Received August 7, 2021, accepted August 23, 2021, date of publication August 25, 2021, date of current version September 2, 2021.

Digital Object Identifier 10.1109/ACCESS.2021.3107914

Design of Broadband Double-Ridge HornAntenna for Millimeter-Wave ApplicationsYUQI HE 1, (Student Member, IEEE), XIAOYUAN ZHAO 1, (Student Member, IEEE),LUYU ZHAO 1,2, (Senior Member, IEEE), ZEXING FAN2, JING-KE WANG2,LU ZHANG2, CHAO NI 3,4, AND WEI-JUN WU31Key Laboratory of Antennas and Microwave Technologies, Xidian University, Xi’an 710071, China2Xi’an Lambda Communication Technology Company Ltd., Xi’an, Shaanxi 710116, China3National Key Laboratory on Electromagnetic Compatibility, China Ship Development and Design Center, Wuhan, Hubei 430064, China4Radar and Signal Processing Laboratory, Electronic Information School, Wuhan University, Wuhan, Hubei 430072, China

Corresponding author: Luyu Zhao (lyzhao@xidian.edu.cn)

This work was supported in part by the National Key Research and Development Program of China under Grant 2019YFF0216603, in partby the Key Research and Development Program of Shaanxi under Grant 2020ZDLGY15-03, and in part by ZTE Cooperation underGrant HC-CN-20191227012.

ABSTRACT In this paper, a broadband double-ridge horn antenna (DRHA) covering 5G millimeter-wave (mm-wave) band is proposed, which maintains stable radiation pattern and gain in the entire band.By redesigning the transition between coaxial and waveguide, there is no splitting or deterioration in theaxis pattern throughout the entire 18-54 GHz frequency band, and the VSWRs are all below 2. Meanwhile,by replacing the H-Side wall of the antenna with metal grid, the variation on 3 dB beamwidth of theradiation pattern also becomemore stable against frequencies. In order to fabricate the prototype successfully,the method of block processing and assembly is adopted, which can ensure high precision machining andmake the assembly simple and easy to operate. Prototype measurement results show that the VSWR of theantenna is in well agreement with the simulation ones, while the gain is stable as well as no pattern splittingin the entire frequency band can be observed. It will find potential applications in 5G mm-wave frequencyband, especially in the field of antenna measurement system as a standard calibration antenna or a probeantenna.

INDEX TERMS Antenna measurement, broadband antennas, double-ridged horn antenna, millimeter waveantennas.

I. INTRODUCTIONAs the international organization for Standardization (3GPP)specifies the frequency bands for 5G NR (new radio) mm-wave usage, equipment or devices for 5G mm-wave com-munication have sprung up [1]. As a necessary means toevaluate the wireless performance of these devices, the mea-surement in the corresponding frequency band must be devel-oped accordingly. In other words, the measurement system isrequired to have the ability to cover the entiremillimeter wavefrequency band with good accuracy and stable performances.

Horn antenna is a commonly used antenna type in themeasurement system, either as standard antennas for cali-bration or as probe antennas for field acquirement. With therequirement of broadband operation, ridged horn is becoming

The associate editor coordinating the review of this manuscript and

approving it for publication was Sandra Costanzo .

widely used as an evolutionary form of horn antennas, whichis formed by adding ridge into horn antenna. Compared tohorn antenna with the same volume, it has wider single-mode bandwidth and comparable radiation characteristics.Therefore, for mm-wave measurement systems, the optimumcandidate of calibration antenna is ridged horn antenna.

The ridged horn antenna is proposed by J. Kerrin 1973 based on the theory of ridge waveguide and hornantenna [2]. Since the 21st century, scholars have done alot of work on ridged horn antenna. In [3]–[5], the oper-ation of the DRHA in the 1-18 GHz band was analyzedusing electromagnetic simulation. The results show that atfrequencies above 12 GHz, the main lobe starts to split withsignificant gain loss. It is pointed out that the presence ofhigher order modes in the waveguide is the main causeof this phenomenon. Then, the academic community hascarried out researches on this issue, especially for patterns

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at higher frequencies. In [6]–[8], by adding wedge structureinto the feed waveguide and replacing the H-side wall ofthe horn with metal grid, the structure of the conventionalDRHA has been modified to reduce the influence of higher-order mode and improve the radiation performance below20 GHz. In [9]–[13], by loading dielectric or metamateriallens at the antenna aperture, the antenna gain is significantlyimproved. However, it deteriorates the matching performanceof the antenna. In addition, [14], [15] analyze the process-ing tolerances of antennas and pointed out that due to thecomplex structure of conventional ridge horn antennas, gapsoccur in the internal structure of the antenna during design,processing and assembly. Therefore, in the design of mm-wave band DRHA, two aspects need to be considered, oneis the electrical performance of antenna such as broadbandoperation, pattern splitting and stable bandwidth, the other isto fabricate the prototype precisely.

