FractalComsExploring the limits of Fractal Electrodynamics for the future telecommunication

technologiesINFORMATION SOCIETY TECHNOLOGIES (IST) PROGRAMME

Quality Factor and Radiation Efficiency Measurement of Genetically Designed Planar Monopoles

WP4 Fractal devices development

Task 4.3: Prototype construction and measurement

J.M. González and J. Romeu

May 2003

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University of Granada applied GA for the design of Koch-like small antennas, developing a Pareto tool to optimize several characteristics of pre-fractal wire antennas simultaneously.

The code was also used to generate Euclidean structures to investigate wether fractal shapes might be the best alternative for the design of efficient antennas with minimum resonant frequency.

NEC was used to carry out the simulations. The space to be filled by the antennas was a rectangle of

dimensions h x w.

University of Granada Request (1)

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Pre-fractal Meander Line ZigzagResonant Freq.

Matching Freq.

Matching

Quality Factor

Efficiency

University of Granada Request (2)

868 MHz

878 MHz

-7.6 dB

13.6

96.7 %

827 MHz

836 MHz

-8.3 dB

12.8

97.1 %

825 MHz

836 MHz

-7.3 dB

14.1

96.7 %

h=6.22 cm

w=1.73 cm

a=6.45 cm a=6.24 cm

Copper wire = 0.2 mm

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Standard electronics printed circuits board photo-etching technology was used to manufacture the monopoles.

FR4 fiberglass substrate: thickness 0.25 mm. Copper strips: 0.29 mm width and 0.25 m thick to have the same electrical section than the wire models.

Monopoles were mounted on a 80 x 80 cm ground plane. Connection to the RF source through an SMA connector.

Fabricated Monopoles (1)

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Meander Line Zigzag /4Pre-fractal

Fabricated Monopoles (2)

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and Q measured using the Wheeler cap method. The input impedance of the AUT is measured at the resonant

frequency of the antenna with and without a cap. The cap is a metallic surface that completely encloses the AUT.

Measurement Procedure (1)

/2

Zin=Rr+R+jXin Zcap=R+jXcap@ at resonance f0:

@ RLC model:

in

capin

r

r

Z

ZZ

RR

R

Re

ReRe

inin

r

X

d

dX

RQ

2

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To accurately model the input impedance of the antenna as an RLC circuit (series or parallel) a rotation on the measured data is applied to the Smith chart plot.

Measurement Procedure (2)

Matching Freq.= Resonant Freq.

Matching Freq.

Free-space input impedance

Resonant Freq.

Rot

atio

n A

ngle

Wheeler cap input impedance

Rotated free-spaceinput impedance

Rotated Wheeler cap input impedance

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The cap used to carry out the measurements was an aluminium cylinder (height: 12.5 cm; diameter: 6 cm).

Measurement Procedure (3)

-12

-10

-8

-6

-4

-2

0

2

1 2 3 4 5 60.1

Re

turn

Lo

ss

es

(d

B)

f (GHz)

TM 0

1,1

TM 0

1,5

TM 0

1,9

TE 1

1,i

TE 2

1,i

radi

anle

ngth

lim

it

Modes inside a cylindrical cap excited by a /4 monopole resonant at 694 MHz and skewed 10º.

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Radiation Efficiency

Measurement Results (1)

t: strip thickness;w: measured strip wideness;w': desired strip wideness;: measured radiation efficiency;': corrected radiation efficiency;simulated: computed radiation efficiency;

Antenna t (mm) w (mm) w' (mm) (%) ’ (%) /4 0,025 0,380 0,290 94,14 92,58 IFS 0,025 0,395 0,290 90,03 87,13

Meander Line 0,025 0,385 0,290 90,76 88,30 Zigzag 0,025 0,430 0,290 90,47 86,79

simulated (%)

98,40 96,70 97,07 96,65

10 log (’/simulated) -0,26 dB -0,45 dB -0,41 dB -0,47 dB

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Measurement Results (2)

Quality Factor

Q: measured quality factor;Qsimulated: computed quality factor;

Antenna Q /4 9,42 IFS 12,67

Meander Line 12,60 Zigzag 13,89

Qsimulated 7,96

13,59 12,76 14,07

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Conclusions

The expected behavior of GA designs was assessed by comparison with a /4 monopole.

Differences between simulated and measured designs: wires / strips ( same electrical section) no connector / SMA connector soldering losses conductivity of copper additional losses

Not corrected effects: contact between cap and ground plane sistematical errors: do not influence measured but Q