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Defence Research and Recherche et developpement 
Development Canada pour la defense Canada 


DEFENCE 



Miniaturized broadband 3-dB / 90° 
and 180°power splitters for GPS/GNSS 
anti-jam systems 

Mathieu Caillet, Michel Clenet, Ala Sharaiha and Yahia M. M. Antar 


Defence R&D Canada - Ottawa Canada 

Technical Memorandum 
DRDC Ottawa TM 2009-270 
February 2010 



Miniaturized broadband 3-dB / 90° and 180° 
power splitters for GPS/GNSS anti-jam 
systems 

Mathieu Caillet 

Royal Military College of Canada 
Michel Clenet 

Defence R&D Canada - Ottawa 
Ala Sharaiha 

Rennes Institute of Electronics and Telecommunications, France 

Yahia M. M. Antar 

Royal Military College of Canada 


Defence R&D Canada - Ottawa 

Technical Memorandum 
DRDC Ottawa TM 2009-270 
February 2010 



Co-author 


Original signed by Michel Clenet 
Michel Clenet 

Approved by 

Original signed by Bill Katsube 

Bill Katsube 
SH CNEW 

Approved for release by 

Original signed by Brian Eatock 

Brian Eatock 
Chairman DRP 


© Her Majesty the Queen in Right of Canada as represented by the Minister of National 
Defence, 2010 

© Sa Majeste la Reine (en droit du Canada), telle que representee par le ministre de la 
Defense nationale, 2010 




Abstract 


This document reports on compact broadband rat-race and branch-line hybrids designed in 
the 1.15-1.6 GHz frequency band using microstrip technology. Two relatively simple de¬ 
sign techniques using a conventional unilayer fabrication process have been investigated. 
The technology described in this document has been specificall developed for design¬ 
ing antenna feeding circuits for GPS/GNSS anti-jam systems, but it can be used for other 
wideband applications. 

A miniaturized two-section 180° hybrid using microstrip space-fillin curves has been de¬ 
signed and fabricated to operate between 1.15 and 1.6 GHz on an FR4 substrate. The 
miniaturized geometry area is 31% of the broadband two-section 180° hybrid area. The 
obtained performance is as good as the conventional geometry. A 3-dB coupling with max¬ 
imum amplitude imbalance of less than 0.15 dB has been measured over the 1.15-1.6 GHz 
band. Over this frequency band, the phase variation is ± 2°. The measured insertion loss 
is approximately 0.9 dB, and is mainly due to the substrate loss. To further reduce the 
footprint of the compact hybrid and the insertion loss, a second circuit has been designed 
on a substrate having a higher dielectric constant and lower loss (CerlO). The area of the 
second circuit is 54% of the hybrid area on the FR4 substrate. The measured performance 
is very similar, except that the insertion loss has been reduced by about 0.4 dB. 

The lumped distributed element method has been applied to miniaturize a two-section 
branch-line hybrid. The obtained geometry area is 65% of the two-section branch-line hy¬ 
brid area and 54% when considering only the width of the circuits. Over the 1.15-1.6 GHz 
band, the maximum coupling imbalance obtained by measurement is 1 dB, and the phase 
variation is 4°. Higher than expected maximum coupling imbalance has been measured 
because this parameter is sensitive to the impedance values of the two-section branch-line. 


Resume 


Le present document porte sur Fetude et la conception de coupleurs hybrides 180° et 90° 
miniatures et large bande concus dans la bande de frequences 1,15-1,6 GHz utilisant la 
technologie microruban. Deux techniques de conception relativement simples faisant ap- 
pel a un processus de fabrication monocouche classique ont ete etudiees. La technologie 
detaillee dans ce document a ete developpee specialement pour la conception de circuits 
d’alimentation d’antennes pour les systemes anti-brouillage GPS/GNSS, mais elle peut etre 
utilisee pour d’autres applications large bande. 

Un coupleur hybride 180° miniature a deux sections utilisant des courbes fractales micro¬ 
ruban a ete concu et fabrique sur un substrat FR4 pour fonctionner entre 1,15 GHz et 1,6 
GHz. La surface de la geometrie miniaturisee represente 31% de la surface du circuit hy¬ 
bride 180° a deux sections. Les performances obtenues sont aussi bonnes que cedes du 


DRDC Ottawa TM 2009-270 


i 



coupleur 180° classique. Un desequilibre du couplage a 3 dB de 0,15 dB maximum a ete 
mesure dans la bande 1,15-1,6 GHz. Dans cette bande de frequences, la variation de phase 
est de ± 2°. Les pertes d’insertion mesurees sont d’environ 0,9 dB et sont attribuable prin- 
cipalement aux pertes du substrat. Pour reduire encore plus la taille du coupleur hybride 
compact et diminuer les pertes d’insertion, un second circuit a ete concu sur un substrat 
ayant une constante dielectrique plus elevee et des pertes plus faibles (CerlO). La surface 
du second circuit represente 54% de la surface du coupleur realise sur le substrat FR4. 
Les perfonnances mesurees sont tres similaires, excepte que les pertes d’insertion ont ete 
reduites d’environ 0,4 dB. 

La methode des elements repartis localises a ete appliquee pour la miniaturisation d’un 
coupleur hybride 90° a deux sections. La surface geometrique obtenue represente 65% 
de la surface du coupleur hybride 90° a deux sections, 54% lorsque la largeur des circuit 
uniquement est prise en compte. Dans la bande 1,15-1,6 GHz, le desequilibre du couplage 
a 3 dB mesure est de 1 dB maximum, et la variation de phase est de 4°. Un desequilibre 
du couplage a 3 dB plus eleve que prevu a ete mesure car ce parametre est sensible aux 
valeurs d’impedance du coupleur hybride 90° a deux sections. 


ii 


DRDC Ottawa TM 2009-270 



Executive summary 

Miniaturized broadband 3-dB / 90° and 180° power 
splitters for GPS/GNSS anti-jam systems 

Mathieu Caillet, Michel Clenet, Ala Sharaiha, Yahia M. M. Antar; DRDC Ottawa 
TM 2009-270; Defence R & D Canada - Ottawa; February 2010. 

Background: Microwave hybrids, such as the branch-line and rat-race conf gurations, are 
important components with many applications in circuits and antenna feed systems. For 
example, dual-orthogonal fed circularly polarized antennas mostly employ external power 
divider feed networks. To achieve a circularly polarized antenna feed structure, quadrature 
and 180-degree hybrids as well as T-junctions and Wilkinson power dividers have been 
successfully used due to their ease of design. However, the power divider circuits are rela¬ 
tively large in size and have limited frequency bandwidth. Their size needs to be reduced 
and their bandwidth improved before they can be considered for use in wideband circularly 
polarized antennas. 

Principal results: A miniaturized two-section 180° hybrid using microstrip space-filing 
curves has been designed and fabricated to operate between 1.15 and 1.6 GHz on FR4 sub¬ 
strate. The miniaturized geometry area is 31% of the broadband two-section 180° hybrid 
area. The obtained performance is as good as the conventional geometry. A 3-dB coupling 
with maximum amplitude imbalance of less than 0.13 dB has been noted by measurement 
over the 1.15-1.6 GHz band. Over this frequency band, the phase variation is ± 2°. The 
measured insertion loss is approximately 0.9 dB, and is mainly due to the substrate loss. 
To further reduce the footprint of the compact hybrid and the insertion loss, a second cir¬ 
cuit has been designed on a substrate having a higher dielectric constant and lower loss 
(CerlO). The area of the second circuit is 54% of the hybrid area on the FR4 substrate. The 
measured performance is very similar, except that the insertion loss has been reduced by 
about 0.4 dB. 

