HELICAL ANTENNASPresented by :- Ankur Soni (10/IEC/007)- Girik Pachauri (10/IEC/016)- Satyendra Singh (10/IEC/049)- Avinash Kumar (10/IEC/062)

INTRODUCTION

A helical antenna is an antenna consisting of a 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.

OPERATIONAL MODES

Helical antennas can

operate in one of two

principal modes :

• Normal Mode

• Axial Mode

NORMAL MODE

In the normal mode or broadside helix, the dimensions of

the helix (the diameter and the pitch) are small compared

with the wavelength. The antenna acts similarly to an

electrically short dipole or monopole, and the radiation

pattern, similar to these antennas is omnidirectional, with

maximum radiation at right angles to the helix axis. The

radiation is linearly polarised parallel to the helix axis.

OPERATION & CONSTRUCTION

They are usually wound in a linear "spiroidal" pattern

(constant parallel spaced turns) providing consistent uniform

radiation as a reduced sized equivalent in respect to the

standard 1/4 wave antenna.

An effect of this type of concertinaed 'reduced size 1/4 wave'

is that the matching impedance is changed from the nominal

50 ohms to between 25 to 35 ohms base impedance.

An example of the type as used in mobile communications is

"spaced constant turn" in which two or more different linear

windings are wound on a single former and spaced so as to

provide an efficient balance between capacitance and

inductance for the radiating element at a particular resonant

frequency.

A common form of

normal-mode helical

antenna is the Rubber

Ducky antenna used in

portable radios. The

loading provided by the

helix allows the antenna

to be shorter than its

electrical length of a

quarter-wavelength.

ADVANTAGES

1. They are typically used for applications where reduced size is

a critical operational factor.

2. These simple and practical "Helicals" were primarily designed

to replace very large antennas.

3. Their reduced size is therefore most suitable for Mobile and

Portable High-frequency (HF) communications in the 1 MHz

to 30 MHz operating range.

AXIAL MODE

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. It radiates circularly polarised radio

waves.

The main lobes of the radiation pattern are along the axis of

the helix, off both ends.

Helical antenna for

WLAN communication,

working frequency app.

2.4 GHz

Axial Antenna

OPERATION & CONSTRUCTION

Since in a directional antenna only radiation in one direction is wanted, the other end of the helix is terminated in a flat metal sheet or screen reflector to reflect the waves forward.

The helix in the antenna can twist in two possible directions: right-handed or left-handed, as defined by the right hand rule. In an axial-mode helical antenna the direction of twist of the helix determines the polarisation of the radio waves: a left-handed helix radiates left-circularly-polarised radio waves, a right-handed helix radiates right-circularly-polarised radio waves.

Helical antennas can receive signals with any type of

linear polarisation, such as horizontal or vertical

polarisation, but when receiving circularly polarised

signals the handedness of the receiving antenna must

be the same as the transmitting antenna; left-hand

polarised antennas suffer a severe loss of gain when

receiving right-circularly-polarised signals, and vice

versa.

The dimensions of the helix are determined by the wavelength λ of the radio waves used, which depends on the frequency. In axial-mode operation, the spacing between the coils should be approximately one-quarter of the wavelength (λ/4), and the diameter of the coils should be approximately the wavelength divided by pi (λ/π).

Axial Mode Helical Antenna

B : Centre SupportC : Coaxial Cable FeedlineE : Spacers/supports for the helixR : Reflector ground planeS : Helical Element

PARAMETERS

The parameters of the helix antenna are defined below.• D - Diameter of a turn on the helix antenna.

• C - Circumference of a turn on the helix antenna (C=pi*D).

• S - Vertical separation between turns for helical antenna.

• α - pitch angle, which controls how far the helix antenna grows in the z-direction per turn, and is given by

• N - Number of turns on the helix antenna. H - Total height of helix antenna, H=NS.

• The input impedance is primarily real and can be approximated in Ohms by :

• The axial ratio for helix antennas decreases as the number of loops N is added, and can be approximated by :

• The gain of the helix antenna can be approximated by :

• The Half-Power Beamwidth for helical antennas can be approximated (in degrees) by :

ADVANTAGES & DISADVANTAGES

Advantages :

• Overall length/height of antenna is reduced.

• They can be easily constructed.

• They produce circularly polarized fields.

Disadvantages :

• Poor reception and transmission properties.

• Bandwidth is too narrow for cellular communication.

APPLICATIONS

• Global Positioning Systems (GPS).

• Wireless LAN.

• Satellite communication.

• Animal tracking.

• Dual ISM (Industry Scientific & Medical) bands.

THANK YOU