Design, Implementation and Comparative Study Slotted Waveguide Antennas

Transcription

1 Design, Implementation and Comparative Study Slotted Waveguide Antennas João Carlos Ferreira Monteiro Instituto Superior Técnico Avenida Rovisco Pais, Lisboa João.carlos.ferreira.monteiro@ist.utl.pt Abstract: Nowadays, wireless networks appear as a tool capable of providing the most varied services (Television, Internet, Phone, etc). Such networks are available to all users, whether in study places (Universities, etc), in recreational sites (bars, shopping malls, etc) or even created by the users themselves for their own entertainment. The growing demands for such networks, has led to the development of projects in this area, as well as optimization of existing recourses in order to make this service more competitive. In this paper a comparative study of two antennas made of rectangular waveguides with slots in two orthogonal Planes, operating in 2,45Gz band, is performed. The antennas have the same number of slots, but the position of the slots differs; one contains the slots in the Plane zx, and the other in the Plane zy. They also differ in the offset parameters; the antenna with slots in Plane zx requires the dimensioning of the offset, while the antenna with slots in zy Plane presents no offset. The antennas analysis will be based on characteristic parameters such as -3dB bandwidth, the gain in different polarization Planes and the SWR. The project has included a simulation study using the CST-MWS software and measurements in an anechoic chamber. 1. INTRODUCTION Currently, wireless communication allows you to establish communication between two devices through the propagation of electromagnetic waves (for example radio waves, infrared light, laser, etc.). In the telecommunications industry, its applicability is remarkable in the transmitters and radio receivers, remote controls, computer networks, among others. In this context, the antennas made of slots assume greater importance in the wireless communication system (Wireless). A slot in a waveguide is a metal radiating element. Their behavior is identical to the functioning of an electric dipole. By analogy, an aggregate of slots has the identical behavior of an aggregation of dipoles. It is in this context that emerges the primary objective of this thesis: the analysis of two antennas composed by waveguides with different positioning of the slots in the guide, so that it can be done a comparison between antennas. The antennas consist of a rectangular waveguide with 4 slots made in the zy plane (antenna 1) and in the zx plane (antenna 2). In this chapter it is possible to understand its mode of functioning. In order to compare two types of antennas, it will be presented their mode of operation, dimensioning, positioning, width and length of the slots to insert in the guide. Waveguide The wave guide used as a basic element of these antennas was produced from an anodized aluminum profile, which results in a low cost for these antennas and also in the ease of manufacture, two important characteristics especially in military applications. These guides were designed so to operate in fundamental mode T1. Figure 1 Rectangular waveguide The measurements of the guides used are shown in Table 1. Table 1 Measurements of the used waveguide Dimensions (mm) Width (a) 37 eight (b) 97 Thickness 1,5 Propagation direction 1

2 Fundamental Mode The fundamental mode of propagation, in the guides with the dimensions mentioned above, is the T1 mode. This mode has its cutoff frequency 1.55 Gz, thus allowing operation at central frequency of work, 2.45 Gz. In table 2 we can observe how to calculate the characteristics of the mode of propagation in the guide. Table 3 Slots Characteristics Antenna ZY Plane XZ Plane L (lenght) W (width) 7 4 ZY Plane Slots Table 2 Waveguide characteristic parameters Generic writing Rectangular waveguide [rad ] [Gz] [mm] [rad ] [mm] Figure 2 lectric field lines, magnetic field and electric current distribution (3) Based on this structure of current lines (Figure 2), the slots were scaled so that they could have a maximum radiation. The current lines have a distribution along the guide according to figure 2. This distribution can be expressed by the following expressions: Type and positioning of the slots The sizing of slots, by other words, the width and length are the same for both antennas. The criterion for the design corresponds to ensure maximum radiation for each slot. A half-wave resonant dipole or a resonant slot has a length of.475 (1). lliott and Kurtz concluded that the length of the slot is given by.483 (2). The same authors, using the curves of Stegen, determined that the slot width is given by: These expressions, as well as the distribution of current lines in Figure 2, are essential to understand the positioning of the antenna with slots in the plane zy. The slots are placed in the zy plane which corresponds to the points of convergence and divergence of the electric power lines (zx plane). The arrangement of the slots along the guide can be seen in Figure 3. owever, for the antenna with slots in the plane zy simulations were performed to optimize the width of the slot. In the following table you can view the measurements of the slots Figure 3 ZY Plane slots 2

