Microwave Antennas

3 GHz 30 GHz
MW region 1 GHz
GH 100 GH
GHz
High Directivity and Broadband
% BW =

OperatingBand ( f H f L )
X 100%
CenterFreq. f 0

1000 M 500 M
X 100% = 66.67%
750 MHz
2000M 1000M
% BW =
X 100% = 66.67%
1500MHz

% BW =

1. Horn Antenna
2. Parabolic Reflector
3. Lens Antenna
Feeding methods Optical

Horn Antenna
Wave guide antenna
Open
O
tterminated
i t d wave guide
id
Radiation poor
Mismatch between w/g and free space

Mouth of waveguide is flared out to improve

radiation
Horn antenna

Horn Antenna
Sectorial Horn Antenna
Sectorial H Plane Horn

Pyramidal Horn

Sectorial E Plane Horn

Conical Horn

Prof. Rizwan Alad, D. D. University,

Nadiad

Pyramidal Horn Antenna

dE

dH

Design of Horn
l
d

= flare angle
D= aperture

L= horn length defined

de
ed ass d
distance
s ce
from end of waveguide
to end of antenna
= path length
diff
difference

Prof. Rizwan Alad, D. D. University,

Nadiad

Design of Horn

D
(L + )2 = L2 + ( )2
2
2
D
L2 + 2 + 2L = L2 +
4
D2
2L =
4
D2
L=
8

0= optimal path length

difference

L
0 =
L
cos( / 2)
0 = 0.1 0.4
Pyramidal
y
Horns 0= 0.25
Conical horns optimal 0= 0.32
which gives,

L = 0.39
Prof. Rizwan Alad, D. D. University,
Nadiad

D2

Analysis of Horn Antenna

HPBW , E

56
=
dE

HPBW , H

67
=
dH

Dir Gain ,
Dir.

D =

7 .5 A

Power Gain , G p =

, A = dH dE

4 .5 A

Prof. Rizwan Alad, D. D. University,

Nadiad

Horn antenna
For a constant length L,
Directivity of the horn increases (beam width decreases)
If aperture (flare angle) becomes large enough so that
aperture phase error () reaches certain value 0,
A phase reversal at the edges of the aperture causes
increase of the side lobes and reduction in the directivity
It follows that the maximum directivity occurs at the largest
aperture for which 0.
Prof. Rizwan Alad, D. D. University,
Nadiad

Horn antenna
For uniform aperture distribution
A very long horn with small flare angle is required
Practical considerations, the horn is desired as short as
possible
Horn antenna use - W frequency, moderate power gain
High power gain, Horn dimension becomes large
Prof. Rizwan Alad, D. D. University,
Nadiad

Parabolic Reflector Antenna

Paraboloidal Antenna

Path Length,
O B+BC=O D+DE=2F
OB+BC=OD+DE=2F
Prof. Rizwan Alad, D. D. University,
Nadiad

Reflector antennas
Increase aperture area to increase its directivity
Gain =

4Ae

Instead of Large direct radiating apertures; use

reflecting surface and a small aperture antenna
called feed
Th high
Thus
hi h directivity
di ti it with
ith a small
ll feed
f d aperture
t
Reflector antennas operate on principles of
geometrical optics (GO)

Paraboloidal Reflector
The geometry of the Paraboloidal reflector has two
valuable features:
All rays leaving the focal point O are collimated
along the reflector
reflectorss axis after reflection.

All path lengths from the focal point to the reflector

and on to the aperture plane are the same and equal
to 2F .

Prof. Rizwan Alad, D. D. University,

Nadiad

radiation Pattern of Parabolic Reflector Antenna

Parabolic reflector

4F / D

0 = 2 tan 1

Feed
F
d should
h ld illuminate
ill i t
the reflector to achieve
maximum efficiency of
the antenna

Parabolic reflector
Reflector design problem involves mainly
Matching of the feed antenna pattern to the
reflector
Focal distance F of a given reflector can be
calculated after measuring its diameter D and its
height H0 :

D2
F=
16H 0

Effect of F/D
D=1
1
0 = 2 tan

4F / D
1

0
F 1
= cot( )
2
D 4
2

D
F=
16H 0
Optimum
F/D = 0.25 to 4

Subtended Angle 0 Focal Length F

Height H0

28 6472
28.6472

0 0022
0.0022

7.1591

0.0087

14

2.0361

0.0307

24

1.1762

0.0531

34

0.8177

0.0764

54

0 4907
0.4907

0 1274
0.1274

64

0.4

0.1562

74

0.3318

0.1884

84

0.2777

0.2251

90

0.25

0.25

94

0.2331

0.2681

104

0.1953

0.3199

Selection of F/D
F/D is a very important design
parameter
Proper illumination is decided by
feed and Feed size is decided by
F/D
Higher F/D, lower the angle of
subtend
Higher
Higher F/D, aperture size of the
feed should be higher
Lower
L
F/D,
F/D the
th feed
f d iis small
ll

