The high-gain substrate-superstrate configuration,
which was proposed to increase the gain in printed circuit antennas,
is applied to dielectric leaky-wave antennas (LWAs) to
improve its frequency response. Analysis of a slitted suspended
dielectric rectangular waveguide is carried out using a full-wave
method. It is proved that the minimum values of the leakage
constant of the leaky-wave mode for the suspended configuration
are related to the high-gain resonance conditions. Moreover, it is
found that the suspended LWA exhibits very small beamwidth
variations in a large frequency bandwidth. It is well-known that
inhomogeneous filled LWAs suffer from variation of beamwidth
as the angle of maximum radiation is scanned with frequency.
The proposed topology can be adjusted so that a flat response of
the beamwidth can be obtained in a large frequency band, while
maintaining the frequency-scanning behavior of the LWA.This work was
supported by Spanish National Projects ESP2001-4546-PE and TIC2000-0591-
C03-03, by Regional Seneca Projects 2002 PB/4/FS/02 and PMPDI-UPCT-
2004, and by the EPSON-Ibérica Foundation

Gómez Tornero, José Luis

Quesada Pereira, Fernando Daniel

Álvarez Melcón , Alejandro

IEEE Microwave and Wireless Components Letters

English

Repositorio Digital de la Universidad Politécnica de Cartagena

Application of the High-Gain Substrate-SuperstrateConfiguration to Dielectric Leaky-Wave AntennasJosé Luis Gómez-Tornero, Fernando Quesada-Pereira, and Alejandro Álvarez-Melcón.Abstract—The high-gain substrate-superstrate configuration,which was proposed to increase the gain in printed circuit an-tennas, is applied to dielectric leaky-wave antennas (LWAs) toimprove its frequency response. Analysis of a slitted suspendeddielectric rectangular waveguide is carried out using a full-wavemethod. It is proved that the minimum values of the leakageconstant of the leaky-wave mode for the suspended configurationare related to the high-gain resonance conditions. Moreover, it isfound that the suspended LWA exhibits very small beamwidthvariations in a large frequency bandwidth. It is well-known thatinhomogeneous filled LWAs suffer from variation of beamwidthas the angle of maximum radiation is scanned with frequency.The proposed topology can be adjusted so that a flat response ofthe beamwidth can be obtained in a large frequency band, whilemaintaining the frequency-scanning behavior of the LWA.Index Terms—Leaky-wave antennas (LWAs), millimeter-waveantennas.I. INTRODUCTIONL EAKY-WAVE antennas (LWA) are a type of traveling-wave antennas based on the excitation of a leaky-wavemode in an open waveguide. Many interesting features makethis type of antenna useful in the millimeter waveband range,as the introduction of low-losses, very large bandwidths, simplefabrication and easy interconnection with standard waveguideand printed technologies [1]. Besides, the radiated beamwidthcan be made very small, obtaining high gains while the beamdirection can be scanned with the frequency from near broad-side to endfire. LWA can be completely hollow or contain adielectric guide. Air-filled LWA present lower losses than di-electric-filled LWA, but the scanning capability is reduced, notbeing able to reach the endfire direction. On the contrary, dielec-tric-filled LWA can scan up to endfire, but have an importantdisadvantage when compared to air-filled LWA: the beamwidthvaries as the beam is scanned with the frequency [1]. This phe-nomenon can be a disadvantage for many applications, wherethe same beamwidth is required for different pointing directions.This letter presents a novel and simple suspended topology,which is applied to a slitted dielectric rectangular waveguideManuscript received July 6, 2004; revised October 7, 2004. This work wassupported by Spanish National Projects ESP2001-4546-PE and TIC2000-0591-C03-03, by Regional Seneca Projects 2002 PB/4/FS/02 and PMPDI-UPCT-2004, and by the EPSON-Ibérica Foundation. The review of this letter was ar-ranged by Associate Editor M. Mrozowski.The authors are with Departamento de Tecnologías de la Información y lasComunicaciones, Technical University of Cartagena, Cartagena 30202, Spain(e-mail: josel.gomez@upct.es).Digital Object Identifier 10.1109/LMWC.2005.845731Fig. 1. Suspended dielectric LWA.