Transitions between Gap Waveguides for use in a Phased Array Antenna fed by a Rotman Lens

Luis Fernando Carrera Suárez1, Diana Verónica Navarro Méndez1, Mariano Baquero-Escudero2, Bernardo Bernardo-Clemente2, Sara Martinez Giner2.

1 DETRI, Escuela Politécnica Nacional, Quito, Ecuador, fernando.carrera@epn.edu.ec, veronica.navarro@epn.edu.ec 2 ITEAM, Universidad Politécnica de Valencia, Valencia, España, mbaquero@dcom.upv.es, berbercl@upv.es,

samargi2@teleco.upv.es

Abstract—This paper presents two slot-coupled vertical transitions between Gap Waveguides placed in different layers; this transitions will be later used in a phased array antenna fed by a Rotman lens in the Ka band. The first proposed transition connects two ridge-gap waveguides by an H-type slot and the second transition uses a twist to connect a ridge-gap waveguide and groove-gap waveguide horizontally polarized. Results show an 11% of bandwidth for the first transition and a 2.6% of bandwidth for the second transition, which may be enough for many applications.

Index Terms—Rotman lens, gap waveguide, metamaterial.

I. INTRODUCTION Nowadays, there is an increasing interest for commercial

applications at millimeter-wave frequencies. The main advantage of these waves is that they cannot travel long distances due to atmospheric absorption [1] and, hence, the wave interferences among adjacent cells are minimized. The most common commercial application areas are [2]:

• Millimeter-wave communications in the 60 GHz frequency band, which is very suitable for high data rate and wide bandwidth indoor WLAN/WPAN communications.

• Automotive traffic control in which millimeter-wave techniques offer the possibility of detecting vehicles and establishing communications among them. The major areas of deployment are: road transportation informatics, microwave Doppler sensing and automotive collision avoidance radar. Such radars are built in frequencies from 60 GHz to 80 GHz [3].

In both types of applications, it is necessary to point the beam in specific directions or sweep the beam in a given area.

Millimeter-wave systems have been developed based on various transmission-line techniques e.g. metal waveguides or printed lines. The first one presents low losses but the bandwidth can be too narrow for certain applications and may suffer losses due to imperfections in metal contacts, whereas printed lines are heavily affected by dielectric losses at these frequencies.

During the last years, a lot of efforts have been done in order to develop transmission lines which combine the low losses of metal waveguides and the flexibility and low cost of

printed lines. Gap Waveguides are suitable candidates for that purpose [4].

Gap Waveguides are implemented without dielectrics and, due to their configuration, a perfect metal contact between their constitutive parts is not needed. Consequently, these lines are free of dielectric losses (presented in printed lines) and do not need a perfect metallic contact in the joints (necessary in standard waveguides).

In a previous report, it was presented the design of a Rotman lens with Ridge Gap Waveguide (RGW) technology for beam scanning applications at millimeter wave frequencies [5]. In that design, the lens was in the same layer as the array antenna, therefore the overall dimensions of the system may be considerable for certain purposes.

In many applications, a low profile lens with small area becomes necessary due to space restrictions. In these cases the concept of multi-layer structure is a suitable solution for size reduction. As a result of this approach, it is necessary to design appropriate transitions to connect the different layers of the structure.

In this work, a slot-coupled vertical transition between two RGW and a twist from RGW to Groove Gap Waveguide (GGW) both at a central frequency of 38 GHz, are presented.

II. GAP WAVEGUIDES Recently, a new metamaterial EBG-based waveguide

technology has been proposed [4]. The Gap Waveguides were developed as an alternative to conventional transmission lines at high frequencies. These waveguides are implemented in a narrow gap between two parallel metal plates by using a textured structure on one of the surfaces.

The gap waveguides are based on the parallel plate cut-off condition between a Perfect Electric Conductor (PEC) layer and a Perfect Magnetic Conductor (PMC) layer, when the separation between them is less than /4. Typically, a set of metallic posts called “bed of nails” is used to implement the PMC surface.

