Chapter 6 Helical Antenna (6 1)

-
to (6-14)
6.1 Introduction...eee**
. . 6-2

6.2 Helical Antenna Structure and Helical Geometry... .6-2

6.3 Normal or Perpendicular Mode of Radiation . .

.... 6-5

6.4 Axial or End Fire Mode of Radiation.. ... 6-8

6.5 Salient Features of Helical Antenna...
...6-13
6.6 Applications of Helical Antenna...
... 6-13
Jenil Thakkar
Chapter 7 Arrays of Dipoles and Anertures
 Helical Antenna
6
Syllabus
Helical geometry, Transmission radiation modes, Practical design considerations, Wide band
characteristics ofhelical antenna.

Contents
6.1 Introduction

5.2 Helical Antenna Structure and Helical Geometry..Dec.-11, Winter-13, ... Marks 7

6.3 Normal or Perpendicular Mode of Radiation...... May-12, Summer-13, 14 Marks 7

64 Axial or End Fire Mode of Radiation.. . .
May-11, 12, Dec.-11,
...
Winter-13, Summer-13,14 Marks 7
6.5 Salient Features of Helical Antenna
66
Applications of Helical Antenna

(6-1)
Jenil Thakkar
Antenna and Wave Propagation 6-2

Helical krlerma
6.1
***
Introduction
.The concept of helical antenna was first introduced by John D. Kras.
3-dimentional structure of which linear and loop antennas arethes Kraus in 1947.
19
l cases
. I t is basically a
simple broadband VHF and UHF antenna which
hich provides ciraila
nro

polarization.
It consists of a thick copper wire wound in the form of a screw thread
helix. orming a
When the helical wire is unwounded on a flat surface it becomes a straight uin
The wire is wounded in such a way that it is wounded as if on a
unitn
cylinder.
When it is observed from any end, the shape observed is circular. Thus the helir
of a helical antenna combines three different geometric shapes like straightlire
circle and a cylinder. Moreover it can be right handed or left handed.
helix radiates in many modes, the modes of special interest a
Eventhough, a

normal mode and axial mode.

In the axial mode, the
maximum radiation is along
the helix axis under condition
that the circumference of the
helix is of the order of one
wavelength as shown in
Fig. 6.1.1 (6). In normal mode,
the radiation is maximum
along the broadside to the mode
Conical

helix axis under condition that mode

(c)
Nomal mode (b) Axial
circumference of the helix (a)
e n n a

the antens
helical
of
is smaller with respect to one Fig. 6.1.1 Different
modes

wavelength. The radiation nornal

mode is illustrated in the Fig. 6.1.1 (a). required for

6 . 1 . 1

6.1.1
When dimensions of the helix circumference exceed hose the Fig.
in
mode and axial mode, a multilobed pattern as showr mode.

observed. Such higher order radiation mode is called conica

Wnter

Geometry
6.2 Helical Antenna Structure and Helical
G
******w.
TU
Dec.
11,
r z r i n

form

t h r e a d

s c r e w

t consists of a o fa
thick copper wire wound in the form o o m

helix as shown in the Fig. 6.2.1.

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 6-3
Weve
Propagation
Helical Antenna
Anensand

neral, the helix is used with ground

There are fferent forms of the
planes.
such as at ground plane,
ground planes
round

or frustrum cavity.
ylindrical cavity Thick copper
eneral the helical is fed with
antenna wire helix
In in which the central
CO-axial transmission line
conductor is connected to the helix at the feed
Doint; while the outer conductor is attached to
the ground plane.
The helical antenna can operate in many
modes but the two important modes are - Ground plane

normal mode and axial mode of operation.

Feed wire
In the normal mode, the field radiated by the
antenna is maximumn in a
plane normal to the Co-axial cable
axis of helix and minimum the along axis.
This mode is also termed as broadside mode.
Fig. 6.2.1 Helical antenna
In the axial mode, the field radiated by the
antenna is maximum in the
plane along the axis. In axial mode there is only one
major lobe with maximum intensity along the axis of the helix. The axial mode is
the most
practical mode and it is
also known as endfire mode.
Oimensions of the helix in the axial mode are not critical. Hence the end fire
or
helical antenna can be used to achieve circular polarization over a wider
bandwidth.
For the space communication the helical antennas suitable
they have wide bandwidth, applications,
are most
as

To
higher directivity and circular polarization.
heli transmit or receive VHF signals through ionosphere generally an array of
antennas isire used. The helical antenna is widely used for space and satellite
helical ennas
Communications.
Considers a helical
Ground plane-
Feed wire
antenna as
shown Coaxial cable
Fig, 6.22 in the d
It
helixbasiofcally consists of a
thick
or
copper w
tubing wound Inner Outer
Helix
Shape of screw on a conductor conductor A= N.S
and used thread
6.2.2 Structure of hellcal antenna
with Fig.
metal a
plate
e
call a flat
called ground plane or ground plate.