(Fig. 1), previously studied in [2], to improve the frequency re-sponse of the leaky-wave mode. It is shown that the new struc-ture synthesizes a flat response for the beamwidth in a large fre-quency band. This phenomenon can be related to the high-gainsubstrate-superstrate configuration presented in [3], devoted toimprove the gain in printed circuit antennas [3]–[5]. LWA do notsuffer from low gains, but this resonant high-gain configurationallows minimizing the variation of the beamwidth for inhomo-geneous filled LWA, as it will be demonstrated.II. THEORY AND RESULTSA full-wave, accurate method of moment (MoM) technique[6], [7] is used to analyze the effect of the suspended config-uration in the slitted dielectric rectangular guide LWA shownin Fig. 1. In the standard configuration [2], the rectangular di-electric guide, of height and width , is not suspended fromthe bottom plane ( 0). The rectangular dielectric guide di-mensions , determine the propagation characteristics ofthe leaky-wave mode which must be responsible for the radia-tion of this antenna. This leaky-wave mode is basically themode of the dielectric rectangular waveguide (the index “ ” cor-responds to dimension “ ”), but perturbed by the slot located atthe top wall in order to make it radiate, and also to control theradiated beamwidth. The leaky-wave mode has a complex prop-agation constant in the longitudinal direction of the open wave-guide(1)Fig. 2 shows the variation with frequency of the phase andattenuation constants for the leaky-wave mode of the non-suspended LWA ( 0 mm). From the plot of , it can be seenthat the leaky-wave mode is a perturbation of the mode ofFig. 2. Phase and attenuation constant for the nonsuspended LWA.Fig. 3. Modification of  and  with the suspended configuration.the dielectric guide; results from [2] are also plotted for compar-ison purposes (note that in [2] is assumed to be infinite). Theattenuation constant decreases as the frequency increased,from large values of below the cut-off region (45 GHz) tozero for the surface-wave regimen (above 60 GHz).From and , the beam direction and the 3-dBbeamwidth of the antenna can be approximately com-puted with the next known formulas [1] (where the antennalength is assumed so that 90% of the injected power is radiated)(2)(3)When the suspended configuration is applied ( 0 mm),the leaky-wave mode is perturbed, showing the behavior plottedin Fig. 3. It can be seen some minimums of at some givenvalues of .This behavior is due to the high-gain resonance phenomenon,described in [3], and more recently related in [8] to the excitationof weakly attenuated leaky-waves. It was demonstrated in [3]that a narrow beam can be obtained at any desired angleusing a two layered topology, if the next resonance conditionsare satisfied(4)(5)Fig. 4. Resonance Conditions for the suspended LWA.being and positive integers, and obtaining a narrower beam(higher gains) at as . In our case, the proposedsuspended configuration uses 1 and 2.56(see Fig. 1). In [8], a leaky-wave analysis was used to explainthis narrow-beam phenomenon. There, it was demonstrated thatthe resonance gain was attributable to the excitation of weaklyattenuated leaky waves on the structure, which were responsiblefor the radiation, therefore leading to a narrow radiated beam.To check that the minimums of shown in Fig. 3 are related tothe resonance conditions, (4) and (5) are rewritten using (2)(6)(7)In order to plot the values of for which these resonanceequations are satisfied, it is easier to use the next functions,which are equal to zero at the resonance conditions(8)(9)Using the variation of and shown in Fig. 3, (8) and (9) canbe computed and plotted, obtaining the results shown in Fig. 4.It can be seen that the minimums of in Fig. 3 correspondto values of at which the resonance condition is fulfilled,which is the most important condition for the high-gain effect[3]. Therefore, it is verified that the simple suspended configura-tion allows to obtain the high-gain resonance phenomenon. Ascommented, there are other ways to obtain high gains in LWA(by weakly attenuating the leaky-wave mode), which are sim-pler than the proposed suspended configuration [1]. However,the suspended configuration in the high-gain condition can beused to achieve a completely novel and very interesting feature.Fig. 5 shows the frequency response of and forthe leaky-wave mode for three different values of and .The original, nonsuspended configuration ( 0 mm,1.59 mm, solid line) is compared with the results obtained forthe suspended LWA with a value of 3 mm correspondingto the first resonance condition shown in Figs. 3 and 4 (3 mm, 1.59 mm, dotted line). It can be seen from the plotof , that the response of the antenna has shifted to higherfrequencies. The dielectric guide thickness can be readjusted toFig. 