IEEE TRANSACTIONS ON EDUCATION, VOL. E-28, NO. 1, FEBRUARY 1985

and second-level automatic element and coordinate generationhas been used in both the circular and rectangular portions.

This problem has been chosen to provide the student withan opportunity to compare FINEL's finite element solution toa closed-form analytic solution. By mounting an image chargecylinder below the ground plane and computing field contri-butions due to each cylinder using Gauss' law and symmetry,he/she can analytically compute the potential at any point ofinterest. For example, FINEL computes a normalized poten-tial at the center of the cylinder of 0.0123, as compared to ananalytic value of 0.0129.

REFERENCES

[11 Z. J. Csendes and J. R. Hamann, "Surge arrestor voltage distributionanalysis by the finite element method," IEEE Trans. Power App. Syst.,vol. PAS-100, pp. 1806-1813, Apr., 1981.

[21 J. L. Davis and J. F. Hoburg, "Wire-duct precipitator field and chargecomputation using finite element and characteristic methods," J. Electro-statics, vol. 14, pp. 187-199, 1983.

[3] J. F. Hoburg and J. L. Davis, "A student-oriented finite element programfor electrostatic potential problems," IEEE Trans. Educ., vol. E-26, pp.138-142, Nov. 1983.

[4] P. P. Silvester et al., "Exterior finite elements for 2-dimensional fieldproblems with open boundaries," Proc. Inst. Elec. Eng., vol. 124, no.12, pp. 1267-1270, Dec. 1977.

James L. Davis, photograph and biography not available at the time of pub-lication.

James F. Hoburg (S'64-S'75-M'75), photograph and biography not availableat the time of publication.

Microstrip Circuits for the Classroom LaboratoryC. L. SAYRE, III, MEMBER, IEEE

Abstract-The procedure and background for an experiment on thedesign of microwave microstrip circuits are presented. An effort is madeto keep the experiment simple and the costs to a minimum so that theexperiment can be easily implemented using the facilities of a school. Thecircuits are fabricated using a copper-tape construction method. The ex-periment is intended for a senior-level course.

INTRODUCTION7 fICROSTRIP circuits are commonly used to make com-lVJponents such as transmission lines, impedance matchingnetworks, resonators, filters, and directional couplers in mi-crowave communication and radar systems. Compared to metalwaveguides, microstrip has the advantages of small size, lightweight, and easy implementation in integrated circuit form.The fabrication is less costly compared to the machining ofwaveguide parts because microstrip circuits are fabricatedusing integrated circuit masks and photolithographic process-ing steps. Because of the prolific use of microstrip, microwaveengineering students should be introduced to its properties.

Manuscript received January 23, 1984; revised July 17, 1984.The author was with Cornell University, Ithaca, NY. He is now with AT&T

Bell Laboratories, Whippany, NJ 07981.

This paper outlines a lab experiment that exposes students tothe fundamentals of microstrip by having them design, con-struct, and test their own circuits.Modern microwave laboratories have been established at

numerous schools including Cornell University, the Universityof Massachusetts [1], the Universidad Estadual de Campinas(Campinas, Brazil) [2], and the University of Maryland [3].Many of these schools use the labs as part of a laboratorycourse on microwave engineering. The basics of microstrip areoften included among the topics covered. For example, atCampinas the students have constructed microstrip circuitsfrom copper-clad substrates using both a photographic tech-nique and by applying a mask directly to the surface of theboard. Both of these techniques will be briefly described inthe section on Circuit Fabrication. Also described is a copper-tape construction method that allows circuits to be quickly fab-ricated with a small amount of tools and materials.

STRUCTURE OF MICROSTRIPMicrostrip is an open boundary waveguide that consists of

a solid conducting layer that forms the ground plane, a dielec-tric substrate, and a pattern of conducting strips (Fig. 1). The

0018-9359/84/0200-0028$01.00 © 1985 IEEE

28

SAYRE: MICROSTRIP CIRCUITS

Strip conductor

Ground planeFig. 1. Structure of a microstrip circuit.

physical dimensions of the strip conductors, the dielectric con-stant of the substrate, and the substrate thickness control thecharacteristic impedance, capacitance, and inductance of thecircuit elements [4], [5]. A wide variety of passive networkscan be implemented by using the strip conductors to form thecircuit elements. In addition, since the top surface is open,active devices such as transistors can easily be mounted in amicrostrip network.

