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1036 PIERS Proceedings, Moscow, Russia, August 1923, 2012

Design of Stripline Structure for Electromagnetic Characterization

at Microwave Frequency

Ellen Yoshie Sudo Lutif1, 2, Anderson Kenji Hirata2,Alberto Jose de Faro Orlando1, and Antonio Carlos da Cunha Migliano1, 2

1Aerospace Technological Institute (ITA), CTA, Brazil2Institute of Advanced Studies (IEAv), CTA, Brazil

Abstract A comprehensive approach to the design of a stripline for EMC testing is given inthis paper. The authors attention has been focused on the design items that are most crucial bythe achievement of satisfactory value of the VSWR and impedance. The characteristic impedanceof the stripline test section should be smoothly matched with the feed and terminations pointsin order to minimize the standing waves. Thereby, the most critical parameters that directlydetermine the physical design of the stripline are impedance matching at the feed port (S11parameter) and transmission between two ports (S21 parameter). An analysis can be performedfor the stripline configuration using a vector network analyzer. A measurement of the reflectionfrom transmission through a material along with knowledge of its physical dimensions providesthe information to characterize electromagnetic waves at microwave frequencies range.

1. INTRODUCTION

Typical striplines are constructed to have an impedance of either 50 or 90 . The ratio betweenthe width of the active conductor and the height of the active conductor and the height of the activeconductor above the ground plane determines the characteristic impedance. The design given inthis paper is focused on the 50 stripline.

Today in communication systems the use of magnetic and dielectric materials exceeds the usualfields of application (randomes, antennae, microwave circuits, . . . ). New components are developedto meet the demand of leading areas. This is the case for materials absorbing the electromagneticenergy, which are used for microwave electromagnetic compatibility (EMC). A vector analyzer isa versatile measurement system, which comprises of a two or four channels for microwave receiverdesigned to process the magnitude and phase of transmitted and reflected waves of the network. It

directly displays the S-parameters of passive and active networks at the desired frequency range.With advancement of technology, VNA are available now with full range of parameters to bemeasured like S-parameters in magnitude (dB)/phase form, real/imaginary form, as well as inthe linear form, VSWR, Group delay, impedance, etc.. When dealing with vector measurementquantities, such as complex reflection and transmission coefficients (i.e., S-parameters) in RF andmicrowave metrology, several important factors need to be considered such as the expression formof the complex quantities (either in the real and imaginary components or magnitude and phasecomponents) and correlation between these components [1].

Earlier the magnitude and phase form of complex S-parameter was selected as the measurand.The uncertainties in the magnitude and phase form of the VNA measurements have been studiedand reported earlier [2]. The mathematical model for determining the measurement uncertaintydepends on the type of measurand. The studies showed an ambiguity in the phase measurement,where phase depends highly on the structure and application of device under test (DUT) as well as

the operating frequency. To avoid the problems during the statistical analysis of complex quantitiesin the magnitude/phase form, the real and imaginary form has been chosen to analyze the complexquantities. In this form, the real and imaginary components of complex S-parameter are correlated,so their covariance also contributes to the uncertainty.

2. THEORY

2.1. Stripline Design

A stripline consists of upper and down grounding plates, and the central conductor. Between thegrounding plates and the central conductor is air or dielectric materials.

The fundamental propagation mode for a stripline is TEM. For the TEM wave propagation ina stripline, the phase velocity is:

vp =c

r(1)

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Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1923, 2012 1037

where r is the dielectric constant of the filling medium and c is the speed of light.If the central conductor is narrow, the interference between the fields at the two edges cannot

be neglected. We may take the central conductor as a cylinder by introducing equivalent diameteras shown in the equation above:

d =

2 1 +t

1 + ln4

t+ 0.51

t

2

(2)where the characteristic impedance can be calculated using the following equation:

Z0 =60r

ln

4b

d

() (3)

2.2. The Stripline for Material Characterization

Open structures can radiate and have a complicated field structure. Measuring the permittiv-ity of lossy materials, circuit boards, thin films, and substrates nondestructively is frequently ofinterest [4].

