Comparative study, building, measurement and simulation of two wi-fi slotted waveguide antennas made by a rectangular guide

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1 Comparative study, building, measurement and simulation of two wi-fi slotted waveguide antennas made by a rectangular guide João Filipe Tavares Rodrigues Instituto Superior Técnico Avenida Rovisco Pais, Lisboa joao.t.rodrigues@ist.utl.pt Abstract: The constant growth in the number of applications using the Internet, especially wireless applications, increases the need to develop new antennas designed to allow the development of Wireless Local Area Networks (WLAN). These applications are also useful for military applications, where the use of WLAN would lead to an improvement of the bit rate of data transmission in real time. The present thesis deals with the different steps of an engineering project. The waveguide slotted antennas were designed, simulated, built, tested in an anechoic chamber and the measured and theoretical results were compared. It is important to make a reference to the simulator used as an extremely important tool in the simulation of an antenna. The structure of this thesis follows the steps of an engineering project where the antennas were developed based on the theoretical concepts, simulated with the appropriate software, tested in an anechoic chamber and all the results were compared. In this study case, the final results presented will be the principal characteristics of the developed antennas: gain, -3dB bandwidth, input impedance and VSWR. Keywords: Simulator (CST MWS), gain, -3dB bandwidth, VSWR, farfield diagram 1. INTRODUCTION This type of antennas is based on a metallic rectangular section waveguide with an array of slots placed along the main axis with an offset which is dimensioned in accordance to the wavelength inside the guide. The mechanism of radiate is based on the Principle of Babinet. The dimensions and locations of the slots are calculated to enhance the radiation in the H Plane. The simulation was made using the CST MWS software and the antennas were tested in an anechoic chamber. 2. THEORETICAL DESIGN OF THE ANTENNAS The waveguide is operating in the fundamental mode TE 10 which is presented to the following characteristics. TABLE 2.1 GUIDE CHARACTHERISTIC figure. FIGURE 2.1 ELECTRIC FIELD, MAGNETIC FIELD AND SURFACE CURRENT [1] In these antennas, the introduction of the slots in the broad face of the guide perturbs the surface current which leads to the creation of an electric field perpendicular to the plane where the slots are placed. Therefore the maximum gain of the antenna is in this direction. Each slot works as an admittance in the equivalent transmission line as shown in the next figure Generic write [rad ] [Hz] [m] [rad ] FIGURE 2.2 ADMITANCES OF THE SLOTS [2] There is a need to create an offset from the center of the guide in order to prevent that the fields would cancel themselves. This offset can be calculated thanks to the following equation [3] Inside the guide the electric field, magnetic field and surface current have the distribution shown in the next The space between the center of each slot is function of the wavelength. In case of the adjacent slots the distance

2 is of and the total length of the antennas is function Where N stands for the number of slots. To prevent the reflections inside the guide the feed is placed from the bottom of the guide as well as the center of the last slot that has a distance of to the top of the guide. The distances can be seen in the following figure. FIGURE 3.2 S 11 (4 SLOTS) FIGURE 3.3 S 11 (8 SLOTS) As we can see, there is a minimum in the 2,45 GHz. Next, it is shown the 3D radiation diagram of both antennas. FIGURE 2.3 POSITION OF THE SLOTS The theoretical design ends with the explanation of the waveguide feed. This is made with a N connector which has a pole of of length. FIGURE 3.4 RADIATION DIAGRAM (4 SLOTS) 3. BUILDING AND SIMULATION The first simulation proved that the scattering parameters were not as good as wanted. Therefore a double stub was designed to improve the S-Parameters. This stub was design recurring to the Smith Chart and optimized with the simulator. The first pole is placed from the feed pole and the second one from the first one. The simulated results are presented only in the presence of this stub. FIGURE 3.5 RADIATION DIAGRAM (8 SLOTS) FIGURE 3.1 STUB First of all, it is important to measure the S-Parameters in the operation frequency. In these diagrams it is represented the results obtained by the CST MWS. We can see the side lobes as well as the maximum gain perpendicular to the guide. Page 2

3 S_11 [db] S11 [db] 4. EXPERIMENTAL MEASUREMENTS To continue the analysis of the slotted waveguide antennas, they were tested in an anechoic chamber in IST. First of all, there is the need to measure the scattering parameters. This measure was made with a Network Analyser. The next figure shows the way that the measure was made. antenna. This analysis is made by rotating the probe antenna from the correct polarization, passing by the position of the cross polarization and finishing again with the correct polarization. Polarization Diagram (4 Slots) FIGURE 4.4 POLARIZATION DIAGRAM (4 SLOTS) Polarization Diagram (8 Slots) FIGURE 4.1 MEASURE OF SCATTERING PARAMETERS The results of the scattering parameters are shown in the following graphics. First, it is shown the S 11 of the antenna with four slots and then the S 11 of the eighth slots antenna S_11 S 11 (4 Slots) 2 2,2 2,4 2,6 2,8 3 FIGURE 4.2 S 11 OF THE 4 TH SLOTTED ANTENNA -4,00-8,00-12,00-16,00 S_11 S 11 (8 Slots) 2 2,2 2,4 2,6 2,8 3 FIGURE S 11 OF THE 8 TH SLOTTED ANTENNA It was said that this antennas rejects the crosspolarization in a high percentage. The following graphics are the real proof of the rejection. First, it was analysed the polarization diagram of each FIGURE 4.5 POLARIZATION DIAGRAM (8 SLOTS) The next step of the analyse is to compare the copolarization with the cross-polarization both in the E and H Plane. The following graphics shows the comparison in the antenna of 4 slots. -80 Hx H FIGURE 4.6 POLARIZATION OF THE 4 TH SLOTTED ANTENNA (H PLANE) Page 3

