A New Substrate Integrated Waveguide Phase Shifter - IEEE Xplore

Proceedings of the 36th European Microwave Conference

A New Substrate Integrated Waveguide Phase Shifter Kheireddine SellalPl', Larbi Talbi(2), Tayeb Denidni(') and Jules Lebel(3)

(1) INRS Telecommunications, Montreal, Quebec, Place Bonaventure, 800 de la Gauchetiere Ouest, bureau 6900, H5A 1K6, Canada (2) Department of Computer Science and Eng., Universite du Quebec en Outaouais, Gatineau, Quebec, 101 rue Saint-Jean Bosco, Case postale 1205, Succursale Hull, J8Y 3G5, Canada (3) Industry Canada, Communications Research Centre, Ottawa, Ontario, 3701 Carling Avenue, K2H8S2, Canada Abstract - This paper presents a new phase shifter based on substrate integrated waveguide (SIW) technique. Phase shifting is achieved by changing the diameter and the position of metallic inductive posts inserted in the SIW substrate. Using a two-post configuration, simulations have been carried out for different diameters and positions which have shown a good agreement with the theory. To prove the concept, prototypes have been fabricated and measured. Preliminary experimental results show clearly that the use of inductive posts alters the phase of the wave traveling in the SIW substrate. Index Terms - Microwave circuits, Microwave phase shifter, Millimeter-wave phase shifter, Phase shifters, waveguides

shifter was implemented by inserting a ferrite toroid into the substrate, replacing one part of it. In this paper, a new phase shifter using SIW and a postbased technique is proposed. A two post configuration is used for the design of the phase shifter and to obtain the desired phase shifts. This paper is organized as follows. Section II describes the structure and provides the basic theory. In Section III the design, simulation and experimental results are presented. Another approach for the design of a SIW phase shifter is given in section IV. Finally, section V presents our conclusions.

I. INTRODUCTION

The cylindrical inductive post in a rectangular waveguide has first been treated by Shwinger [1] and the results are given in Markuvitz's waveguide handbook [2]. Many authors [1] have treated the problem using different methods, but they all agreed that the cylindrical inductive post is equivalent to a T-network high pass filter whose elements are function of the diameter and the position of that cylindrical inductive post in the waveguide. Moreover, in a switched high-pass/low-pass phase shifter [3], the high pass filter shows a phase advance, meaning that an inductive post in a rectangular waveguide affects the phase of the propagating wave. In fact, the postbased technique has been implemented and a phase change has been obtained [4]. Recently, the concept of the Substrate Integrated Waveguide (SIW) has been proposed [8], in which an "artificial" waveguide is synthesized and constructed with linear arrays of metallized via-holes or posts embedded in the same substrate. The connection between the waveguide and the planar circuits is provided via transitions formed with a simple matching geometry between both structures [14]-[17], thus providing a compact and low cost platform. This new SIW concept allowed for the design of microwave and millimeter-wave circuits such as filters [5] and antennas[6]. However, only one design of a phase shifter has been presented using this technique [7]. In that work, a phase

2-9600551-6-0 (D 2006 EuMA

II. STRUCTURE AND THEORY

As shown in Fig. 1, the insertion of a cylindrical metal post in a standard rectangular waveguide (RW) is equivalent to a high-pass filter. The elements of that equivalent circuit vary with the position x and the diameter d of that metal post. Equations representing this relationship are given in [2]. From [3], the parameter S21 and its phase are given by the following equations (1) and (2)

2

2(1-XbX)+j(Xb -2X 0 = tan -1Xb +2Xa -Xb X L-

2(l1-XbXa )

+XbX2)

(1)

(2) -i

where Xa and Xb are the reactance and the susceptance of the high-pass filter, respectively. The dependence of both Xa and Xb on the position and the diameter of the metal post makes the phase shift O a function of these same parameters.

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September 2006, Manchester UK

W=0.38mm and Wt=1.6mm, as illustrated in Fig. 2. Fig. 3 shows a photograph of the designed phase shifter.

Equivalent circuit -jXa -

-jXa Fig. 3.

-i

Photograph of the SIW phase shifter.

In order to study the new phase shifter we have inserted two inductive posts into the SIW structure and carried out simulations using the HFSS EM simulator and its optimetrics

feature [10]. By varying the position and the diameter of the inductive posts inside the SIW waveguide we have obtained the plot shown in Fig. 4. As one can notice, moving the inductive posts inside the structure results in changing the phase of the incident wave.