When the dielectric is solid (as opposed to being air), as is usually the case, the speed atwhich the wave travels along the transmission line (velocity of propagation) is reduced, as is thewavelength [6, 7]. The actual stripline wavelength () is equal to the free space wavelength (0)divided by the square root of the relative permittivity (r):

=0r

(4)

To emphasize the importance of the dielectric constant to the physical size of stripline, the tablebelow shows five frequencies and their wavelengths in air and in two types of dielectrics [5].

The Table 1 shows how the dielectric constant of the measured material increases, the requiredsize of the stripline components may be reduced [810]. Because the dielectric constant controlsthe wavelengths in the stripline circuit, it is a critical property in all applications; however, thethickness of the dielectric is often of equal importance. The characteristic impedance (Z0) afundamental design parameter for all stripline circuits depends on the dielectric constant [1113],the width and thickness of the conductor, and the thickness of the dielectric layers.

This structure of the stripline with two ground planes as shown in Figure 1 has a much higherquality factor than the microstrip line. Also, this stripline structure is very useful for broadband

Table 1: Wavelength versus frequency at different materials.

Frequency (GHz) 0 (air) in inch (r = 5) in inch (r = 9) in inch

0.50 23.60 10.5 7.87

1.00 11.80 5.27 3.93

3.00 3.93 1.75 1.31

5.00 2.36 0.99 0.79

12.00 0.98 0.44 0.33

Figure 1: Stripline structure cross-section connected with a vector network analyzer.

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1038 PIERS Proceedings, Moscow, Russia, August 1923, 2012

circuits, since it can be modeled by assuming TEM propagation and using a standard lossy trans-mission line model [14]; the transmission line can be characterized by a characteristic impedanceZ0 and a complex propagation constant = a +j. Using this model the properties can be foundby time or frequency domain measurements [15, 16]. This stripline technique depends on the factthat the conductor loss and the dielectric loss vary differently with frequency in order to separatethe loss terms from the total attenuation.

A cross section of the stripline configuration is shown in Figure 1. It depicts a narrow, flat

strip of perfect conductor sandwiched between two outer layers. The outer surfaces of the dielectricsheets are faced with perfect conductor. The circuit metallization is located in the middle of thelayers. Metallic plates are located at the top and bottom of the structure, resulting in a striplinestructure.

3. RESULTS

Relative complex permittivity (permittivity) of printed circuit (PC) board and substrate materialis a critical parameter that affects circuit performance.

Characterizing this parameter at RF is becoming more important because of increased clockfrequencies used in todays high speed computers. In addition, performance of dielectric materialsat RF is equally important for wireless communication circuits and components. The goal wasto perform a physical design of stripline according to the ISO standard [3] by which the improve-

ments ofS11

and S21

parameters were achieved by an application of the experience from numericalsimulations.The calibration of the cables assures a perfect matching with 50 Ohms in the frequency of 0 GHz

until 12 GHz. The results of VSWR show dimensional resonance in frequency range, accordingFigure 4 and Figure 5. According to the results the work area changes a lot in the frequency range,

Figure 2: Reflection coefficient as a function of fre-quency.

Figure 3: Reflection coefficient as a function of fre-quency.

Figure 4: VSWR as a function of frequency. Figure 5: VSWR as a function of frequency.

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Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1923, 2012 1039

accordind Figure 2 and Figure 3. The electromagnetic sensibility is related with low reflectionsmaller than 10 dB and transmission near than 0 dB.

4. CONCLUSION

The goal of this paper was to provide with the comprehensive design study of stripline with focuson the achievement of satisfactory level of the VSWR and reflection coefficient at the extendedfrequency range from 0 GHz to 12 GHz. The VSWR of stripline model was lower than by the

commercial equipment in the upper frequency range from 1 GHz to 4 GHz.

ACKNOWLEDGMENT

The authors wish to thank the laboratory of electromagnetic systems at Advanced Study Institutefor infrastructure offered. And, they wish to thank Microwave Department at Aerospace Techno-logical Institute for the realization of this work.

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