A compact two-section branch-line hybrid has been designed by applying the lumped dis¬ 
tributed element method. The obtained geometry area is 65% of the two-section branch¬ 
line hybrid area and 54% when considering only the width of the circuits. Over the 1.15- 
1.6 GHz band, the maximum coupling imbalance obtained by measurement is 1 dB, and 
the phase variation is 4°. Higher than expected maximum coupling imbalance has been 
measured because this parameter is sensitive to the impedance values of the two-section 
branch-line. 

Significance of results: The proposed broadband rat-race hybrid design is among the most 
compact circuits present in the literature. Only two other conf gurations have a smaller 


DRDC Ottawa TM 2009-270 



footprint: a rat-race employing a frequency-independent coplanar waveguide phase in¬ 
verter [1] has a layout that has been reduced by 75%, and a design based on artif cial 
transmission lines [2] allowing 91% of reduction. However, these designs include plated 
thru holes and very thin printed lines which require a sophisticated fabrication process. 
The advantage of the design proposed in this work is that it uses a simple method and the 
circuit is very easy to fabricate. When considering a 3-dB coupling with maximum ampli¬ 
tude imbalance of 0.5 dB and a phase variation of 10°, the measured frequency band of the 
proposed rat-race design goes up to 50%. 

The footprint of the proposed broadband branch-line hybrid is equivalent to the most com¬ 
pact circuit found in the literature. A design based on artif cial transmission lines [2] allows 
a 73% reduction. Again, this circuit requires a sophisticated fabrication process, as opposed 
to the investigated design. If allowing a 3-dB coupling with maximum amplitude imbal¬ 
ance of 1 dB and a phase variation of 4°, the frequency band of the proposed branch-line 
design increases up to 47%. 

The technology described in this document has been specif cally developed for design¬ 
ing antenna feeding circuits for GPS/GNSS anti-jam systems, but it can be used for other 
wideband applications. 

Future work: Investigation could be carried out on a recently proposed method called dual 
transmission lines [3]. This miniaturization technique allows for a 64% size reduction, 
and is as simple as the ones used in this work. Moreover, a unilayer printed circuit is 
required, as in the two compact hybrid presented here. Additionally, some work to reduce 
the imbalance coupling of the two-section branch-line hybrid would be of interest. Finally, 
the 90° and 180° hybrids could be integrated further using multi-layer Low Temperature 
Co-f red Ceramic (LTCC) technology to achieve a very compact footprint for the circuits. 


IV 


DRDC Ottawa TM 2009-270 



Sommaire 


Miniaturized broadband 3-dB / 90° and 180° power 
splitters for GPS/GNSS anti-jam systems 

Mathieu Caillet, Michel Clenet, Ala Sharaiha, Yahia M. M. Antar; DRDC Ottawa 

TM 2009-270; R & D pour la defense Canada - Ottawa; fevrier 2010. 

Contexte : Les coupleurs hybrides hyperfrequences, comme la confguration des cou- 
pleurs en anneau et en quadrature, sont des composants importants qui comptent de nom- 
breuses applications dans les circuits et les systemes d’alimentation d’antenne. Par exemple, 
les antennes a polarisation circulaire alimentees en deux positions orthogonales emploient 
souvent des reseaux d’alimentation comportant des diviseurs de puissance externes. Pour 
realiser un reseau d’alimentation d’antenne a polarisation circulaire, les coupleurs hybrides 
180° et en quadrature ont ete employes avec succes, ainsi que les diviseurs de puissance 
Wilkinson et a jonction en T en raison de la facilite de conception. Toutefois, les diviseurs 
de puissance ont une taille relativement importante et ont une bande de frequence limitee. 
II est done necessaire de reduire leur taille et d’ameliorer leur bande de frequence avant 
d’envisager de les integrer a des antennes a polarisation circulaire large bande. 

Resultats : Un coupleur hybride 180° miniature a deux sections utilisant des courbes frac- 
tales microruban a ete concu et fabrique sur un substrat FR4 pour fonctionner entre 1,15 
GHz et 1,6 GHz. La surface de la geometrie miniaturisee represente 31% de la surface du 
circuit hybride 180° a deux sections. Les performances obtenues sont aussi bonnes que 
cedes du coupleur 180° classique. Un desequilibre du couplage a 3 dB de 0,15 dB maxi¬ 
mum a ete mesure dans la bande 1,15-1,6 GHz. Dans cette bande de frequences, la variation 
de phase est de ± 2°. Les pertes d’insertion mesurees sont d’environ 0,9 dB et sont attri- 
buable principalement aux pertes du substrat. Pour reduire encore plus la taille du coupleur 
hybride compact et diminuer les pertes d’insertion, un second circuit a ete concu sur un 
substrat ayant une constante dielectrique plus elevee et des pertes plus faibles (CerlO). La 
surface du second circuit represente 54% de la surface du coupleur realise sur le substrat 
FR4. Les performances mesurees sont tres similaires, excepte que les pertes d’insertion ont 
ete reduites d’environ 0,4 dB. 

La methode des elements repartis localises a ete appliquee pour la miniaturisation d’un 
coupleur hybride 90° a deux sections. La surface geometrique obtenue represente 65% 
de la surface du coupleur hybride 90° a deux sections, 54% lorsque la largeur des circuit 
uniquement est prise en compte. Dans la bande 1,15-1,6 GHz, le desequilibre du couplage 
a 3 dB mesure est de 1 dB maximum, et la variation de phase est de 4°. Un desequilibre 
du couplage a 3 dB plus eleve que prevu a ete mesure car ce parametre est sensible aux 
valeurs d’impedance du coupleur hybride 90° a deux sections. 


DRDC Ottawa TM 2009-270 


v 



Portee des resultats : La confguration proposee pour le coupleur hybride 180° large 
bande fait partie des geometries les plus compactes parmi les circuits presents dans la 
litterature. II n’y a que deux autres conf gurations qui ont une taille plus faible : un cou¬ 
pleur hybride 180° employant un inverseur de phase en ligne coplanaire independant de la 
frequence [1] a une taille reduite de 75%; une conception basee sur des lignes de transmis¬ 
sion artif cielles [2] permet d’obtenir une reduction de 91%. Ces conceptions comprennent 
cependant des trous metallises et des lignes microruban tres f nes, ce qui exige un proces¬ 
sus de fabrication perfectionne. L’avantage de la methode proposee dans le present travail, 
est qu’elle fait appel a une methode simple et que le circuit est tres facile a fabriquer. Si un 
desequilibre du couplage a 3 dB de 0,5 dB maximum et une variation de phase de 10° sont 
toleres, la bande de frequence mesuree pour le coupleur hybride 180° propose est de 50%. 

La geometrie du coupleur hybride 90° large bande propose est equivalente a celle du circuit 
le plus compact que Ton peut trouver dans la litterature. Une conception basee sur des 
lignes de transmission artif cielles [2] pennet une reduction de 73%. La encore, ce circuit 
requiert un processus de fabrication perfectionne par rapport a la geometrie etudiee. Si un 
desequilibre du couplage a 3 dB de 1 dB maximum et une variation de phase de 4 sont 
acceptables, la bande de frequences de la conf guration proposee pour le coupleur hybride 
90° est de 47%. 