CIRCUIT FABRICATION

There are several techniques that are used to fabricate mi-crostrip circuits including photographic processes, the directmask technique, and the copper-tape technique. The conven-tional method is to apply a mask to one of the copper surfacesand then etch out the microstrip lines. When high resolutionand repeatability are required, a photographic processing tech-nique is commonly used [6]. In this process, a negative iscreated that defines the circuit paths and elements. Next, aphotoresist is applied to the copper surface and then the sur-face is exposed to light through the negative. After this, theboard is developed and etched. A solution of ferric chloride,an etchant commonly used in making printed circuit boards,is frequently used to do the etching. Etching removes the un-wanted copper from the surface of the board with the remain-ing pattern of copper forming the circuit paths. After the re-maining photoresist s removed and the board is cleaned, themicrostrip circuit is ready for testing. The process can be sim-plified by applying the mask directly to the copper surfacewith tape or India ink. The direct mask technique requiresfewer materials (i.e., no photoresist, developer, or light sourceare required) and takes fewer steps than the photographic pro-cess; however, the resolution of the direct mask technique isnot as good. While working on a research project at CornellUniversity, I found that about 1 h is needed to produce a mi-crostrip circuit using the direct mask technique.On the other hand, a copper-tape construction method pro-

vides an even quicker technique for making circuits. With thecopper-tape method, a circuit can be constructed in less than15 min. To make a circuit, first one side of a copper-clad mi-crowave substrate is etched completely clean of copoer. Thiscan be done using ferric chloride. Next, the strips that formthe transmission lines and circuit elements are cut from thecopper tape and applied directly to the substrate. The solidcopper layer on the reverse side of the board forms the groundplane. The circuit is now ready to have connectors attached orfor mounting in a test fixture. Measurements of transmissionlines made from copper tape indicate that their performance

Transmission feed lines(copper tape)

Resonant section(copper tape)

Dielectric tape

Ground plane Dielectric substrate

Fig. 2. Resonator constructed using copper and dielectric tape.

Fig. 3. Test equipment configuration.

is comparable to that of etched transmission lines. In resonatorcircuits, a high degree of coupling between the resonant sec-tion and transmission feedline can be achieved using the sand-wich method of forming gaps as shown in Fig. 2. In formingthe gap, the resonant section and feedline are separated by apiece of dielectric tape such as Temp-R-Tape [7]. The widthof the gap (or amount of overlap for low insertion loss circuits)will adjust the amount of coupling.

THE CLASSROOM EXPERIMENT

The Solid State Microwave Systems class at Cornell Uni-versity covers both the theory and application of microwavedevices and circuits. Classroom lectures are supplemented byweekly lab experiments where students learn the techniques ofmicrowave measurements. An experiment using one 90-minlab period was designed where the students constructed andtested several microstrip circuits that operated in the 1-5 GHzregion. It was decided to have the students build resonatorssince the design calculations are straightforward and resona-tors are easily constructed with copper tape. In order to intro-duce the experiment, a lecture covering the theory of micro-strip and the lab procedure was presented. Graphs ofmicrostrip characteristic impedance versus strip width and ef-fective dielectric constant versus strip width were distributedas design aids. The students were instructed to do the calcu-lations for their resonators before coming to the lab. In thefirst part of the experiment, the students measure the fre-quency response of a transmission line and a resonator that areprovided for them (see Fig. 3). The measurements are madewith a network analyzer which the students are familiar withfrom previous experiments.

29

IEEE TRANSACTIONS ON EDUCATION, VOL. E-28, NO. 1, FEBRUARY 1985

Zz7ZZZZZA Z Th//,-log

(aI) (b) (c)Fig. 4. Microstrip resonators. (a) Side-coupled resonator, (b) ring resonator,

and (c) series-coupled straight resonator.

