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					An Introduction to Antennas for Wireless Communications
Karim M. Nasr
nasrk@cs.man.ac.uk

School of Computer Science

CS6242

Base Station Antennas

Mobile Phones λ/4 monopole •Losses by human body

Where is the antenna ??

K.NASR

CS6242

Intro. to Antennas 23/2/2006

•Six mounted antennae t cover 3 sectors with diversity
•Downtilting/ electrical tilt • Omnidirectional versus directional

K.NASR

CS6242

Intro. to Antennas 23/2/2006

WLAN Access Point

Two Antennas ?? Where is the antenna ??
K.NASR CS6242 Intro. to Antennas 23/2/2006

The Electromagnetic Spectrum

Mobile Communications and WLAN
K.NASR CS6242 Intro. to Antennas 23/2/2006

K.NASR

CS6242

Intro. to Antennas 23/2/2006

Antenna Definition
The antenna (aerial, EM radiator) is that part of a transmitting or receiving system, which radiates or receives electromagnetic waves. An antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line). An important property of an antenna is the ability to focus and shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions. Its main purpose is to convert the energy of a guided wave into the energy of a free-space wave (or vice versa) as efficiently as possible, while in the same time the radiated power has a certain desired pattern of distribution in space.
K.NASR CS6242 Intro. to Antennas 23/2/2006

Main Antenna Types
1) Wire antennas (dipoles, monopoles, folded dipoles, loops, helices) Up to 1 to 2 GHz

(One of many classifications)

2) Aperture antennas (horns, waveguides) 1 to 20GHz

K.NASR

CS6242

Intro. to Antennas 23/2/2006

Main Antenna Types (ctd.)
3) Microstrip (patch) antennas ~ 10 GHz or more

4) Reflector antennas and lenses (radio astronomy and satellite comms.) Very high gains > 30 dBi
K.NASR CS6242 Intro. to Antennas 23/2/2006

Main Antenna Types (ctd.)
5) Arrays

Other Classifications: • By size: 1. electrically small, 2. resonant antennas, 3. electrically large • Broadband or Narrowband
K.NASR CS6242 Intro. to Antennas 23/2/2006

Electromagnetic Propagation

λ = c / f ; c = 3 x 108 m/s
K.NASR CS6242 Intro. to Antennas 23/2/2006

Basic Theory of Operation
The length of the antenna structure plays an important role. The antenna is a resonant device

Zg Zant Zant =(Ra+Rl) +j Xa

• Reciprocity • Matching (Maximum Power Transfer)
K.NASR CS6242 Intro. to Antennas 23/2/2006

Radiation Zones
• Reactive Near Field Zone • Radiating (Fresnel) Near Field Zone (Pattern still depends on distance) • Far Field (Fraunhofer) Zone (plane waves) ( Pattern is independent on the distance) e.g. for an antenna of 1m at 900MHz = 6 m

K.NASR

CS6242

Intro. to Antennas 23/2/2006

Main Antenna Parameters
1) Radiation Pattern (Field / Power Patterns) The radiation pattern (or antenna pattern) is the representation of the radiation properties of the antenna as a function of space coordinates. (squeeze power into a beam)

K.NASR

CS6242

Intro. to Antennas 23/2/2006

Radiation Pattern of a half wave dipole Zant = 73+j 42.5 Ω Rant (λ/4 monopole): ~37 Ω

Rant (folded dipole): ~292 Ω (Twin Cable : 300 Ω )

The Larger the antenna (or array), the narrower the beam hence more power can be concentrated.
K.NASR CS6242 Intro. to Antennas 23/2/2006

Main Antenna Parameters (ctd.)
2) Directivity D 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 (isotropic).
D (θ,φ) = S (θ,φ) / S ref , S ref (isotropic) = Total Power / 4 π r2 ; D isotropic = 1 •Since all real antennas will radiate more in some directions than in others, you can say that gain is the amount of power you can reach in one direction at the expense of the power lost in the others. When talking about gain it is always the main lobe that is discussed. Notes on Gain:

