antenna design 2014

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Antenna Design

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Outline

Introduction

Fundamental Antenna Parameters

Design Methodology

Examples:

microstrip patch antennas

slot antennas

Yagi-Uda antennas

others antennas

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Introduction

The antenna (aerial, EM radiator) is a device, which radiates or

receives electromagnetic waves

The antenna is the transition between a guiding device (transmission

line, waveguide) and free space (or another usually unbounded medium)

Conductor or group of conductors used either for radiating

electromagnetic energy into space or for collecting it from space

The electromagnetic radiation from an antenna is made up of two

components, the E field and the H field

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Antennas are reciprocal devices: properties are similar both in

the transmitting mode and the receiving mode

(example: radiation pattern)

The electrical and electromagnetic characteristics of an antenna

apply equally, regardless of whether you use the antenna for

transmitting or receiving.

Microwave circuit

(electric power)

Free space

(electromagnetic

power)

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Matching device from a transmission line to the free

space and vice versa

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Types of antenna

Wire Antennas

Aperture Antennas

Microstrip & printed

Antennas

Reflector Antennas

Lens Antennas

Array Antennas

dipole monopole loop

pyramidal horn

slot

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Types of antenna

Wire Antennas

Aperture Antennas

Microstrip & printed

Antennas

Reflector Antennas

Lens Antennas

Array Antennas

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Types of antenna

Wire Antennas

Aperture Antennas

Microstrip & printed

Antennas

Reflector Antennas

Lens Antennas

Array Antennas

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The space surrounding the antenna is divided into three regions

according to the predominant field behavior (the boundaries

between the regions are not distinct)

Reactive near-field region

Radiating near-field region

Far-field (Fraunhofer) region

(transverse EM wave)

where:

D is the largest dimension of the antenna

λ is the wavelength

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Near field Far field

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Feeding transmission lines

rectangular metal waveguide microstrip line

coplanar waveguide coaxial line parallel-wire line

E

electric field

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Fundamental Antenna Parameters

Radiation pattern

Pattern beamwidth

Directivity

Input impedance

Antenna gain

Frequency bandwidth

Microwave circuit

(electric power)

Far field

(electromagnetic

power)

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Radiation pattern

The radiation pattern (RP)

(or antenna pattern) is the

representation of the radiation

properties of the antenna as a

function of space coordinates

The pattern can be a 3-D plot

(both θ and ϕ vary), or a 2-D plot

A 2-D plot is obtained as an

intersection of the 3-D one with a

given plane, usually a const θ=

plane or a . const ϕ= plane that

must contain the pattern’s maximum Spherical coordinate system

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Radiation pattern

An isotropic antenna that radiates

at an equal strength to all directions is a

good reference antenna but is not

realizable in practice.

Omnidirectional antenna is an

antenna, which has a non-directional

pattern in a given plane, and a

directional pattern in any orthogonal

plane.

Directional antenna is an antenna,

which radiates (receives) much more

efficiently in some directions than in

others.

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describe the antenna

resolution properties

Radiation pattern

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Radiation pattern:

polarization

E

electric field

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Directivity

Directivity of an antenna in a given direction is the ratio of the radiation

intensity in this direction and the radiation intensity averaged over all

directions.

The radiation intensity averaged over all directions is equal to the total power

radiated by the antenna divided by 4 π. It is a measure of the antenna’s ability

to focus the energy in one or more specific directions.

directivity of an isotropic source = 1 (0 dBi)

isotropic

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Input impedance (return losses)

ZA= RA+ j XA

RA is the antenna resistance

XA is the antenna reactance

RA= RL + Rrad

Rrad is the radiation resistance

RL is the loss resistance

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Input impedance (return losses)

The antenna input impedance is frequency dependent. Thus, it is

matched to its load in a certain frequency band. It can be influenced

by the proximity of objects, too.

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Input impedance (return losses)

Resonant antenna Wideband antenna

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Antenna gain

The gain G of an antenna is the ratio of the radiation intensity U in a

given direction and the radiation intensity that would be obtained, if the

power fed to the antenna Pin were radiated isotropically.

