Microwave Antenna - Universiti Malaysia...

Microwave Antenna

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Microwave Antenna

Chapter 5

Microwave Antenna

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Types of Microwave Antenna

1. Horn antenna– Sectoral E– Sectoral H

2. Parabolic antenna 3. Microstrip antenna

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Long waves 30-300 kHz 10-1 km

Medium waves (MW) 300-3000 kHz 1000-100 m

Short waves (SW) 3-30 MHz 100-10 m

Very high frequency (VHF) waves 30-300 MHz

0.3-30 GHz*

10-1 m

100-1 cm

Millimeter waves 30-300 GHz 10-1 mm

Submillimeter waves 300-3000 GHz 1-0.1 mm

Frequency Wavelength l

Infrared (including far-infrared) 300-416,000 GHz 104-0.72 mm

* 1 GHz = 1 gigahertz = 10 Hertz or cycles per second,+ 1 mm = 10-6 m.

Microwaves

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Why Microwaves ?

Radio equipment are classified under VHF, UHF & Microwaves.

VHF and UHF radios used when few circuits are needed and narrow bandwidth.

Earlier equipment were large in size and use Analog Technology.

Recently Digital Radio with better efficiency is being used.

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Microwave Use• Lower bands are already occupied

• Now we have better electronics, and modulation schemes

Advantages of Microwave Utilization:

• Antennas are more directive—better beam control.

• Wider operating bandwidth.

• Smaller size elements

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Terrestrial Microwave

• Used for long-distance telephone service .• Uses radio frequency spectrum, from 2 to 40 GHz .• Parabolic dish transmitter, mounted high .• Used by common carriers as well as private networks .• Requires unobstructed line of sight between source and

receiver .• Curvature of the earth requires stations (repeaters) ~30 miles

apart .

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Microwave Applications

• Television distribution .• Long-distance telephone transmission .• Private business networks .

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Wireless Technologies

• Microwave – Microwave systems transmit voice and data through the atmosphere

as super-high-frequency radio waves.

• One particular characteristic of the microwave system is that it cannot bend around corners; therefore microwave antennas must be in "line of sight" of each other.

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• The following are some of the characteristics of the microwave system:– High Volume – Long distance transmission – Point to point transmission – High frequency radio signals are transmitted from one terrestrial

transmitter to another – Satellites serve as a relay station for transmitting microwave signals

over very long distances. See image next slide

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• Low-Orbit Satellite and Microwave Transmission

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Microwave Spectrum

• Range is approximately 1 GHz to 40 GHz– Total of all usable frequencies under 1 GHz gives a reference on the

capacity of in the microwave range.

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Microwave Systems

• Microwave communication is line of sight radio communication.

• Antenna types for directive antennas, or broadcasting are omi-directional antennas

• Radio Transmission: the speech signals are converted to EM.

• Power is transmitted in space towards destination.• EM waves are intercepted by receiving antennas and

signal power is collected.

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Microwave Impairments

• Equipment, antenna, and waveguide failures.• Fading and distortion from multipath reflections.• Absorption from rain, fog, and other atmospheric conditions.• Interference from other frequencies.

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Microwave Engineering Considerations

• Free space & atmospheric attenuation.• Reflections.• Diffractions.• Rain attenuation.• Skin affect• Line of Sight (LOS)• Fading• Range• Interference

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Horn antenna

Sectoral ESectoral H

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Introduction

• 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, sectoralH-plane, pyramidal, conical, etc.

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Application Areas

• 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.• Used for making electromagnetic interference measurements.

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Rectangular Sectoral Horn Antenna

• It categorized into following two types:– Sectoral H-plane horn antenna: the flaring is along the direction of

magnetic field i.e. H- field.– Sectoral E-plane horn antenna: the flaring is along the direction of

electric field i.e. E - field.

• pyramidal horn antenna type where flaring is made along H-plane and E-plane directions both. It has shape of truncated.