3 ZX Plane Slots Tabela 4 Simulação da antena sem stub 3D Plane Plane S 11 (db) -6,25 VSWR 2.9 Gain (dbi) 1,1 1, 1, SLL (db) - 4,25 6,28 Figure 4 ZX Plane slots Figure 4 shows that the slots are displaced from the longitudinal axis of the face of the antenna, this deviation is called offset. The next step is to calculate this factor. Starting from the initial formula of Stevenson (2): where represents the conductance of the slot and the represents the conductance of the guide. lliot, with the help of curves Stegen, made some adjustments, so the previous equation has acquired the following form: -3dB bandwidth - 1,3 147,7 As you can see in the table, the values of S11 ( db) and VSWR (2.9) reflect the misfit of the antenna. Later the antenna was adapted using a stub, first it was scaled using the Smith Chart and then optimized by successive simulations. For this antenna the stub is at 4mm from the top of the guide and has a depth of 13mm. We then performed the analysis of the results obtained by simulation of the antenna already adapted Figure 5 S 11 (Simulation) Through the analysis of figure 5 it can be seen that the antenna has a value of S11 of db, which results in a VSWR of These values can state that the antenna is adapted. Using the above equation it is possible to determine the value of the disregarding the value of. Then this value is used, replacing it in the equation, along with the and other parameters, and so we can determine the offset that is given by d. 2. SIMULATION AND XPRIMNTAL MASURMNTS Figure 6 Radiation diagram ( Plane) ZY Plane Slots Antenna Initially it was used in the simulations, an non adapted antenna. The results are presented in Table 4. Figure 7 Radiation diagram ( Plane) 3

4 S 11 [db] S 11 [db] Analyzing figure 6, we can observe that for the plane the antenna has a main lobe with approximately 11dB of gain and a level of secondary lobes (NLS) of -3.7 db. So is it possible to analyze the figure 7, where the gain has a value of 1dB and an NLS of xperimental measures -5-1 S11-15 Frequency [Gz] Figure 8 S 11 (xperimental) In figure 8 we can observe that the experimental S11 value is very close to-15db. This value is very close to the minimum acceptable value which is-15db. Despite it is slightly above the desired value is considered acceptable S Polarization and Cross polarization x Figure 9 Polarization ( Plane) Polarization and Cross polarization x Figure 1 Polarization ( Plane) Through the analysis of the previous figures we can conclude that, in both planes of polarization, the cross-polarization always has a value far below to the normal polarization. Therefore the antenna rejects the cross-polarization Plane Gain Figure 11 Gain ( Plane) Figure 12 Gain ( Plane) In the figures 11 and 12, in blue is the distribution of gain along the azimuth, in red is the maximum value obtained and in green the level of secondary lobes. It is worth noting the high amplitude of secondary lobes, which can cause interference in the radiation. The summary of the results obtained in the two previous figures as well as the other experimental measurements is present in Table 5. Table 5 xperimental measures Plane S 11 (db) -13,8 VSWR 1.52 Plane Gain (dbi) 1,75 1,61 SLL (db) 5,18 6,48-3dB bandwidth Simulation and xperimental results comparison Max SLL Plane Gain Max Comparison S Frequency [Gz] SLL NA CST Figure 13 Comparison S 11 4

5 The values of S11, experimental and simulated, don't have the same value, showing a difference of about 3dB. This may be due to the manual adjustment of the stub, and to the difficulty in placing the same to a precise depth of penetration in the guide Gain comparison ( plan) Figure 14 Comparison ( Plane Gain) Gain comparison ( plan) Figure 15 Comparison ( Plane) The figures above show that the distribution of the gain in plan or in the plan is coincident almost always throughout the graph. It can be concluded that the antenna, for these two parameters, presents itself as an ideal antenna. For a better understanding of the comparison between simulations and experimental results is presented in Table 6. Table 6 Comparison (Resume) Simulation results CST CA CST CA xperimental results Plane Plane Plane Plane the stub, in other words, a non adapted antenna. The results can be viewed in the following table. Table 7 Antenna simulations without stub 3D Plane S 11 (db) -6,59 Plane VSWR 2,76 Directivity 12,6 (dbi) 1 12,6 12,6 Gain(dBi) 11,4 9 11,5 11,5 SLL (db) - -13,8-18,7-3 db bandwidth - 19,9º 74,1º As we can see, the parameter values of the S11 and of the VSWR are distant from the intended, respectively-15db and 1.5, therefore, the antenna is not adapted. This fact forced the design of a stub (dimensioned similarly to the dimension to the antenna above). The stub appears as a characteristic length of 31mm to 4mm and is placed at the top of the guide. After the adjustment of the antenna, new simulations were conducted. The results of these simulations are presented below. Figure 16 S 11 (Simulation) The figure 16 shows the distribution of the S11 over the frequency, the value of this for the working frequency (2.45 Gz) is db, which implies a VSWR of The values obtained confirm the good adaptation of the antenna. S11(dB) ,8 VSWR 1,28 1,52 Gain(dBi) 1,8 1,8 1,75 1,61 SLL (db) 5,1 7,1 5,18 6,48-3dB Bandwidth 1,3 147, Figure 17 Radiation diagram ( Plane) ZX Plane slots Antenna Simulations Like the previous study, for this antenna it was also made simulations for an antenna without Figure 18 Radiation diagram ( Plane) 5

