Microwave Antenna
Chapter 5
1 Microwave Antenna Types of Microwave Antenna
1. Horn antenna – Sectoral E – Sectoral H 2. Parabolic antenna 3. Microstrip antenna
2 Microwave Antenna
Frequency Wavelength l
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 10-1 m
Microwaves 0.3-30 GHz* 100-1 cm
Millimeter waves 30-300 GHz 10-1 mm Submillimeter waves 300-3000 GHz 1-0.1 mm
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. 3 Microwave Antenna 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.
4 Microwave Antenna 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
5 Microwave Antenna 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 .
6 Microwave Antenna Microwave Applications
• Television distribution . • Long-distance telephone transmission . • Private business networks .
7 Microwave Antenna 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.
8 Microwave Antenna Wireless Technologies
• 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
9 Microwave Antenna Wireless Technologies
• Low-Orbit Satellite and Microwave Transmission
10 Microwave Antenna 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.
11 Microwave Antenna 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.
12 Microwave Antenna 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.
13 Microwave Antenna Microwave Engineering Considerations
• Free space & atmospheric attenuation. • Reflections. • Diffractions. • Rain attenuation. • Skin affect • Line of Sight (LOS) • Fading • Range • Interference
14 Microwave Antenna
Sectoral E Sectoral H
Horn antenna
15 Microwave Antenna 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, sectoral H-plane, pyramidal, conical, etc.
16 Microwave Antenna 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.
17 Microwave Antenna 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.
18 Microwave Antenna
19 Microwave Antenna
20 Microwave Antenna Dimensions of E-plane
21 Microwave Antenna E-plane Sectorial
22 Microwave Antenna
E-Plane Sectoral Horn- (Radiated Fields)
b2 s = 1 b1 [4.1] 8λρ sin(θ ) [4.2] 1 λ
1+ cosθ = + 4πU 64aρ 2 Eθ [E(dB)] 20log10 max 1 [4.3] 2 DE = = F(t) Prad πλb1 64aρ b b = 1 C 2 1 + S 2 1 DE = directivity for the E-plane [4.4] πλb1 2λρ 2λρ S = sine Fresnel function 1 1
23 Microwave Antenna Universal curve – E plane
24 Microwave Antenna Dimensions of H-plane
25 Microwave Antenna H-plane Sectorial
26 Microwave Antenna
H-Plane Sectoral Horn (Radiated Fields)
• The directivity for the H-plane sectoral horn
4πU max 4πbρ2 2 2 DH = = ×{[C(u)− C(v)] + [S(u)− S(v)] } [4.8] Prad a1λ 1 λρ a [4.9] = 2 + 1 u 2 a1 λρ2 1 λρ a [4.10] = 2 − 1 v 2 a1 λρ2
[4.11] a1 ≈ 3λρ2 27 Microwave Antenna Universal Curve H-Plane
28 Microwave Antenna E- and H-Plane Patterns of the E-Plane Sectoral Horn
E-Plane H-Plane
00
300 0
) 30 10 dB down (
20
0 600 60
Relative power 30
30 20 10 900 900
1200 1200
0 1500 150
1800 29 Microwave Antenna E- and H-Plane Patterns of the H-Plane Sectoral Horn
E-Plane H-Plane
00
300 300 10
20
0 600 60
Relative power ( dB down ) 30
30 20 10 900 900
1200 1200
0 1500 150
0 180 30 Microwave Antenna E and H-Plane Patterns
E-Plane
H-Plane 00
300 300 10
20
0 600 60
Relative power ( dB down ) 30
30 20 10 900 900
1200 1200
0 1500 150
1800 31 Microwave Antenna E- and H-Plane Patterns of The Conical Horn Antenna
E-Plane H-Plane 00
300 300 10
20
0 600 60
Relative power ( dB down ) 30
30 20 10 900 900
1200 1200
0 1500 150 1800 32 Microwave Antenna Pyramidal Horn
• The combination of the E-plane and H-plane horns and as such is flared in both directions.
33 Microwave Antenna Dimensions of Pyramidal
34 Microwave Antenna 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.
35 Microwave Antenna
36 Microwave Antenna 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).
37 Microwave Antenna Other horn antenna types
• Multimode Horns • Corrugated Horns • Hog Horns • Biconical Horns • Dielectric Loaded Horns
38 Microwave Antenna References 1. 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
39 Microwave Antenna 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.
40 Microwave Antenna
PARABOLIC ANTENNA
41 Microwave Antenna 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…
42 Microwave Antenna 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 2. An Astro TV dish antenna
Figure 1. The "big dish" antenna of Stanford University. 43 Microwave Antenna
• 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.
