Introduction to Antennas & Radiating Systems

Austin Peay State University

William CookeDepartment of Physics & Astronomy

J. Caleb WherryDepartment of Computer Science

Outline

2

I. Introduction

II. What is An Antenna?

i. Transmission Lines

ii. Free Space

iii. The Antenna

III. How An Antenna Works

i. Radiating from Accelerating Particles

ii. Electric and Magnetic Fields

iii. Radiation Patterns

IV. The /2 Short Dipole

V. Conclusion

Introduction

3

What is An Antenna?

4

Transmission Line/Guided PathA transmission line is a path in which a current flows. This line can be a

typical circuit, coaxial cable, or any other material with a well-defined, complex impedance.

Free SpaceFree space is made up of fields and void of other matter. These fields carry

energy in the form of waves which can travel freely throughout.

AntennaAn antenna is the interface between the above transmission lines and free

space. It takes the current from the transmission lines and converts it to waves which get transmitted into free space, and vice versa.

How An Antenna Works

5

How An Antenna Works

6

How An Antenna Works

7

Image Source: http://en.wikipedia.org/wiki/Dipole

Field Patterns

8

Field Pattern Lobes

9

HPBW: half power beam width

Power Pattern

10

Main lobe axisMain lobe

Minor lobes

][1

)(0

BES

Back lobes

11

12

)(

0 )]2

(2

sin[)]([ c

rtj

ezL

IzI

Retarded Current distance z away from poles:

z dz

Integrating over all dz’s to get E & H components of far field:

0

coscos[ ] cos( )[ ] 2 2[ ]

2 sin120

L Lj I

Er

H E Z E

13

]sin

)2

cos(]2cos

cos[[

2

][ 0

LL

r

IjE

We can then find the Power by integrating the above field over a large surface of the Poynting vector:

20

2

0 0

2* 30sin IddrHEPrad

0

2

020

230 R

IIPrad

Radiation Resistance:

730R

Conclusion

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Antennas are all around us! Examples listed here are only a small subset of a rich field of study.

Antennas’ shape are of great importance in the theory of transmission and receiving of antennas (Optimization theory – Genetic Algorithms).

A better understanding of how antennas work gives us an understanding of the electromagnetic radiation into or

out of our electronic device.

Enjoy your summer!

References

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Griffiths, D.J. Introduction to Electrodynamics. 3rd ed. Prentice-Hall: Upper Saddle River, NJ. 1999.

Kraus, J.D. Antennas. 2nd ed. McGraw-Hill: New York.1988.

Questions &| Comments

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Questions &| Comments?