Normal Mode Helical Antenna

Small Dipole:

Small Loop:

Therefore, Axial Ratio is:

For Circular Polarization, AR = 1

2 2

22E SSARE C C

4

jkrokI Se

E j sinr

2

2

2

4

D jkrok I e

E sinr

2C S

D

S

D

S

Design of Normal Mode Helical Antenna

Radiation Resistance (Rs)2

21(790) 0.6 2

IavR h R

s sIo

AR = 2 Sλ / C λ2

= 2x0.01/0.04 2

= 12.5 = 21.94 dB

Axial Ratio (AR)

For Infinite Ground Plane:

Wire length ≈ λ / 4 – text book

> λ / 4 – in reality

Feed is tapped after one turn for impedance matching

Normal Mode Helical Antenna (NMHA) on Small Circular Ground Plane

NMHA Design on Small Circular Ground Plane

Resonance Frequency 1.8 GHz

Wavelength 166 mm

Spacing = 0.027λ 4.5 mm

Diameter of Helix = 0.033λ 5.5 mm

No of Turns (N) 7

Pitch Angle (α) 14.6 Degree

Length of Wire = 0.75λ 124.5 mm

Effect of Ground Plane Size on NMHA

As ground plane radius increases from λ/30 to λ/20, resonance

frequency decreases and the input impedance curve shifts upward.

NMHA designed for 1.8 GHz and rwire = 1.6 mm (λ/100)

Effect of Wire Radius on NMHA

As radius of wire decreases from λ/80 to λ/120, its inductance

increases so resonance frequency of NMHA decreases and its input

impedance curve shifts upward (inductive region).

NMHA designed for 1.8 GHz and rg = 5.5 mm (λ/30)

Effect of Wire Radius on Bandwidth of NMHA

Fabricated NMHA on Small Ground Plane and its Results

Horn Antennas

Prof. Girish KumarElectrical Engineering Department, IIT Bombay

gkumar@ee.iitb.ac.in

(022) 2576 7436

Horn Antennas

H-Plane Sectoral HornE-Plane Sectoral Horn

Pyramidal Horn Conical Horn

TE10 mode in Rectangular Waveguide

TE10 mode in Rectangular Waveguide

Rectangular Waveguide

b

a

For Fundamental TE10 mode: E-Field varies

sinusoidally along ‘a’ and is uniform along ‘b’

X-Band Waveguide WR90 (8.4 to 12.4 GHz):

a = 0.9” and b = 0.4”

Cut-off Wavelength = 2a = 2 x 0.9 x 2.54 = 4.572 cm

Cut-off Frequency = 3 x 1010 / 4.572 = 6.56 GHz

E-Plane Sectoral Horn Antenna

E-Plane Sectoral Horn: Side View

≈

E-Plane Sectoral Horn: Directivity Curve

ρ1 6 10 20 100

b1 3.46 4.47 6.32 14.14

Max. Directivity:

E-Plane Sectoral Horn: Max. Phase Error

Maximum Directivity occurs when

which gives ‘s’ approximately equal to:

δmax = 90°

δmax = 2πs, where≈

Maximum Phase error occurs when y’ = b1 / 2

Phase Error too high:

Not Recommended

E-Plane Sectoral: Universal Pattern

E-Field for s = 1/4 (δmax = 90°)

E-Field for s = 1/8 (δmax = 45°) - Recommended

H-Plane Sectoral Horn Antenna

δmax = 2πt, whereMaximum Phase

error at x’ = a1 / 2

H-Plane Sectoral Horn: Directivity Curve

Max. Directivity: ρ2 6 10 20 100

a1 4.24 5.48 7.75 17.32a1 3λρ2

H-Plane Sectoral Horn: Max. Phase Error

Maximum Directivity occurs when

which gives ‘t’ approximately equal to:

δmax = 135°

δmax = 2πt, where

Maximum Phase error occurs when x’ = a1 / 2

Phase Error too high:

Not Recommended

a1 3λρ2

H-Plane Sectoral: Universal Pattern

E-Field for t = 1/4 (δmax = 90°)

E-Field for t = 1/8 (δmax = 45°)

Recommended

max. phase

error between

45° and 90°

Pyramidal Horn Antenna

Side ViewTop View

Pyramidal Horn Antenna

Condition for Physical Realization:

Pyramidal Horn: Design Procedure

Directivity of

Pyramidal Horn

Antenna can be

obtained using

Directivity

curves for E-and

H-Planes

Sectoral Horn

antenna

Alternatively

Pyramidal Horn Design Steps

Pyramidal Horn Design: Example

Pyramidal Horn Design: Example (Contd.)

Pyramidal Horn Design: Example (Contd.)

Optimum Dimensions vs. Directivity

Gain (dBi)

aEλ

aHλ

Lλ