as the “normal mode”. In general, the radiation field of this mode is elliptically polarized in all directions. But, under particular conditions, the radiation field can be circularly polarized. Because of its small size compared to the wavelength, the normal-mode helix has low efficiency and narrow bandwidth. pattern is similar to these antenna is omnidirectional,with the maximum radiation at right angle to the helix axis

AXIAL MODE When the circumference of a helix is of the order of one wavelength, it radiates The antenna functions as a directional antenna radiating a beam off the ends of the helix,along the antenna’s axis.This radiation mode is referred to as “axial mode”. The radiation field of this mode is nearly circularly polarized about the axis. The sense of polarization is related to the sense of the helix winding.

In an axial mode helical antenna the direction of twist of the helix determine the polarization of the radio waves :a left handed helix radiates left –circularly –polarized radio waves,a right handed helix radiates right circularly polarized radio waves

Radiation pattern for axial mode helix.

Radiation pattern for normal mode helix

APPLICATION Global system(GPS) Wireless lan Satellite communication Animal tracking Dual ISM (industry scientific & medical) bands.