There are three different ways to implement those waveguides: the first one named Ridge Gap Waveguide, use a metal ridge among the bed of nails to guide the field along a particular path. All wave propagation in other directions is prohibited due to the high surface impedance in the textured

This work has been supported by the Spanish Ministry of Science and Innovation under the projects TEC2010-20841-C04-01 and CSD2008-00068, project PROMETEO/2011/061 and the Government of Ecuador (SENESCYT)

The 8th European Conference on Antennas and Propagation (EuCAP 2014)

978-88-907018-4-9/14/$31.00 ©2014 IEEE 774

surface; the structure of this waveguide is shown in Fig. 1a. The other two realizations of these waveguides are the vertical and horizontal Groove-Gap Waveguides. In this case, the field propagates inside a groove created in the textured surface instead of along the top of a ridge. Depending on the geometry of the groove, it is possible to propagate vertical (GGW-VP) or horizontal (GGW-HP) polarizations. The configuration of these waveguides is illustrated in Fig. 1b and Fig. 1c, respectively.

a)

b)

c)

Fig. 1. Types of Gap Waveguides. a)RGW, b) GGW-VP, c) GGW-HP

III. DESING OF TRANSITIONS As mentioned above, the area occupied by the phased array

antenna system (formed by an array antenna fed by a Rotman lens) may be reduced using a multilayer structure. The lens body is placed in the bottom metal layer whereas the array antenna is on a top metal layer. The input and output lines of the lens are implemented with RGW technology. Due to this configuration, it is necessary to design a slot-coupled transition to feed the array antenna. Fig. 2 illustrates the proposed system configuration.

Fig. 2. Two layer phased array antenna fed by a Rotman lens.

The array antenna is formed by untilted slots placed in the narrow wall of a GGW-HP. These slots are feed by parasitic dipoles located in a plane parallel to the waveguide´s wall at a certain distance of the slots. The parasitic dipole is tilted with respect to the slot so that the relative angle between them determines the amount of coupling [6]. Thereby, it is necessary to design a transition acting as a twist from vertical polarization of the E field in the RGW to horizontal polarization of the E field in the GGW.

The most important part in designing Gap Waveguides is to determine the lower and upper cut-off frequencies of the stop-band. These frequencies are function of the geometrical parameters of the periodic surface.

Therefore, the design of a Gap Waveguide starts with the selection of the appropriate dimensions of their constitutive elements. The geometrical parameters that determine the cut-off bandwidth are:

• The distance from pins to the upper metal plate, i.e., the gap height (ha).

• The height of the pins (hp). • The period of the pins, i.e., the distance between

their centers (p). • The width of the square pin (wp). • The geometry of the lattice (rectangular,

triangular). In the present work we have chosen a stop-band between

33GHz and 59GHz and a rectangular geometry of the lattice. This bandwidth is obtained by varying the other parameters. The optimal dimensions are: p = 1.25 mm, wp = 0.45 mm, hp = 1.95 mm, ha = 0.395mm.

As an example, the dispersion diagram for the RGW is plotted in Fig. 3. It is easy to see that only one mode is propagating in the waveguide. This mode is a quasi-TEM mode that propagates within the metal ridge and the upper-metal plate.

Fig. 3. Types of Gap Waveguides. a)RGW, b) GGW-VP, c) GGW-HP

A. Vertical transition between two RGW The first designed transition connects two RGW placed on

different layers: A solution to this problem has already been presented in [7]. The current design has the same conceptual idea. The geometry of the transition is depicted in Fig. 4. As can be observed in Fig. 4a, the lower ridge ends in a bed of

The 8th European Conference on Antennas and Propagation (EuCAP 2014)

775

nails which is acting as a high-impedance surface. This structure is equivalent to an open-circuit and, hence, the slot must be placed at a distance d= gRGW/4 away from the end of the ridge in order to obtain a short-circuit condition. Due to the current distribution on the upper lid surface, the slot is placed perpendicular to the ridge. Fig. 4b depicts the upper layer with the RGW. A longitudinal cut of the transition and a detail of the H-type slot are shown in Fig. 4c and Fig. 4d, respectively. The optimized dimensions are b=2.7mm, c=0.8mm, d=1.85mm, e=0.55mm.

a)

b)

c)

d)

Fig. 4. Details of the slot-coupled vertical transition. a)Lower Layer (RGW), b) Upper Layer (RGW), c) Longitudinal cut, c) Coupling slot.