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Dimensions of E-plane

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E-plane Sectorial

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E-Plane Sectoral Horn- (Radiated Fields)

1

21

8λρbs = ( )θ

λsin1b

( )[ ]

+

+=2cos1log20 10

θθ dBEE ( )

+

=

==

1

12

1

12

1

1

2

1

1max

2264

644

λρλρπλρ

πλρπ

bSbCb

a

tFb

aPUDrad

E

[4.1] [4.2]

[4.3]

[4.4]DE = directivity for the E-planeS = sine Fresnel function

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Universal curve – E plane

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Dimensions of H-plane

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H-plane Sectorial

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H-Plane Sectoral Horn (Radiated Fields)

• The directivity for the H-plane sectoral horn

( ) ( )[ ] ( ) ( )[ ]{ }22

1

2max 44 vSuSvCuCab

PUDrad

H −+−×==λρππ

21

2

1

1

2

2

1

1

2

3

21

21

λρ

λρλρ

λρλρ

≈

−=

+=

a

aa

v

aa

u

[4.8]

[4.9]

[4.10]

[4.11]

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Universal Curve H-Plane

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00

030

060

090

0120

0150

0180

0150

0120

090

060

030

102030

10

20

30Rel

ativ

e po

wer

(dB

dow

n)

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

E-PlaneH-Plane

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

E-PlaneH-Plane

00

030

060

090

0120

0150

0180

0150

0120

090

060

030

102030

10

20

30Rel

ativ

e po

wer

(dB

dow

n)

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E and H-Plane Patterns

E-Plane

H-Plane00

030

060

090

0120

0150

0180

0150

0120

090

060

030

102030

10

20

30Rel

ativ

e po

wer

(dB

dow

n)

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00

030

060

090

0120

0150

0180

0150

0120

090

060

030

102030

10

20

30Rel

ativ

e po

wer

(dB

dow

n)

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

E-PlaneH-Plane

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Pyramidal Horn

• The combination of the E-plane and H-plane horns and as such is flared in both directions.

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Dimensions of Pyramidal

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Design Procedures

• The pyramidal horn is widely used as a standard to make gain measurements of other and as such it is often referred to as a standard gain horn.

• To design a pyramidal horn, one usually knows the desired gain G0 and the dimensions a, b of the rectangular feed waveguide.

• The objective of the design is to determine the remaining dimensions (a1, b1, ρe, ρh, Pe, and Ph) that will lead to an optimum gain.

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Exercice

• Design an optimum gainX-band (8.2–12.4 GHz) pyramidal horn so that its gain(above isotropic) at f = 11 GHz is 22.6 dB. The horn is fed by a WR 90 rectangular waveguide with inner dimensions of a = 0.9 in. (2.286 cm) and b = 0.4 in. (1.016 cm).

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Other horn antenna types

• Multimode Horns• Corrugated Horns• Hog Horns • Biconical Horns • Dielectric Loaded Horns

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References1. A.W. LOVE “The Diagonal Horn Antennas” microwave J.,

Vol. V, pp. 117-122, Mar. 1962

2. Constantine A. Balanis, ‘Antenna Theory, Analysis and Design’ 2nd Ed., Wiley,1997

3. D.M Pozar, ‘Directivity of Omnidirectional Antennas’ 1993

4. R.E Collin, ‘Antennas and Radiowave Propagation’ McGraw-Hill , 1985*

5. Samuel Silver, ‘Microwave Antenna Theory And Design’ McGraw- Hill , 1949

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Example Question• Given an E-plane horn antenna parameters as ρ1 = 6λ, b1 =

3.47λ and a = 0.5λ. Compute (in dB) its pattern at θ = 0°, 10°and 20° using the results of universal patterns for E-plane.

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PARABOLIC ANTENNA

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Terrestrial Microwave Antennas for Point-To-Point Communication

• Terrestrial microwave antennas generate a beam of RF signal to communicate between two locations.

• Point-To-Point communication depends upon a clear line of sight between two microwave antennas.

• Obstructions, such as buildings, trees or terrain interfere with the signal.

• Depending upon the location, usage and frequency, different types can be utilized.

• We will address the basic characteristics of these various types…

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Parabolic Reflector Antenna

• The most well-known reflector antenna is the parabolic reflector antenna, commonly known as a satellite dish antenna. Examples of this dish antenna are shown in the following Figures.

Figure 1. The "big dish" antenna of Stanford University.

