The width and length of those microstrip lines control the coupled power amplitude to the patches. Likewise, each subarray is fed by means of a vertic

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IEEE MELECON 26, May 16-19, Benalmádena (Málaga), Spain Monopulse Waveguide Patch Array Antenna in 37 GHz Band Manuel Sierra Pérez, Pedro Rodríguez Fernández, José Luis Masa Campos, José Luis

1 IEEE MELECON 26, May 16-19, Benalmádena (Málaga), Spain Monopulse Waveguide Patch Array Antenna in 37 GHz Band Manuel Sierra Pérez, Pedro Rodríguez Fernández, José Luis Masa Campos, José Luis Fernández Jambrina Department: Señales y sistemas de Radiocomunicación. E.T.S.I. de Telecomunicación (Universidad Politécnica de Madrid). Madrid (Spain). [email protected] II. ANTENNA STRUCTURE Abstract This paper presents a circularly polarized patch array in 37 GHz frequency, integrated in a narrow band application (BW < 1%) for signal detection. The designed array has to fulfill some specific requirements related to its radiation pattern (monopulse in the horizontal pattern and cosecant shape in the vertical one); other significant requirements are: circular polarization (RHCP with axial ratio under 3 db), low side lobes (-2 db below the main lobe) in the sum pattern of the monopulse, and high gain (greater than 32.5 db). I. INTRODUCTION Due to the high operation frequency and gain, a lossless scheme is required. A waveguide feeding provides it in a satisfactory way. The power coupling mechanism from the waveguides to the radiating elements is done through a new concept, which is based on microstrip lines located inside the waveguides. The radiating element is a modified circular patch easily manufactured on printed circuit. Circular polarization has been forced by a proper design of the dimensions of the perturbation elements inserted in the standard circular patch. Three different modules have been projected (Fig. 1), namely: (1) the radiating circuit (patches and microstrip coupling lines), (2) the waveguide feeding network and (3) the monopulse network in charge of obtaining the right phase configuration in each subarray to get a sum pattern or a difference pattern for a monopulse working in the horizontal plane; if the outputs of the network present the same phase a sum pattern is obtained, however, if there is a phase difference of 18º between the two outputs in the left part of the circuit and the two outputs in the right part, the difference radiation pattern is achieved. Antenna ANTEN Vertical feeding waveguides Cosecant feeding network Waveguide connection to the monopulse - beam forming network Coupling slots between radiating and feeding waveguides Radiating waveguide Figure 1. : Antenna structure. The first module of the antenna (1) consists of four subarrays fed, named horizontal/radiating waveguides (Fig. 1) (module 2). Inside these radiating waveguide rectangular microstrip coupling lines have been placed. They carry the power to each radiating element (Fig. 2) through a metallic via hole Red Sum Patch Mono-pulse beam forming ReDifference diferenc Phases in sum Phases in difference Figure 2. : Detailed horizontal waveguide and coupling microstrip lines /6/$2. 26 IEEE 324

The width and length of those microstrip lines control the coupled power amplitude to the patches. Likewise, each subarray is fed by means of a vertical/feeding waveguide (Fig. 3). 2.65 mm S/2.25 mm.

Detailed vertical waveguide with its slots to feed one subarray The field from those vertical waveguides is coupled to the horizontal ones by slots placed in every junction wall between them.

2 The width and length of those microstrip lines control the coupled power amplitude to the patches. Likewise, each subarray is fed by means of a vertical/feeding waveguide (Fig. 3) mm S/2.25 mm.25 mm Figure 5. :Detailed modified circular patch (radiating element). Figure 3. Detailed vertical waveguide with its slots to feed one subarray The field from those vertical waveguides is coupled to the horizontal ones by slots placed in every junction wall between them. The monopulse network (3), placed in the bottom side of the structure, presents two different inputs related to the obtaining of the sum or difference pattern, as well as four outputs corresponding to each subarray. Connection between the monopulse network and the feeding network (vertical waveguides) is carried out by a hybrid based waveguide circuit. Over the radiating circuit a foam layer will be placed to fasten it. Covering the antenna it s placed a radome taken into account in the design process to minimize the reflection coefficient at the working frequency (Fig. 4). The perturbations generate two orthogonal modes and, as the dimensions of the additional elements are adjusted to the optimum value both modes are excited in equal amplitude and 9º out of phase at the centre frequency, getting the rotation of the field. With the dimensions shown in Fig. 4 the following axial ratio is obtained (Fig. 5). Figure 6. : Axial ratio IV. FEEDING NETWORK As it was mentioned previously, the antenna feeding network was implemented by using waveguide technology, due to losses requirements. The main advantage of this technology is lossless design; however it has some drawbacks compared to PCB design as bigger cost and size. Fig. 6 shows a transverse view of the feeding network: Figure 4. : Picture of the antenna. III. RADIATING ELEMENT The radiating elements are printed patches on a thin dielectric plate with a 2.17 dielectric constant. The resultant circuit is placed over the top walls of every single horizontal waveguide (Fig. 1). The main characteristic of the patches used in this design is the circular polarization of the radiated field. To get this sort of polarization a modified circular patch has been used (Fig. 4). 325

Foam >6mm Radome Fiber glass.5mm -2 db from the main lobe level in the sum pattern. The difference radiation pattern must be over the sum pattern except in the pointing angular zone (6º).