La technologie detaillee dans ce document a ete developpee specialement pour la concep¬ 
tion de circuits d’alimentation d’antennes pour les systemes anti-brouillage GPS/GNSS, 
mais elle peut etre utilisee pour d’autres applications large bande. 

Recherches futures : II serait interessant d’analyser une methode proposee recemment 
appelee methode des lignes de transmission doubles [3], Cette technique de miniaturisa¬ 
tion permet une reduction de la taille de 64% et est aussi simple que celle utilise dans le 
present travail. En outre, un circuit imprime monocouche est requis avec cette technique, 
comme dans le cas des deux coupleurs hybrides compacts proposes dans le present docu¬ 
ment. De plus, des travaux visant une reduction du desequilibre du couplage a 3 dB pour 
le coupleur hybride 90° a deux sections pourraient presenter un interet. Enf n, les cou¬ 
pleurs hybrides 90° et 180° pourraient etre integres davantage a l’aide de la technologie 
des ceramiques a plusieurs couches cuites a faible temperature (LTCC) pour obtenir une 
taille de circuit tres compacte. 


VI 


DRDC Ottawa TM 2009-270 



Table of contents 


Abstract. i 

Resume. i 

Executive summary. iii 

Sommaire. v 

Table of contents .vii 

List of f gures. ix 

1 Introduction. 1 

2 Miniaturized broadband rat-race hybrid . 2 

2.1 Broadband two-section 180° printed hybrid. 2 

2.2 Compact broadband 180° printed hybrid. 4 

2.2.1 Moore 2nd-iteration conventional rat-race hybrid. 4 

2.2.2 Design and simulations. 4 

2.2.3 Fabrication and measurements. 5 

2.2.4 Implementation on a substrate having a higher dielectric 

constant and lower losses. 10 

2.3 Concluding remarks. 11 

3 Miniaturized broadband 90° printed hybrid. 14 

3.1 Broadband two-section branch-line hybrid. 14 

3.2 Compact broadband branch-line hybrid using lumped distributed elements 17 

3.2.1 Miniaturization method. 17 

3.2.2 Design and simulations. 18 

3.2.3 Fabrication and measurement results.21 

3.3 Concluding remarks.24 

DRDC Ottawa TM 2009-270 vii 
























4 Conclusions and perspectives.26 

References.27 


viii 


DRDC Ottawa TM 2009-270 





List of figures 


Figure 1: Broadband two-section 180° printed hybrid on 60-mil FR4 substrate ... 2 

Figure 2: Broadband two-section 180° printed hybrid simulation results (a) 


S-parameters and (b) phase outputs. 3 

Figure 3: Fabricated Moore 2nd-iteration rat-race hybrid on 30-mil FR4 

substrate. Horizontal size of the board: 63.5 mm (2.5 in.). 5 

Figure 4: Measured results of the Moore 2nd-iteration rat-race hybrid 

S-parameters: (a) magnitude and (b) phase outputs. 6 


Figure 5: Implementation of the miniaturized broadband two-section 180° hybrid 
from the fractal hybrid, (a) AB and AA’ fractal hybrid curves are used 
as X g /4 and X g /2 lines, respectively, (b) The f rst section is constructed 


by connecting Z 4 and Z 5 lines, (c) Following the same procedure, the 
second section is then built by assembling Z\ and Zi lines, (d) Finally, 
the central line Z 3 is meandered in the room in the middle of the two 
sections and ports are placed to complete the implementation. 7 

Figure 6 : Miniaturized broadband two-section 180° hybrid simulation results (a) 

S-parameters and (b) phase outputs. 8 

Figure 7: Compact broadband two-section 180° hybrid prototype. Edge size of 

the square board: 63.5 mm (2.5 in.). 9 


Figure 8 : Measured S-parameters of the compact broadband two-section 180° 
hybrid. Solid lines correspond to the simulated S-parameters, markers 
are for the measured results. 9 

Figure 9: Measured output magnitude and phase difference of the compact 
broadband two-section 180° hybrid. Solid lines correspond to the 
simulated S-parameters, markers represent the measured data. 

A mag(S/[i — S 31 ), x angle(S/n — S 31 ). 10 

Figure 10: Compact broadband two-section 180° hybrid prototype on 25-mil 

CerlO substrate. Edge size of the square board: 63.5 mm (2.5 in.) .... 11 

Figure 11: Miniaturized broadband two-section 180° hybrid on 25-mil CerlO 

substrate measurement results (a) S-parameters and (b) magnitude and 
phase output difference. Solid lines correspond to the simulated 


S-parameters, markers represent the measured data. 12 

DRDC Ottawa TM 2009-270 ix 












Figure 12: Broadband 180° hybrid designs on same scale. 13 

Figure 13: Broadband two-section branch-line hybrid geometry. 14 

Figure 14: Simulation results of the broadband two-section branch-line hybrid: (a) 

S-parameters and (b) phase outputs. 15 

Figure 15: Measurement results of the broadband two-section branch-line hybrid: 

(a) S-parameters and (b) magnitude and phase output difference. Solid 
lines correspond to the simulated S-parameters, markers represent the 
measured data. 16 

Figure 16: Size reduction scheme using lumped distributed elements. 

(a) Conventional transmission line, (b) Equivalent transmission line 

with a series transmission line and two open stubs. 18 

Figure 17: Miniaturized broadband two-section branch-line hybrid geometry using 

a 30-mil FR4 substrate. 19 

Figure 18: Simulation results of the miniaturized two-section branch-line hybrid: 

(a) S-parameters and (b) phase outputs.20 

Figure 19: Miniaturized broadband two-section branch-line hybrid prototype using 

a 30-mil FR4 substrate.21 

Figure 20: Measurement results of the miniaturized two-section branch-line 
hybrid: (a) S-parameters and (b) magnitude and phase output 
difference. Solid lines correspond to the simulated S-parameters, 
markers represent the measured data.22 

Figure 21: Simulated (a) Sn and (b) magnitude output difference, as a function of 

the bl impedance of the two-section branch-line hybrid.23 

Figure 22: Miniaturized broadband two-section branch-line hybrid parameters. ... 23 

Figure 23: Simulated magnitude output difference considering the actual 

dimensions of the fabricated miniaturized broadband branch-line 
prototype.24 

Figure 24: Hybrid designs on same scale using 30-mil FR4 substrate.25 


x 


DRDC Ottawa TM 2009-270 















1 Introduction 


Microwave hybrids, such as the branch-line and rat-race conf gurations, are important com¬ 
ponents with many applications in circuits and antenna feed systems. For example, dual- 
orthogonal fed circularly polarized antennas mostly employ external power divider feed 
networks. To achieve a circularly polarized antenna feed structure, quadrature and 180- 
degree hybrids as well as T-junctions and Wilkinson power dividers have been successfully 
used due to their ease of design. However, the power divider circuits are relatively large in 
size and have limited frequency bandwidth. Their size needs to be reduced and their band¬ 
width improved before they can be considered for use in wideband circularly polarized 
antennas. 

Publications on miniaturized wideband 90-degree hybrids for various applications are nu¬ 
merous, as was reported most recently [2, 4, 5], but compact wideband 180-degree hybrids 
are less popular. Many methods have been developed to reduce the size of the rat-race hy¬ 
brid. Reduced ring perimeter [6], folded lines [7], artif cial transmission lines [8], periodic 
stepped-impedance resonator structure [9], coupled lines [10] and lumped elements [11] 
are miniaturization strategies employed to reduce the footprint of the microstrip rat-race 
hybrid. Using the same principle as of folded lines, an approach based on the use of space- 
f lling curves have been proposed by Ghali and Moselhy [12, 13]. It allows one to greatly 
reduce the occupied area of hybrids using a relatively simple and symmetric structure. 