Fig. 1. A metal post inserted in a rectangular waveguide ant its equivalent circuit.

By combining the preceding theory with the SIW concept, the porposed phase shifter will have the structure shown in Fig. 2. This phase shifter is composed of respectively two parallel arrays of via holes, two parallel plates and two metal posts. The parallel plates and the arrays of via holes are used as a waveguide while the position and the diameter of the metal posts in tyhe substrate are used to alter the phase of the incident wave.

140r

D=0.3mm D=0.4mm D=0.5mm D=0.6mm

120 /

100 u)

0,

a)

cn 0

a)

806040 20-

0 -20

-40 -60

.4

0

0.5

1

1.5

2

Offset (mm)

2.5

3

3.5

4

Fig. 4. Phase of S21 as a function of the diameter and the position of the inductive posts.

Fig. 2. Structure of the SIW phase shifter using two posts. III. DESIGN, SIMULATION AND EXPERIMENTS

The SIW phase shifter was designed following the rules given in [7[-[9]. The latter used ROGERS RT/Duroid 5880 substrate of relative permittivity er = 2.2. The diameter of the via holes is d=0.77mm and the pitch is p=1.52mm, with a width a=5.57mm and a height b=0.508mm. The parameters of microstrip to SIW transitions were found to be lt=1.778mm,

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Moreover, for a given "offset", using different post diameters produced different phase shifts. Thus a good agreement between simulation results and theory was observed. Considering the phases for an acceptable level of return loss SIt (-OdB), the maximum offset was limited to 1.2mmfor a phase range between 1.20 and 600. To validate the proposed concept we have designed prototypes for three bits, 11.25°, 22.5° and 45°, as they are usually used in the phase progression in a phased array [12]. Fig. 5 to Fig. 7 show simulation versus experimental results. Simulations have shown a return loss better than 19dB, an

insertion loss better than 0.2dB and phase shifts of 11.820, -10 22.20 and 44.480, respectively. The bandwidth was 0.35% about the design frequency of 28GHz for a phase error of ±30. -20 The experimental results, in the other hand, have shown a -30 return loss better than 12.5dB and insertion loss better than -40 5.9dB for phases of -44°, -19.4° and -13.52° respectively, a -50 coupled with a bandwidth of 0.3%. these results we notice that SIt Comparing agrees fairly -60 well with the simulation. The measurements point to a slight -measured, 11.25° -70 shift towards higher frequencies, but only about 2%. This -simulated, 11.25° -80 measured, 22.5° may be due to a difference in the dielectric constants of the simulated, 22.5° -90 actual and simulated. material, measured, 45° simulated, 45° Considering bits 11.250 and 22.50, the differences between -100 27.4 27.6 27.8 28 28.2 28.4 28.6 actual and simulated insertion losses S21 is 1.6 dB for the frequency (GHz) first and 2.6dB for the second. However, for bit 450, the difference is 5.6dB which needs further investigation and may Fig. 5. Experimental vs. simulated return loss for the three be attributed to several factors such as: considered bits. 1) conductor loss that was not included in the simulation, 2 2) an assumed loss tangent that applied at a lower frequency, f=IOGHz, and that likely increases at higher 0 frequency, 3) a calibration of the vector network analyzer where -2 -_ -= -2~~~~~~~ standards made of alumina were used while the prototypes -4 were built using duroid material. Phase shift errors, in the other hand, may be due to slight -4 errors in positioning of the via holes, representing the inductive posts, during the fabrication process. Simulation settings have to be verified as well. -measured, 11.25° -10 _ Although we notice some differences between experimental -simulated, 11.25° -measured, 22.5° -12and simulation results, the concept of using inductive posts in -simulated, 22.5° a SIW to obtain phase shifting has been proven and this is measured, 45° -14 simulated, 45° clearly shown in Fig. 7. Cf

-

- - - -

-

-

-

-

27.8

IV. COMPARISON WITH ANOTHER APPROACH

Fig. 6.

27.85

27.9

27.95 28 28.05 frequency (GHz)

28.1

28.15

28.2

Experimental vs. simulated insertion loss for the three

An alternate approach in designing a SIW phase shifter is considered bits. to use structures with different lengths. Using this approach we have simulated three SIW structures of length LI, L2 and 60 L3, respectively, where Li