The simulated S-parameters of the transition are plotted in Fig. 5. As can be observed the S11 parameter is below -20dB between 36.4GHz and 40.6GHz (relative bandwidth of 11%), and the S21 parameter is above -0.35dB within the same frequency band.

Fig. 5. Simulated S-parameters for the slot-coupled vertical transition.

B. Twist between RGW and GGW-HP The second proposed transition connects a RGW in the

lower level and a GGW-HP in the upper level. As mentioned

in section II, since the array antenna has been designed using GGW-HP, it is necessary to design a transition to twist the vertical polarization of the E field in a RGW to the horizontal polarization of the E field in the GGW-HP.

The design principle of this transition is the same as the previous one. In the lower level, the RGW ends in a bed of nails that is equivalent to an open-circuit and, hence, the slot must be placed at a distance d1= gRGW/4 to achieve a short-circuit condition. In the upper layer, the slot is at d2= gGGW/4 away from the end of the GGW-HP. The geometry of the model is illustrated in Fig. 6. The optimized dimensions are: d1=2.5mm, d2=3mm, f=4.4mm, g=0.5mm, =18 degrees.

a)

b)

c)

d)

Fig. 6. Details of the slot-coupled transition between RGW and GGW. a) Lower layer (RGW), b) Upper layer (GGW), c9 longitudinal cut, d) coupling slot.

Fig. 7. Simulated S-parameters for the transition.

The simulated S-parameters of the transition are shown in Fig. 7. The S11 parameter is below -20dB between 37.5GHz and 38.5GHz (relative bandwidth of 2.6%), which is enough

The 8th European Conference on Antennas and Propagation (EuCAP 2014)

776

for many applications. Also, the S21 parameter is above -0.3dB within the same frequency band.

IV. CONCLUSIONS In this paper, two transitions between Gap Waveguides

have been presented. In the first transition (between two RGW in different layers) the simulated results show an excellent behavior of the design.

A return loss better than 20dB, in the frequency range from 37.5 GHz to 38.5GHz, has been obtained. A prototype is currently under construction, to validate the simulations.

In the second transition (between RGW and GGW-HP) the simulated return loss is better than 20dB from 37.4GHz to 38.5GHz. The simulated bandwidth is enough for many applications. Future current will be focused in increasing the obtained bandwidth.

V. REFERENCES [1] A. Vander Vorst, "Millimetre-Wave Atmospheric Propagation and

System Implications," in 16th European Microwave Conference, 1986., 1986, pp. 19-30.

[2] H. H. Meinel, "Commercial applications of millimeterwaves: history, present status, and future trends," IEEE transactions on Microwave Theory and Techniques, vol. 43, pp. 1639-1653, 1995.

[3] Y. Yamada, S. Tokoro, and Y. Fujita, "Development of a 60 GHz radar for rear-end collision avoidance," 1994, pp. 207-212.

[4] P. S. Kildal, "Three metamaterial-based gap waveguides between parallel metal plates for mm/submm waves," in 3rd European Conference on Antennas and Propagation, 2009. EuCAP 2009. , 2009, pp. 28-32.

[5] F. C. Suárez, D. N. Mendez, and M. Baquero-Escudero, "Rotman lens with Ridge Gap Waveguide technology for millimeter wave applications," in 7th European Conference on Antennas and Propagation (EuCAP), 2013, 2013, pp. 4006-4009.

[6] S. Martinez Giner, A. Valero-Nogueira, J. Herranz Herruzo, and M. Baquero Escudero, "Excitation of untilted narrow-wall slot in groove gap waveguide by using a parasitic dipole," in 7th European Conference on Antennas and Propagation (EuCAP), 2013, 2013, pp. 3082-3085.

[7] H. Kirino and K. Ogawa, "A 76 GHz multi-layered phased array antenna using a non-metal contact metamaterial waveguide," IEEE Transactions on Antennas and Propagation, vol. 60, pp. 840-853, 2012.

The 8th European Conference on Antennas and Propagation (EuCAP 2014)

777