Figure 2. An AstroTV dish antenna

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• Parabolic reflectors typically have a very high gain (30-40 dB is common) and low cross polarization.

• They also have a reasonable bandwidth, with the fractional bandwidth being at least 5% on commercially available models, and can be very wideband in the case of huge dishes (like the Stanford "big dish" , which can operate from 150 MHz to 1.5 GHz).

• The smaller dish antennas typically operate somewhere between 2 and 28 GHz. The large dishes can operate in the VHF region (30-300 MHz), but typically need to be extremely large at this operating band.

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The basic structure

• It consists of a feed antenna pointed towards a parabolic reflector. The feed antenna is often a horn antenna with a circular aperture.

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• Unlike the dipole antenna which are typically approximately a half-wavelength long at the frequency of operation,

• The reflecting dish must be much larger than a wavelength in size. • The dish is at least several wavelengths in diameter, but the

diameter can be on the order of 100 wavelengths for very high gain dishes (>50 dB gain).

• The distance between the feed antenna and the reflector is typically several wavelengths as well.

• This is in contrast to the corner reflector, where the antenna is roughly a half-wavelength from the reflector

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Ga (dBi) = 10 log10 η [ 4 π Aa / λ2 ]

Where:Ga = Antenna Directive Gain (Catalog spec)η = Aperture Efficiency (50-55%)Aa = Antenna Aperture Areaλ = Wavelength (speed of light / frequency)

Parabolic AntennaDirective Gain in dBi

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Geometry of Parabolic Dish Antenna

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Antenna Diameter

Freq

uenc

y2 ft

(0.6m)4 ft

(1.2m)6 ft

(1.8m)8 ft

(2.4m)10 ft

(3.0m)12 ft

(3.7m)15 ft

(4.5m)2 GHz 19.5 25.5 29.1 31.6 33.5 35.1 374 GHz 25.5 31.6 35.1 37.6 39.5 41.1 43.16 GHz 29.1 35.1 38.6 41.1 43.1 44.6 46.68 GHz 31.6 37.6 41.1 43.6 45.5 47.1 49.1

11 GHz 34.3 40.4 43.9 46.4 48.3 49.9 51.815 GHz 37 43.1 46.6 49.1 51 52.6 NA18 GHz 38.6 44.6 48.2 50.7 NA NA NA22 GHz 40.4 46.4 49.9 NA NA NA NA38 GHz 45.1 51.1 NA NA NA NA NA

Typical Parabolic Antenna Gain in dBi

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Radiation Pattern Concept

Antenna Under Test

Source Antenna

Antenna Test Range

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Standard Parabolic Antenna

Shielded Antenna GRIDPAK®

AntennaFocal Plane Antenna

Basic Antenna Types

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• Grid Reflector• Low Wind load• Single Polarized• Below 2.7GHz• Shipped in Flat,

Lightweight Package

GRIDPAK Antenna

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• Basic Antenna• Comprised of

– Reflector– Feed Assembly– Mount

Standard Parabolic Antenna

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• Deeper Reflector• Edge Geometry• Improved F/B Ratio• Slightly Lower Gain

Focal Plane Antenna

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• Absorber-Lined Shield• Improved Feed System• Planar Radome• Improved RPE

Shielded Antenna

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f

D

f/D = 0.250

Antenna f/D Ratio

f

D

f/D = 0.333

Standard & Shielded Antennas

Focal Plane Antennas

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Scattering

Diffraction

Spillover

Unwanted Signals

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Direction of Signal

Direction of Signal

Direction of Signal

Shielded AntennaFocal Plane AntennaStandard Parabolic Antenna

Front to Back Ratio

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MICROSTRIP ANTENNA

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Microstrip Antenna

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IntroductionAdvantages of Microstrip Antenna

• Low profile• conformable to planar and non-planar surface• simple and inexpensive to manufacture using modern printed-circuit technology• mechanical robust when mounted on rigid surfaces, compatible with MMIC designs• very versatile in terms of resonant frequency, polarization, patterns and impedance.