Enhancement of the frequency bandwidth of the branch-line and the rat-race hybrid is also 
a subject of interest as the bandwidth demands of broadband applications keep increasing. 
Diverse approaches have been proposed to increase the bandwidth of the branch-line hy¬ 
brid: addition of two half-wavelength serial branches or one serial branch and one open 
stub being both a half-wavelength long [14], multi-section broadband impedance trans¬ 
forming [15, 16], or use of mixed distributed and lumped distributed elements [11, 17]. 
Several techniques have been reported for the improvement in bandwidth of the rat-race 
hybrid: addition of a ffth port [18], use of crossovers [19, 20, 21], coplanar waveguide 
phase inverter [1] or vertically installed planar circuit structure [22], However, these de¬ 
signs require metallic tape or bonding wires, plated thru-holes or the installation of a verti¬ 
cal substrate, which complicates the fabrication. 

This report proposes compact broadband rat-race and branch-line hybrids using single layer 
printed technology in the 1.15-1.6 GHz frequency band. Two relatively simple design tech¬ 
niques are suggested in this work. The technology described in this document has been 
specif cally developed for designing antenna feeding circuits for GPS/GNSS anti-jam sys¬ 
tems, but it can be used for other wideband applications. In section 2, the conf guration of 
the proposed compact wideband rat-race coupler is presented. The design, simulation and 
measurements are reviewed. Section 3 is dedicated to the implementation of the miniatur¬ 
ized wideband branch-line coupler. The detailed design considerations and experimental 
results will be presented, followed by discussion and conclusions in section 4. 


DRDC Ottawa TM 2009-270 


1 



2 Miniaturized broadband rat-race hybrid 


A broadband miniaturized microstrip circuit designed to evenly split the power and provide 
180° of phase difference between the two outputs is reported in this section. A broadband 
two-section 180° printed hybrid was studied frst. Then, a miniaturization method based 
on fractal geometries [12, 23] has been proposed to reduce the size of the broadband two- 
section rat-race hybrid. Simulation results, prototype and characterization are presented. 
All simulations have been performed considering material losses. The proposed miniatur¬ 
ization method is applicable to larger multi-section rat-race hybrids. 

2.1 Broadband two-section 180° printed hybrid 

Conventional hybrids have a limited frequency bandwidth. For instance, a rat-race hybrid 
with equal power division has a bandwidth of about 25%. It is well known that the op¬ 
erating bandwidth can be greatly increased using multisection hybrids, as shown in [15] 
for the branch-line coupler. Thus, a two-section 180° hybrid is proposed in this work to 
enhance the bandwidth of the conventional rat-race. A two-section 180° hybrid consists of 
cascading two conventional rat-race hybrids, as shown in Figure 1 for a design on a 60-mil 
FR4 substrate ( E r = 4.4, tan8 = 0.02). 


61.4 mm 



Figure 1: Broadband two-section 180° printed hybrid on 60-mil FR4 substrate 

The broadband two-section 180° printed hybrid includes three vertical X g /2 and four hor¬ 
izontal Xg/4 lines whose impedances Z, are def ned to have a good broadband matching. 
This forms a four-port network with a 180° phase shift between the two output ports. The 
distance between the output ports is X g /2. Figure 1 shows the geometry and the conven¬ 
tions for impedances and ports. If the input is applied to port 1, the power will be evenly 
split into two components with a 180° phase difference at ports 3 and 4, and port 2 will be 
isolated. 


2 


DRDC Ottawa TM 2009-270 


















The hybrid could be seen as a four-port impedance transforming structure. The set of 
impedances Z, has been optimized using a linear analysis tool, the circuit module of An- 
soft Designer [24] in this case. The impedances required to have a good matching and 
balanced coupling over a 50% bandwidth are Z\ — 66.1 Q., Z 2 — 56.7 Q., Z 3 = 30.8 Q, 
Z 4 = 52.9 Q., and Z 5 = 74.1 Q. for a 50 Q. port characteristic impedance. Simulation re¬ 
sults of S-parameters obtained on a 60-mil FR4 substrate (£ r = 4.4, tg 6 = 0.02) with the 
dimensions of Figure 1 are presented in Figure 2(a). The electromagnetic module of An- 
soft Designer has been used to achieve the simulations. The isolation (S 21 ) is greater than 
35 dB between 1.15 and 1.65 GHz. Phase data are given in Figure 2(b). Between 1.15 
and 1.6 GHz, a 3-dB coupling with maximum amplitude unbalance of less than 0.2 dB is 
obtained and the phase variation is ± 1.5° over the frequency band. The insertion losses 
are in the range of 0.9 dB. 




Figure 2: Broadband two-section 180° printed hybrid simulation results (a) S-parameters 
and (b) phase outputs. 


DRDC Ottawa TM 2009-270 


3 















2.2 Compact broadband 180° printed hybrid 

The space-filing curves method has been considered to reduce the size of the conventional 
rat-race hybrid. In [13], three space-filing curves, falling into the family of the fractal 
geometries, were compared for the design of compact hybrids. These curves are Moore, 
Sierpinski, and Minkowski constructions. Minkowski’s curves are limited to the f rst itera¬ 
tion because the number of segments is very high in the case of the 2nd and 3rd iterations. 
The footprint of Sierpinski’s curves is a little bit larger than Moore’s. Thus, Moore’s curves 
allow the best reduction in area and have been selected here to reduce the size of the rat- 
race hybrid. The second iteration of the Moore’s geometry has been applied to each of the 
two sections of the broadband 180° hybrid to reduce its size. 

2.2.1 Moore 2nd-iteration conventional rat-race hybrid 

A compact conventional rat-race hybrid has been previously investigated in [23] using the 
Moore 2nd-iteration space-f lling curve. Design infonnation and results are reported here 
for comparison purposes. 

This hybrid coupler has been implemented on a 30-mil FR4 substrate (£, = 4.4, tan 8 = 
0 . 02 ) to obtain a reasonable width for the microstrip lines, which is required to build the 
fractal geometry without additional quarter-wave transformers. The Moore 2nd-iteration 
space-filing curve includes 68 segments, and the length of a segment has been set to 
2.6 mm. The widths of 50 and 70.7 Q. lines for the selected substrate are 0.77 mm and 
1.45 mm, respectively. 

A prototype has been fabricated (Fig. 3), and the measured results for magnitude and phase 
are shown in Figure 4. From 1.15 to 1.6 GHz, the maximum magnitude difference between 
the two outputs is 1 dB, and the phase difference varies by 22° (from 192° to 170°). 

2.2.2 Design and simulations 

To miniaturize the two-section 180° hybrid from Section 2.1, the vertical k ? /2 and hori¬ 
zontal X g /4 lines can be taken from the compact rat-race hybrid described in Section 2.2.1. 
As shown in Figure 5(a), the fractal section AB corresponds to a length of X g /4, and the 
fractal section AA’ is X g /2 long. Two AB and one AA’ fractal lines are connected together 
to build the frst section of the hybrid, with respective impedances Z 4 and Z 5 (Fig. 5(b)). 
The same procedure is applied to shape the second section of the two-section rat-race com¬ 
posed of sections of impedance Z\ and Z 2 (Fig. 5(c)). Finally, the central line of impedance 
Z 3 has to be inserted to complete the implementation of the design. Due to the relatively 
low impedance (Z 3 = 30.8 O), and the resulting wide line, the meandering has been spread 
using all the room in the middle of the two sections. Figure 5(d) shows the complete circuit. 
The ports are placed in the same way as the wideband two-section rat-race hybrid. 


