What is Helical Antenna : Working & Its Applications

According to IEEE standard, an antenna or radiator can be defined as a medium that is used to transmit and receive radio waves. There are different types of antennas available which are categorized into different types based on the application like horn antenna, dipole, patch, broadband, log-periodic, etc. Among them, helical antenna is one of the types of broadband antenna which is also called helix antenna. This is one of the most primary, realistic & straightforward antennas which are designed with conducting wire-wound in a helical structure form. So, this article discusses an overview of a helical antenna and its working with applications.

What is Helical Antenna?

Definition of helical antenna: The simplest antennas which are used widely in ultra-high frequencies are known as helical antennas, so this antenna works in VHF & UHF ranges. These antennas are designed with conducting wire in a helix shape. This antenna has some unique characteristics like wide bandwidth, high gain & circular polarization.

The frequency range of this antenna ranges from 30MHz0-3GHz. This antenna is used in space communication such as satellite relays and also in radio astronomy, wireless networking & satellite communications.

Features

The salient features of a helical antenna include the following.

• This is a simple antenna used for circular polarization.
• It is utilized in different bands like VHF &UHF.
• It is most commonly used in axial mode.
• It is not chosen in normal mode where efficiency and beamwidth are small in this mode.
• Its design is very simple & has maximum directivity.
• In axial mode, it is a wideband antenna.
• If the axial ratio is zero then, linear horizontal polarization occurs.
• If the axial ratio is infinite then linear vertical polarization occurs.
• If the axial ratio is one, then circular polarization occurs.

Design of Helical Antenna

The construction of the helical antenna is shown below. This antenna can be designed with a conducting wire-like thick copper wire wounded in helical form & it is connected to the ground plate through a feeder line. This antenna is very simple to design and it provides waves that are polarized circularly, so it is used in different communication-based applications where satellite relays are involved.

The above diagram depicts a helical antenna system that needs wider outdoor space. Here, one helix end is connected to the middle conductor of the cable whereas the external conductor is simply connected to the ground plate which is made with the conductor. This antenna is fed by a coaxial cable which can be functioned in axial & normal modes.

We know that the helical antenna operates in two modes where the deciding factor of these modes is the diameter of the helix & spacing between two consecutive spins.

Suppose, the length of a single turn of the helical antenna is ‘L’, the spacing between consecutive turns is ‘S’, then the whole antenna’s length will be NS. The helix circumference is denoted with C that is given as ‘πD’. Here, ‘D’ is the helical spring diameter.

When we consider the one unrolling turn of the helix antenna, then the different parameters which are related to it can be defined through the following triangle. The helical antenna’s radiation mainly depends on the turn spacing, diameter & pitch angle. So, the pitch angle is the existing angle in between a line tangent to the helix wire & plane normal to the helix axis.

So, tan α = S/C = S/ πD =>

Pitch angle in a Helical Antenna α = tan^-1 (S/πD)

Where,

‘D’ is the helix diameter.

‘S’ is the turn spacing

‘α’ is the pitch angle.

Therefore, finally, we can conclude that the antenna radiation characteristics are modified by changing the above parameters based on the wavelength.

How does a Helical Antenna Work?

Once a conductive wire in an antenna is excited through a coaxial cable otherwise two-wire transmission line then-current supplies throughout the conducting wire. So, this causes the field lines generation & emission of radiations. It is important to know that a helical antenna may emit in different modes, so modes of operation of the helical antenna are normal mode and axial mode which are discussed below.

Normal Mode

In this type mode, both the pitch & the diameter of the helix antenna are tiny as compared to the wavelength. So, its operation is analogous to an electrical short dipole or monopole. In radiation of normal mode, the field of radiation is normal to the helix axis so, radiated signals are polarized circularly.

This mode has low efficiency & narrow bandwidth, so used mainly for compact antennas for portable, two-way mobile radios & UHF TV broadcasting antennas. The helical antenna’s radiation pattern is a combination of both the loop & short dipole antennas.

The radiation pattern of the helical antenna in the normal mode of radiation mainly depends on the diameter values and its turn spacing. The main disadvantages of normal mode operation are, narrow bandwidth, low radiation efficiency thus, it is not frequently used.

Axial Mode

In this mode, both the pitch and diameter of the helix antenna are similar to the wavelength, so it works like a directional antenna. Not like the normal mode, there is no simple solution available to decide the radiation properties in this axial mode. Thus for this mode, experimentally determined analytical & numerical methods are used to conclude these factors. This mode is frequently used through earth-based stations within satellite communications systems. This mode of operation is used more practically.

Formulas

The following formulas are used for calculating helical antenna gain, diameter, space between coils, wire length, half-power beamwidth, beamwidth first nulls, and appearance.

Helical Antenna Gain (G) = (10.8+10 x log (number of turns x space between coils)) / log10

Diameter (D) = wavelength/3.14

Space between coils (S) = Space between coils x wavelength

Length of wire (L) = Number of turns x √(wavelength^2 + Space between coils^2

Half Power Beam Width of antenna = 52 / (√(Number of turns x space between coils))

Beam Width First Nulls of antenna = 115 / (√(Number of turns x Space between coils))

Aperture = (10(Antenna Gain / 10) x wavelength^2 ) / (4 * 3.14)

Advantages

The advantages of a helical antenna include the following.

• Design is simple.
• Directivity is high.
• Wide bandwidth.
• Circular polarization can be obtained.
• It can be used at VHF and HF bands.
• Robust construction.
• When it uses a circularly polarized pattern then it is acceptable through both vertical & horizontal polarized antenna types.

Disadvantages

The disadvantages of a helical antenna include the following.

• Its size is larger so occupies more space.
• The efficiency mainly depends on the number of turns so, because of the number of turns, the efficiency will be decreased.
• High cost.

Applications

The applications of helical antennae include the following.

• These antennas are applicable in satellite & space probe communications because of their circular polarization of the transmitted electromagnetic waves & maximum directivity.
• A single or array of helical antennas are used for transmitting & receiving VHF signals.
• Used for satellites at Earth stations.
• Used for telemetry links through ballistic missiles.
• Communication can be established between the moon & the earth.
• Helical antennas are used in many satellites like data relay and weather.
• This antenna is used for transmitting & receiving VHF waves, especially for ionospheric propagation.
• It is used for different communications like radio astronomy, space telemetry, satellite, and space.

Thus, this is all about an overview of the helical antenna and its working with applications. These are very simple antennas where the conducting wire of this antenna is wounded in helical form & connected to the ground plate through a feeder line. The polarization of the helical antenna is circular and also has high directivity, so used in satellite & space communications.