broadside coupler

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BROADSIDE SUSPENDED STRIPLINE 3 dB COUPLERS Carlos Valerio Borges Maciel Daniel F. M. Argollo Humberto Abdalla Jr. Universidade de Brasilia Department of Electrical Enginnering P.O. Box 04591 70 910 - 970 - Brasilia - DF - Brazil ABSTRACT This work presents a design procedure for broadside suspended stripline 3 dB couplers, based on a set of analytic and empirical equations. Using this formulation, a C language program for PC compatible microcomputers was developed. The software shows, from the design parameters, the dimensions of the structure as well as the theoretical transmission and coupling curves. An 8.3 GHz centered coupler was assembled and measured based on the software results. INTRODUCTION The Broadside Suspended Stripline (BSS) is an inhomogeneous line consisting of two superimposed conducting strips at each side of the substrate. The substrate, supporting the metal strips, is placed symmetrically between the two ground planes, leaving an empty space in both substrate sides (fig. 1). Choosing a thin low dielectric constant substrate allows to obtain strong coupling without mechanical restrictions. The designed structures are placed in rectangular metallic boxes to improve isolation. The box dimensions should be chosen in order to inhibit spurious mode propagation. The cutoff frequency of the first higher order mode in rectangular waveguide, the TE,, mode, should be greater than the design frequency. This guarantees that the propagation occurs on the conducting strip [l]. Fig. 1 - Broadside suspende stripline directional coupler. 117

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Page 1: Broadside Coupler

BROADSIDE SUSPENDED STRIPLINE 3 dB COUPLERS

Carlos Valerio Borges Maciel Daniel F. M. Argollo Humberto Abdalla Jr.

Universidade de Brasilia Department of Electrical Enginnering

P.O. Box 04591 70 910 - 970 - Brasilia - DF - Brazil

ABSTRACT

This work presents a design procedure for broadside suspended stripline 3 dB couplers, based on a set of analytic and empirical equations. Using this formulation, a C language program for PC compatible microcomputers was developed. The software shows, from the design parameters, the dimensions of the structure as well as the theoretical transmission and coupling curves. An 8.3 GHz centered coupler was assembled and measured based on the software results.

INTRODUCTION

The Broadside Suspended Stripline (BSS) is an inhomogeneous line consisting of two superimposed conducting strips at each side of the substrate. The substrate, supporting the metal strips, is placed symmetrically between the two ground planes, leaving an empty space in both substrate sides (fig. 1). Choosing a thin low dielectric constant substrate allows to obtain strong coupling without mechanical restrictions. The designed structures are placed in rectangular metallic boxes to improve isolation. The box dimensions should be chosen in order to inhibit spurious mode propagation. The cutoff frequency of the first higher order mode in rectangular waveguide, the TE,, mode, should be greater than the design frequency. This guarantees that the propagation occurs on the conducting strip [l].

Fig. 1 - Broadside suspende stripline directional coupler.

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ANALYSIS EQUATIONS

In a BBS directional coupler, the dielectric substrate between the two strips results in an odd mode phase velocity smaller than the even mode one. (vo < ve), For that reason, the odd mode eletrical length of the coupled line is greater than the even mode one.

A general analysis of this coupler, taking into account the unequal phase velocities was made by Dalley [2], who showed that an imperfect input impedance match it is necessary to have:

tan e, = tan e, (1)

Because 8, is greater than 8, in this structure we can write: 0, = e, + n

This condition gives:

Knowing that: -

where h, is the free space wavelength. The coupled section length 1 can be determined by PI:

and, consequently: 71 e, =

(7)

SYNTHESIS PROCEDURE

Under the excitation of the even and odd modes, the BBS field distribution (fig 2) shows that the coupling is made by the odd mode. It is also seen that the field distribution for the even mode is the same of the inverted microstrip and of a isolated shielded microstrip for the odd mode.

From these equivalences, since the substrate has a low dieletric constant:

The even mode dielectric constant E,, can be considered the same as that of the inverted microstrip with value approximately 1 ; The odd mode impedance Z,, and the effective dielectric constant E,, are equal to the isolated shielded microstrip impedance Z, and the dieletric constant E. This permits

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the evaluation of the odd mode effective dieletric constant and the strip width W, using the shielded isolated microstrip equations.