44 Microwave Antenna 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.
45 Microwave Antenna
• 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
46 Microwave Antenna Parabolic Antenna Directive Gain in dBi
2 Ga (dBi) = 10 log10 η [ 4 π Aa / λ ]
Where:
Ga = Antenna Directive Gain (Catalog spec) η = Aperture Efficiency (50-55%) Aa = Antenna Aperture Area λ = Wavelength (speed of light / frequency)
47 Microwave Antenna Geometry of Parabolic Dish Antenna
48 Microwave Antenna Typical Parabolic Antenna Gain in dBi
Antenna Diameter 2 ft 4 ft 6 ft 8 ft 10 ft 12 ft 15 ft (0.6m) (1.2m) (1.8m) (2.4m) (3.0m) (3.7m) (4.5m) 2 GHz 19.5 25.5 29.1 31.6 33.5 35.1 37 4 GHz 25.5 31.6 35.1 37.6 39.5 41.1 43.1 6 GHz 29.1 35.1 38.6 41.1 43.1 44.6 46.6 8 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 51.8 15 GHz 37 43.1 46.6 49.1 51 52.6 NA 18 GHz 38.6 44.6 48.2 50.7 NA NA NA Frequency 22 GHz 40.4 46.4 49.9 NA NA NA NA 38 GHz 45.1 51.1 NA NA NA NA NA
49 Microwave Antenna Radiation Pattern Concept
Antenna Under Test
Antenna Test Range
Source Antenna
50 Microwave Antenna Basic Antenna Types
Standard Shielded Antenna Focal Plane GRIDPAK® Parabolic Antenna Antenna Antenna
51 Microwave Antenna GRIDPAK Antenna
• Grid Reflector • Low Wind load • Single Polarized • Below 2.7GHz • Shipped in Flat, Lightweight Package
52 Microwave Antenna Standard Parabolic Antenna
• Basic Antenna • Comprised of – Reflector – Feed Assembly – Mount
53 Microwave Antenna Focal Plane Antenna
• Deeper Reflector • Edge Geometry • Improved F/B Ratio • Slightly Lower Gain
54 Microwave Antenna Shielded Antenna
• Absorber-Lined Shield • Improved Feed System • Planar Radome • Improved RPE
55 Microwave Antenna Antenna f/D Ratio
f f
D D
f/D = 0.333 f/D = 0.250
Standard & Shielded Focal Plane Antennas Antennas
56 Microwave Antenna Unwanted Signals
Scattering Spillover
Diffraction
57 Microwave Antenna Front to Back Ratio
Direction of Direction Direction Signal of Signal of Signal
Standard Parabolic Antenna Focal Plane Antenna Shielded Antenna
58 Microwave Antenna
MICROSTRIP ANTENNA
59 Microwave Antenna Microstrip Antenna
60 60 Microwave Antenna Introduction
Advantages 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
61 Microwave Antenna
Disadvantages: • low efficiency • low power • poor polarization purity • poor scan performance • spurious feed radiation very narrow bandwidth
62 Microwave Antenna Basic Characteristic
L1
Figure 1: Microstrip antenna and coordinate system
63 Microwave Antenna 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
64 Microwave Antenna Geometry
• Substrate (ɛr) : – 2.2 ≤ɛr≤12. – Thick substrate whose lower dielectric constant provides good antenna performance (better efficiency, larger bandwidth, loosely bound field for radiation into space) but at the expense of larger element size. – Thin substrate with higher dielectric constants are desirable for microwave circuitry because they require tightly bound fields to minimize undesired radiation and coupling, and lead to smaller element size; but greater loss, less efficiency, smaller bandwidth
65 Microwave Antenna BASIC CHARACTERISTICS
Figure 2: Respective shapes of microstrip patch elements
66 Microwave Antenna FEEDING METHODS
Figure 4a.3: typical feeds for microstrip antennas 67 Microwave Antenna FEEDING METHODS
Equivalent circuits for typical feeds 68 Microwave Antenna 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%.
