Slide 1 Mohamed El-Hadidy Duisburg/UWB weekly 14.12.2005 Antenna Fundamentals & UWB Antenna M.Sc. Eng. Mohamed El-Hadidy Duisburg-Essen Universität –

Slide 1Mohamed El-Hadidy

Duisburg/UWB weekly14.12.2005

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Antenna Fundamentals & UWB Antenna

M.Sc. Eng. Mohamed El-Hadidy

Duisburg-Essen Universität – Nachrichtentechniksystem

UWB weekly



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Contents

1 -General Introduction / Objective

2 -Directional Coupler Configurations

Delta Circuit Configuration

Discontinuities

Star Circuit Configuration

Potential Divider Configuration

3 -Comparison of Electromagnetic Properties

4 -Conclusion

5 -References

Antenna Fundamentals & UWB Antenna



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The head containsThe eyes and the antennae grow from the head.

The Antenna

The Antennae



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Introduction

• UWB communication systems transmit pulses instead of modulated sine waves– Signal occupies a very broad BW (1Hz~2GHz)– Antenna can no longer be optimized at the carrier

frequency (no carrier in UWB!)– Frequency-independent antenna is needed

DSP



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Challenges in UWB Antenna Design

• EM aspects of UWB communication systems have not been studied adequately– Most of the conventional antenna analyses

assume harmonic time dependent (not the case in UWB)

– Time-domain EM analysis/simulation are needed• Issues in UWB antenna design

– Efficient pulse generation/reception– Pulse dispersion problem– Matching/Ringing problem



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System Design Perspective

• UWB antenna is not likely to be a purely resistive load and may strongly influence the transmitter circuits– Antenna/circuit co-design is necessary

• Efficient pulse-shape design– Taking pulse-shape design into account adds one

more dimension to improve the performance of the antenna

Pulse Generator

BondingWire

TransmissionLine

Antenna



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Antenna Specifications

• Requirements of UWB antenna– 2-Dimensional– Omni-directional field pattern– Small size– Low cost

• Possible candidates– Dipole antenna– Loop antenna– Microstrip antenna– …



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Dipole Antenna

Feed point

• Consists of two straight wires

– Simple scheme, easy to analyze, mechanism is well-known

• Popular in narrow-band systems• “Humps” in frequency domain • Resistively loaded dipoles exhibit

very broad BW since reflection on the antenna is suppressed, but– Radiation efficiency is reduced– Termination is a problem



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Loop Antenna

• Circular turns of wire– To meet the 2D geometry spec

only 1 turn is used• Used for AM radio• Radiate normally/axially if the loop is

small/large relative to a wavelength • A modified version, Large Current

Radiator, is adopted by Aether Wire & Location, Inc., an UWB localizer company. Large radiation power can be delivered, but it’s shape is 3D

Input



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• Metallic patches sit on a dielectric substrate• Usually made on PCB• Low profile, conformable to various surfaces,

inexpensive, durable, but narrow-band• Modify the shape to broaden the bandwidth, e.g.

bowtie antenna

Microstrip Antenna

Antenna Patch

Dielectric substrate

ground



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• Things people care about– Directivity– Radiation efficiency– Radiation bandwidth– Polarization…

Antenna Parameters



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Antenna in Communication Systems

• At Receiver– E-field at the Rx is translated to a voltage source– By reciprocity theorem, Zant,rx=Zant,tx

• At Transmitter– Antenna is modeled as a passive circuit component;real part in it determines the radiated power (if s=¥)– Current distribution in the antenna determines Erad



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Traditional Antenna Design

• Designed for narrowband systems• Assume time-harmonic (steady-state sinusoidal)– Phasor is applied (d/dt=jw), Maxwell’s equations become more friendly.– Drive the antenna by cos(wt), radiate cos(wt+q1), and receive cos(wt+q2)– Matching is trivial à make it resonate



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Challenges in UWB Antenna Design

UWB means very broad bandwidth (DC~2GHz)• Phasor can no more be applied– Maxwell’s equations can’t be simplified• Waveform dispersion– Redefine directivity• Ultra-wideband matching– Ringing might happen– High radiation efficiency is hard to achieve• Flat frequency response



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UWB Antenna Requirements



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Transmission Line Basics



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VSWR



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Achieving Broader Bandwidths

For instance, it is well known that thickening a dipole leads to a broader bandwidth.