4 Ex E FIGURE 4.7 POLARIZATION OF THE 4 TH SLOTTED ANTENNA (E PLANE) Now, it is shown the same comparison to the eighth slotted antenna Hx H FIGURE 4.8 POLARIZATION OF THE 8 TH SLOTTED ANTENNA (H PLANE) E Ex FIGURE 4.9 POLARIZATION OF THE 8 TH SLOTTED ANTENNA (E PLANE) One of the goals of this dissertation was to analyse the principal parameters of the antenna. Those parameters are shown in the next graphics, where the red curve represents the maximum gain and the green one represents the level of the more influent side lobe. First, the 4 th slotted antenna. 4 Slots Gain - H Plane 210,41 1-3, H FIGURE 4.10 RADIATION DIAGRAM (4 SLOTS H PLANE) 4 Slots Gain - E Plane 210, , E FIGURE RADIATION DIAGRAM (4 SLOTS E PLANE) To continue, the same analyse in the eighth slotted antenna. 8 Slots Gain - H Plane 15,00 5,00 0,85-25,00 13,29 H FIGURE 4.12 RADIATION DIAGRAM (8 SLOTS H PLANE) Page 4

5 S_11 [db] S11 [db] 8 Slots Gain - E Plane , ,97 E FIGURE 4.13 RADIATION DIAGRAM (8 SLOTS E PLANE) To conclude the analyse of the measurements made in the anechoic chamber, the principal parameters are resumed in the following tables. TABLE 4.1 PARAMETERS (4 SLOTS) H Plane E Plane S 11 (db) -19,74 VSWR 1,23 Gain(dBi) 12,41 12,35 (db) -14,12, TABLE PARAMETERS (8 SLOTS) H Plane E Plane S 11 (db) -13,84 VSWR 1,51 Gain(dBi) 15,29 14,97 (db) -12,44-27, It is perceptible the difference of 2 db to what was expected in the simulated results. This problem was also detected in other dissertation and it is expected that this difference is the result of a bad calibration of the software used with the anechoic chamber. 5. COMPARATIVE ANALYSIS AND DISCUSSION Pretending to compare the simulated results with the measured ones, the next graphics show the simulated curve in red and the measured result in the blue curve. As in the previous chapter, the analyse stars with the measure of the scattering parameters and then proceed to the E and H Plane. Comparison S 11 (4 Slots) -1-2 S_11 (NA) S_11 (CST) 2,4 2,45 2,5 FIGURE 5.1 S 11 COMPARISON (4 SLOTS) -1-2 Comparison de S 11 (8 Slots) 2,4 2,45 2,5 S_11 (NA) S_11 (CST) FIGURE 5.2 S 11 COMPARISON (8 SLOTS) In the first case (4 slots) the S 11 measured is even better than the simulated one, but in the eighth slotted antenna there was, probably, a small imprecision in the adjust of the stub leading to a worse S 11. Next, it is shown the comparison of the principal parameters of the antennas FIGURE 5.3 COMPARISON (4 SLOTS H PLANE) Page 5

6 FIGURE 5.4 COMPARISON (4 SLOTS E PLANE) 15,00 5,00-25,00-35,00 FIGURE 5.5 COMPARISON (8 SLOTS H PLANE) FIGURE 5.6 COMPARISON (8 SLOTS E PLANE) In the previous graphics, the measured curve has a increment of 2 db to cover the problem explained in the previous chapter. The curves are very similar, which proves the good simulation made. There is a brief resume of the principal parameters in the next tables. TABLE 5.1 COMPARISON (4 SLOTS) Simulation Real H E H Plane E Plane Plane Plane S 11 (db) -19,34-19,74 VSWR 1,24 1,23 Gain(dBi) 12,51 12,51 12,41 12,35 (db) -13,80-18,70-14,12,20 19,90º 74,10º 20º 73º TABLE 5.2 COMPARISON (8 SLOTS) Simulation Real H E H Plane E Plane Plane Plane S 11 (db) -16,82-13,84 VSWR 1,34 1,51 Gain(dBi) 15,44 15,44 15,29 14,97 (db) -12,80-16,20-12,44-45,07 9,80º 79,10º 8º 81º 6. FINAL CONCLUSIONS, PERSPECTIVES OF FUTURE WORK AND ORIGINAL CONTRIBUTIONS The constant growth of the use of wireless communications leads to the need of a new type of antennas. This concept says that these antennas should be easy and cheap to build and they could be very useful in WLAN. There is also a possibility to use these antennas with military purposes. In these cases, the portability of the antenna is very important. One of the greatest problems during the period of elaboration of the dissertation was the length of the work that a kind of project asks for. To proceed the study of the slotted waveguide antenna, it is suggested that future dissertation would analyse different kind of feed, the usage of circular polarization instead of horizontal polarization, which can be achieved by using crossed slots and also slots in the narrow face of the guide. There were two major original contributions in the dissertation. The first one was the choice of the feed used. This choice was made even though the scattering parameters were worst but the gain higher. The second and most important one was the design of the double stub which allows the user to regulate the penetration of the poles inside the guide. Page 6

7 ACKNOWLEDGMENT The author would like to thank Prof.ª Maria João Marques Martins and Prof. José Luís Gonçalves Correia da Mata for their assistance. REFERENCES [1] ELLIOTT, R. S. ANTENNA THEORY DESIGN. NEW JERSEY: [S.N.], [2] WADE, P. Microwave Antenna Book. [S.l.]: [s.n.], [3] SILVER, S. (Ed.). Microwave Antenna Theory and Design. 1ª. ed. [S.l.]: McGraw Hill Book Company, Inc, Page 7