Paper submitted to The European Conference on Antennas and Propagation 2022 (EuCAP), 27 March - 1 April 2022, Madrid (Spain).This paper presents a circular-polarized gap-waveguide-based compact monopulse array antenna for millimeter-wave tracking applications at Ka-band (29 to 31 GHz). The gap waveguide planar monopulse comparator network is integrated in a single layer with a 2x2 corporate-fed network combining ridge gap and groove gap waveguides. Radiating cavities consist of cubes with chamfered corners. Preliminary results show a bandwidth of 2 GHz with input reflection coefficients better than -20 dB for both ports of the antenna. In addition, the isolation between ports is greater than 50 dB. The design allows for scalability to build higher gain arrays from the antenna presented in this communication.This work is part of the projects PID2019-107688RB-C22 and PID2019-103982RB-C43 funded by the Spanish Ministry of Science and Innovation MCIN/AEI/10.13039/501100011033

Ferrando-Rocher, Miguel

English

Repositorio Institucional de la Universidad de Alicante

A Gap Waveguide-Based 2×2 Circularly-polarizedMonopulse Antenna ArrayMiguel Ferrando-Rocher∗†‡,∗University of Alicante, UA, Alicante, Spain† Antennas and Propagation Lab, APL, Valéncia, Spain‡Microwave and Applied Computational Electromagnetics Group, GMECA, Alicante, Spain, miguel.ferrando@ua.esAbstract—This paper presents a circular-polarized gap-waveguide-based compact monopulse array antenna formillimeter-wave tracking applications at Ka-band (29 to 31 GHz).The gap waveguide planar monopulse comparator network isintegrated in a single layer with a 2×2 corporate-fed networkcombining ridge gap and groove gap waveguides. Radiating cavi-ties consist of cubes with chamfered corners. Preliminary resultsshow a bandwidth of 2 GHz with input reflection coefficientsbetter than −20 dB for both ports of the antenna. In addition,the isolation between ports is greater than 50 dB. The designallows for scalability to build higher gain arrays from the antennapresented in this communication.Index Terms—antennas, array, circular polarization, gapwaveguide technology, monopulse antenna.I. INTRODUCTIONAntennas for satellite communications on the move (SOTM)are currently of great interest. Antennas mounted on a movingplatform provides a controlled heading that enables a geosyn-chronous satellite to communicate with the ground [1]. Anaspect that it is important to study is what happens to thecommunication of SOTM antennas in the event of changesin elevation or even in the event of unexpected vibrations. Inthese cases in which the antenna could lose communication,a monopulse tracking method would be effective for antennapointing control on the vehicle.In the literature, countless examples of monopulse an-tennas can be found. Reflector antennas, for example, aregood candidates, thanks to their ease of design and goodperformance [2]-[3]. However, bulky structures are usuallynot suitable for implementation in modern communicationssystems. Alternatively, substrate-based antennas can be a low-profile and low-cost solution [4]-[5]. However, the need forlow-loss substrates, and a narrow bandwidth are some of thedrawbacks of this SIW or microstrip antennas. A trade-offsolution between these two alternatives are full-metal planararray antennas, which can provide high-gain performancetogether with compact and low-profile structure for monopulseapplications. Of course in this case they will always bebulkier structures than substrate-based antennas but less thanreflectors, so they are intermediate solutions.In the past, mainly hollow waveguide feeding networkshave been one alternative to solve the aforementioned issues.Nevertheless, achieving good electrical contact among theFig. 1. Schematic of the feeding network and radiating elements of the unitcell.Fig. 2. Top view and perspective view of the antenna without and with thelid.building blocks is not especially easy at millimeter-wavefrequencies.From 2010s, the use of Gap Waveguide (GW) technologyhas become popular, as it can ensure the confinement ofthe field in non-contact metal structures under certain con-ditions [6]-[20].This is a draft version−50 0 50−30−20−100Angle (degrees)Normalizedamplitude(dB)29 GHz30 GHz31 GHz(a)−50 0 50−30−20−100Angle (degrees)Normalizedamplitude(dB)29 GHz30 GHz31 GHz(b)−50 0 50−30−20−100Angle (degrees)Normalizedamplitude(dB)29 GHz30 GHz31 GHz(c)−50 0 50−30−20−100Angle (degrees)Normalizedamplitude(dB)29 GHz30 GHz31 GHz(d)Fig. 3. Simulated normalized radiation patterns (a) ϕ=0◦ (port 2); (b) ϕ=90◦ (port 2); (c) ϕ=0◦ (port 1); (d) ϕ=90◦ (port 1).. Under these premises, this paper presents a circularly-polarized single-layer Ka-band monopulse slot array antenna,whose corporate-feed network and cavities are housed in a bedof nails, characteristic of the GW. The design and simulatedresults are presented in the following.II. 2×2 UNIT CELL DESIGNThe unit cell design is presented in Fig. 1 and Fig. 2. Itconsists of 4 antennas placed in a two-dimensional manner,composing a 2×2 array. The total array size is 17 mm×16mm×16 mm. The radiating cavities are cube-shaped, cham-fered along two of their edges. This type of cavities havealready been used in this working