Antenna gain includes:

Mismatch losses

Losses in the transmission line

Losses in the antenna: dielectric losses, conduction losses,

polarization losses

Gain ≤ Directivity Gain = η Directivity

η – antenna efficiency [%]

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Frequency bandwidth (FBW)

This is the range of frequencies, within which the antenna characteristics

conform to a specified standard.

Antenna characteristics, which should conform to certain requirements,

might be: input impedance, radiation pattern, beamwidth, polarization,

side-lobe level, gain, beam direction and width, radiation efficiency. Often,

separate bandwidths are introduced: impedance bandwidth, pattern

bandwidth, etc.

broadband antennas narrowband antennas

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Design Methodology

1. Analytical estimation of the main layout dimensions at the

central operating frequency

2. Optimization of the antenna performances in terms of radiation

pattern, antenna gain and reflection losses using intensive

electromagnetic simulations

3. Manufacturing and testing of the antenna demonstrators

4. Design and optimization of the antenna structures for a given

application, integrated with other circuit element into a receiver

or transmitter front-end.

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Planar antennas

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End-fire radiation pattern

Broadside radiation pattern

Types of planar antennas

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Electromagnetic simulations

IE3D Zeland Software Inc., Freemont, CA, full wave,

Method-of-Moments (MoM) simulator

performs electromagnetic analysis for arbitrary 3-D

planar geometry maintaining full accuracy at all

frequencies.

the electromagnetic analysis includes dispersion,

discontinuities, surface waves, higher order modes,

metallization loss and dielectric loss

optimization engine that allows the using of multiple

objective function

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Electromagnetic simulations

MoM electromagnetic simulators are, according to

their solution domains, divided into two groups:

Open boundary Green’s function formulations;

Close boundary Green’s function formulations

(Sonnet, AWR Microwave Office, etc).

exact boundary conditions for most antennas and

many different RF and microwave circuits.

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Duality in Maxwell’s equations

The electromagnetic (EM) field is described by two sets of quantities,

which correspond to each other in such a manner that substituting the

quantities from one set with the respective quantities from the other set

in any given equation produces a valid equation (the dual of the given

one).

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Duality in Maxwell’s equations

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Slot antenna Dipole antenna

Similar radiation characteristics …

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Slot antenna Dipole antenna

… but different input impedances

≈ 73 Ω ≈ 495 Ω

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Duality in Maxwell’s equations

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Microstrip patch antennas

small, light, and suitable for integration and mass-production

typically rectangular, half-wave-long patch

easy to be integrated into antenna arrays

rectangular microstrip antenna fed by a microstrip line

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Analytical Design

Input: 2 < εr < 12; fo; h

1. For good radiation efficiency:

2. Effective dielectric constant:

3. Fringing effect:

4. Patch length:

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Input impedance

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Aperture Coupled Microstrip Antenna

Coaxial line feed antenna substrate dielectric constant: bandwidth and

radiation efficiency of the antenna

antenna substrate thickness: bandwidth and coupling level

microstrip patch length: resonant frequency of the antenna

microstrip patch width: resonant resistance of the antenna

feed substrate dielectric constant: good microstrip circuit

slot length: coupling level, impedance matching

length of tuning stub: matching

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Microstrip antenna arrays

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Microstrip antenna arrays

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Slot antennas

bi-directional radiation pattern

CPW feed line

easy to be integrated into antenna arrays and with active devices

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Slot antennas

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Slot antennas

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(Quasi-) Yagi-Uda antennas

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Quasi-Yagi antennas

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Quasi-Yagi antennas

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Other planar antennas

Frequency independent antennas (very wide bandwidth)

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V antenna linear tapered slot antennas (LTSA)

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Conclusions

For a good antenna design:

Understand the main antenna parameters

Start using simple analytic formulas

Optimize the antenna layout using electromagnetic

simulations and multiple parameter objective function

Test the antenna demonstrator before designing and

manufacturing complex front-ends

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References

C.A.Balanis, “ANTENNA THEORY – Analysis and Design”,

Second Edition, John Willey & Sons Inc., 1997

A.V. Raisanen, A.Lehto, “Radio Engineering for Wireless

Communication and Sensor Applications”, Artech House Inc.,

2003

T.A.Milligan, “Modern antenna design”, McGraw-Hill, 1985