Note:• MMIC is a type of integrated circuit (IC) device that operates at microwave frequencies (300

MHz to 300 GHz). These devices typically perform functions such as microwave mixing,power amplification, low-noise amplification, and high-frequency switching

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Disadvantages:• low efficiency• low power• poor polarization purity• poor scan performance• spurious feed radiation very narrow bandwidth

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Basic Characteristic

Figure 1: Microstrip antenna and coordinate system

L1

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Geometry

• Height:– h<<< λo usually 0.003 λo≤h≤0.05 λo above a ground plane

• Radiation Pattern:– Its pattern maximum is normal to the patch (broadside radiator).– Properly choosing the mode (field configuration) of excitation beneath the patch.

• For rectangular patch, length (L):– L is usually λo/3 < L < λo/2

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Geometry

• Substrate (ɛr) :– 2.2 ≤ɛr≤12.– Thick substrate whose lower dielectric constant provides good antenna

performance (better efficiency, larger bandwidth, loosely bound field forradiation into space) but at the expense of larger element size.

– Thin substrate with higher dielectric constants are desirable for microwavecircuitry because they require tightly bound fields to minimize undesiredradiation and coupling, and lead to smaller element size; but greater loss, lessefficiency, smaller bandwidth

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BASIC CHARACTERISTICS

Figure 2: Respective shapes of microstrip patch elements

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FEEDING METHODS

Figure 4a.3: typical feeds for microstrip antennas

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FEEDING METHODS

Equivalent circuits for typical feeds

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Microstrip line

• Microstrip line is a conducting strip, usually much smaller width compared to the patch. Easy to fabricate, simple to match by controlling the inset position and rather simple to model. But as the substrate thickness increases, surface waves and spurious feed radiation increases and limit the bandwidth typically 2 – 5%.

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Microstrip Line

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Coaxial-line feed

• Inner conductor of the coax is attached to the radiation patch while the outer conductor is connected to the ground plane

• Easy to fabricate and match and has low spurious radiation• But narrow bandwidth and difficult to model especially for

thick substrates (h > 0.02 λo)• Both microstrip and coax line produce cross-polarized radiation

for higher order modes

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Coaxial Probe Line

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Aperture-coupled feed

• The most difficult to fabricate and narrow bandwidth• Easier to model and has moderate spurious radiation• The aperture coupling consists of two substrates separated by a

ground plane. On the bottom side of the lower substrate there is a mictrostrip feed line whose coupled to the patch through a slot on the ground plane separating the two substrates

• Typically a high dielectric material is used for bottom substrate and thick low dielectric constant material for the top substrate

• The ground plane in between is to isolate the feed from the radiating element and minimizes interference of spurious radiation for pattern formation and polarization purity

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Proximity coupled

• Largest bandwidth 13%• Easy to model and has low spurious radiation• Difficult to fabricate• The length of the feeding stub and the width to line ratio of the

patch can be used to control the match

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Method of analysis

• Transmission line: the easiest but less accurate and difficult to model coupling

• Cavity: more accurate but more complex and is rather difficult to model coupling

• Full wave: very accurate, very versatile and can treat single elements, finite and infinite arrays, stacked elements, arbitrary shaped elements and coupling. But most complex model

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Transmission Line Model

Microstrip line and its electric field lines, and dielectric constant geometry

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Fringing effects

• Because the dimensions of the patch are finite along the length and width, the fields at the edges of the patch undergo fringing

• The amount of fringing is a function of the dimensions of the patch and the height of the substrate

• Most of the electric field lines reside in the substrate and parts of some lines exist in air, thus an effective dielectric constant εeff is introduced to account for fringing and the wave propagation in the line

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( )

( )LLL

hW

hW

hL

Wh

hW

eff

eff

eff

rreff

r

∆+=

+−

++

=∆

+

−+

+=

>>

−

2

8.0258.0

264.03.0412.0

1212

12

1

11

21

ε

ε

εεε

ε[4a.1]

[4a.2]

[4a.3]

[4a.4]

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• Effective dielectric constant is a function of frequency, as the frequency increases most of the electric field lines concentrate in the substrate

• Therefore the mictrostrip line behaves more like a homogenous line of one dielectric (only the substrate), and the effective dielectric constant approaches the value of the dielectric constant of the substrate

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Effective dielectric constant versus frequency for typical substrate

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Figure 4a.7: physical and effective lengths of rectangular microstrip patch

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• Because of the fringing effect, electrically the patch of the mictrostrip antenna looks greater than its physical dimensions

• Its length have been extended on each by a distance ΔL

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Transmission Line Model• Design

– Specify: εr , fr (in Hz) and h– Determine W, L– Design procedure

• For an efficient radiator, a practical width that leads to good radiation efficiencies is below.

