Helical Antenna
 Broadband (2:1) Antenna used in VHF (30-300 MHz,
FM radio broadcasting, television broadcasting, twoway land mobile radio systems ,long range data
communication up to several tens of kilometers
with radio modems, amateur radio, and marine
communications. Air traffic control communications
and air navigation systems) and UHF (300MHz and
3 GHz, television broadcasting, cell phones, satellite
communication including GPS, personal radio
services including Wi-Fi and Bluetooth, walkietalkies, cordless phones, ) ranges.
 Provides Circular Polarization
 Electromagnetic radiator is that of a conducting wire
wound in the form of a screw thread forming a helix
 The helix is used with a ground plane
 Diameter of ground plane should be 3λ/4
 The ground plane can also be cupped in the form of
a cylindrical cavity
If one turn of helix is uncoiled
D= diameter of helix
C= circumference of helix
S=spacing between turns
α= pitch angle
Lo= length of one turn
N= number of turns
L= axial length/ helix length
2a= diameter of helix conductor
L = NS
total helix length
total length of the wire
where
Pitch Angle
When α = 0, the spacing between turns=0 making it a loop
antenna of N turns
When α = 900 the diameter of helix = 0 the helix reduces to a
linear antenna
00<α < 900 – true helix is formed
The radiation characteristics of the antenna can be varied by controlling the size of its geometrical properties
compared to the wavelength. The input impedance is critically dependent upon the pitch angle and the size of the
conducting wire, especially near the feed point, and it can be adjusted by controlling their values. The general
polarization of the antenna is elliptical. However circular and linear polarizations can be achieved over different
frequency ranges.
Modes of operation
1. Normal Mode (broadside mode)- radiation is intense in the direction normal to the axis of the helix ( perpendicular)
and null along the axis similar to small loop or a small dipole
2. Axial Mode ( end fire mode)- radiation along the axis of the helix
Because an elliptically polarized antenna can be represented as the sum of two orthogonal linear components in timephase quadrature, a helix can always receive a signal transmitted from a rotating linearly polarized antenna. Therefore
helices are usually positioned on the ground for space telemetry applications of satellites, space probes, and ballistic
missiles to transmit or receive signals that have undergone Faraday rotation by traveling through the ionosphere.
Normal Mode
To achieve the normal mode of operation, the dimensions of the helix are usually small compared to the wavelength
(NL0 ≪ λ0).
The geometry of the helix reduces to a loop of diameter D when the pitch angle approaches zero
and to a linear wire of length S when it approaches 90◦. Since the limiting geometries of the helix
are a loop and a dipole, the far field radiated by a small helix in the normal mode can be described
in terms of E𝜃 and E𝜙 components of the dipole and loop, respectively
Combination of N small loops and N short dipoles connected together in series
 The fields are obtained by superposition of the fields
from these elemental radiators.
 The planes of the loops are parallel to each other and
perpendicular to the axes of the vertical dipoles.
 The axes of the loops and dipoles coincide with the
axis of the helix.
Small loop of diameter D and short dipole of length S
Due to small dimensions current is constant
two components are in time-phase quadrature, a necessary but not sufficient condition for circular or elliptical
polarization.
AR = 0 when EѲ = 0 representing linear horizontal polarization – helix becomes horizontal loop i.e S =0
AR = infinity when D=0, making EФ = 0 , helix becomes vertical dipole- linear vertical polarization
AR = 1 when
Resulting in CP
Drawbacks of normal mode
1. Due to small dimensions radiation efficiency is low
2. Narrow BW
Design A five turn helical antenna in the normal mode at 400 MHz. The spacing between turns is λ/50. It is desired
that the antenna possesses CP. Determine the
1. Circumference of the helix ( in λ and meters)
2. Length of a single turn ( in λ and meters)
3. Overall length of the helix ( in λ and meters)
4. Pitch angle in degrees
Axial Mode of Helical Antenna
Pattern Multiplication:
In an array
total field= Element factor X Array Factor
Element factor: Depends on type of element
Array factor: depends on 1. distance between the elements
2. Progressive phase shift
3. Number of elements
φ = K d cos θ + β
φ
sinN 2
AF =
φ
N sin 2
For AF to be maximum φ = 0
φ = K d cos θ + β = 0
β = −K d cos θ
For end fire case Ѳ = 0/ 180
β = −kd/ + kd
Helical Antenna
To achieve axial mode of operation D and S should be large fractions of wave length
To achieve CP
4/3
3
λ
4
4
3
<C< λ
fu/fl = 3/4 = 16/9= 1.78 ~ 2 which satisfies Broadband requirement
Phase difference between opposite points of the loop is zero.
So in far-field helix gives maximum radiation due to reinforcement
Helix is considered as an array of identical turns
placed along Z axis
S- distance between elements
φ = K S cos θ + β = 0
β = −K S cos θ
Helix radiates along 0 direction
β = −K S
Phase shift between elements
As C~λ current travelling through the loop introduces additional delay of 2π
β = −K S − 2π
Velocity Factor: velocity of wave travelling through helix conductor < velocity of wave in free space
P= v/c
If for one turn of length on helical conductor phase shift is
β = −𝑘1 𝐿𝑜
β = −K S − 2π = −𝑘1 𝐿𝑜
K S + 2π = 𝑘1 𝐿𝑜
φ = K S cos θ + β = 0
φ = KScosθ − k1Lo
φ = K Scosθ −
k1Lo
Lo
= K Scosθ −
k
P
Specifications of a helical antenna designed at a center frequency of 475 MHz are HPBW=39 deg, pitch angle= 12.5 deg
Circumference of one turn is equal to one wave length at the center frequency.
Calculate
1. The number of turns needed
2. Directivity in dB
3. Axial Ratio
4. Over what range of frequencies will these parameters remain relatively constant
5. The input impedance at the design frequency and at the ends of the band.
• References
• C. A. Balanis, Antenna Theory: Analysis and Design (3rd eds.), John Wiley & Sons, Hoboken, NJ,
2005.
• www.wikipedia.org