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 6-4
Antenne and Weve Propagation
elical Antenma
.The helix can be described by using following symbols
N Number of turms
D Diameter of helix (center to center)

C Cireumference of helix = n D

S Spacing between two turns measured center to center

A = Axial length

= Length of one turn

a =
Pitch angle
d Diameter of conductor
Basically the symbols for diameter and circumference are of imaginary cylinde
with surface passing through center line of helix.
The helix is fed by a coaxial cable. One end of the helix is connected to the cene
or inner conductor of the cable and outer conductor is connected to the
gruu MOLntere
plane.
The mode of the radiation of the
antenna depends on the diameter of tne
D, the spacing between turn S
which is measure between two centres or he tuns
The circumference of the
helix is denoted by C and it is
equal to Tt.
.The numberof turns are denoted
Then the axial by N.
length A =NS. Length of -S
one
complete turn is denoted by L. The
spacing of the helix from the
is denoted
by I.
ground plane
If we unroll one turm of helix on a
surface, then the plane C nD
between turns circumference, spacing
(S), tum
pitch angle () can be length (L) and
other related to each
through the
triangle
Fig. 6.2.3. terminology as
shown in the
Then we can write, OOyfor

terminoiooln
=s2+C2 S2
L Fig. 6.2.3 Trlangle
unrolled
on d

.The pitch angle is defined

+(TD)2=
u m of hellx Wi

and the as the angle between a line tangent

as,
plane normal to the axis of between a line tane c a n b ee p e

helix. The pitch angle then

TECHNICAL
PUBLICATIONS" An up thrust for know dg Jenil Thakkar
 Propagation 6-5
A t tenne
enn
and
Vave Helical Antenna

tana= nD
w **************** **********

a tan D
************************* w www..
wwwww.

In general, a helícal antenna can radiate in many modes. But the most
modes of radiation are as follows. important
Normal or perpendicular mode of radiation.
Axial or End fire or Bean mode of radiation.

Review Questions
******
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1. Draw the geometry of a helical antenna and

explain its physical parameters.
GTUWinter 13, Marks 7
2. Drw the
figure of helix with its associated dimensions
showing the relationship between
circumference, spacing, turn length and pitch angle of helix. GTU
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * .* mraououps*osen ons *os nsnansns seos oseno*s*s nos e*s s s ns s* * * *nsae * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * e* *Dec.-11,
* * * * * * * * * * * * * * * * *Marks
*****

* * * * * * * * * * *3* * * * *
***
*ww

6.3 Nomal or Perpendicular Mode of Radiation

GTU May-12, Summer-13, 14
In this mode of helical antenna, the radiation is
maximum in broadway direction
Le. Normal or perpendicular to the axis of the
helix, hence the mode is called
normal or
to the helix
perpendicular mode of radiation. The radiation in the direction normal
axis is circularly wave. polarized
his mode of radiation can be obtained if the helix dimensions are made very
a s compared with radiation is N.S <« But with this mode of radiation, the
andwidth of antenna becomes narrow and the radiation
very less. efficiency also becomes
The
radiation pattern of a helical antenna in a normal mode is the combination of
the quivalent radiation from a short dipole located on the same helix axis and
aSmall loop which is coaxial with the same helix axis. This is the condition
obtained for different
pitch angle.
hen pitch and when
angle a =0, helix corresponds to a loop a = 0 then it
heli ds to a linear dipole. Basically these are the limiting conditions of the
helix.
Ror a helix
lbo with fixed to zero, the helix turns out to be a
diameter with S tending
loop while the helix with fixed S and D tending to zero turns out to be short
dipole as shown in the Fig. 6.3.1 (a) and (6).