For example, an antenna with a ratio l/d =5000 has an acceptable bandwidth of about 3%, which is a small fraction of the center frequency.

An antenna of the same length but with a ratio l/d =260 has a bandwidth of about 30%. This would correspond to a bandwidth of approximately 2.0 GHz for a center frequency of 6.5 GHz, which is still not sufficient for the entire UWB bandwidth of 7.5 GHz.



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There are also several known antenna topologies that are said to achieve broadband characteristics, such as the horn antenna, biconical antenna, helix antenna and bowtie antenna.

Achieving Broader Bandwidths



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Babinet’s Equivalence Principle

Z1Z2 = η2/4

This principle can be used to achieve impedance matching throughout frequency,

ZA = ZB= η/2

For all frequencies



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Rumsey’s Theory of frequency independent geometry

r=F(θ, φ)

r’ = KF(θ, φ)

KF(θ, φ) = F(θ, φ + C)

(dK/dC)F(θ,φ) = K∂F(θ,φ)/ ∂φ

1/K (dK/dC) = (1/r) ∂r/∂φ

The general solution for the surface r = F(θ,φ) of the antenna:

r = F(θ,φ) = eaφ f(θ) where a = 1/K (dK/dC)



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Equiangular Spiral Slot Patch Antenna

This spiral curve can be derived by letting f’(θ) = Aδ(π/2 – θ), where A is constant and δ is the three dimensional Dirac deltaFunction.

Letting θ = π/2, r = Aea(φ-φ0), where A= roe-aφ0. The representation of r in wavelengths,

rλ = Aea(φ-φ1), where φ1 = (lnλ)/a.



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Diamond Dipole Antenna

The diamond dipole antenna configuration follows from theory that thickening a dipoleincreases its impedance bandwidth.

Thickening a dipole spreads the energy throughout the dipole, therefore lowering its resonant Q value. Thin dipoles are analyzed theoretically under the assumption that all of the energy of the dipole is located within a few wire radii of the antenna.



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If this assumption holds true (as it does for thin wire antennas), a TEM transmission line approximation can be applied to the analysis of these antennas.

However, this assumption breaks down as the antenna thickness is increased. Also, it becomes much harder theoretically to solve Maxwell’s equations for more complex shapes.

Simulation tools and empirical results attest to the claim that thickening a wire antenna increases its bandwidth.




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This theory mentioned that antennas in a loop configuration with sharp corners at the edges provide current nulls at the edges, which leads to lower standing wavelength ratios (SWR) at antiresonant frequencies.

This therefore leads to broader bandwidth.

Sharp Corners Theory



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Curved Solid Diamond Dipole

Curved Wire Diamond Dipole

Sharp-Edged Wire Diamond Dipole




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Diamond Dipole Antenna VSWR



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Circular Disc Monopole Antenna



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Circular Disc Monopole Antenna



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VSWR Characteristics



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Impulse Response



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Single Ended and Differential Elliptical Monopole Antennas (SEA and DEA)

Elliptical Monopole Antennas



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Adjustment for ellipticity is achieved by defining

L = 2*y radius (cm) & r = (x radius)/4 (cm).




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Commercial UWB Antennas

3.1—10 GHz Ultra-Wideband AntennaCommercial UWB Applications

FeaturesBased on Patent-pending Antenna ElementSmall and CompactDesigned for Low-cost Applications



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Documents

Slide 1 Mohamed El-Hadidy Duisburg/UWB weekly 14.12.2005 Antenna Fundamentals & UWB Antenna M.Sc. Eng. Mohamed El-Hadidy Duisburg-Essen Universität –