2011 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media,

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1 2011 IEEE. Pesonal use of this mateial is pemitted. Pemission fom IEEE must be obtained fo all othe uses, in any cuent o futue media, including epinting/epublishing this mateial fo advetising o pomotional puposes, ceating new collective woks, fo esale o edistibution to seves o lists, o euse of any copyighted component of this wok in othe woks.

2 Pape Submitted to the IEEE Antennas and Wieless Popagation Lettes 1 Enhancing Fequency-Scanning Response of Leaky-Wave Antennas Using High Impedance Sufaces Maía Gacía-Vigueas, Student Membe, IEEE, José Luis Gómez-Toneo, Membe, IEEE, Geoge Goussetis, Membe, IEEE, Andew R. Weily, Membe, IEEE, and Y. Jay Guo, Senio Membe, IEEE Abstact The use of High-Impedance Sufaces (HIS) to incease the fequency-scanning sensitivity of hollow leaky-wave antennas (LWAs) is pesented. The LWA consists of a hollow ectangula waveguide with one of its naow walls eplaced by a Patially Reflective Suface (PRS) and it is loaded with a metallodielectic HIS. Theoetical esults based on a simple Tansvese Equivalent Netwok illustate the physical mechanism esponsible fo the impovement, and they ae veified by expeiments on a pototype woking in the 11GHz-16GHz band. beam at a fixed fequency [2]. Howeve, fequency scanned antennas may lead to highe font-end complexity due to the boade bandwidth needed to pefom the scanning. Theefoe, it is desiable to incease the antenna fequency sensitivity (i.e, how quickly the beam angle scans as the fequency is vaied) in ode to educe the bandwidth of the cicuity associated with fequency-scanned LWAs. Index Tems Leaky-wave antennas, fequency beam scanning, high impedance sufaces. F I. INTRODUCTION REQUENCY-beam scanning is a well-known popety of leaky-wave antennas (LWAs) [1]. Due to the dispesive natue of a leaky-mode complex popagation constant k(f), the associated highly-diective main-beam elevation angle RAD can be fequency scanned accoding to the following elations [1]: k( f ) ( f ) j( f ) (1) ( f ) RAD( f ) acsin (2) k whee is the leaky-mode phase constant, is its leakage ate, k 0 is the fee-space wave numbe, and RAD is measued fom the boadside diection. LWAs offe an attactive mechanism to fequency scan the adiated beam of electically lage apetues by simply feeding a leaky-mode, thus avoiding moe expensive feeding netwoks associated with aay antennas. Fequency-scanned LWAs ae also much simple compaed to electonically-scanned phased-aays, which scan the adiated Manuscipt eceived May 30, This wok has been suppoted by Spanish National poject TEC C03-02/TCM, Regional Seneca poject 08833/PI/08, and by Spanish scholaship Salvado de Madaiaga (ef. PR ). M. Gacía-Vigueas and J.L. Gómez-Toneo ae with the Depatment of Communication and Infomation Technologies, Technical Univesity of Catagena, Catagena Spain (phone: ; maia.gacia@upct.es, josel.gomez@upct.es). G. Goussetis is with the Institute of Electonics Communications and Infomation Technology, Queen s Univesity Belfast, BT3 9DT, UK. A.R. Weily and Y.J. Guo ae with the CSIRO ICT Cente, PO Box 76, Epping, NSW 1710, Austalia. 0 a) b) Fig. 1. a) Hollow LWA and b) LWA loaded with HIS (a=h=11mm, S=5mm, D=1.13mm, є=2.2, LPRS=10mm, LHIS=9mm, P=1.5mm, Q=0.5mm). The fequency-scan behavio of LWAs depends on the dispesive esponse of the associated leaky-mode phase constant (f), as stated by Eq.