B34.4.

A WIDE-BAND 3-DB COUPLER WITH A VERY TIGHTLY COUPLEDCROSS-SECTION OF MICROSTRIP LINES

Krzysztof Sachse

Technical University of Wroclaw, Wroclaw, Poland

1* Introduction

The 3-dB quadrature hybrid couplers are commonly used as integ-ral components in microwave integrated circuits. The multioctavebandwidth for these couplers can be achieved by designing them oncoupled lines I. The major problem encountered in the developmentof these couplers designed on microstrip lines is the realisationof the tightly coupled section. This problem may be partially sol-ved by means of a floating potential conductor applied on the topof a dielectric overlay 2, or by means of a shield used as a "re-entrant" coupled section of microstrip lines 3.

In the paper the connection of these two methods /Fig. 1/ usedin order to achieve a very tightly coupled cross-section of micro-strip lines is described. The analysis and the calculations of thestructure are presented. The experimental results of the 35dB hyb-rid couplers are shown.

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Fig, 1. Very tightlyr coupled cross-seetionof microstrip lines

S "R "R b u" ^,w, wR ,1u uu ,b Iu ,u -, -R -u w-- |",u l,| u |uThe author wishes to express his acknowledgment to Professor Dr.

Marian Suski for his assistance and encouragement as well gratefulto the Centre of Semiconductors at Wroclaw which support this work.

Analysis and calculations

It is well known that the analysis of the couplers can be car-ried out by means of its even- and odd-mode impedance /or capaci -

tanoe/. For the odd-mode, a zero potential of additional conductorsshould be assumed, since they are symmetrically located above andunder the strips. The odd-mode capacitance will be considerably in-creased if H1 will be sufficiently small. For the even-mode, thepotential of the upper conductor will be close to the potential ofthe strips, so that the even-mode capacitance should not be changedsubstantially by the presence of this conductor. The small value ofthis capacitance will be attained by means of using a lower oonduc-tor, since the capacitance between this conductor and ground isconneoted in series with the remaining ones.

This approach should be treated as an electrostatic boundary va-lue problem. It is based upon the solution of the Laplace; equationand appropriate boundary conditions and then upon the calculationof the capacitances. The computerised program based on finite-difo-ference teohniquo 4 using ALGOL for ODRA-1204 digital machine waswritten.Numerical calculations for various values of dimensions andfor the empty and inhomogenous dielectric-filled space were carriedout. But not all the results are presented here /lack of space!/The curves from Fig. 2 present only the even and odd-mode impedan-oes for B/H = 0.05 and for the empty and dielectric-filled space.

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aOt __02_L4 o.s1 0.2. 0.3 0. 0.S 0.1 0.2 0.3'wv/H 301 . W/H

Fig. 2. Even- and odd-mode impedancesfor coupled microstrip lines

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3. Experimental results

In order to present the application of the structure for micro-wave integrated circuits,a number of couplers were made and tested:

351. Single section 3-dB couplersThe even- and odd-mode impedance of these couplers should be 121

and 20.7 ohms/for a input impedance of 50 ohms/. From Fig. 2 we canfind the following values necessary for the couplers design: A/W=oo1W/H=o.15, H1/H-0.03, S/H=0.05,W1=2W, Ho=H and H2=0.5H. The couplerswere designed for 1.5-, 2.5- and 3.0 GHz. They were made of aluminasubstrate /1.5 mm thickness/, alina sheet,,and mica interlay whichseparates the microstrip lines from the upper conductor. In Fig. 3,a coupler and the measured response of the coupling are shown.

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FA.EQ.- CwHZ4Fig. 5. Single section 5-dB coupler and measured response

Basing upon series of measurements it was stated that the para-meters of couplers in one octave bandwidth are as follows: typicalisolation: 25 d.B, mar. VSWR: 1.2, amplitude inbalance:, 0.5 dB, andinsertion loss: 0.5 dBo.

5.2. Single section 1o5-dB couplerThe even- and odd-mode inpedance of this coupler should be 170.2

and 14.6 ohms. From Fig. 2 we can find the value as follows: /W=5,W/H=0.25, H1/H=0.0355 S/_H0.05,W1=2W, H0=H and H2=0.5H. The couplerwas designed for the 2.5 GHz frequency In Fig. 4, the measured

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______ Fig. 4. Measured responseof 1.56dB coupler

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response of the coupling is shown /the measured VSWR is less than1.2 and the typical isolation is 20 dB/.

3*3. Three-section 3-dB hybrid couplerThe three-section symmetrical 3-dB coupler was designed for the

1-4 GHz frequency band. In the design 3 ± 0.20 dB coupling, 5.83 :1bandwidth,and the normalized even-mode impedance of 1.208 and 3.41,were used i. It consists of two-end section which are 14.6-dB coup-lers and of a center section which is the previously tested 1.5-dEcoupler. All these are quarter wavelength long at 2.5 GHz. Thecoupler is made of alumina substrate which is printed from both si-des, two auxiliary alumina sheets, and mica interlay. In Fig. 5,the top and bottom side view of the coupler with the auxiliary she-ets removed in order to expose the microstrip lines, and the measu-red response of the coupling are shown.

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_1 * _gS Q ~~~~ \\

0.25 4.0 .0 3.0 4.0 4.7SFR.EQ.e - CKZ

Fig. 5. Three-section 3-dB coupler and measured response

The coupling performance is good in the 14 GHz frequency band,however, the insertion loss are not so small as to be negligible.The remaining measured parameters are: maz.VSWR: 1.3, typical iso-lation: 20 dB and minimums 17 dB.

References:1. E.G. Cristal, L. Young, Theory and Tables of Optimum Symmetrical

TEM-lLMode Coupled-Transmission Directional Couplers, IEsE Trans.MTT, , 5, 544558, 1965

2. K*C. Wolters, P.L. Clar, C.W. Stiles, Analysis and experimentalevaluation of distributed overlay structures in microwave integ-rated circuits, 1968 G-MTT Symp. Dig., 123-130

3. E.G. Cristal, Nonsymmetrical Coupled Lines of Reentrant Cross-Section, I Trans. MbTT, j 9, 529-550, 1967

4. H.E. Green, The Numerical Solution of Some Important Transmis-sion-Line Problems, I TEETrans MTT, j, 5, 676-692, 1965