G.A. Casula, G. Mazzarella and G. Montisci The Elliot procedure for the design of a waveguide slot array is extended to the case of partially filled radiating waveguides. The dielectric slab lies on the top of the radiating waveguide, i.e. on the slotted wall. The proposed solution prevents some of the most common drawbacks of waveguide slot arrays. Results are validated against a commercial FEM software.
Introduction: Waveguide slot arrays have a long history, their use dating at least from the 1950s, therefore their pros and cons from both the mechanical and the electromagnetic standpoint are quite well known [1]. Their main advantages are high efficiency, polarisation purity and considerable mechanical strength, but there are some drawbacks. In this Letter we propose a possible solution for some of the most common drawbacks of waveguide slot arrays. (i) A waveguide slot array has no flexibility since, once an array is realised, its electromagnetic behaviour cannot be changed. Actually, if the antenna requirements change, the array itself must be changed. Moreover, the array realisation can require, mainly in the K-band, expensive procedures, such as electro-erosion. (ii) Some antenna applications, such as RADAR, require handling high transmitted power and the main limitation, when using waveguide slot antennas, is the reduced dielectric strength of the air. (iii) The spacing between radiating slots in a waveguide slot array is fixed at half the guided wavelength [1], which corresponds, in an empty waveguide, to about 0.7–0.8 l0, l0 being the free-space wavelength. As a consequence, the number of array elements is limited and this can prevent realisation of a ‘good’ aperture distribution or radiation pattern. To achieve a structure that is flexible and easier to realise, as a solution for (i), the upper waveguide wall can be replaced with a copper-clad laminate [2]. The slots are etched on the metal layer using one of the standard technologies for printed antennas. The electromagnetic behaviour of the array depends only on this slotted copperclad laminate, which can be easily replaced at a small fraction of the total array cost. Moreover, such an array can be realised more easily and with the same, or even higher, accuracy than usual waveguide slot arrays. The copper layer could be placed above [2] or below [3] the dielectric laminate, and a metallic frame is required to sustain the laminate (see Fig. 1a). However, if the copper layer is below the laminate, this frame will cause almost unpredictable diffraction effects (owing to both space and surface waves). Therefore, we propose to insert the dielectric inside the waveguide. In this way, the slots radiate in an open half-space, exactly as in a standard array. copper-clad laminate t a
h
b-h a
slotted waveguide wall
h b-h b dielectric layers
slotted waveguide wall t
h a
Design guidelines: Fig. 2 shows the array geometry. Each radiating waveguide is fed by a resonant inclined coupling slot [5]. The array input port is placed at one side of the feeding guide, and the opposite side is short-circuited at a distance of half a wavelength from the last coupling slot. The alternate phase feeding is achieved by using coupling slots with the same tilt angle, spaced half a wavelength apart. feeding waveguide
input port
metal layer/waveguide wall
Fig. 2 Array geometry with alternate phase feeding
To investigate the properties of the proposed solution, the Elliot procedure for the design of a waveguide slot array [1] has been extended to the case of partially filled radiating waveguides fed with alternating phases. The slot self-admittance is calculated by using the MoM procedure described in [2]. The internal mutual coupling between adjacent radiating waveguides has been neglected in the design procedure. This means that, in the array model, each radiating waveguide has been considered separated from the adjacent ones by metallic sidewalls, i.e. each waveguide has its own dielectric slab (Fig. 1c). Therefore, the cases in Figs. 1a and b can both be studied using the model in Fig. 1c, with proper choice of the thickness of the internal slot region, i.e. the metal thickness in the copper-clad laminate (Fig. 1a) or the waveguide wall thickness (Fig. 1b).
dielectric layer
t a
waveguide slotted wall (inside the radiating waveguide or in the external half-space region). Therefore, the etched copper-clad laminate, proposed as a solution for (i), with the dielectric inside the waveguide, can be used also to maintain the pressurisation. As an alternative, if the pressurisation requires a more robust structure, the copper-clad laminate can be replaced by a dielectric slab, placed inside the waveguide, stuck to a thick slotted wall (Fig. 1b). The dielectric inside the waveguide also reduces the guided wavelength, and consequently the array spacing, providing a solution for (iii). Of course, the simpler array configuration is the one with a single dielectric layer, common to all the radiating waveguides. Therefore, the proposed configuration consists of a metallic ‘comb-like’ structure, formed by the bottom and sidewalls of the waveguides, covered with a copper-clad laminate (Fig. 1a) or with a dielectric slab stuck to the slotted waveguide wall (Fig. 1b). In both cases, it is required that the radiating waveguides are fed with alternating phases [4] to prevent the internal coupling between adjacent radiating waveguides, which is produced by the common dielectric layer.