(2). Paticulaly, LWAs based on dielectic-filled waveguides pesent highe fequency sensitivity and highe ange of scanned angles than LWAs based on hollow ai-filled waveguides [3-5]. Fo a ectangula waveguide of height H with elative pemittivity opeating in the petubed TE 01 leaky-mode such as the one shown in Fig,1, and assuming small petubation conditions, the lowe and uppe fequencies of opeation fo boadside ( RAD=0º) and endfie ( RAD=90º) scanning can be appoximated by: c0 1 fbroadside (3) 2H f ENDFIRE c0 2H 1 f 1 BROADSIDE 1 As can be seen by (4), the bandwidth needed to scan fom boadside to endfie is educed as is inceased, while hollow waveguides ( =1) would equie vey lage bandwidths to (4)

3 Pape Submitted to the IEEE Antennas and Wieless Popagation Lettes 2 appoach angles close to endfie, thus limiting the scanning ange in pactice. On the othe hand, dielectic-filled LWAs have highe losses associated with the dielectic medium and they suffe fom highe changes in the beamwidth as fequency is vaied [1]. Fo these easons, it would be desiable to ceate a LWA based on a hollow waveguide to minimize the ohmic losses and beamwidth dispesion, but with inceased fequency sensitivity to pefom the scanning in the minimum bandwidth. With this main objective in mind, we pesent a new LWA based on a hollow ectangula waveguide with its top naow wall eplaced by a Patially Reflective Suface (PRS) (as descibed in [6]) by adding a High Impedance Suface (HIS) [7] in the bottom naow wall, as illustated in Fig.1. In Section II, the effect of the HIS on the leaky-mode fequencydispesion will be theoetically studied by using an efficient TEN (Tansvese Equivalent Netwok) developed in [6], illustating how the HIS inceases the fequency sensitivity of the oiginal hollow-waveguide LWA. Expeimental esults on manufactued pototypes ae epoted in Section III, validating the poposed mechanism to impove the LWA fequencyscanning esponse. Finally, Section IV pesents the conclusions of this wok. II. ANALYSIS OF HOLLOW LWA LOADED WITH HIS Figue 1-a shows a LWA based on a hollow ectangula waveguide with a metallodielectic PRS fomed by pinted dipoles [6]. The highly-eflective PRS allows the popagation of a TE 01-type leaky-mode with a weak leakage ate, poviding a lage adiating length and theefoe high diectivity. The dispesion with fequency of this leaky-mode can be analyzed by using a simple but accuate TEN developed in [6]. The dimensions of the stuctue to opeate in the 15GHz fequency band ae also shown in Fig.1. Figue 2 shows how the pointing angle of this LWA is scanned fom RAD=5º at f=13ghz to RAD=65º at f=26ghz. If the waveguide is filled with a dielectic medium (fo instance Teflon, with =2.2), and the cavity height H is deceased fom H=11mm to H=9.5mm in ode to opeate in a simila fequency band, a moe sensitive scanning esponse is obtained, which is fom RAD=5º at f=11ghz to RAD=90º at f=15ghz, thus inceasing the ange of scanned angles up to endfie and educing the bandwidth, in accodance with (3-4). Fig. 2. Leaky-mode fequency dispesion fo the LWA of Fig.1-a with and without dielectic filling to obseve the change in the fequency-scanning. To avoid the use of dielectic and its associated losses, a metallodielectic HIS fomed by a gounded aay of pinted dipoles [6,7] can be added at the bottom wall of the hollow leaky waveguide as shown in Fig.1-b. The HIS stongly modifies the bounday conditions seen by the incident TE 01- type mode[6]. Paticulaly, the length of the HIS dipoles (L HIS in Fig.1-b) can be designed to ceate an Atificial Magnetic Conducto (AMC) condition which ultimately causes the