(a) Even mode @) Inverted Microstrip

@) Odd mode (d) Covered Microstrip

Fig. 2 - Field distribution equivalence on the structures.

The design procedure is here described:

Knowing the coupling factor c and considering that Zo = J Z X , (4) gives:

1. Odd mode impedance Zoo calculation.

2. Evaluation of the strip width W and the odd mode effective dieletric constant

h/2, where h is the BBS substrate height. These values are calculated with the shielded isolated microstrip [3] replacing h by

3. Calculation of the coupled section length 1 and the even mode electrical length 8,. Since E,, = 1:

(9) J-0 E =

2 (JEeo - 1)

and

After obtaining the above parameters, it should be verified that the approximation is satisfactory. This can be done in the following way: - with the value of K, K = antlog(-c/20), and the value of 8, (lo), V,N , is computed by (3). - if V,N, < antlog(-c/20), c is modified in a iterative way until V,N, is approximately equal to an antilog(-c/20).

The access strips are isolated suspended striplines and are calculated with empirical equations [4].

This procedure was implemented in a software that gives, from the specified design parameters, the structure layout and the transmission and coupling curves. Figs. 3 and 4 show some of the screens of the program for an 8.3GHz centered 3dB ccupler in a 8mm x 4mm box.

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Project : BBSSTRIP

Author : Carlos U,B. Haciel: Daniel Argollo: Humberto Abdala J r , Title : Broadside Stripline Coupler

Deuice : Suspended Str ipLine

Device’s parameters

Center frequency : 8,38 GHz Height h : 8 ,588 mm Waue length : 36.14 mm Size of the guide (AXE) : 8.88 x 4.88 mm Line impedances : 58.88 Q

Coup le factor : 3,88 dB

Impedances IQ 1

20 = 58.88 W line = 4,7712 W coupler = 2,7516 L coupler = 39.5992 Fc guide = 18.75 GHz

Physical parameters

Dieletric constant Er : 2.17

press any key...= Fig. 3 - Software output screen.

Suspended StripIine(3 dB) : Transnxissian and Coupler Plots

f/fc cfc=8.30 GHzl <ESC> to quit

Fig. 4 - Theoretical transmission and coupling curves.

EXPERIMENTAL RESULTS

In order to verify the proposed design, an 8.3GHz centered 3dB coupler was constructed from the software results. Although the BBS technology is more suitable for the millimetric wave band and the software has no frequency restriction, the prototype was assembled to the X band (due to equipment restrictions). Table 1 shows the measured parameters, fig. 5 shows the assembled device performance, and fig. 6 shows the experimental results obtained.

1 2 0

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TABLE I - MEASURED PARAMETERS

Coupling

Directivity

3 dB

19 dB

I I I s o l a t l o n

Band

22 dB II 1 7 GHz (20 5 %)

Fig. 6 - Assembled device.

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I _ . .

Fig. 7 - Coupler frequency response.

CONCLUSIONS

In this work a design procedure for a 3dB coupler was described and implemented. From the coupler specifications, a software gives all the necessary elements to assembly the device. The similarity among the theoretical and experimental results validates the proposed procedure.

REFERENCES

[2] .I. 1;. IlalIe\ : "A Stripline I)ircctional Coupler Utilizing a Non-homopeneos Dielectric bledium": IEEE 'l'rans MT'I'; 1-01 M'I"1' 17: pp 705-7 1 1 :

[3] S.1,. March: "1:mpiricnl 1:ormulas !'or Inipedance and Effecti1.e Dielectric Constant of Col-ered Microstrip l'or IJse in Coniputer Aided Design im Microstrip Circuits": Proc. 1 I* European Microivave COIII. pp 67 1-676; 198 1 :

[4] W. Yunyi. (;. Kaiijun. "Synthesis lkpa t ions for Shielded Suspended Substrate Microstrlp and Broadside Couplcd Striplinc": 1f;f:l:. M'I'T-S Digest. pp. 33 1-334: 1988.

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