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  • 1

    Design, Implementation and Comparative Study Slotted

    Waveguide Antennas

    Joo Carlos Ferreira Monteiro

    Instituto Superior Tcnico

    Avenida Rovisco Pais, 1 1049-001 Lisboa

    Joo.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 2,45GHz 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 TE10.

    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

    Height (b) 97

    Thickness 1,5

    Propagation

    direction

  • 2

    Fundamental Mode

    The fundamental mode of propagation, in the

    guides with the dimensions mentioned above, is

    the TE10 mode. This mode has its cutoff

    frequency 1.55 GHz, thus allowing operation at

    central frequency of work, 2.45 GHz.

    In table 2 we can observe how to calculate the

    characteristics of the mode of propagation in the

    guide.

    Table 2 Waveguide characteristic parameters

    Generic

    writing

    Rectangular waveguide

    [rad ]

    [GHz]

    [mm]

    [rad ]

    [mm]

    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 0.475 (1). Elliott and Kurtz concluded that the length of the slot is given by

    0.483 (2).

    The same authors, using the curves of Stegen,

    determined that the slot width is given by:

    However, 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

    Table 3 Slots Characteristics

    Antenna ZY Plane XZ Plane

    L (lenght) 59 59

    W (width) 7 4

    ZY Plane Slots

    Figure 2 Electric 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:

    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.

    Figure 3 ZY Plane slots

  • 3

    ZX Plane Slots

    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. Elliot, with the help

    of curves Stegen, made some adjustments, so

    the previous equation has acquired the

    following form:

    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 EXPERIMENTAL MEASUREMENTS

    ZY Plane Slots Antenna

    Initially it was used in the simulations, an non

    adapted antenna. The results are presented in

    Table 4.

    Tabela 4 Simulao da antena sem stub

    3D H Plane E Plane

    S11 (dB) -6,25

    VSWR 2.9

    Gain (dBi) 10,01 10,0 10,0

    SLL (dB) - 4,25 6,28

    -3dB

    bandwidth

    - 10,3 147,7

    As you can see in the table, the values of S11 (-

    6.26 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 40mm 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 S11 (Simulation)

    Through the analysis of figure 5 it can be seen

    that the antenna has a value of S11 of -18.25

    dB, which results in a VSWR of 1.279. These

    values can state that the antenna is adapted.

    Figure 6 Radiation diagram (H Plane)

    Figure 7 Radiation diagram (E Plane)

  • 4

    Analyzing figure 6, we can observe that for the

    H 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 10dB and an NLS of -3.7.

    Experimental measures

    Figure 8 S11 (Experimental)

    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.

    Figure 9 Polarization (E Plane)

    Figure 10 Polarization (E 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.

    Figure 11 Gain (E Plane)

    Figure 12 Gain (H 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 Experimental measures

    H Plane E Plane

    S11 (dB) -13,8

    VSWR 1.52

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