6 S11 [db] In Figure 17 it is possible to identify a main lobe with a range of 12.5 db, there are also visible two side lobes prominent in relation to the others, this results in an S of db. The SLL has a main lobe with -3dB bandwidth of On the plane (Figure 18) the antenna has a maximum gain of 12.5 db and an SLL of db. In this plane, the main lobe with -3dB bandwidth of xperimental measures S 11 (4 Fendas) Frequency [Gz] Figure 19 S 11 (xperimental) Looking at Figure 19, it appears that the S 11 has a value of around 2dB. This value is below- 15dB, which fact attests to the good design of the stub, and the consequent good antenna adaptation. Polarization and Cross polarization Figure 2 Polarization ( Plane) S_11-1 x Polarization and Cross polarization x Max 2 NLS SLL Figure 22 Gain ( Plane) Figure 23 Ganho (Planeo ) The gains of the different planes of radiation are shown in Figures 22 and 23. At first, we can see that there is a main lobe, the maximum amplitude of it is 12.41dB, flanked by two side lobes. The green line represents the SLL, which has a value of dB. In the second Figure SLL is much smaller than the previous one, which means that, in this Plan, the secondary lobes will cause less interference than Plan ones. In the following table, we can see the analyzed results so far and the remaining parameters in the analysis. Table 8 xperimental results (Resume) Plane S 11 (db) -19,74 VSWR 1,23 Plane Gain (dbi) 12,41 12,35 SLL (db) -14,12-3,2-3 db Bandwidth Plane Gain (4 Slots) Azimut [º].. Máximo Plane Gain (4 Slots) 2.. Max 1 SLL NLS Máximo Figure 21 Polarization ( Plane) According to the figures analysis, the values of the cross-polarization component distribution are always lower than the normal polarization. In plan, for some azimuths the crosscomponent is higher than the normal polarization. owever it may be said that this antenna rejects the cross-polarization. 6

7 S 11 [db] S11 [db] Simulation and experimental measures comparison Comparison S 11 (4 Slots) -25 Frequency [Gz] Figure 24 Comparison S 11 In Figure 24 we can see the correlation between experimental value and the value obtained by simulation. For this parameter the objectives have been met. Figure 25 Comparison ( Plane) Figure 26 Comparison ( Plane) S_11 (NA) S_11 (CST) CST e CA gain comparison 2 Ganho CA 1 Ganho CST CST e CA gain comparison 2 Ganho CA 1 Ganho CST Azimuth[º] For the plane (Figure 25) experimental and simulated values are nearly coincident. In the plane (Figure 26) there are small variations, particularly in the area of the side lobes, however, the remaining values of the graph are almost coincident. The following table presents a summary of comparisons made between experimentally values and values obtained by the simulations done. Table 9 Comparison (Resume) Simulation Real Plane Plane Plane S 11 (db) -19,34-19,74 VSWR 1,24 1,23 Plane Gain (dbi) 12,51 12,51 12,41 12,35 SLL (db) - 13,8-3 db Bandwidth - 18,7-14,12-3,2 19,9º 74,1º 2º 73º 3. ANTNNAS COMPARISON Presented the two antennas, which are an integral part of this study, it is time to compare them taking into account the parameters used to analyze them individually. Results obtained by simulations The first parameter to be analyzed will be the standing wave ratio (S11). -1 Antennas S 11 (simulation) -3 Frequency [Gz] Figure 27 Antennas Comparison (Simulation - S 11 ) From the viewpoint of results obtained using simulations, the antennas have very close values of S 11, this values are approximately equal to - 2dB. owever, the zx plane slots antenna has a better adaptation, due to theirlower S Antennas Gain ( plane) Azimut [º] Figure 28 Antennas Comparison ( Plane gain-simulation) 7