4 


DRDC Ottawa TM 2009-270 




* * 


OROC QTTRVfl / RMC 
JUNE 2007 


Figure 3: Fabricated Moore 2nd-iteration rat-race hybrid on 30-mil FR4 substrate. Hori¬ 
zontal size of the board: 63.5 mm (2.5 in.) 

Planar EM simulation results of the broadband compact rat-race hybrid obtained on a 30- 
mil FR4 substrate are presented in Figure 6. The designed coupler is 43.5 mm long and 

33.1 mm wide. The return loss is greater than 14 dB, the isolation (Shi) is greater than 35 dB 
between 1.1 and 1.65 GHz. Over this frequency band, a 3-dB coupling with maximum 
magnitude imbalance of less than 0.2 dB is obtained and the phase variation is 5° over the 
frequency band. 

If the considered area of the hybrids is a.b, with a and b representing the lengths of the hor¬ 
izontal and vertical sides, the obtained miniaturized geometry area is 31% of the broadband 
two-section 180° hybrid area. 

2.2.3 Fabrication and measurements 

As for the miniaturized rat-race circuit described in section 2.2.1, a 30-mil FR4 substrate 
has been considered for building a compact broadband two-section 180° hybrid. Figure 7 
presents the fabricated prototype of the broadband fractal hybrid. Experimental results 
demonstrate that the magnitude of the S-parameters is consistent with the simulated data 
(Fig. 8). The measured bandwidth def ned by a 3-dB coupling with a maximum amplitude 
imbalance of less than 0.5 dB is 50%, from 1.05 to 1.75 GHz. Over this frequency band, 
the phase variation is ± 5° (Fig. 9), the isolation is greater than 25 dB and the return loss is 
equal to or greater than 10 dB. Additionally, the experimental output amplitude difference 
is in accordance with the simulated one, having a maximum imbalance of 0.15 dB between 

1.1 and 1.7 GHz. The insertion losses are in the range of 1 dB. 


DRDC Ottawa TM 2009-270 


5 


0 




(b) 

Figure 4: Measured results of the Moore 2nd-iteration rat-race hybrid S-parameters: 
(a) magnitude and (b) phase outputs 


6 


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Portl 


Port3 



Figure 5: Implementation of the miniaturized broadband two-section 180° hybrid from 
the fractal hybrid, (a) AB and A A’ fractal hybrid curves are used as X g / 4 and A.^,/2 lines, 
respectively, (b) The frst section is constructed by connecting Z 4 and Z 5 lines, (c) Follow¬ 
ing the same procedure, the second section is then built by assembling Z\ and Zi lines, (d) 
Finally, the central line Z 3 is meandered in the room in the middle of the two sections and 
ports are placed to complete the implementation. 


DRDC Ottawa TM 2009-270 


7 














0 




Figure 6: Miniaturized broadband two-section 180° hybrid simulation results (a) S- 
parameters and (b) phase outputs 


8 


DRDC Ottawa TM 2009-270 

















Figure 7: Compact broadband two-section 180° hybrid prototype. Edge size of the square 
board: 63.5 mm (2.5 in.) 



Figure 8: Measured S-parameters of the compact broadband two-section 180° hybrid. 
Solid lines correspond to the simulated S-parameters, markers are for the measured results. 


DRDC Ottawa TM 2009-270 


9 





















Figure 9: Measured output magnitude and phase difference of the compact broadband 
two-section I8ff hybrid. Solid lines correspond to the simulated S-parameters, markers 
represent the measured data. A mag(S^\ — S 31 ), x angle[S^\ — S 31 ). 

Detailed comparison of simulation and measured results are given in Table 1. 



Amplitude S 31 /S 41 (dB) 

Phase diff. (°) 


Simu. 

Meas. 

Simu. 

Meas. 

1.15 GHz 

-3.'79/-3.93 

-3.86/-3.85 

178.5 

177.7 

1.35 GHz 

-3.74/-3.85 

-3.79/-3.85 

180.0 

178.8 

1.6 GHz 

-4.07/-3.98 

-4.09/-3.96 

181.2 

179.4 


Table 1: Comparison of simulated and measured results for the miniaturized broadband 
two-section 180° hybrid. 


2.2.4 Implementation on a substrate having a higher dielectric 
constant and lower losses 

The interesting results in terms of size and performance obtained for the design of the 
broadband two-section 180° hybrid on the FR4 substrate encouraged the authors to try to 
further reduce the footprint of the hybrid. An easy solution to achieve this is to use a higher 
dielectric constant substrate. Furthermore, the chosen substrate has lower losses, which is 
also of interest to reduce the insertion losses of the broadband rat-race hybrid (in the range 
of 1 dB using the FR4 substrate). 

Rigorously using the same design method, a two-section 180° hybrid has been designed 
on a 25-mil CerlO substrate (£ r = 9.5, tanS = 0.0035). Figure 10 shows the fabricated 
prototype. 


10 


DRDC Ottawa TM 2009-270 










Figure 10: Compact broadband two-section 180° hybrid prototype on 25-mil CerlO sub¬ 
strate. Edge size of the square board: 63.5 mm (2.5 in.) 

Planar EM simulation and measured results of S-parameters are presented in Figure 11(a). 
Output phase and amplitude difference are given in Figure 11(b). One can notice that 
there is a frequency shift of about 100 MHz between simulation and measurement results. 
This can be explained by the fact that the dielectric constant of the used substrate is quite 
high, and thus fabrication tolerances are increased. Also, the lines are thin, and thus more 
sensitive to fabrication tolerances. 

Between 1.15 and 1.6 GHz, the return loss is greater than 17 dB, and the isolation is better 
than 30 dB. A 3-dB coupling with maximum magnitude imbalance of less than 0.1 dB is 
obtained and the phase variation is 3° over the frequency band. 

The footprint of the two-section 180° hybrid printed on the CerlO substrate is 54% the 
area of the same hybrid circuit on FR4. The insertion loss is around 0.6 dB using the 
CerlO substrate which is 0.4 dB lower than the design on the FR4 substrate. Thus, both the 
size and the insertion losses have been reduced using a substrate having a higher dielectric 
constant and lower loss. 

2.3 Concluding remarks 

A miniaturized two-section 180° hybrid using microstrip lines has been designed to operate 
between 1.15 and 1.6 GHz on FR4 substrate. To miniaturize this circuit, the 2nd-iteration 
Moore’s space-filing curve has been used. The miniaturized geometry area is 31% of the 
broadband two-section 180° hybrid area. The obtained performances are as good as the 
conventional geometry. A 3-dB coupling with maximum amplitude unbalance of less than 
0.15 dB has been noted by measurement over the 1.15-1.6 GHz band. Over this frequency 
band, the phase variation is ± 1°. The measured insertion losses are in the vicinity of 1 dB. 


DRDC Ottawa TM 2009-270 


11 





Figure 11: Miniaturized broadband two-section 180° hybrid on 25-mil CerlO substrate 
measurement results (a) S-parameters and (b) magnitude and phase output difference. Solid 
lines correspond to the simulated S-parameters, markers represent the measured data. 