band in other works [28]-[30], although in those cases they were actually cylindricalcavities. The separation between the antennas is 8.1 mm onthe horizontal axis and 7.6 mm on the vertical axis. Sincethe central working frequency of the antenna is 30 GHz, theelements are spaced 0.81λ0 and 0.76λ0, respectively.These four cavities are fed by a combination of differ-ent GW waveguides: ridge (RGW) and groove (GGW) gapwaveguides. Building distribution networks combining bothwaveguide types, provides an interesting phasing arrangementthat was first exhibited in [31] and later widely used [32]-[34].In a monopulse antenna, a sum pattern and a differencepattern are desired. That is, a pattern with a radiation max-imum in the broadside direction (ϕ=0◦, θ=0◦), and anotherpattern presenting a null in that same direction. In order toexcite the antennas in such a way to achieve these two typesof patterns, the distribution network comes into play. Port 2,which is a port located at the bottom of the antenna, feedsthe central GGW through a coupling slot located right in thecenter of the structure. The field, thanks to its orientation,barely couples into the lateral GGW as will be demonstratedin the simulations section. Thus, the entire signal is distributedevenly to the 4 RGWs that finally excite the four chamferedcavities. In this way the sum diagram is obtained.Regarding the difference pattern, it is achieved by excitingport 1, connected to the lateral GGW. The first divider, basedon an E-plane GGW splitter, introduces a 180◦ phase shifton both sides of the divider. After that, the field is evenlydistributed, but this phase shift is no longer undone and it iswhat makes the two upper cavities have a 180◦ phase-shiftwith respect to the lower ones. In this way the differencepattern is obtained.A linearly-polarized (LP) version of this antenna was firstlyand briefly described in [35]. Here the antenna concept hasbeen redesigned to generate circular polarization (CP) by re-placing the cylindrical cavities with square ones and thanks tothe chamfered edges. Interestingly, despite this new attractivefeature for the antenna, its dimensions are even smaller thanthe LP version, thus achieving a design as compact as that oneand with the advantage of providing CP.III. SIMULATED RESULTSPreliminary results for this cell are presented below. First,Fig. 3 shows the diagrams in the main cuts (ϕ=0◦ and ϕ=90◦)for both ports. The sum pattern is the one obtained for port2, and normalized diagrams with stable behavior are obtainedfor different frequencies. The maximum directivity achievedin both cases is 14.5 dBi for the sum diagram and 11.5 dBifor the difference. As expected, the difference diagram has3 dB less than the sum diagram. The difference diagram that2This is a draft version28 28.5 29 29.5 30 30.5 31 31.5 32−60−40−200Frequency (GHz)S-parameters(dB)S11 S12 S21 S22Fig. 4. Simulated S-parameters of the antenna28.5 29 29.5 30 30.5 31 31.5012345Frequency (GHz)Axialratio(dB)Fig. 5. Simulated axial ratiomakes sense to analyze is the ϕ=90◦ cut. To obtain a differencepattern at the ϕ=0◦ cut would imply a third port and a morecomplex network than the one presented here, which could beaddressed in a larger antenna.Fig. 4 illustrates the S-parameters preliminary results. Verygood matching is obtained in this unit cell for both ports(<-20 dB). Likewise, as noted in the previous section, thedecoupling between the input ports is considerable, with anisolation of more than 50 dB. Finally, a key performance ofthis antenna is its circular polarization. Recall that it has notbeen necessary to add an extra layer as is the usual solutionfound in the literature. In this case the radiating cavities excitedby a network integrated in a single layer is able to providecircular polarization as shown in Fig. 5. Although this unitcell does not stand out for a large axial ratio (AR) bandwidth,it at least meets the typically accepted thresholds of an ARbelow 3 dB for the working band (29 GHz to 31 GHz).IV. CONCLUSIONA compact 2×2 monopulse Ka-band antenna with circularpolarization is presented. A combination of ridge gap andgroove gap waveguides has been used to build the monopulsenetwork comparator. The working frequency is from 29 GHzto 31 GHz, a typical band for transmitting antennas in SOTMapplications. The axial ratio remains below 3 dB for the bandof interest. These promising preliminary results should bevalidated, ideally on a larger antenna, which would be possiblethanks to the scalability of the presented design.ACKNOWLEDGMENTThis work is part of the projects PID2019-107688RB-C22 and PID2019-103982RB-C43 fundedby the Spanish Ministry of Science and InnovationMCIN/AEI/10.13039/501100011033.REFERENCES[1] G. Cho and G. T. Kim, “Monopulse tracking performance of a satcomantenna on a moving platform,” Journal of electromagnetic engineeringand science, vol. 17, no. 3, pp. 120–125, 2017.[2] N. Rezazadeh and L. 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A Gap Waveguide-Based 2x2 Circularly-polarized Monopulse Antenna Array

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A Gap Waveguide-Based 2x2 Circularly-polarized Monopulse Antenna Array

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