Conclusion of Single
g Prime Focal Reflector
High gain antenna
Optimum F/D like 0.25 and 0.4:
location of the feed is large distance away from
the reflector

Suppo t g feed
Supporting
eed becomes
beco es longer
o ge and
a d thicker
t c e
Larger plumbing loss, less efficiency
Structurally poor
For such cases, the feed can be conveniently
placed using dual reflector antenna

Aperture
p
antenna : Reflector Antenna

Circular aperture Parabolic reflector

Feed at the focus of reflector
High Gain
Hi h P
High
Power A
Applications
li ti

Antennas have (maximum) gain G

related to the effective aperture area
as follows:

Gain =

4Ae

Where: Ae is effective aperture area.

Reflector Antenna Gain

G=

Ae

Ae = A p

= product of the eficinecy terms related to following

Aperture
p
blockage
g efficiency
y
Illumination taper Efficiency
Defocusing Efficiency
Cross polarization Efficiency
Non-uniform amplitude/phase distributions Efficiency
Prof. Rizwan Alad, D. D. University,
Nadiad

Spillover Efficiency
The spillover efficiency is simple to understand.
This
hi measures the
h amount off radiation
di i from
f
the
h
feed antenna that is reflected by the reflector. Due
to the finite size of the reflector,
reflector some of the
radiation from the feed antenna will travel away
from the main axis at an angle
g ggreater than , thus
not being reflected.
p
byy movingg the
This efficiencyy can be improved
feed closer to the reflector, or by increasing the
size of the reflector.

 Cross Polarization - The loss of gain due to

cross polarized (non-desirable)
cross-polarized
(non desirable) radiation
A
Aperture
t
Bl k
Blockage
- The
Th feed
f d antenna
t
( d the
(and
th
physical structure that holds it up) blocks some of
the radiation that would be transmitted by the
reflector.
Non-Ideal Feed Phase Center - The parabolic
dish has desirable pproperties
p
relative to a single
g
focal point. Since the feed antenna will not be a
point source, there will be some loss due to a nonperfect phase center for a horn antenna.

 Primary antenna isotropic antenna,

FNBW = 140(/D)o
Directivity, D = 4/2*Ae
If Ae = Ap,
p,
Directivity, D = (D/)2 = 9.87(D/)2
Now primary antenna, half wave dipole antenna
Ae = 0.65A
0 65Ap
Directivity, D = 6.389(D/)2

Cassegrain Feed

Prof. Rizwan Alad, D. D. University,

Nadiad

Dual reflector Antenna

Hyperbolic sub reflector Cassegrain

Elliptical sub reflector Gregorian
Higher effective F/DF/D Lesser spreading loss
Efficiencies are on the order of 65-70%,
10% higher
than for front-fed systems
Prof. Rizwan Alad, D. D. University,
Nadiad

Offset Parabolic Reflector Antenna

Single offset parabolic reflector antenna

Aperture projected
on bore-sight

Reflector outline

Focus

Prof. Rizwan Alad, D. D. University,

Nadiad

Important design
parameters: F/D
and offset angle,

Dual offset Reflector Antenna Configuration

Very compact as lesser F/D

Gregorian Configuration

Cassegrain configuration

Prof. Rizwan Alad, D. D. University,

Nadiad

Dual offset Reflector Antenna Configuration

Offset Parabolic Reflector Antenna

Reduces aperture blocking effects
Blockage
g due to supporting
pp
g struts eliminated

Frequency reuse: higher levels of isolation between orthogonal

pol.
pol
Mutual
M t l coupling
li
between
b t
feed
f d elements
l
t via
i the
th reflector
fl t is
i
reduced
Shaping of these reflectors results in higher efficiency and lower
side lobe levels
Prof. Rizwan Alad, D. D. University,
Nadiad

Prof. Rizwan Alad, D. D. University,

Nadiad

Prof. Rizwan Alad, D. D. University,

Nadiad

Typical satellite antenna patterns and coverage zones

Antenna for the global beam is usually a waveguide horn with reflector
Scanning beams and shaped beams require phased array antennas or reflector
antennas with phased array feeds