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 Antenna and Wave Propagation 6-6

The resultant field

Helical Anten
is dependent on Z
the ratio of the
field strengths or
the amplitudes of
the two
components. The a=0,S0
X a 90°,D-0
amplitudes of the
components (a) Loop
actually depend on (b) Short dipole
the pitch Fig. 6.3.1 Limiting conditions on helix
angle a.
Even though the
radiation patterns of the
polarizations are at right angles. loop and short dipole are
Hence the same, their
polarized or elliptically resulting field is either
circulary
small, the radiation frompolarized. As it
depends on the
pitch
dominant and when a is angle the radiaton
the loop is a, when a s
from the short
dipole is dominant. So in the large,
polarizations are linear. limiting conditions, botn
Consider a helix in Z
spherical
co-ordinate system as shown in
the Fig. 6.3.2.

Considering that the

helical
antenna is
made up of
of small number
loops and short
dipoles arranged in series
that loop diameter such
helix diameter D
and
equal to
dipole equallength
the short of
to the
spacing between two helix S. X
The far field of
given by,
a small loop is
Fig. 6.3.2 Helical
antenna with spherical
co-ordinates
E 120 n.sin 0
A . . .6.3.1)
where
r
Retarded current, A
Distance at a
=

A
point, m
Area of
loop =

A=
Wavelength in m.
TECHNICAL
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 Propagation
6-7 Helical Antenna
Wave
and
Antenna

Similarly
the far field of the short dipole is given by,
60 Tsin 0 S
Eg i - I 6.3.2)
S = dL= Length of the dipole
where

above equations, it is clear that two fields in

enim are
phase quadrature (phase of
oPTo get axial ratio (AR) of the elliptical polarization, we should take ratio of
macritudes of the field due to short dipole to that due to the loop. Hence the
axial ratio is given by,

60t [] sin
AR = Eel SA
E 120t sin, A 2TA

Substituting value of A as we get,

AR = SA 4SA

( 2nD2

AR 2S
wwwiM niininsainnimaan.
:
wwwwww

Now depending on values of AR, we get three conditions.

Condition 1: When AR =0, the elliptical polarization becomes linear horizontal
polarization,
ã o n 2: When AR =
o, the elliptical polarization becomes linear vertical
polarization,
Condition 3: When AR 1, the elliptical polarization becomes circular
Thus polarization.
COndition for the circular polarization is given
by
AR =1=Eel 2S
ie. ETD2
Hence, we can El El
write,
2SA= T2D2

TECHNICAL PUBLICATIONs"- An up thrust for knowledge

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 6-8
Antenne and Wave Propagation
*********** ******tneee*********** u
Helical Antenna
S D2
ie S 2
wwwwwwwwwwwww.wwww. * ***** *****
633
C Circumference = TD
where,
Hence the pitch angle for the circular polarization is given by,

n2D2
2
a =

tan TDtan TD

a tan-1 TD
2tan .6.3.4)
2 wwww.www

where, C = Circumference =TD

1taual
Thus the resultant radiation
pattern for the helical antenna in
the normal mode can be obtained
by superposing the field patterns
of the loop and the dipole for Helix axis
circular polarization as shown in w
6.3.3
Fia, 6.3.3
Fig. Normal mode radiation pattem
Normal mode
the Fig. 6.3.3.
circular polarization for helical
antenna liea antenns
Review Questions
********************************** ******************************************

1. Derive interrelation between specing circumferences turn length diameter jor 1

treating an array. GTU: Summer-14,
Marks
made

One
2. Define and
explain two modes of helical antenna. Enlist the design steps for
with the help of necessary May-12, Marks 7
formulae. GTU
3. For helical antenna, discuss briefly i) Different modes of operation, i)
Des
GTUSummer13, Marks
******************** ******************** * **************

****************************************** e ***************************o * .* ssoesanneo***********

6.4 Axial or End Fire Mode of Radiation 14

Summer:13
-13,
GTU May-11, 12, Winter
The helical antenna radiating field maximum in the end fire dir ction ie. along
axis of the helix is called e in
thne
ar. circular o
axial mode end fire mode helical a
or
With the axial mode
nearly circular. radiation, the polarization of the
wav n o d es

The main difference in axia

the radiation pattern of the normal mode and in
norzn

that in the axial

mode, radiation is maximum along the e while "
axis
while

mode, radiation is maximum in the direction normal to ne axis.