radiating waveguides
Design of slot arrays in waveguide partially filled with dielectric slab
b c
Fig. 1 Side view of radiating waveguides a ‘Comb-like’ structure with copper-clad laminate as ‘cover’ b ‘Comb-like’ structure with dielectric slab and slotted wall as ‘cover’ c Array model used in synthesis procedure
The pressurisation of the waveguide internal region (which increases the dielectric strength of the air) provides a possible solution for (ii). However, the waveguide pressurisation requires use of a dielectric slab that acts like a ‘cap’ for the radiating slots and therefore must lie on the
Results and discussion: To assess the synthesis procedure, a number of planar arrays, with different size and aperture distribution, have been designed and then analysed using an FEM commercial code (Ansoft HFSS 10). The results show, for all cases tested, very good agreement with the design goals on the far-field pattern. As an example, the case of a 10 5 planar array with a 10-element Taylor distribution (20 dB sidelobes) in the E-plane and a 5-element uniform distribution in the H-plane, has been described in detail. Standard WR90 waveguides have been considered, at the operating frequency of 9 GHz. The thickness of the dielectric slab is h ¼ 0.508 mm and its dielectric permittivity is 2.2. The thickness of the waveguide slotted wall t has been chosen equal to 1 mm. The simulated (HFSS) far-field patterns for the designed array (for both the array with a common dielectric layer (Figs. 1a and b) and its approximate model (Fig. 1c)) are shown in Figs. 3 and 4, and in Fig. 5 the frequency response is plotted. Very good agreement with the design goals and good agreement between the curves (i) and (ii) in Figs. 3–5 both validate our design procedure and demonstrate that the use of alternate phase feeding allows neglect, in the design procedure, of the effects of the mutual coupling between the radiating waveguides.
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Therefore, the configurations in Figs. 1a and b, which are easier to realise, can be reasonably designed by neglecting the effect of the mutual coupling between adjacent radiating waveguides, provided that alternate phase feeding is used. 25 20 15 gain, dB
10 SSL = –21.2 dB
5 0 –5 –10 –15
(i)
(ii)
–20 –90 –75 –60 –45 –30 –15 0 15 30 45 60 75 90 angle from broadside, deg
Fig. 3 Far-field pattern (E-plane) (i) array with common dielectric layer (ii) approximate model with separate radiating waveguides Nominal pattern is Taylor with 20 dB sidelobes
G.A. Casula, G. Mazzarella and G. Montisci (Dipartimento di Ingegneria Elettrica ed Elettronica, Universita` di Cagliari, Piazza d’Armi, Cagliari 09123, Italy)
References 1
20 gain, dB
# The Institution of Engineering and Technology 2006 28 April 2006 Electronics Letters online no: 20061336 doi: 10.1049/el:20061336
E-mail:
[email protected]
25
15
Conclusion: The use of radiating waveguides partially loaded by a dielectric layer can be a useful solution to overcome some of the main drawbacks of waveguide slot arrays and to improve their performances. This configuration presents a number of advantages over standard waveguide arrays and allows simpler and less expensive realisation. The design procedure presented is very accurate and effective, though based on an approximate array model, which neglects the effect of the internal mutual coupling between radiating waveguides owing to the common dielectric layer. An extensive validation against a commercial FEM software fully assesses the proposed procedure.
2 SSL = –12.3 dB
(i)
3
10 5
(ii)
4
0 –5 –90 –75 –60 –45 –30 –15 0 15 30 45 60 75 90 angle from broadside, deg
Fig. 4 Far-field pattern (H-plane)
5
Elliot, R.S.: ‘Antenna theory and design’ (Prentice-Hall, Englewood Cliffs, NJ, 1981) Montisci, G., and Mazzarella, G.: ‘Full-wave analysis of a waveguide printed slot’, IEEE Trans. Antennas Propag., 2004, 52, pp. 2168–2171 Mazzarella, G., and Montisci, G.: ‘A rigorous analysis of dielectriccovered narrow longitudinal shunt slots with finite wall thickness’, Electromagnetics, 1999, 19, pp. 407–418 Kimura, Y., Hirano, T., Hirokawa, J., and Ando, M.: ‘Alternating-phase fed single-layer slotted waveguide arrays with chokes dispensing with narrow wall contacts’, IEE Proc., Microw. Antennas Propag., 2001, 148, (5), pp. 295–301 Rengarajan, S.R., and Shaw, G.M.: ‘Accurate characterization of coupling junctions in waveguide-fed planar slot arrays’, IEEE Trans. Microw. Theory Tech., 1994, 42, (1–2), pp. 2239–2248
(i) array with common dielectric layer (ii) approximate model with separate radiating waveguides Nominal pattern is one of uniform array 0 (i)
return loss, dB
–5 –10
(ii)
–15 –20 –25 –30 –35 8.7
8.8
8.9 9.0 9.1 frequency, GHz
9.2
9.3
Fig. 5 Frequency response (i) array with common dielectric layer (ii) approximate model with separate radiating waveguides
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