4700 4740

f (MHz)4780 4820 4860 4900 4940 4980

l l

Fig. 5. Copper-tape resonator.

For the second part of the experiment, several resonatorswere constructed from copper tape and Epsilam-10 circuitboards. The circuit boards, which were 1.5 in square with a

substrate thickness of 0.025 in, had been prepared before classby etching away one side of the copper. Several designs in-cluding a side-coupled resonator, a ring resonator, and a se-

ries-coupled resonator [8] were suggested as possible circuits(see Fig. 4). The next step was for the students to mount theresonators in a test fixture and measure the frequency re-

sponse. Problems observed in testing the resonators were oftenfixed by adding or moving pieces of copper tape. In measuringa resonator the parameters of interest are the resonant fre-quency, the midband insertion loss, the Q-factor, and the half-power bandwidth [9]. The resonant frequency is determinedby the length of the resonant section. A resonance is observedwhen the length of the resonant section is one-half of a guidedwavelength (or a multiple thereof). The frequency response ofa typical series-coupled straight resonator constructed in thislab is shown in Fig. 5. It has a resonant frequency of 4.82GHz, a midband insertion loss of 6 dB, a bandwidth of 150MHz, and a loaded Q-factor of 32. The students were askedto compare the measured resonant frequency to the theoreticalresonant frequency that is calculted from the physical dimen-sions of the resonator. The resonator in Fig. 5 has a theoreticalresonant frequency of 4.33 GHz which differs by 10 percentfrom the measured value. The measured and calculated dataof the experiment were included in a lab report submitted fol-lowing the completion of the experiment. Most of the lab

groups constructed and tested two or three resonators. Veryfew problems were encountered in running the lab and mostof the students quickly grasped the concepts behind designingsimple microstrip circuits.

MATERIALSIn order to fabricate circuits using the copper-tape tech-

nique, only a few materials are needed. Copper-clad laminatessuch as Epsilam-10 [10] and RT-Duroid [11] work well as cir-cuit boards since they are easy to cut and process and are mod-erately priced. Also required is a copper tape with adhesivebacking such as the X1181 copper tape of the 3M Company[12] or the E-Z Circuit EZ302501 copper tape of BishopGraphics Inc. [13]. For dielectric tape the Temp-R-Tape typeK104 (0.001 in thick) of the Chamberlin Rubber Companyworks well [7]. For connectors, an APC-7-to-microstrip con-

nector [14] is instrumentation quality and excellent for mea-

surements; however, they are expensive. A more moderatelypriced alternative is an SMA-to-microstrip connector or an

SMA connector with a solder tab [14]. Additional materialsthat are needed include: an X-ACTO knife, steel ruler, ferricchloride solution, and a magnifier.

CONCLUSIONSA review of the lab reports submitted by the Cornell stu-

dents indicates that the experiment successfully demonstratedthe basic properties of microstrip circuits. The copper-tapeconstruction technique enabled the students to build simplemicrostrip circuits with a minimum of time and material.

7IE UII I¢iLtU

0-

IS211(d B)

-5-

30

LI-- I. 5 In, li

r

t4- 1.ol 't n --)-i o.0 4 'tr%

10o

IEEE TRANSACTIONS ON EDUCATION, VOL. E-28, NO. 1, FEBRUARY 1985

ACKNOWLEDGMENTThe author would like to thank Prof. G. C. Dalman for his

assistance during the course of this project. The author wouldalso like to thank C. Carver for typing the manuscript.

REFERENCES

[1] R. E. McIntosh, "A modern undergraduate microwave engineering lab-oratory," IEEE Trans. Educ., vol. E-13, pp. 110-114, Aug. 1970.

[2] A. J. Giarola and D. A. Rogers, "A successful approach to teachingmicrowave-integrated-circuit techniques," in Proc. 1978 Frontiers inEduc. Conf:, Oct. 1978, pp. 105-108.