the he
TECHNICAL PUBLICATIONs". An up thrust for knowedge Jenil Thakkar
 Propagetion 6-9
and
Wave Helical Antenna
Antenna

narameters of the helical antenna which decide the mode of radiation are
hetween two turns (s) and the diameter of helix (D). When these two
s p a c

kers are of the order of one wavelength to achieve axial mode of radiation.
parame

ote thatin the normal mode, the dimension N.S<«

wal mode radiation of the helical antenna is more important because most of
The axia
the practical antennas produce similar radiation patterns with the features like
broad and directional beam in axial direction and minor lobes at oblique angles.
. This mode is possible in the helical antenna if the circumference is selected of the
order of one wavelength and spacing is selected approximately equal t o a s

shown in the Fig. 6.4.1 (a) or two wire transmission line as shown in the
Fig. 6.4.1 (b).
axial mode, the Maximum
.For the radiation
pitch angle a varies
from 12 to 18. The
optimum pitch angle
is 14. Two wire
transmission
The terminal line
impedance is resistive Ground
plane N8
at the resonant
Outer
frequency and it conductor
Coaxial
becomes reactive at
Inner
conductor
cable
higher and lower Maximum
frequencies. The
(a) (b)
radiation
terminal impedance is
given by, Fig. 6.4.1 Helical antenna in axial mode

**************************************
140C
R ..(6.4.1)
******************************************

hhelix al mode, the beam width both depend on the

antenna gain and length of
i.e. NS.T The beamwidth between half power points is given by,
aaAsAA***naA*********a******

HPBW -
5223 ..(6.4.2)
*** s ***** CVN-S degrees
******************** **sssnetns******************

milarly
t h the
e beam
width between
rsannsngono
first nulls is given Dy

BWFN = degrees
************sn s s ****************

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 Antenna and Wave Propagation 6-10
Helica Antenna
The maximum directive gain in the axial mode is given by,

15NSC2
Gp a3
.6.4.3
The axial ratio (AR) is given by,
1
AR 1 2N
.6.44)
.The helix can have either
right handed pitch or
left handed pitch. Then
accordingly the circular
polarization may be right handed or left handed.

Moreover, a receiving helical antenna with right

handed pitch can not receive left handed
circularly polarized wave and vice versa.
The
Fig. 6.4.2 Radiation pattem of
radiation pattern for axial mode is helical antenna
helical antenna in axial
axial mode
mode
the Fig. 64.2.
as shown in

Review QuestionsS

1. wwwwwwnmwwwwwwmwwwwwnwwwwwwwwwwwwwwww
Define and
explain two modes of helical wwwwwwwwwwww

mode with the antenna. Enlist the design steps for it m one
help of necessary formulae. uny
2. For helical
antenna,
GTU May-12, Marks 1
discuss briefty i) Different modes of operation, i) consideratioms
Desigm cons
3.
Explain practical design considerations GTU: Summer-13, Marke 7
for the monofilar axial mode helical arnteru
..re.........

* * * * * * * *on* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *a s se* * * e
Examples based s s s arne* * * * * * * * * * * * * * GTU May-11
Helical Antenna
Example 6.4.1 Calculate
one wavelength. directivity of 20 turn helix with a=12 and circuera
Solution: The
directivity of helical antenna is
15NSC2 given by
3GDnax
where, N Turns =

of helix 20, C = =

By property, Circumference A, S = acing between tum

Spacng
tan a

TECHNICAL
PUBLICATIONS"- An up thrust for knowledge
Jenil Thakkar
 Popagation 6-11 Helical Antenna
Wave
and
Antenna

S C tan d =
Mtan 12°) =
0.2125 a

15x 20x0.2125A XA) =

63.75
GDmax 3
Calculate HPBW, axial ratio, gain and power pattern for a 16 turn helical1
Example 6.4.2

beam
antenna witha circumference of A and spacing between turns to be 0.25
GTU WIinter-13, Marks 7
Salution: Given N = turns = 16, C =, S= 0.25 A

Beamwidth between half power points is given by,

5C
21N3S 2 2
1
HPBW c NST NS/ -52 26.85
*16 (0.25)
The axial ratio is given by,

1
AR 1+N 2(16) 1.03125

ii) The maximum directive

gain in axial
mode is given
by,
C = 5 NS C2 15 (16) (0.25)
12 =
60
3 (a) Helical antenna with
(b) Radiation patterm
16 turns in axial mode
The helical antenna in axial mode
and its Fig. 6.4.3
adiation pattern are as
shown in the Fig. 6.4.3.
64.3 Calculate the directipity, half power beamwidth and
nelitx
opeating in an axial mode with following dimensions terminating resistance for
Number of turns 9 i Operating
i) Circumfere freguency 2 2 GHz
Cumference 0.8 2 iv) Distance between turns 0.2
=