leaky-mode to ente a suface-wave egime (/k 0>1) [6], thus inceasing the LWA fequency scanning sensitivity up to endfie. This fact is shown in Fig.3-a, whee theoetical leakymode fequency dispesion esults obtained using the TEN ae given fo diffeent illustative scenaios (without HIS and with L HIS=5mm and L HIS=5.5mm). b) Fig. 3. a) Leaky-mode fequency dispesion fo the hollow LWA of Fig.1 with diffeent HIS dipoles length b) Reflection phase pesented by the HIS. As shown in Fig.3-a, the beam-scanning cuve becomes moe sensitive to fequency change when the HIS is pesent. Moeove, by popely tuning the length of the HIS dipoles L HIS [6], one can incease this sensitivity futhe. To undestand this effect, we adopt the bouncing plane wave intepetation fo waveguide popagation [9] and plot the eflection phase, HIS, expeienced by the wave tavelling along the leaky waveguide at the HIS plane. Fig.3-b plots HIS vesus fequency fo the diffeent scenaios of Fig.3-a. In the absence of the HIS, the bounday condition at this plane coesponds to a metallic wall and the eflection phase has a constant value of +180º fo all fequencies and associated scanned angles RAD. When the HIS is intoduced, the eflection phase expeienced at this bounday, HIS, apidly changes with fequency fom HIS=+140º to HIS=-180º, as shown in Fig.3-b. Paticulaly, the AMC condition ( HIS=0º) can be tuned to lowe o highe fequencies by modifying the a)

4 Pape Submitted to the IEEE Antennas and Wieless Popagation Lettes 3 HIS dipoles length L HIS [7]. As pesented in Fig.3-b, the AMC condition is set to 17.5GHz fo L HIS=5mm and to 15.8GHz fo L HIS=5.5mm. Fo claity we note that the angle of incidence of the bouncing wave at the HIS plane, which is equal to the pointing angle of the adiated beam, θ RAD, at these points is 68.5º and 67º espectively. This AMC esonance condition establishes the fequency in which the leaky-mode pointing angle RAD is pushed to high values (aound RAD=70º in Fig.3- a), eventually obtaining the endfie diection ( RAD=90º) when fequency is inceased, and finally enteing the suface-wave egime (/k 0>1 with an imaginay value of RAD [1]). In this way, by tuning the AMC esonance one can make the endfie adiation fequency to be located close to the boadside adiation fequency (which is aound 11GHz fo the HISloaded LWAs, as can be seen in Fig.3-a), hence educing the bandwidth needed to scan the beam. This is the eason why the boadside to endfie fequency ange is educed to [11GHz- 18GHz] when L HIS=5mm, and to [11GHz-16GHz] when L HIS=5.5mm (see Fig.3-b). Fig.4, the field is confined in the AMC dielectic slab at 16GHz with exponentially deceasing amplitude in the aifilled cavity egion, as it coesponds to a suface-wave [1]. III. EXPERIMENTAL RESULTS To expeimentally confim the effect of the HIS in the impovement of the leaky fequency-scanning esponse, thee hollow LWAs wee fabicated accoding to the dimensions of Fig.1. The fist of them does not use any HIS, while the othe two LWAs wee loaded with metallodielectic HIS with L HIS=5mm and L HIS=5.5mm, espectively. A pictue of the manufactued pototype is shown in Fig.5. Figue 6 shows the fequency-scanning cuves of the thee manufactued LWAs, measued in the 10GHz-20GHz fequency ange. The leakymode esults fom 10GHz to 26GHz obtained fom the TEN ae supeimposed fo compaison, showing good ageement between theoy and expeiments, and thus confiming the mechanism to enhance the scanning sensitivity and the ange of scanned angles. f=11ghz f=15ghz f=15.8ghz S D z H LA x D Fig. 4. Leaky-mode