Helical Antennas

Lecture: 24 Design of Antennas-II Dr. Yogesh Kumar Choukiker

School of Electronics Science Engineering Microwave and Photonics Division VIT University, Vellore, India

Dr. Yogesh Kumar Choukiker

Helical Antenna 

A conducting wire that is wound in the form of a screw thread along with a ground plane (a metallic conductor of specific shape and dimensions) is referred as helical antenna

Cupped Ground Plane 1


Helical Antenna 

Geometrically a helix can be characterized as a wire having N turns of diameter D and a spacing or pitch of S between each turn. The total length of the antenna is then L = NS



While total length of the wire used to wind the coil is given as Ln = NL0 = N (S 2+C 2 ). Where C =  D is the circumference



Another important geometrical parameter of helical antenna is the pitch angle , which is angle between the tangent to helix and plane  to its axis

 S  1  S    tan    tan     D   C  1

2


Helical Antenna If  = 0 0, we have a spiral antenna

If  = 90 0, we have a wire antenna

Conducting wire

Axis of helix

If 00 <  < 90 0, we have a Helical antenna

3


Helical Antenna 



The radiation characteristics of a helical antenna can be controlled by adjusting the geometrical parameters. For example, the input impedance of the antenna is critically dependent on pitch angle and size of the wire at input terminals.

There are typically two modes of operating the helical antenna, namely Normal or Broadside Mode

4

Axial or End-Fire Mode


Helical Antenna-Normal Mode 



To have a normal or broadside mode of operation, the dimensions of the helical antenna must be small compared to the wavelength i,e, NL0 <  0

We know that, when

  00  Helix  loop of diameter D &   90  Helix  Dipole of length S 0

5



Since the limiting geometries of a small helix operating in normal mode are a loop and a dipole, we can assume its far field to be having E  and E  components respectively



The far-field of a small helical antenna operating in normal mode can be obtained by superposition of the fields from both the elemental radiators Dr. Yogesh Kumar Choukiker


The equivalent circuit of a helical antenna in normal mode is

Small Helical Antenna

Loop antenna Short dipole

6





Since the helical antenna dimensions are small, the current is assumed to be constant and the relative field due to N dipoles and loops can be assumed to be approximately equal to fields from single loop and dipole

The far-zone electric field of a short dipole is

I 0 Ske  jkr

E   j 

4 r

The far-zone electric field of a small loop is

I 0  D / 2 k e 2

E     Now ‘Axial ratio’ can

AR  7

sin 

E  E 

2

 jkr

sin 

4r be defined as



4S

 kD

2



2 S

 D 

2



2 S

C 

2



A wide range of axial ratios can be obtained by varying D and S (0  AR   ). There are three special cases



If AR = 0, which means |E  | = 0. The helical antenna behaves like a loop antenna and is linearly polarized, with horizontal polarization.



If AR =  , which means |E  | = 0. The helical antenna behaves like a dipole antenna and is linearly polarized, with vertical polarization.



If AR = 1, which means |E  | = |E  |. The helical antenna behaves like a loop antenna and is circularly polarized.

AR  1 

 8

2 0 S

C 

2

2 0 S

 D 

2

1

1 C  2S  0

& tan  

S

 D

 D  tan    0 Choukiker Dr. Yogesh2 Kumar

Helical Antenna-Axial Mode 

The axial or end-fire mode is more practical and widely used in helical antenna applications. The bandwidth and efficiency of broadside mode operation is limited due to restriction on antenna size which should be less than the wavelength. No such restriction exists for axial or end-fire mode.



There is only one major lobe in this mode of operation and it lies along the axis of helix. To excite this mode D & S should be large fractions of wavelength. Since dimensions of the antenna are not very critical in this mode of operation, it achieves high bandwidths



For circular polarization, the circumference C, pitch S and the pitch angle  should satisfy the limits

3 4 9



C

 0

4

 0

3

4

 , S 

, 12    14 0

0


Helical Antenna 

The input impedance of a helical antenna in axial mode is

 C  R  140    0  

The half power beam width is given as

  0 3 / 2   HPBW  52  C NS   

The first null beam width is given as

  0 3 / 2   FNBW  115  C NS     10

And the directivity as

 C 2 S  D0  15 N  3      


Helical Antenna 

The axial ratio for increased directivity is

AR  

2 N  1 2 N

The normalized far-field pattern is given by

sin N / 2      E  sin  cos sin / 2  2 N   L0  where   k 0  S cos    p   

Where p is the wave velocity on the helix relative to the free-space

p  11

L0 /  0

S /  0   1


Problems Problems 18. Design a five turn helical antenna which at 400 MHz operates in the normal mode. The spacing between the turns is 0/50. It is desired that the antenna possesses circular polarization. Determine (a) Circumference of the helix (b) Length of a single turn (c) Overall length of the entire helix (d) Pitch angle

12

19. A helical antenna of 4 turns is operated in the normal mode at a frequency of 880 MHz. The length L of the helical antenna is 5.7 cm and the diameter of each turn is 0.5 cm. Determine (a) Spacing S between the turns (b) Length L0 of each turn (c) Overall length Ln of entire helix (d) Axial ratio of helix Dr. Yogesh Kumar Choukiker (e) Primary polarization of helix. Explain.

Problems Problems 20. It is desired to design an optimum end-fire helical antenna radiating in the axial mode at 100 MHz whose polarization axial ratio is 1.1. Determine (a) Directivity (b) HPBW (c) Input Impedance (d) VSWR when connected to a 50  line (e) Wave velocity on the helix 21. Design a 5 turn helical antenna which at 300 MHz operates in the axial mode and possesses circular polarization in the major lobe. Determine (a) near optimum circumference (b) spacing for near optimum pitch angle design (c) Input impedance (d) HPBW, FNBW, Axial ratio, Directivity

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22. Design a Rhombic antenna to operate at a frequency of 30 MHz Dr. Yogesh Kumar Choukiker with the angle of elevation 300, w.r.t ground

Problems 23. A 16 turn helical beam antenna has a circumference of , and a turn spacing of /4. what is HPBW, axial ratio and directivity 24. Design a 6 element Yagi – Uda antenna to operate at 500 MHz. Give the dimensions and spacing between all elements


Helical Antennas

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