8 S 11 [db] Figure 29 Antennas Plane Gain (Simulation) In Figure 28 we can observe two main lobes, one in the red curve and another in the blue curve, but the lobe that belongs to the red curve has a greater bandwidth than the blue curve. This characterizes the antenna with slots in the plane zy as more directive. owever the antenna with slots in zx plane has a higher gain, of approximately 2dB. In the plane, Figure 29, the roles are inverted, the antenna with slots in the plane zx is more directive in the result of a narrower main lobe, the antenna with slots in zy plane is nearly isotropic, due to a larger width of the lobe main. In the following table can have a perception of all parameters examined in this comparison. Table 1 Comparison (Simulations) ZX Plane slots antenna Planeo Planeo ZY Plane slots antenna Planeo S 11(dB) -19,34-18,25 VSWR 1,24 1,28 Planeo Gain (dbi) 12,51 12,51 1,8 1,8 SLL (db) -13,8-18,7-5,1-7,1-3dB Bandwith Plane Gain(Simulation) xperimental measures Azimut [º] 19,9 74,1 1,3 147,9 Similar to the comparison between values obtained using simulations, a comparison was made based on experimental results obtained for both antennas Antennas S 11 (experimental) Frequency [Gz] Figure 3 Antennas Comparison (xperimental S 11 ) Contrary to what happened with the measurements obtained by simulation, the experimental measurements of the two antennas have different values. This difference has a value of about 5 db. This is due to the fact that adapting the antenna with slots in zy plane was not expected. The manual adjustment of the stub, and the difficulty in setting it may be the reason for this bad adaptation Figure 31 Plane xperimental Gain comparison Plane xperimental Gain Azimut [º] plane experimental gain Azimut [º] Figure 32 Plane experimental Gain As experimental and simulated values in relation to gain, practically match for both antennas, the analysis to be made to the figures 31 and 32 matches with the analysis already made to the figures 28 and 29. The main conclusions of this analysis are the higher directivity, in the plane, of the antenna with slots in the plane zy. owever the antenna with slots in zx plane is more directive in the plane, 8

9 where its competitor plan is practically isotropic. Table 11 present all values obtained in the experimental values comparisons. Table 11 Comparison (xperimental measures) ZX Plane slots antenna ZY Plane slots antenna Plane Plane Plane Plane S 11(dB) -19,74-13,8 VSWR 1,23 1,52 Gain (dbi) 12,41 12,35 1,75 1,65 SLL (db) -14,12-3,2-5,18-6,48-3dB Bandwidth CONCLUSIONS The development of this work had as the initial objective, the design, construction and analysis of two individual antennas. This analysis was divided into two main parts, results obtained by simulation comparisons and results obtained experimentally comparisons. Figure 34 3D radiation diagram (zy Plane antenna - Plane) Through observation of the previous graphics, Figure 33 and Figure 34, there are differences in the main lobe, the Figure 6.1 show a higher bandwidth main lobe than the Figure 6.2. This allows us to say that the zy plane slots antenna is more directive, in the plane, than the zx plane slots antenna It is still possible to see that the level of side lobes is greater in the zy plane slots antenna. Subsequently, the objective was focused on a comparison between antennas, while keeping in mind all the results obtained in the individual analysis. Then, it is possible to see the 3D radiation diagrams the antennas under study. Figure 35 3D radiation diagram (zx Plane antenna - Plane) Figure 33 3D radiation diagram (zx Plane antenna - Plane) Figure 36 3D radiation diagram (zy Plane antenna - Plane) 9

10 Analyzing the figures 35 and 36, corresponding to the 3D radiation diagrams of the antennas under study, according to a different perspective, we can conclude that the antennas have different behaviors. Comparing the main lobes, like the study for the figures 6.1 and 6.2, is possible to verify that the antenna with slots in the plane zx has an inferior bandwidth when compared with the bandwidth of the antenna with slots in the plane zy. For the antenna with slots in zy plane, we can say that it is almost isotropic in this plane of radiation. This situation can be solved by placing a metal net in the rear of the antenna. Generally, it was found that the antenna with slots in the zx plane is presented, accounting for all parameters in the analysis, as the most viable option. The parameters which are highlighted and that gave advantage to the antenna with slots in the plane zx were the gain, the level of side lobes and the coefficient of stationary wave. Although both antennas are easy to build, the antenna with slots in the plane zy presents greater ease of construction because they do not need an offset dimension. owever, there are features common to both antennas, such as low cost and robustness. This last feature makes them an option to be taken into account to perform in military communications. RFRNCS 1. Kraus, John D. Antennas. s.l. : McGraw-ill Book Company, Wade, Paul. Microwave Antenna Book Faro, M. de Abreu. Propagação e Radiação de ondas lectromagnéticas. s.l. : Técnica AIST,