5. Frequency response of the LWA for different values ofH andD (mm).Fig. 6. Optimization of the beamwidth response by varying H (D =2.58 mm).center the leaky-wave mode response at 50 GHz ( 3 mm,2.58 mm, dashed line).Of much interest is the frequency behavior of the beamwidth, seen in Fig. 5. It can be seen that the standard frequencydecreasing curve of for the nonsuspended LWA changes toa quasioscillatory shape for the high-gain configuration. This isdue to the resonant nature of this arrangement, showing a valleyat a given frequency, which is followed by a peak. For lowerfrequencies ( 45 GHz), suddenly increases due to thecut-off typical response, leading to high values for , whichhave no physical meaning since the mode can neither propagatenor radiate. Also, in the surface-wave region becomes zerosince there is no radiation. The radiation region of the LWA islocated between these two limits (from 45 GHz to 60 GHz).The idea is to use the resonant behavior of the high-gain con-figuration to obtain a flat response of in the radiation regionof the LWA. Fig. 6 shows that this can be done by adjusting thevalue of to an optimum suspension height, which in our caseis 2.2 mm. Validation results from HFSS 3D simulationsare also plotted with circles in Fig. 6, showing good agreementwith the results obtained with the two-dimensional (2-D) MoMtechnique used in this letter. A flat beamwidth response is ob-tained from 48 GHz to 53 GHz, with a nearly constant valueof , while maintaining unchanged the original LWAscan angle ability for different values of ( varies from 10at 48 GHz to 43 at 53 GHz). Below and above the optimumvalue of , the response of is either too oscillatory or itdecreases linearly with a too step slope, respectively. It can beseen that a second-order effect occurs when varying ; this isthat the cut-off frequency of the leaky-wave mode is lowered asis increased. This can be corrected by modifying to centerthe response back to the desired frequency band, as previouslydone.III. CONCLUSIONIn this letter, it has been shown that the beamwidth-frequencyresponse of a dielectric LWA can be strongly modified by usinga simple suspended configuration. This arrangement can makethe leaky-wave mode work in a high-gain resonance condition,obtaining an oscillatory response for its beamwidth as the fre-quency is varied. By adjusting the thickness of the dielectricguide and the height of the suspension region, the beamwidthvariation can be minimized in a certain frequency band, ob-taining a constant beamwidth for a wide range of scanning an-gles. This is an important new feature for dielectric-filled LWAs,since it is desirable in many applications to maintain the samebeamwidth for different beam scan-angles.REFERENCES[1] A. A. Oliner, “Leaky-wave antennas,” in Antenna Engineering Hand-book, 3rd ed, R. C. Johnson, Ed. New York: McGraw-Hill, 1993, ch.10.[2] P. Lampariello, F. Frezza, and A. A. Oliner, “The transition regionbetween bound-wave and leaky-wave ranges for a partially dielec-tric-loaded open guiding structure,” IEEE Trans. Microw. Theory Tech.,vol. 38, no. 12, pp. 1831–1836, Dec. 1990.[3] D. R. Jackson and N. G. Alexopoulos, “Gain enhancement methods forprinted circuit antennas,” IEEE Trans. Antennas Propag., vol. 33, no. 9,pp. 976–987, Sep. 1985.[4] C. S. Lin, S. S. Zhong, J. H. Shi, and Y. Wang, “Gain enhancementtechnique for microstrip antennas,” in Proc. IEEE Antennas Propag.,vol. 36, Jul. 1988, pp. 905–910.[5] X.-H. Shen, G. A. E. Vandenbosch, and A. Van de Capelle, “Studyof gain enhancement method for microstrip antennas using momentmethod,” IEEE Trans. Antennas Propag., vol. 43, no. 3, pp. 227–231,Mar. 1995.[6] J. L. Gómez and A. A. Melcón, “Non-Orthogonality relations betweencomplex-hybrid-modes: An application for the leaky-wave analysis oflaterally-shielded top-open planar transmission lines,” IEEE Trans. Mi-crow. Theory Tech., vol. 52, no. 3, pp. 760–767, Mar. 2004.[7] , “Radiation analysis in the space domain of laterally-shieldedplanar transmission lines. Part I: Theory,” Radio Sci., vol. 39, no. 3, pp.1–10, Jun. 2004.[8] D. R. Jackson and A. A. Oliner, “A leaky-wave analysis of the high-gainprinted antenna configuration,” IEEE Trans. Antennas Propag., vol. 36,no. 7, pp. 905–910, Jul. 1988.

Application of the high-gain substrate-superstrate configuration to dielectric leaky-wave antennas

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Application of the high-gain substrate-superstrate configuration to dielectric leaky-wave antennas

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