We must realize that with this configuration the horizontal waveguides are not totally closed because of the thickness of the substrate of the coupling microstrip lines; therefore each horizontal

choosing a vertical waveguide whose half wavelength was equal to the distance between two consecutive horizontal waveguides ( gv /2 = 7 mm. ). Fig.

3 Foam >6mm Radome Fiber glass.5mm -2 db from the main lobe level in the sum pattern. The difference radiation pattern must be over the sum pattern except in the pointing angular zone (6º). 3mm 6 7 mm Radiating Waveguides 5 6 mm Feeding Waveguide Figure 7. : Waveguides dimensions. The dimensions of the horizontal waveguides are selected to work with only one propagating mode. We must realize that with this configuration the horizontal waveguides are not totally closed because of the thickness of the substrate of the coupling microstrip lines; therefore each horizontal waveguide is fed with opposite phase respect to the contiguous one, in order to generate a virtual short-circuit that electromagnetically closes the waveguide; this can be done in a simple way by choosing a vertical waveguide whose half wavelength was equal to the distance between two consecutive horizontal waveguides ( gv /2 = 7 mm. ). Fig. 8 shows a picture of the manufactured feeding network where we can see the horizontal waveguides that distribute the power among the radiating elements through the microstrip coupling lines. The slots to couple power from every single vertical waveguide to the corresponding horizontal one are also shown. Figure 9. Monopulse radiation pattern. In the vertical plane a cosecant shape is needed for the radiation pattern (Fig. 1) Figure 1. Cosecant radiation pattern. The radiating patches must be fed in order to achieve the radiation pattern in the two main planes. The feeding amplitude will be controlled by the dimensions of the microstrip coupling lines in the horizontal plane (Fig. 11) and by the length of every slot and its distance to the centre of the vertical waveguide in the vertical plane (Fig. 12). Figure 8. :Feeding network V. RADIATION PATTERNS The design requires a monopulse radiation pattern in the horizontal plane (Fig. 9), with secondary lobes level under Figure 11. Microstrip coupling lines. 326

Figure 12. Slots in printed circuit. In the case of the cosecant radiation pattern (vertical array) the feeding phase must also be controlled.

Furthermore, patches separation in a same horizontal waveguide has been selected to be 3 grad/4 to minimize both reflections between microstrip lines and grating lobes appearance; however, the same

13 shows the complete array where the visual effect of the patch rotation is observed. Figure 13. Patch array. VI.

4 Figure 12. Slots in printed circuit. In the case of the cosecant radiation pattern (vertical array) the feeding phase must also be controlled. It hast to be different between consecutive horizontal waveguides. It is carried out by turning the patches around its feeding point an appropriate angle (in vertical direction). Furthermore, patches separation in a same horizontal waveguide has been selected to be 3 grad/4 to minimize both reflections between microstrip lines and grating lobes appearance; however, the same feeding phase in the radiating elements of the horizontal array is needed, so an additional patch rotation must be done (in horizontal direction). Fig. 13 shows the complete array where the visual effect of the patch rotation is observed. Figure 13. Patch array. VI. MONOPULSE NETWORK In order to get the monopulse functioning in the horizontal plane, a waveguide network has been designed (Fig. 14). Figure 14. Waveguide monopulse networkt. Figure 15. Measured output phases-(a)sum input, (b) diff. input.. The structure presents four outputs each for the corresponding subarray. The hybrid circuit together with the control of the waveguides length obtain the desired phase difference at the outputs depending on the excited input (Fig. 15). Using one of the inputs the same feed phase is obtained for both horizontal semi-arrays, which leads to a sum pattern. If the other input is excited, a phase difference of 18º will be obtained, resulting in a difference radiation pattern. REFERENCES [1] Manuel Sierra Castañer, Manuel Sierra Pérez, María Vera Isasa, José Luis Fernández Jambrina, Low-Cost Monopulse Radial Line Slot Antenna, IEEE Trans. on Antennas and Propagat., vol. 51, no2, pp , Feb. 23 [2] J. R. James, P.S. Hall, Handbook of Microstrip Antennas, IEE ELECROMAGNETIC WAVES SERIES 28, Peter Peregrinus Ltd., 1989 [3] J.L. Masa, M. Sierra Pérez, A New Feeding Structure For Radial Line Planar Match Antenna, Proceedings of Symposium on APS, pp , Monterey, California, USA, June 24 [4] K. Sakakibara, Y. Kimura, J. Hirokawa, M. Ando, N. Goto, A Two- Beam Slotted Leaky Waveguide Array for Mobile Reception of Dual- Polarization DBS, IEEE Trans. on Vehicular Technology., vol. 48, no1, pp. 1-7, Jan [5] C.G Montgomery, R. H. Dicke, E.M. Purcell, Principles of Microwave Circuits vol. 8, M.I.T. Radiation Laboratory Series, Boston Technical Publishers, Inc., pp , Ed.1964 [6] Y. Kimura, T. Hirano, J. Hirokawa, M. Ando, Alternating- Phase Fed Single-Layer Slotted Waveguide Arrays with Chokes Dispensing With Narrow Wall Contacts, IEE. Proc. Microw. Antennas Propag., vol. 148, no.5, Oct

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