• V0 is the free space velocity of light.

12

212

21 0

00 +=

+=

rrrr fv

fW

εεεµ[4a.5]

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Transmission Line Model• Determine the effective dielectric constant of

the microstrip antenna• Once W is found, determine the extension of

the length ΔL Eq. [4a.3]• The actual length of the patch can be

determine by using Eq. [4a.6]

Lf

voLf

Leffreffr

∆−=∆−= 22

22

1

00 εεµε [4a.6]

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Experimental models of rectangular and circular patches

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Transmission Line Model• Conductance

– Each radiating slot is represented by a parallel equivalent admittance Y (conductance G and susceptance B).

( )

( )[ ]121212

00

01

0

20

01

111

,,101ln636.01

120

101

2411

120

BBGGYY

hhkWB

hhkWG

jBGY

===

<−=

<

−=

+=

λλ

λλ

[4a.7]

[4a.7a]

[4a.7b]

[4a.8]

Both slots are identical

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>>

<<

=

00

0

2

01

1201

901

λλ

λλ

WW

WW

G [4a.9]

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Rectangular microstrip patch and its equivalent circuit transmission line model

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Figure 4a.10: Slot conductance as a function of slot width

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Resonant Input Resistance

• The total admittance at slot #1 (input admittance) is obtained by transferring the admittance of slot #2 from the output terminals to input terminals using the admittance transformation equation of transmission lines

• Ideally, two slots should be separated by λ/2, but because of fringing effect the separation less than λ/2.

• The approximate length : 0.48 λ < L < 0.49 λ• The transformed admittance of slot #2 becomes

1

~

2

1

~

2

~

1122

~

2~

BB

GG

jBGBjGY

−=

=

−=+=

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Transmission Line Model• Total Resonant Input

Admittance is real thus the resonant input impedance is also real

( )

∫∫ ⋅×=

±=

===

=+=

S

in

inin

in

in

dsHEV

G

GGR

GR

YZ

GYYY

*212

012

121

1

121

Re12

1211

2~ [4a.10]

[4a.11]

[4a.12]

[4a.13]

No mutual coupling Effect

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• Rin is the resonant input by taking into account mutual effectsbetween the slots, while Zin is without mutual effects.

• (+) sign is used for modes with odd (antisymmetric) resonantvoltage distribution beneath the patch and between slot while (-)sign is used for modes with even(symmetric) resonant voltagedistribution

• G12 is mutual conductance in terms of the far-zone fields• E1 is electric field radiated by slot #1, H2 is the magnetic field

radiated by slot#2, Vo is the voltage across the slot, Jo is the Besselfunction

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( ) θθθθ

θ

ππ

dLkJ

Wk

G 3000

0

212 sinsincos

cos2

sin

1201

∫

=

≥

+++

≤

+

=1

444.1ln667.0393.1

120

14

8ln60

0

00

00

0

hW

hW

hW

hW

hW

Wh

Z

eff

eff

c

ε

πε

[4a.14]

[4a.15]

Zc is microstrip-line feed characteristic impedanceWo = width of microstrip line

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Recessed microstrip line feed and variation of normalized input resistance

Inset feed technique

yo is recessed distanceWo is microstrip line width

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TRANSMISSION LINE MODEL

( ) ( )

−

+

+

±== 0

210

22

21

21

02

1210 sinsincos

21 y

LYBy

LYBGy

LGGyyR

ccin

πππ

( ) ( )

( )

==

±==

02

02

1210

cos0

cos2

1

yL

yR

yLGG

yyR

in

in

π

π

[4a.16]

[4a.16a]

Where Yc=1/Zc,Since for most typical microstrips G1/Yc<<1 and B1/Yc<<1, so:

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