antenna basics 4

8/8/2019 Antenna Basics 4

1/27

15 Feb 2001 Property of R. Struzak 1

Antenna Fundamentals (4)

R. Struzak

[email protected]

School on Digital and Multimedia Communications Using Terrestrial and Satellite Radio Links

The Abdus Salam International Centre for Theoretical Physics ICTP Trieste (Italy) 12 February 2 March 2001


2/27


Note: These materials may be used for study,research, and education in not-for-profitapplications. If you link to or cite these materials,

please credit the author, Ryszard Struzak. Thesematerials may not be published, copied to orissued from another Web server without theauthor's express permission. Copyright 2001Ryszard Struzak. All commercial rights arereserved. If you have comments or suggestions,please contact the author [email protected].


3/27


Summary Slide Antenna Measurements

Antenna Calibration Anechoic Chamber

TEM Cells

3D Radiation Pattern Measurements Conclusion


4/27


Transmission vs. Reception


5/27


Transmit/ Receive Equivalence Reciprocity theorem.

The radiation pattern and radiation resistance of an

antenna is the same when it transmits and when itreceives, if no non-reciprocal devices are used.

Does not apply to active antennas


6/27


Antenna Measurements


7/27


Radiation Pattern Measurements (1) Measured antenna in

receiving mode

The antenna is rotated(or the radiowavesource is movedaround)

The power received

(output voltage) isregistered vs. thedirection angle(azimuth, elevation)

Measured antenna in

transmitting mode

The antenna is rotated(or the field-strength

meter is moved

around)

The field-strength isregistered vs. the

direction angle

(azimuth, elevation)


8/27


Radiation Pattern Measurements (2) Laboratory

Special test site

Open field Anechoic chamber

Near-field / Far field

calculation

Scaling

Field

In-situ measurements

Measuring instrumentsin car, balloon,

aeroplane, or helicopter

Actual distance /

standard distance

problem Environmental effects


9/27


Electric Field Measurement Dipole antenna

Balance matching

Impedance matching


10/27


Wideband Antennas Impedance and

radiation pattern of

antenna are frequencydependent

Wideband antennas

Conical antennas

Equi-angular antennas

Log-spiral antennas

Log-periodic antennas


11/27


Impedance Matching For maximum power transfer

the load impedance must matchthe source impedance:

RLOAD = RSOURCE

XLOAD = -XSOURCE Transmission line must

terminate in its characteristicimpedance

The balanced/ unbalancedmode-continuity must beassured or a transformer (balun)must be used

BALANCED

UNBALANCED

P/2


12/27


Magnetic Field Measurement Loop antenna

Screen against

electric component

Slot


13/27


Calibration


14/27


Gain Measurements: 2 Antennas Reciprocity method

2 identical antennas are

used: one as thetransmitting antenna

and another as

receiving antenna

The ratio of the power

received to power

transmitted is

measured

!

!!

!

P

T

T

P

r

P

P

G

GGG

r

GGPP

T

R

RT

RTTR

4

4

2


15/27


Gain Measurements: 3 Antennas The 3-antenna method can

be used to calibrate 3

arbitrary antennas.

3 measurements are made,

giving 3 equations with 3

unknown gains

It is the only method

applicable to active

antennas that cannot be

used in transmit mode.

2

3113

2

3223

2

2112

4

4

4

r

r

r

T

P

T

P

T

P


16/27


Calibrating Test Antennas (1) Simulation of free-

space conditions

Removing the reflectedray by using absorbers

Exploiting directivity(radiation nulls)

Practical with vertical

polarization Does not require

anechoic chamber


17/27


Calibrating Test Antennas (2) Exploiting reflection

Using conducting

surface Adjusting antenna

height to receive

maximum

Practical withhorizontal polarization

Does not require

anechoic chamber


18/27


Measurements in the Field Relative (comparative)

measurements using

an auxiliary antenna ofknown radiation

pattern eliminate the

distance dependence

Auxiliary antenna

Antenna

under test


19/27


Far-Field Conditions1. R >> (P/2T)

2. R >> 2L2 / P

O

R

R+H

H

L

H > (L2 /8) * (16/ P)

Important when dealing with

radiation nulls

R


20/27


Example Antenna diameter = 2 m

1. 300 MHz (P = 1 m) P/2T= 1/ 6.28 ~ 0.16 m

2L2/ P = 8 / 1 = 8 m

2. 3000 MHz (P = 0.1 m) P/2T= 0.1/6.28 ~ 0.016 m

2L2/ P = 8 / 0.1 = 80 m


21/27


Anechoic Chamber Needs

Benefits

Problems

Practical examples

and applications

Photographs


22/27


TEM Cells Needs

Benefits Problems

Practical examples and applications

Double-polarization TEM Cell at theauthors institute (photographs)


23/27


3DRadiation Pattern Measurements Needs

Benefits

Problems

Flying radio-measurement laboratory

of the authors institute (photographs)


24/27


Conclusion Antenna: substantial element of radio link

We have just reviewed Basic concepts

Radio wave radiation physics

Elementary radiators

Antenna systems Antenna measurements


25/27


For Further Readings Kraus JD: Antennas (1998)

Stutzman WL et al.: Antenna Theory and

Design (1981)

Johnson RC: Antenna Engineering

Handbook (1993)


26/27


Computer Programs NEC (Numerical

Electromagnetics

Code) Free software:

NEC Archives:

(www . gsl . net /

wb6tpu /swindex .

html). More than 300

NEC matches.

Pozar D. Antenna Design

Using Personal Computers

Li et al., Microcomputer

Tools for Communication

Engineering


27/27


Thank you for your attention

antenna basics 4

Documents