12 


DRDC Ottawa TM 2009-270 
























To further reduce the footprint of the compact hybrid and the insertion loss, a second cir¬ 
cuit has been designed on a substrate having a higher dielectric constant and lower losses 
(CerlO). The area of the second circuit is 54% the area of the hybrid on the FR4 substrate. 
The measured performance is very similar, except that the insertion losses have been re¬ 
duced to about 0.6 dB. 

Figure 12 presents the two hybrid designs on the same scale allowing for direct size com¬ 
parison with the broadband two-section 180° hybrid. 


FR4 FR4 CerlO 



20 mm 

Figure 12: Broadband 180° hybrid designs on same scale. 


DRDC Ottawa TM 2009-270 


13 






3 Miniaturized broadband 90° printed hybrid 


A broadband miniaturized microstrip circuit designed to evenly split the power and provide 
90° phase difference between the two outputs is proposed in this section. A two-section 
hybrid has been investigated to enhance the frequency bandwidth response. To reduce the 
size of this circuit, an equivalent transmission line model including open stubs has been 
used as presented in [11]. Simulation results, prototype and characterization are reported 
here. All simulations have been performed considering material losses. The proposed 
miniaturization method is applicable to larger multi-section branch-line hybrids. 

3.1 Broadband two-section branch-line hybrid 

Conventional hybrids have a limited frequency bandwidth. For instance, a branch-line 
hybrid with equal power division has a bandwidth of about 15%. To increase the operating 
frequency band of the branch-line hybrid, a two-section geometry (Fig. 13) can been used. 
The impedances have been chosen from [15] to have the weakest coupling imbalance and 
the wider frequency bandwidth. The impedances are: a\ — 102.88 f>, a2 = 25.26 Q. and 
bl = 26.95 a. 


bl 


bl 



Figure 13: Broadband two-section branch-line hybrid geometry 

Planar EM simulation results with the geometry of Figure 13 using a 30-mil FR4 substrate 
are shown in Figure 14. Over the 1.15-1.6 GHz band, the maximum coupling imbalance is 
0.55 dB and the phase variation is ± 2°. 

A prototype has been fabricated using a 30-mil FR4 substrate with the dimensions of Fig¬ 
ure 13, and the measured results for magnitude and phase are shown in Figure 15. From 
1.15 to 1.6 GHz, the maximum magnitude difference between the two outputs is 0.6 dB, 
and the phase difference is 1°. 

Detailed comparison of simulation and measured results are given in Table 2. 


14 


DRDC Ottawa TM 2009-270 











(b) 

Figure 14: Simulation results of the broadband two-section branch-line hybrid: (a) S- 
parameters and (b) phase outputs 


DRDC Ottawa TM 2009-270 


15 




















0 




(b) 


Figure 15: Measurement results of the broadband two-section branch-line hybrid: (a) S- 
parameters and (b) magnitude and phase output difference. Solid lines correspond to the 
simulated S-parameters, markers represent the measured data. 


16 


DRDC Ottawa TM 2009-270 























Amplitude S 31 /S 41 (dB) 

Phase diff. (°) 


Simu. 

Meas. 

Simu. 

Meas. 

1.15 GHz 

-4.08/-3.55 

-3.94/-3.35 

-92.2 

-91.1 

1.35 GHz 

-3.62/-3.91 

-3.43/-3.79 

-87.9 

-90.2 

1.6 GHz 

-4.02/-3.47 

-3.727-3.68 

-90.6 

-91.4 


Table 2: Comparison of simulated and characterized results for the broadband two-section 
branch-line hybrid 


Based on this geometry, the lumped distributed elements miniaturization technique de¬ 
scribed in [11] can be applied to reduce the footprint of the coupler. The design using this 
technique is presented in section 3.2. 

3.2 Compact broadband branch-line hybrid using 
lumped distributed elements 

The space-f lling curves method does not allow to reduce the footprint of the two-section 
branch-line hybrid as much as for the two-section rat-race coupler. Thus, the lumped dis¬ 
tributed elements miniaturization technique has been selected because the size reduction 
is among the best throughout the different techniques enumerated in the introduction of 
this report. Furthermore, it is quite easy to use for designing a branch-line hybrid and the 
circuit fabrication requires only a conventional unilayer process, like for the design of the 
broadband rat-race hybrid. 

The miniaturization method applied for the size reduction of the two section branch-line 
hybrid is f rst described in this section. The design of a compact wideband branch-line 
hybrid is then presented. 

3.2.1 Miniaturization method 

The lumped distributed elements method consists of shrinking a conventional transmission 
line having a Z c characteristic impedance and a 0 electrical length (Fig. 16(a)) by replacing 
it by a shorter transmission line, of characteristic impedance Z s and an electrical length of 
0,s, and two open stubs having a Z 0 \ characteristic impedance and a Q 0 \ electrical length 
(Fig. 16(b)). This represents its equivalent transmission line model. 

Two design equations are needed to f nd the parameters of the equivalent transmission 


DRDC Ottawa TM 2009-270 


17 



line [11]: 


B 0 1 = 

Zs = 


cos 0 V — COS 0 
Z c sin 0 
Z c sin 0 
sin 0^ 


where B () \ is the input admittance of the open stubs def ned by 


(3.1) 

(3.2) 


j B 0 1 = j tan Q 0 \/Z 0 \. (3.3) 

The ratio 0. s /0 represents the size reduction factor. Knowing 0, 0. v is f xed to have the de¬ 
sired reduction. The equivalent transmission line of Figure 16(b) is a flter, so the cutoff 
frequency has to be considered. The authors of [11] show that 0 = 30°, 0, = 12.5° associ¬ 
ated with 50 D. stubs allows wideband applications as well as high cutoff frequency. Thus, 
the quarter-wavelength lines would be split into 3 segments. 


3.2.2 Design and simulations 

0 and 0 S have been chosen to be 30 and 12.5° respectively. For each horizontal microstrip 
line of the two-section branch-line hybrid (Fig. 13), B 0 \ and Z s can be calculated using 
equations 3.1 and 3.2. From B () \, the impedance and length of stubs are found using equa¬ 
tion 3.3 and choosing Z 0 \. The initial values of the distributed lumped elements are given 
in Table 3. Then, three sections are used for each quarter-wave length line. To respect the 
geometry of the three-branch hybrid, each two parallel stubs are combined in a single stub. 
Finally, the values have to be adjusted to match the desired performances. 

The proposed design of the miniaturized 90° hybrid obtained on a 30-mil FR4 substrate 
is shown in Figure 17. Planar EM simulation results of S-parameters with the dimensions 
of Figure 17 are presented in Figure 18(a). Phase data are given in Figure 18(b). Over 
the 1.15-1.6 GHz bandwidth, a 3-dB coupling with maximum amplitude unbalance of less 
than 0.3 dB is obtained and the phase variation is ±1.4°. The obtained geometry area is 
65% of the two-section branch-line hybrid area and 54% when considering only the width 
of the circuits. 



Zoi - e 0l 

h S ' S I- 1 - 

I ^9 } - 


(a) 


(b) 


z 0l , e 0 


Figure 16: Size reduction scheme using lumped distributed elements, (a) Conventional 
transmission line, (b) Equivalent transmission line with a series transmission line and two 
open stubs. 


18 


DRDC Ottawa TM 2009-270 




Z, 

62.4 fl 

B 0 \ 

0.0082 5 

ZoX 

50 fl 

0ol 

22.22° 


Table 3: Choice of lumped distributed element’s values forZ c = 26.95 D. and 0 = 30° for 
e, = i2.5°. 