[3] K. Zaki and R. W. Newcomb, "A microwave-circuits laboratory withmultilevel educational objectives," IEEE Trans. Educ., vol. E-20, pp.108-111, May 1977.

[4] T. C. Edwards, Foundations for Microstrip Circuit Design. New York:Wiley, 1981, p. 44.

[5] K. C. Gupta et al., Microstrip Lines and Slotlines. Dedham, MA:Artech, 1979, p. 87.

[6] W. A. Vossberg, "Stripping the mystery from strip line laminates," 3MCompany, application note.

[7] Temp-R-Tape, Chamberlin Rubber Company, Rochester, NY.[8] T. C. Edwards, Foundationsfor Microstrip Circuit Design. New York:

Wiley, 1981, p. 184.[9] "Cu-Tips 7: Test method for apparent substrate dielectric constant, ef-

fective microstrip dielectric constant, and unloaded Q of Epsilam-10,"3M Company, application note.

[10] Epsilam-10 Microwave Substrate, 3M Company, Electronic ProductsDivision, St. Paul, MN.

[11] RT-Duroid Microwave Substrate, Rogers Corp., Chandler, AZ.[12] 3M Company, Industrial Electrical Products Division, St. Paul, MN.[13] Bishop Graphics, Inc., Westlake Village, CA.[14] Omni-Spectra Inc., Merrimack, NH.

Clifford L. Sayre, HI (S'78-M'83) photograph and biography not availableat the time of publication.

Exercises for an Introductory Image Processing ClassBRUCE C. WHEELER, MEMBER, IEEE

Abstract-A user-friendly computer program has been written for use

with large introductory image processing classes. Associated exercisesgive students hands-on insight into many techniques, including spatialand frequency domain filtering, smoothing, and sharpening, as well asseveral medical imaging applications.

INTRODUCTIONS INCE advanced image display and processing equipment

is very expensive, large class instruction in image pro-cessing is usually restricted to theoretical classroom material.Students, especially those who are average or below averagein academic ability or prior training, suffer a lack of intuitiveunderstanding of the material due to the lack of "hands-on"experience. In contrast to the expensive, inaccessible, ad-vanced display equipment available in research facilities, thecentral computing facilities can provide students with imagesof adequate quality for instructional purposes. A properlystructured, "user-friendly" image processing control programcan provide students with significant digital image processingexperience at a basic level. For the past two years such a pro-gram has been used as a basis for a series of exercises whichcomplemented a first course in image processing at the Uni-versity of Illinois at Urbana-Champaign.

Manuscript received January 9, 1984; revised April 4, 1984.The author is with the Department of Electrical Engineering and the

Bioengineering Faculty, University of Illinois at Urbana-Champaign, Urbana,IL 61801.

COURSE CONTENT

This one-semester course, "Computer Processing of Med-ical Images," is taught to juniors and seniors in electrical en-gineering and bioengineering, with a few graduate and medi-cal students in attendance. The prerequisite is an introductionto systems analysis, including only a modest amount of Fou-rier transform theory. The first half of the course heavily em-phasizes digital signal processing aspects of image processingand is drawn from the first five chapters of the text by Gon-zalez and Wintz [1]. The exercises described below concen-trate on the material from this half of the course. The secondhalf of the course emphasizes medical applications, includingnuclear medicine, ultrasound, computed tomography, and nu-clear magnetic resonance imaging. Students also complete fourexercises on an Ohio Nuclear nuclear medicine imaging sys-tem to which access is restricted.

INSTRUCTIONAL IMAGE PROCESSING SYSTEM

The objective for the software was to provide a simple, eas-ily used, yet highly flexible program for manipulation of im-ages. Students should be able not only to complete the as-signed exercises but also to experiment with a variety of relatedtechniques discussed in lecture. Students do no programming,except for special projects, and are, instead, free to concen-trate on the image processing manipulations.

The master program was written in Fortran IV for the Uni-versity's CDC Cyber 174. The only installation dependent fea-

0018-9359/85/0200-0031 $01.00 © 1985 IEEE

31