EIGHTEEXTH i'i.-iTIOSAI~.R-\DfO SCIESCE COSFERESCE March 25-29 2001,3Iansou~ Univ., Egypt

Six-Port Reflectometer Realization using Two Microstrip Three-Section Couplers

A.S.S.hfohra Microstrip Dept., Eiectronics Research Institute, Dokki, Cairo, EgVpt

Abstract

The single-section couplers are narrow band devices, so in order to.. broadcn the useful bandwidth, three-section couplers are used. In this papcr a system of two of three-section couplers is used to construct a six-port reflectometer. The realized six-port reflectometer is calibrated and used to measure different unknown complex terminations. The results of thc six- port reflectometer is found to Le in good agreement with the measurements of the vector network analyzer (HP-85 1OC) with an error not exceeding 4% iii magnitude and 4' in phase in the frequency range (2.4-4.4GHZ).

1.Introduction

The three-section coupler is a tliree-quaner wavelength coupler. Tlic analysis is simplified somewhat when one recognizes dwt the coupler is analytically similar to the quarter-wave transformer [ 1-21. Another way of the analysis of the three-section coupler is carried out by taking the advantages of the four fold symmetries of the slructures. Fig.1. and the even and odd mode escitations technique[3]. A six port reflectometer is analyzed and designed by using two of the prescribed three-section coupler. The system was first simulated by ready-made software package and then fabricated and tested. The measured S-parameters gave good agreement with the theoretical one. The sis-port was calibrated with a matched load and tliree positions of a sliding short circuit [I] in order to obtain the calibration constants.

2.Analysis and Design of a Three-Section Coupler

Escitations are chosen [3] so that the symmetry plane aa. Fig. 1, corresponds to an electric wall (short circuit) or a magnetic wall (open circuit). The Same is done for the case of symmetry plane bb corresponding to an electric or iiiagnetic wall. When bb corresponds to an electric wall, tire mode of propagation on tlrc coupled lines is the odd mode that has a cliaracteristic impedance Zoo and propagation constant Po. When bb corresponds to a magnetic wall, the mode of propagation is the even mode that has a characteristic impedance Z,, and propagation constant pe. The propagation constants are different because the medium is nonhomogeneous. By considering the four different escitations for the three-scction coupler and after some derivations the transmission and coupling parameters are [3]:

EIGHTEENTH S.ATIO.";AL.RADIO SCIENCE COSFERESCE March 27-29 2001,:)Imsour~ Univ., Egypt -TI

Design Drocedure 1.Define the required overall coupling (CO) 2.From Eq.(4). the corresponding values of the coupling coefficients for thc center and the outer seaions can be calculated . 3.From Eq.(j) the even and odd characteristic impedances arc cbtained 4.Using any rcady-made software, the corresponding physical dimensions can be calculated 5.From Eqs. (1-2). the S-Parameters can be calculated.

The abovc analysis was used to evaluate a -20 dB three-section masimally flat coupler on RT/Duroid 6006 (E,=& 15 , H=0.025") at 3.4 GHz as a center frequency, by using some relations [3] and a ready-made software, the following results are obtained:

Outer Sections Center Section Z,, = 50.62Q

Z,, = 49.38R Z,? = 56.665S2

Zwz = 44.118R C, = -38.13dB C2 = -1 8.09dB

=.928mm W2 =.910mm S, = 3.841" S2 =.668mm L, = d, = 7.29Smm L, = 2d2 = 9.663"

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WI EIGHTEENTH S.ATIOXAL.R4DIO SCIENCE COSFEREKCE March 27-29 2001,313nsoura Univ., Egypt

-~ 3.Six-Port Reflectometer Theory

A six-port reflectometer was designed by using two of the above three-section couplers, Fig.2 each has -20 dB as a coupling coeficient. By using the transmission and coupling relations ( Ke-je* and jC,e-jee , respectively, for each directional coupler, n=1,2) and after some mathematical simplification, Fig.2. the power relationships obtained as followv:

I

where j3 is the value of the sliding short circuits connected at port 2 and r, is the reflection coeflicient of the DUT port. There are different solutions to make these relationships suitable to represent the sis-port reflectometer idea with changing the lengths of the connection bctwveen the two couplers (8, , 82 ), and thc couplers connecting ends (a, ) by taking: a, = 6, = 62 = nI2 and C, = C, = -2OdB Then. (6) can be reduced to:

By clloosing three values of p (0,120,240" ) three equations of tlie fonn of (7) will be obtained and thc ccnters of tlie circles represented by these equations are: (1.02,O). (-0.5 4-0.8833) and (-0.5 40.8833) rcspectively. Clearly. these centers are uniformly distributed around the origin.