Antennas are passive. Gain means the power radiated /received is distributed over 3D space in a non uniform way: some directions must have gain below isotropic for others to have gain above isotropic.
K.NASR CS6242 Intro. to Antennas 23/2/2006

3) Directivity “ D”versus Gain “G” ( Antenna Efficiency : e)

• Mismatch losses • TL losses • Dielectric , Conduction and Polarisation losses

Omni (isotropic) : 0 dBi

G = e . D (dBi)
K.NASR CS6242

Dipole: 2.15 dBi =0 dBd 15 m Dish: 70 dBi
Intro. to Antennas 23/2/2006

Main Antenna Parameters (ctd.)
4) Impedance Matching, VSWR , Return Loss

5) Antenna Bandwidth: Broadband Antenna?
Thicker elements only serve to widen antenna bandwidth and up the power handling capability of an antenna system.
K.NASR CS6242 Intro. to Antennas 23/2/2006

Main Antenna Parameters (ctd.)
6) Polarisation The polarisation is the figure traced by the time varying field vector at a given point. (orientation of the field)

Linear

Circular

Elliptical

Generally, the polarization of the receiving antenna is not the same as the polarization of the incident wave. This is called polarization mismatch.
K.NASR CS6242 Intro. to Antennas 23/2/2006

Polarisation Loss Factor ( PLF ) or Polarisation efficiency

K.NASR

CS6242

Intro. to Antennas 23/2/2006

Typical Specifications

Testing : Anechoic Chamber
K.NASR CS6242 Intro. to Antennas 23/2/2006

Diversity
• The simplest diversity technique, spatial diversity, involves the use of two or more receive antennas at a base station, WLAN access point or mobile terminal that are separated by at least the coherence distance to guarantee statistically independent fading. • This relatively low-cost approach yields significant performance improvement by taking advantage of the statistical likelihood that when the phasor sum of paths reaching one antenna is close to zero, it is unlikely to be close to zero at the other antenna. When one antenna is in a fade, the other one will generally not be. If directive antennas are used, space or angle diversity can be implemented by pointing the antennas in different directions. • Interference ????

K.NASR

CS6242

Intro. to Antennas 23/2/2006

SMART (ADAPTIVE) ANTENNA SYSTEM BASIC IDEA
SIGNAL
RF DOWNCONVERSION

ARRAY PROCESSOR
A/D W1

RF DOWNCONVERSION

A/D

W2

∑

O/P
GENERATE ERROR SIGNAL

INTERFERENCE
RF DOWNCONVERSION

A/D

Wn

N elements can null out N-1 interferers
HOW THE PERFORMANCE SHOULD IMPROVE:
• BEAM STEERING ----- FOCUS ENERGY TOWARDS DESIRED USERS

ADAPTIVE ALGORITHM
DOA Estimation

RANGE EXTENSION

•NULL STEERING ------ ISOLATE COCHANNEL USERS

QUALITY /CAPACITY IMPROVEMENT HIGHER DATA

• OPTIMUM COMBINING / SPACE TIME PROCESSING ----- REDUCE MULTIPATH FADING & ISI RATES

•SDMA ------- MULTIPLE SIGNAL IN THE SAME BANDWIDTH
K.NASR CS6242

HIGHER DATA RATES Intro. to Antennas 23/2/2006

MIMO Multiple Input Multiple Output Systems
Exploit the rich multipath via space time coding and transmitting and receiving using multiple antennas

Tx

Rx

K.NASR

CS6242

Intro. to Antennas 23/2/2006

Example of a MIMO WLAN System

K.NASR

CS6242

Intro. to Antennas 23/2/2006

References
• J. D. Kraus, “Antennas” , McGraw Hill, 1988 • W. L. Stutzman and G. A. Thiele, “Antenna Theory and Design”, Wiley, 1998

K.NASR

CS6242

Intro. to Antennas 23/2/2006


				
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