69 Microwave Antenna Microstrip Line
70 Microwave Antenna 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
71 Microwave Antenna Coaxial Probe Line
72 Microwave Antenna 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
73 Microwave Antenna 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
74 Microwave Antenna 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
75 Microwave Antenna Transmission Line Model
Microstrip line and its electric field lines, and dielectric constant geometry 76 Microwave Antenna 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
77 Microwave Antenna Transmission Line Model W h > 1 [4a.1]
ε r > 1 − 1 ε +1 ε −1 h 2 [4a.2] ε = r + r 1+12 eff 2 2 W W (ε + 0.3) + 0.264 ∆L eff h = 0.412 [4a.3] h W (ε − 0.258) + 0.8 eff h
Leff = L + 2∆L [4a.4]
78 Microwave Antenna
• 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
79 Microwave Antenna Transmission Line Model
Effective dielectric constant versus frequency for typical substrate 80 Microwave Antenna Transmission Line Model
Figure 4a.7: physical and effective lengths of rectangular microstrip patch 81 Microwave Antenna Transmission Line Model
• 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
82 Microwave Antenna 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 the1 free space velocity2 ofv light. 2 W = = 0 [4a.5] 2 fr µ0ε 0 ε r +1 2 fr ε r +1
83 Microwave Antenna 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]
1 vo L = − 2∆L = − 2∆L [4a.6] 2 fr εeff µ0ε0 2 fr εeff
84 Microwave Antenna Transmission Line Model
Experimental models of rectangular and circular patches
85 Microwave Antenna Transmission Line Model • Conductance – Each radiating slot is represented by a parallel equivalent admittance Y (conductance G and susceptance B). [4a.7] Y1 = G1 + jB1
W 1 2 h 1 = − < [4a.7a] G1 1 (k0h) 120λ0 24 λ0 10 W h 1 B1 = [1− 0.636ln(k0h) ] < [4a.7b] 120λ0 λ0 10
Y2 = Y1,G2 = G1, B2 = B1 [4a.8] Both slots are identical 86 Microwave Antenna Transmission Line Model
2 1 W W << λ λ 0 90 0 [4a.9] G1 = 1 W >> λ W 0 120 λ0
87 Microwave Antenna Transmission Line Model
Rectangular microstrip patch and its equivalent circuit transmission line model
88 Microwave Antenna Transmission Line Model
Figure 4a.10: Slot conductance as a function of slot width 89 Microwave Antenna 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 approximateY 2length= G2 + : 0.48j B2 =λG< 1L− < jB0.491 λ ~ • The transformedG admittance2 = G1 of slot #2 becomes ~ B2 = −B1 90 Microwave Antenna Transmission Line Model • Total Resonant Input Admittance is real thus the resonant input impedance~ is Y = Y + Y = 2G [4a.10] also real in 1 2 1 1 1 [4a.11] No mutual coupling Zin = = Rin = Effect Yin 2G1 1 Rin = [4a.12] 2(G1 ± G12 ) 1 = × * ⋅ [4a.13] G12 2 Re ∫∫ E1 H2 ds V0 S
91 Microwave Antenna
• Rin is the resonant input by taking into account mutual effects between the slots, while Zin is without mutual effects. • (+) sign is used for modes with odd (antisymmetric) resonant voltage distribution beneath the patch and between slot while (-) sign is used for modes with even(symmetric) resonant voltage distribution • 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 Bessel function
92 Microwave Antenna Transmission Line Model
k W sin 0 cosθ π 1 2 3 [4a.14] G12 = J 0 (k0 Lsinθ )sin θdθ 120π 2 ∫0 cosθ
60 8h W W ln + 0 0 ≤1 ε W0 4h h eff [4a.15] Zc = 120π W 0 ≥1 h W0 W0 ε eff +1.393+ 0.667ln +1.444 h h
Zc is microstrip-line feed characteristic impedance Wo = width of microstrip line 93 Microwave Antenna Transmission Line Model
Inset feed technique
yo is recessed distance Wo is microstrip line width
Recessed microstrip line feed and variation of normalized input resistance 94 Microwave Antenna TRANSMISSION LINE MODEL
Where Yc=1/Zc, Since for most typical microstrips G1/Yc<<1 and B1/Yc<<1, so:
1 π G 2 + B2 π B π = = 2 + 1 1 2 − 1 2 [4a.16] Rin (y y0 ) cos y0 2 sin y0 sin y0 2(G1 ± G12 ) L Yc L Yc L
1 2 π Rin (y = y0 ) = cos y0 [4a.16a] 2(G1 ± G12 ) L π = R (y = 0)cos2 y in L 0
95 Microwave Antenna
96