Unit -III

UNIT III

APERTURE ANTENNAS

Dr.T.V.Padmavathy Professor/ECE

RMKCET

Presentation Outline

 Introduction

 Field Equivalence Principle  Horn Antennas

 E-plane Sectoral Horn

 E- and H-Plane Patterns of the E-Plane Sectoral Horn  E- and H-Plane Patterns of the H-Plane Sectoral Horn  E and H-Plane Patterns

 E- and H-Plane Patterns of The Conical Horn Antenna  Pyramidal Horn

 Reflector Antennas

Introduction

 Aperture antennas emit electromagnetic waves through an opening

(or aperture).

 Aperture antennas can be used directly as a

 source antenna for radiation pattern measurements

 point-to-point radio communication links

 feeds in reflector antenna

 aircraft and spacecraft application

 Aperture antenna to be efficient and have high directivity

Field Equivalence Principle

Statement

Horn Antennas

 Flared waveguides that produce a nearly uniform phase front larger

than the waveguide itself

Constructed in a variety of shapes such as sectoral E-plane, sectoral

H-plane, pyramidal, conical, etc

Applications

 Used as a feed element for large radio astronomy, satellite

tracking and communication dishes

 A common element of phased arrays

Used in the calibration, other high-gain antennas

E-plane Sectoral Horn

 Fields expressions over the horn are similar to the fields of a TE₁₀ mode for a rectangular waveguide with the aperture dimensions of

a and b₁.

 difference is in the complex exponential term, parabolic phase error,.





1 1 2

cos

)

,

(



y

x

e

a

E

y

x

E













_













1 1 2 cos ) , (    y k j

x x e

a E

y x

H   

     _    

 /(2 )

1 1 2 sin ) , (   

 j ky

z x e

a ka

jE y

x

H   

Pyramidal Horn

Other Horn Antenna Types

 Multimode Horns

 Corrugated Horns

 Hog Horns

 Biconical Horns

Reflector Antennas

 Reflectors are used to concentrate flux of EM energy radiated/

received, or to change its direction

 Usually, they are parabolic (paraboloidal).

 The

first parabolic

Heinrich Hertz in 1888.

 Large reflectors have high gain and directivity

 Are not easy to fabricate

 Are not mechanically robust

 Typical reflectors are:

 Plane and corner Reflectors

Principles of Parabolic Reflectors

Simplest reflector antenna

consists of two components

• Reflecting surface

• Smaller feed

A paraboloidal surface is described

by the equation

a

z

F

F







'

)

(

4





is the distance from a point A to the focal point O

'



 The axisymmetric paraboloidal reflector

is entirely defined by the respective

parabolic line – the diameter D and the

focal length F.

 When F/D approaches infinity, the

reflector becomes flat

 When F / D = 0.25, the focal point lies

in the plane passing through the reflector’s rim.

Unit -III Aperture Antennas

Calculating the Focal Point

 The optimal location for the antenna feed, or receiver is mainly depends on the focal point.

 The focal point of the parabolic reflector can be calculated :

F= D2 _{/ 16d}

Where

F = focal point

D = Diameter of the dish d = depth of dish

Parabolic Antenna Types

Standard Parabolic Antenna

Shielded

Antenna Focal Plane

Antenna

Features of Parabolic Antenna Types

Standard Parabolic Antenna

 Basic Antenna

 Comprised of

 Reflector

 Feed Assembly

 Mount

Shielded Antenna

 Absorber-Lined Shield

 Improved Feed System

 Protection Against Ice, Snow

and Dirt

Focal Plane Antenna

 Deeper Reflector

 Edge Geometry

 Slightly Lower Gain

GRIDPAK Antenna

 Grid Reflector

 Low Wind load

 Below 2.7GHz