Figure 17: Miniaturized broadband two-section branch-line hybrid geometry using a 30- 
mil FR4 substrate. 


DRDC Ottawa TM 2009-270 


19 










0 



Frequency (GHz) 
(a) 



Figure 18: Simulation results of the miniaturized two-section branch-line hybrid: (a) S- 
parameters and (b) phase outputs. 


20 


DRDC Ottawa TM 2009-270 





















3.2.3 Fabrication and measurement results 


As described in section 3.2.2, a 30-mil FR4 substrate was used to fabricate the proposed 
compact broadband branch-line hybrid. Figure 19 presents the fabricated prototype of the 
broadband distributed lumped elements hybrid. The measured bandwidth def ned by a 3- 
dB coupling with a maximum amplitude imbalance of less than 1 dB is 47%, from 1.05 
to 1.7 GHz (Fig. 20(b)). Over this frequency band, the phase variation is 4°, the isolation 
is greater than 15 dB and the return loss is equal to or greater than 13 dB. Measurement 
results are reported in Fig. 20. 



Figure 19: Miniaturized broadband two-section branch-line hybrid prototype using a 30- 
mil FR4 substrate. 

Experimental results demonstrate that the magnitude of the S-parameters is consistent with 
the simulated data. However, the experimental output amplitude difference is not in ac¬ 
cordance with the simulated one, having a maximum imbalance of 1 dB between 1.15 and 
1.6 GHz. 

The difference between simulation and measurement of the output amplitude difference is 
greater than expected. This can be explained by the fact that this parameter is very sensitive 
to the values of the impedances in the two-section branch-line hybrid, as a parametric study 
shows it in Fig. 21. Moreover, the microstrip lines of the fabricated prototype have been 
measured (Fig. 22 and table 4) and a simulation has been carried out considering the actual 
dimensions of the fabricated circuit. Figure 23 shows the obtained results. One can see that 
the ripples of the output magnitude difference are larger than the original simulated design. 

Detailed comparison of simulation and measured results are given in Table 5. 


DRDC Ottawa TM 2009-270 


21 






(b) 

Figure 20: Measurement results of the miniaturized two-section branch-line hybrid: (a) 
S-parameters and (b) magnitude and phase output difference. Solid lines correspond to the 
simulated S-parameters, markers represent the measured data. 


22 


DRDC Ottawa TM 2009-270 
























x S„ - b 1= 28.9 Q (3.27 mm) 

_ S - b 1= 28.3 ft (3.37 mm) 

.S - b =27.7 ft (3.47 mm) 

- S n - b 1= 27.1 ft (3.57 mm) 

$ S - ^ =26.5 ft (3.67 mm) 
—A— S - b 1 =26.0 ft (3.77 mm) 



1 1.2 1.4 1.6 1.8 

Frequency (GHz) 


X S 41 -S 31 - b 1 =28.9 £1 (3.27 mm) 

- S 41 -S 31 - b 1= 28.3 £2 (3.37 mm) 

.S 41 - S - b i= 27.7 G (3.47 mm) 

- S 4 -S - b i= 27.1 £2 (3.57 mm) 



O 

-2 1 - 1 - 1 - 1 - 1 -i— 

1 1.2 1.4 1.6 1.8 

Frequency (GHz) 


(a) (b) 

Figure 21: Simulated (a) Su and (b) magnitude output difference, as a function of the bl 
impedance of the two-section branch-line hybrid. 


L Z5 L Z4 



Figure 22: Miniaturized broadband two-section branch-line hybrid parameters. 


DRDC Ottawa TM 2009-270 


23 




















Parameter 

Designed (mm) 

Fabricated (mm) 

Wzi 

0.35 

0.32 

fzi,Z3 

32.00 

32.03 

W Z2 

1.45 

1.41 

fz2 

13.80 

13.81 

W Z3 

4.07 

4.03 

W Z4 

3.10 

3.07 

L/a 

4.60 

4.59 

VTz5 

1.45 

1.44 

Lz5 

12 

11.97 


Table 4: Comparison of designed and fabricated dimensions for the compact two-section 
branch-line hybrid. 



Figure 23: Simulated magnitude output difference considering the actual dimensions of 
the fabricated miniaturized broadband branch-line prototype. 

3.3 Concluding remarks 

Two microstrip broadband 90° hybrids have been simulated around 1.37 GHz on a 30- 
mil FR4 substrate using different geometries. The f rst one is the broadband two-section 
branch-line hybrid. The size is 56 x 36 mm 2 and the 3-dB coupling with maximum am¬ 
plitude unbalance of less than 0.5 dB has been noted from 1.15 to 1.6 GHz. Over this 
frequency band, the phase variation is 4°. 

To reduce the size, the lumped distributed element’s method has been investigated. The 
obtained geometry area is 65% of the two-section branch-line hybrid area and 54% when 


24 


DRDC Ottawa TM 2009-270 














Amplitude S 31 /S 41 (dB) 

Phase diflf. (°) 


Simu. 

Meas. 

Simu. 

Meas. 

1.15 GHz 

-3.90/-3.58 

-3.42/-3.88 

-91.2 

-91.0 

1.35 GHz 

-3.67/-3.87 

-3.20/-4.19 

-88.7 

-90.0 

1.6 GHz 

-4.01/-3.90 

-3.69/-4.06 

-89.6 

-91.1 


Table 5: Comparison of simulated and measured results of the compact two-section 
branch-line hybrid. 


considering only the width of the circuits. Over the 1.15-1.6 GHz band, the maximum 
coupling imbalance is 1 dB, and the phase variation is 4°. Higher than expected coupling 
imbalance has been measured because this parameter is sensitive to impedance variation, 
and therefore to the line width fabrication tolerances. 

Figure 24 presents the two different designs on the same scale allowing for direct size 
comparison. 



1 


V 



15 mm 

Figure 24: Hybrid designs on same scale using 30-mil FR4 substrate. 


DRDC Ottawa TM 2009-270 


25 














4 Conclusions and perspectives 


This document reports on compact broadband rat-race and branch-line hybrids designed 
for operation in the 1.15-1.6 GHz frequency bandwidth using microstrip technology. Two 
relatively simple design techniques using a conventional unilayer fabrication process have 
been suggested. The technology described in this document has been specif cally devel¬ 
oped for designing antenna feeding circuits for GPS/GNSS anti-jam systems, but it can be 
used for other wideband applications. 

A miniaturized two-section 180° hybrid using microstrip lines has been designed using 
an FR4 substrate to operate between 1.15 and 1.6 GHz. To miniaturize this circuit, the 
2nd-iteration Moore’s space-filing curve has been used. The miniaturized geometry area 
is 31% of the broadband two-section 180° hybrid area. The obtained perfonnance is as 
good as the conventional geometry. A 3-dB coupling with maximum amplitude unbalance 
of less than 0.13 dB has been measured over the 1.15-1.6 GHz band. Over this frequency 
band, the phase variation is ± 1°. The measured insertion losses are about 1 dB. To further 
reduce the footprint of the compact hybrid and the insertion loss, a second circuit has 
been designed on a substrate having a higher dielectric constant and lower losses (Taconic 
CerlO). The area of the second circuit is 54% of the area of the hybrid on the FR4 substrate. 
The measured performance is very similar, except that the insertion loss has been reduced 
to about 0.6 dB. 