&Six-Port Reflectometer Realization

Two three-section couplers arranged as shown in Fig.2 are used to construct a sis-port reflectometer, in which a sliding short circuit connected to ports 2, two power meters connected to ports 3 and 4, a matched load (50 ohm) connected to port 5 , while the source Connected to port 1 and the DUT to port 6. The system w a s realized on RTDuroid 6006 (~,=6.15 , H=0.025") at 3.4 GHz center frequency, Fig.3. The Simulated (simulated by IE3D software package) and the measured S-parameters (Using HP8510C at Nationai Institute for Standards (NIS)) are illustrated in Figs.l-6. It is clear from these figures that, the coupling coefficients S12, SI3 vaned around -20 dB by 1 dE3. while the reflection at input port is around -25 dB or less. Also good isolations and transmission were obtained.

5.Six-port Reflectometer Calibration

With choosing suitable tliree different values ofp, (7) could be rewvritten in the form of the sis-port traditional equations as:

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/B\ EIGHTEESTH SATIOXAL~R4DiO SCIENCE COSF'ERESCE March 27-29 2001,5Iansoura t'niv., Egypt

- - where A, C and E arc three real calibration constants and E. D and F are the coi~espondin2 three comFlex calibration constants. The six-port reflectometer was calibrated with a matched load in addition to three positions of a sliding short circuit connected to the DUT port. respectively, and ai L e same time the power readings at ports 3 and 4 recorded for thc tluee diirerent values of p . The calibration constants calculated by using the calibration procedure of [3-5). The realized system was used to measure Werent unlaiown terminations and the same unknowns measured using vector network analyzer (HP 85lOC) at NIS. The realized system gave good

results with error not esceeding 4% in magnitude and 4' in phasc in the operating band (2.4-4.4GHz). A comparison between measurements at 3.4 GHz is illustrated in Table-1 as an example.

Table 1 Comparison between 6PR&VNA measurements at F=3.4 GHZ

Load TyQe

Sliding short circuit positions

0.9 1426 L:-19.28

6.Conclusion

The analysis of using two microstrip three-section couplers as a sis-port reflectometer is illustr3ted. The system is realized on Teflon substrate RTDuroid (E , = 6.15, H = 0.025") at 3.4 GHz as center frequency and then calibrated with pcrfect matched termination and different three positions of a sliding short circuit so, tlic calibration constants could be calculated. The mcasuremei.ts of the sis-port reflectometer gave good agreement with tlic incasurcments of the vcctor network analyzer Orp8510C) with errors not exceeding 4% in magnitude

and 4' in phase especially in the frequency range (2.4-1.4 GHz).

References

[ 1jG.Gristal. Theory and tables of optimum symmetrical TEM-Mode coupled transniission line dinxtional coupler, IEEE, MIT-13, PP.544-558, 1965 [Z]G.Matthaei, Microwave filters, impedance matching networks and couplcd structures, Artech I.ouse, chapter- 14, 1980. [3]A.S.S.Mohra. "Sis-port reflectometer", PbD. thesis, Ain shames Univ.. Fac.of Eng., Cairo, Egypt, Junc 2000. [4]N.A.El-Deeb. "The calibration and performance of a microstrip 6-port rcflectometer", IEEE,IvflT-3 1.. PP.509-514. July 1953. [S]L.Qiao, Improved implementation of four standard procedure for calibrating six-poit reflectometer, IEEE,IM- 44. PP.632-636. 1995.

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p-p-l EIGHTEEXTH S.ATIONAL.RADI0 SCIEXCE COSFEREXCE March 27-29 2001,Mansour~ Univ., Egypt

I 1 ' I I I d 3

I 1 b 2

d, v/ $,,, d, /p /I\ 4

Fig. 1 Three sections coupler

Fig2 Sis-port reflectometer consisting of two three-section couplers

DUT I/P r

s.s.c M*L-y< U, 4

Fig3 The realized six-port reflectometer (scale

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L

1: 1)

-1 EIGHTEEMTH S.ATtOXAL RADIO SCIEZU’CE COSFERESCE March 27-29 2001,Mansoura Univ., Egypt

0-0° I-- -10.00

Y

5 $ -20.00

f- Sl1 Moawred

-4- Sl1 S i u h t o d

-+ S16 Measured

-+ S16 Simulated

A

m E. e E e al

m

%

40.00 I I 1 1 I I 2.50 3.00 3.50 4.00

Frequency (GHz)

Fig4 The measured and simulated SI 1. SlG Panmetcrs.

0.00

-5.00

-10.00

-15.00

-20.00

-25.00

Legend Tdle

+ ~ t 2 Measured

++- 512 Simulated

++ S14Measured

4 S14 Simulated

I 1 1 I I ‘ I I 2.50 3.00 3.50 4.00

Frequency (GHz)

Fig.5 The measurcd and simulated S12, S14 Parameters.

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p)7) EIGHTEESTH SATIOSAL.RADI0 SCEXCE COSFEREXCE %larch 27-29 2001,~Ianroura Univ., Egypt

0.00

-10.00

-20.00

-30.00 I I I I 2.50 3.00 3.50 4.00

Frequency (GHz)

Fig.6 The measured and simulated S13, SI5 Parameten.

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