Reflector Antenna

Reflector antenna:

-reflectors are widely used to modify the radiation patterns of a radiating element.

Microwave antenna:

UHF and SHF bands are resp. 300-3000MHz, and 3000-30000MHz but the microwave region extends from 1GHz to 100 GHz. The transmitting and receiving antennas in microwave frequencies are directive with high gain and narrow BW in both horizontal and vertical planes.

-As the frequencies increases, the wavelength decreases and thus it becomes easier to construct and antenna system that are large in terms of wavelength and which therefore can be made to have greater directivity.

Types of reflectors:

(i). Rod reflector:

-mainly used in yagi-antenna

-placed behind the driven elements

- L = λ/2

- it offers inductive reactance and contributes in increasing the gain

-main disadvantage is: it alters impedance of driven element

(ii). phase reflector:

 -simplest reflector to direct electromagnetic energy in desired direction

-But it is difficult to collimate the energy in forward direction

Fig: phase reflector

-In fact polarization of the primary antenna and its position with respect to the reflecting surface is used to control the pattern characteristics, impedances, power gain, and directivity of complete system

(iii). Corner reflector:  

-is a reflecting object, which consists of two or three, mutually intersecting, conducting flatter surfaces

-Dihedral forms of corner reflector are frequently used in antenna

-Trihedral forms with mutually perpendicular surfaces are used as radar targets

Where,

d = aperture size, l = length

d1 = spacing between vertex and feed point location

α – angle (inclined angle)

-A corner reflector is designed to improve the collimation of electromagnetic energy in the forward direction and to eliminate radiation in back and side direction

Features:

(a). most preferred value of α = 90°

(b). d1 is increased if α is decreased and vice-versa in order to improve efficiency

(c). when α is small, gain is increased by increasing the length of sides of reflector

(d). used as passive target for radar and communication application to return the signal exactly in same direction by choosing α = 90°

-due to this unique feature, most of defense-ships and vehicles are designed with minimum sharp corners to reduce the chances of their detection by enemies’ radars.

-also used in home television antennas

# Parabolic Reflector:

-It is the reflector antenna, which has the shape of paraboloid and employs the properties of parabola

Fig: Different types of parabolic feeder

-parabolic reflectors are  based on the geometric optical principles

-A parabola may be defined as the locus of a point, which moves in such a way that it’s distance from the fixed point called focus plus its distance from a straight line called directrix is constant

                                                 Fig: Geometry of parabolic reflector

The parabola is a two-direction plane curve

OF=Focal length = f

O = Vertex

F = Focus

 OO´ = Axis of parabola

 AB = directrix

By the definition of parabola, we have

FP + PP´ = FQ + QQ´ = FS + SS´ = constant (K)

Where K = a constant depends on the shape of parabola curve

The eqⁿ of parabola is

 y² = 4 fx

-the open mouth (D) of parabola is known as Aperture

-the ration of focal length to aperture size

i.e. . (f/D) known as ‘f over D ratio’ is an important characteristic of parabolic reflector and its value usually varies between 0.25 to 0.50