The lumped distributed element’s method has been applied to miniaturize a two-section 
branch-line hybrid. The obtained geometry area is 65% of the two-section branch-line hy¬ 
brid area and 54% when considering only the width of the circuits. Over the 1.15-1.6 GHz 
band, the maximum coupling imbalance is 1 dB, and the phase variation is 4°. However, 
higher than expected maximum coupling has been measured because this parameter is sen¬ 
sitive to impedance variation, and therefore to the line width fabrication tolerances. 

Future work in miniaturization could investigate the use of the recently proposed method 
called dual transmission lines [3], This miniaturization technique allows 64% size reduc¬ 
tion, and is as simple as the ones proposed in this study. Moreover, a unilayer printed circuit 
is required, like the two compact hybrids presented here. Additionally, some work to re¬ 
duce the imbalance coupling of the two-section branch-line hybrid would be of interest. 
Finally, the 90° and 180° hybrids can be integrated using a multi-layer Low Temperature 
Co-f red Ceramic (LTCC) technology to achieve a very compact footprint of the circuits. 


26 


DRDC Ottawa TM 2009-270 



References 


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Rat-Race Hybrid Using a Novel CPW Inverter, IEEE Trans. Microw. Theory Tech., 
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[2] C.-W. Wang, T.-G. Ma and Yang, C.-F. (2007), A New Planar Artif cial 
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[3] C.-W. Tang, M.-G. Chen and Tsai, C.-H. (2008), Miniaturization of Microstrip 
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[4] Tang, C.-W. and Chen, M.-G. (2007), Synthesizing Microstrip Branch-Line 
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[5] C.-W. Wang, T.-G. Ma and Yang, C.-F. (2007), Miniaturized Branch-Line Coupler 
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[6] Kim, D. I. and Yang, G.-S. (1991), Design of New Hybrid-Ring Directional Coupler 
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[8] Eccleston, K. W. and Ong, S. H. M. (2003), Compact planar microstripline 
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[9] J.-T. Kuo, J.-S. Wu and Chiou, Y.-C. (2007), Miniaturized Rat Race Coupler With 
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[12] Ghali, H. and Moselhy, T. A. (2004), Miniaturized Fractal Rat-Race, Branch-Line, 
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[13] Ghali, H. and Moselhy, T. A. (2004), Design of Fractal Rat-Race Coupler, in Proc. 
of Inti. Microw. Symp. Dig. MTT-S, 1, 323-326. 


DRDC Ottawa TM 2009-270 


27 



[14] Mayer, B. and Knochel, R. (1990), Branch-line couplers with improved design 
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[19] K. S. Ang, Y. C. Leong and Lee, C. H. (2002), A New Class of Multisection 180° 
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[20] M.-H. Murgulescu, P. Legaud E. Penard, E. Moisan and Zaquine, I. (1994), New 
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[24] Designer v. 3.5 (2007), Ansoft Corp., www.ansoft.com. 


28 


DRDC Ottawa TM 2009-270 



3. 


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TITLE (The complete document title as indicated on the title page. Its classification should be indicated by the appropriate 
abbreviation (S, C or U) in parentheses after the title.) 


Miniaturized broadband 3-dB / 90° and 180° power splitters for GPS/GNSS anti-jam systems 


4. AUTFIORS (Last name, followed by initials - ranks, titles, etc. not to be used.) 

Caillet, M.; Clenet, M.; Sharaiha, A.; Antar, Y.M.M. 


5. DATE OF PUBLICATION (Month and year of publication of 

6a. 

NO. OF PAGES (Total 

6b. NO. OF REFS (Total 

document.) 


containing information. 

Include Annexes, 

Appendices, etc.) 

cited in document.) 

February 2010 


42 

24 


7. DESCRIPTIVE NOTES (The category of the document, e.g. technical report, technical note or memorandum. If appropriate, enter 
the type of report, e.g. interim, progress, summary, annual or final. Give the inclusive dates when a specific reporting period is 
covered.) 


Technical Memorandum 


8. SPONSORING ACTIVITY (The name of the department project office or laboratory sponsoring the research and development - 
include address.) 

Defence R & D Canada - Ottawa 

3701 Carling Avenue 

Ottawa, Ontario K1A 0Z4 Canada 


9a. PROJECT NO. (The applicable research and development 
project number under which the document was written. 

Please specify whether project or grant.) 

15en01 

9b. GRANT OR CONTRACT NO. (If appropriate, the applicable 
number under which the document was written.) 

10a. ORIGINATOR'S DOCUMENT NUMBER (The official 

document number by which the document is identified by the 
originating activity. This number must be unique to this 
document.) 

DRDC Ottawa TM 2009-270 

10b. OTHER DOCUMENT NO(s). (Any other numbers which may 
be assigned this document either by the originator or by the 
sponsor.) 

11. DOCUMENT AVAILABILITY (Any limitations on further dissemination of the document, other than those imposed by security 
classification.) 

(X) Unlimited distribution 

( ) Defence departments and defence contractors; further distribution only as approved 


( ) Defence departments and Canadian defence contractors; further distribution only as approved 
( ) Government departments and agencies; further distribution only as approved 
( ) Defence departments; further distribution only as approved 
( ) Other (please specify): 


12. DOCUMENT ANNOUNCEMENT (Any limitation to the bibliographic announcement of this document. This will normally correspond 
to the Document Availability (11). However, where further distribution (beyond the audience specified in (11)) is possible, a wider 
announcement audience may be selected.) 

Unlimited 




13. ABSTRACT (A brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highly 
desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the 
security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). 

It is not necessary to include here abstracts in both official languages unless the text is bilingual.) 

This document reports on compact broadband rat-race and branch-line hybrids designed in the 
1.15-1.6 GHz frequency band using microstrip technology. Two relatively simple design tech¬ 
niques using a conventional unilayer fabrication process have been investigated. The technol¬ 
ogy described in this document has been specifically developed for designing antenna feeding 
circuits for GPS/GNSS anti-jam systems, but it can be used for other wideband applications. 

A miniaturized two-section 180° hybrid using microstrip space-filling curves has been designed 
and fabricated to operate between 1.15 and 1.6 GHz on an FR4 substrate. The miniaturized 
geometry area is 31% of the broadband two-section 180° hybrid area. The obtained performance 
is as good as the conventional geometry. A 3-dB coupling with maximum amplitude imbalance 
of less than 0.15 dB has been measured over the 1.15-1.6 GHz band. Over this frequency band, 
the phase variation is ± 2°. The measured insertion loss is approximately 0.9 dB, and is mainly 
due to the substrate loss. To further reduce the footprint of the compact hybrid and the insertion 
loss, a second circuit has been designed on a substrate having a higher dielectric constant and 
lower loss (Cerl 0). The area of the second circuit is 54% of the hybrid area on the FR4 substrate. 
The measured performance is very similar, except that the insertion loss has been reduced by 
about 0.4 dB. 

The lumped distributed element method has been applied to miniaturize a two-section branch¬ 
line hybrid. The obtained geometry area is 65% of the two-section branch-line hybrid area and 
54% when considering only the width of the circuits. Over the 1.15-1.6 GHz band, the maximum 
coupling imbalance obtained by measurement is 1 dB, and the phase variation is 4°. Higher than 
expected maximum coupling imbalance has been measured because this parameter is sensitive 
to the impedance values of the two-section branch-line. 


14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and could 
be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as 
equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords 
should be selected from a published thesaurus, e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. 
If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.) 

180° power splitter 
90° power splitter 
hybrids 
broadband 
miniaturization 
microstrip technology 





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