Solution: For GTU: Dec. 11. Marks 7

a helix
N
operating in axial mode,
Number of turns =9, f= Operating
frequency =2 GHz =
2x10° Hz
Circumference of helix = 0.8 , S =Distance between turns = 0.2a
i) The
directivity is given by,
D 15NSC2 15x9x(0.2)x(0.84)- 17.28
) The a3 3
wavelength can be obtained as,
3x108 = 0.15 m
f2x109
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 6-12
Antenna and Wave
Propagation
Helical Antena
Hence the half power
beamwidth is given by,
3
52/3
52
HPBW CIN.S 089x0.2
52
= 108.33

il) The axial ratio is given by,

AR 1 =1+ 1.0555

iv) The terminal resistance is given by,

R
140 C
140(0.8) 112 2
Example 6.4.4 A helix operates 2 GHz has dimensioms: S = 5 cm, d = 10 cm, N = 20. Find
out 3 dB beamuidth and directioity. GTU: Summer-14,Maks3
3x108
Solution: = = 0.15 m
2x109
C Td =
n(10x10) 0.3142 m =

3 dB beamwidth means half power beamwidth. For helical antenna HPBW 16 E

Oven

by
HPBW 52TNS
3 52 (0.15)3
= 9.62°
0.3142 1 20(5x102)
i) The directivity (maximum directive gain) is given by,
D15NSC 15(20)(5x10-2)(0.3142)2 = 438.7628
13
(0153
Examples for Practice
Example 6.4.5: For a 20 turn
helical antenna circumert
at 33 GHz with
at
operating CH

10 cm and U operating at
spacing between the turns 0.3 A is LAns, :900,21.29
Calculate the
directivity
and s.:90
Example 6.4.6: Calculate half power beamwiatn. h v i n g

in dB the
a 10° and directivity of a helical arnten
circumference equal to one waveleng 15.9843

39.6675,
d

LAns. :
EXample 64.7: A helical antenna of0 1
u l l beamvridth,a y
m

with turm
Z0furns and operates at 1000spacing of 0.0 ull
d i a m e t e

power beamwidth and MHz. Find null

directive gain. Also find AR 51L81 1
27.1978°27.2"
LAns.: 60.149°,
*********w****e

TECHNICAL
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 Wave Propagation
6-13 Helical Antenna
Antenna
a n a d h

Features of Helical Antenna

Salient
6.5 Sa
antenna is used for circular polarization.
Helical
i The helical antenna is used most widely in VHF and UHF bands.

axial mode of helical antenna is most widely used.

i)The
anena in axial mode has larger bandwidth but in normal mode bandwidth
both are small.
and efficiency
I s construction is simple and directivity is higher.
H axial ratio AR = 0, linear horizontal polarization is resulted. For AR = oo, linear
vertical polarization is resulted. For AR = 1, circular polarization is resulted.

Review Question
*wwww.wwwwwwwww.owwwwwwww

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1. Write salient features of helical antenna.
Mwwwwwwwmmw
www.wmwn

66 Applications of Helical Antenna

1. The dimensions of the helix are critical in the normal mode and it limits the
bandwidth of anternna and also radiation
the helix
efficiency is limited. But for axial mode,
dimensions are not very critical. Hence the bandwidth and radiation
efficiency, both are greater. Hence axial mode helical antennas are used to achieve
Circularly polarized waves over extremely wide bandwidth.
AS the axial mode helical
antennas have wide
bandwidth, the directivity and gain
are greater. Also
they able to produce circularly polarized waves over greater
are
aWidth, they are extensively used in the space communication systems. Such
as

3. A
transmitting telemetry data from moon to the earth.

Or
single helical antenu or an
array of helical antennas are useful in transmitting
4.
receiving VHF signals through the iornosphere.
The helicai
space probe antenr
aremost extensively used in the satellite communication and
communication.

Review Ouestion
1.
** so Write applicati
*******
of helical antenna.
******************* . **n********************* ... ..... aeaansaaan osnaseannas*******************************************************************************

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