electic field distibution inside the HIS-loaded LWA fo diffeent fequencies (LHIS=5.5mm). Fo the case L HIS=5.5mm, Fig.4 pesents the leaky-mode electic field inside the LWA cavity at fou fequencies of the scanning ange. The elated values of HIS ae highlighted with cicles in Fig.3-b. At 11GHz, the HIS behaves appoximately as a gounded dielectic slab ( HIS=+140º in Fig.3-b) and a petubation of the usual waveguide TE 10 mode occus in the LWA cavity (the cavity height H coesponds to halfwavelength of the esonant fields in the tansvese diection, see Fig.4). The hollow cavity height H is designed using (3) to povide nea boadside adiation (waveguide close to cutoff) at this fequency ( RAD0º in Fig.3-a). The effect of the HIS stats to appea as fequency is inceased, apidly deceasing HIS and inceasing RAD due to the AMC esonance as shown in Fig.3. At 15GHz, RAD has been inceased to +48º. At 15.8GHz, the HIS pesents AMC esonance condition ( HIS=0º in Fig.3-b) which manifests in maximum aveage tangential electic fields intensity at the HIS inteface (Fig.4). As a esult of the AMC condition the effective cavity height is inceased [6], making the scanned angle ise suddenly ( RAD70º at 15.8GHz in Fig.3-a) with espect to the case without HIS ( RAD40º in Fig.3-a). When fequency is augmented fom this point, the leaky-mode passes by the endfie condition ( RAD90º at 16GHz in Fig.3-a) and eventually tansfoms into a suface-wave (which does not adiate). As illustated in PRS HIS a) b) Fig. 5. Photogaph of manufactued LWA pototype, a) Metallic waveguide hosting the pinted-cicuit dipole-based PRS (eady to be inseted at the top) and HIS (at the bottom) b) Whole LWA stuctue at the anechoic chambe. Fig. 6. Theoetical and measued fequency-scanning esponse of hollow LWAs showing the effect of the HIS. a S H LA

5 Pape Submitted to the IEEE Antennas and Wieless Popagation Lettes 4 Fig.6 shows that the HIS-loaded LWA with L HIS=5.5mm povides beam scanning fom RAD=4º to RAD=67º in the fequency ange 11GHz-16GHz, while the LWA without HIS has a pooe fequency sensitivity (fom nea boadside to RAD=65º in the fequency ange 13GHz-26GHz). The theoetical and measued nomalized adiation pattens fo the HIS-loaded LWA with L HIS=5.5mm and a adiating length L A=200mm (9.2λ 0 at 14GHz) ae plotted in Fig.7-a and Fig.7- b, espectively. Good ageement between the theoetical and measued adiation pattens is obseved, showing the scanning of the main beam elevation angle RAD. The beamwidth boadens close to boadside (f=11ghz in Fig.7) due to the incease of the leakage ate (see /k 0 cuve in Fig.3-a fo L HIS=5.5mm and f=11ghz) [1]. Fom 12GHz to 15GHz, the expected fequency-beam steeing is obtained. At f=16ghz, a seconday lobe appeas in the angle -67º as a esult of the decease in the leakage ate (see /k 0 cuve in Fig.3-a fo L HIS=5.5mm and f=16ghz), which educes the LWA adiation efficiency and ceates a eflected leaky-wave pointing at a mioed angle with espect to the main beam [1]. Radiation to pointing angles highe than +70º is limited by the appeaance of gating lobes due to highe-ode modes [1,2,8]. The measued insetion ohmic losses of the fabicated hollow LWA loaded with HIS is in the ange of 1.5dB fo the entie fequency band, while fo the case of a simila LWA based on a waveguide filled with Teflon ( =2.2, tan=0.005) the insetion losses ae in the ode of 6dB, as demonstated in [3]. b) Fig. 7. a) Theoetical and b) measued adiation diagam at diffeent fequencies to show the fequency beam-scanning of the designed HIS-loaded LWA (LHIS=5.5mm). a) IV. CONCLUSION This wok has demonstated the impovement in the fequency scanning sensitivity of a hollow LWA due to the insetion of a HIS. The new topology inceases the fequency scanning sensitivity, thus educing the equied bandwidth to scan the main beam fom nea boadside to nea endfie. It avoids the use of dielectic-filled waveguides, thus minimizing the associated dielectic losses. Theoetical esults of the leaky-mode dispesion with fequency have been obtained fom an efficient Tansvese Equivalent Netwok to illustate the physical mechanism. The HIS must be designed to intoduce an AMC esonance condition close to the cut-off of the leaky-mode, so that the boadside and the endfie adiation fequencies ae located in close poximity. Expeimental esults on fabicated pototypes have also been epoted, showing vey good ageement with theoy. The designed HISloaded LWA shows a scanning ange of [5º,67º] in a bandwidth fom 11GHz to 16GHz, achieving a fequency sensitivity of 12.4º/GHz. A simila LWA without HIS pesents a scanning ange of [7º,65º] in the fequency ange fom 13GHz to 26GHz, which has a fequency sensitivity of 4.5º/GHz. Theefoe, the fequency sensitivity is almost thee times highe when the HIS is added to the hollow LWA. A dielectic-filled LWA with simila fequency sensitivity as the HIS-loaded ai-filled LWA would intoduce 4dB highe insetion losses, subsequently deceasing the LWA adiation efficiency. It should be noted that the fequency-scanning enhancement shown in this wok can be extended to any type of ai-filled LWAs (see examples in [1,4,5]) due to the fact that the HIS is able to petub the leaky-mode by intoducing a stongly dispesive AMC bounday condition. REFERENCES [1] A.A. Oline, Leaky-wave antennas, in Antenna Engineeing Handbook, 3d ed, R.C. Johnson, Ed. New Yok, McGaw-Hill, 1993, Ch. 10. [2] R.C. Hansen, Phased Aay Antennas, John Wiley & Sons, New Yok, [3] J.L. Gómez, G.Goussetis, A.Feesidis, and A.A.Melcón, "Contol of leaky-mode popagation and adiation popeties in hybid dielecticwaveguide pinted-cicuit technology: expeimental esults", IEEE Tans. Antennas Popagat., vol.54, No.11, pp , Nov [4] P. Lampaiello, F. Fezza, H. Shigesawa, M. Tsuji and A.A. Oline, A vesatile leaky-wave antenna based on stub-loaded ectangula waveguide: Pat III compaisons with measuements, IEEE Tans. Antennas Popagat, vol.46, no.7, pp , July [5] M. Tsuji, T. Haada, H. Deguchi, and H. Shigesawa, Fequencyscanning antennas with low sidelobes using stub-loaded idgeectangula leaky waveguides, 2003 IEEE Topical Confeence on Wieless Communication Technology, pp , [6] M. Gacía-Vigueas, J.L. Gómez-Toneo, G. Goussetis, J.S. Gómez- Diaz, and A. Álvaez-Melcón, A modified pole-zeo technique fo the synthesis of waveguide leaky-wave antennas loaded with dipole-based FSS, IEEE Tans. Antennas Popag., in pess. [7] G. Goussetis, A.P. Feesidis, J.C. Vadaxoglou, Tailoing the AMC and EBG Chaacteistics of peiodic metallic aays pinted on gounded dielectic substate, IEEE Tans. Antennas Popag, vol. 54, No. 1, pp , Jan Commented [guo013 1]: I don t think the est is necessay as we ae ending the pape.

6 Pape Submitted to the IEEE Antennas and Wieless Popagation Lettes 5 [8] T. Zhao, D. R. Jackson, J. T. Williams, and A. A. Oline, Geneal fomulas fo 2-D leaky-wave antennas, IEEE Tans. Antennas and Popag., vol. 53, No. 11, pp , Nov [9] G. Goussetis, J.L Gómez-Toneo, A.P. Feesidis, and N. Uzunoglu, Atificial impedance sufaces fo educed dispesion in antenna feeding systems, IEEE Tansactions Antennas and Popagation, in pess.