In this paper, a broadband and high performance dualridged horn antenna covering 5Gmm-wave band is proposed.There is no splitting in the axis radiation pattern over theentire band of interest. On the basis of the initial antennastructure, by introducing a wedge-shaped structure into thefeed structure and replacing the H-plane sidewall with ametal grid, a final antenna structure with good matching andradiation characteristics is obtained. Furthermore, in orderto obtain a prototype with excellent performance, the blockmethod is used to ensure the machining accuracy and reducethe difficulty of assembly and feeding. All the simulationresults are obtained using Ansys HFSS [16].

The rest of this paper is organized as follows. In Section II,a commonly used structure of DRHA is presented and ana-lyzed numerically. Through the simulation results, the short-comings of the initial structure are reflected. In Section III,the evolution of the initial antenna structure is presented.In Section IV, the final antenna prototype is fabricated andmeasured to obtain reflection coefficient, antenna gain, andradiation patterns etc. Section V concludes the whole paper.

II. INITIAL ANTENNA DESIGNIn this section, we design an industrially commonly useddouble-ridged horn antenna as the initial antenna in the firstplace, which covers the frequency range of 18-54 GHz. Thenthe shortcomings of this antenna are revealed through sim-ulation, which will be overcame by measures presented insubsequent sections.

A. INITIAL DRHA DESIGNThe E-plane section view of ridged horn antenna is shownin Fig. 1, its main structure is divided into the followingtwo parts: the feed part and the horn part [17]. The feedpart contains the transition of coaxial to waveguide, whichconsists of a coaxial line with the impedance of 50 ohm,a straight waveguide with a stepped ridge and a shorting platethat blocks the backward transmission of electromagneticwaves. The horn section consists of a rectangular waveguidewith a gradually opened cross-section and two exponentially

tapering ridges along the inner wall of the horn, which radi-ates electromagnetic waves into free space.

FIGURE 1. Diagram of E-plane profile of the initial DRHA.

Ridge waveguide is formed by introducing ridge structureinto the normal waveguide. Its relevant parameters such ascutoff frequency, main mode operating bandwidth and char-acteristic impedance are directly related to the size of theintroduced ridges. At the end close to the shorting board,the ridge is stepped to form the back cavity, the size of whichcan be adjusted to tune the return loss of the antenna in abroadband sense.

As shown in Fig. 1, the end of the ridge is circularized,which is to further reduce the reflection of the edge diffrac-tion effect on the horn aperture. From the perspective of theDRHA, the ridge profile is formed by three curves connectedto each other: the sidewall, the exponential taper line andthe circular arc. Among them, the expression equation of theexponential curve is given in (1).

x (z) = C1epz + C2 (1a)

C1 =x2 − x1

epz2 − epz1(1b)

C2 =x1epz2 − x2epz1

epz2 − epz1(1c)

where (x1, z1) and (x2, z2) correspond to the coordinatesof points A (0.2, 0) and B (7, 26.5) in Fig. 1, respectively.The value of parameter p is 0.02, which is regarded as theexponential factor of ridge curve expression.

When we feed the ridged horn antenna, the inner conductorof the coaxial line is fed through the first ridge and thenconnected to the other ridge on the opposite side, and theouter conductor is shorted to the horn wall. Polytetrafluo-roethylene (PTFE) with a relative dielectric constant of 2.1 isused to separate the inner and outer conductors in this paper,to prevent deformation of the inner conductor when it is verythin from contacting the outer conductor and causing a shortcircuit condition.

In addition, the coaxial line is located in the center ofthe wide edge of the straight waveguide, about a quarterwavelength (Ch + h0) from the shorting plate. The specificvalues of each parameters in the figure are shown in Table 1.The structure of the initial DRHA is given in Fig. 2, and the

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FIGURE 2. Structural configuration and feed details of the initial antenna.

TABLE 1. Structural parameters of the initial DRHA (unit: mm).

overall dimensions of the antenna are 38 × 25 × 38 mm3(x × y × z).

B. DRHA SIMULATION RESULTSThe initial antenna is modeled and simulated using commer-cial electromagnetic simulation software (High FrequencyStructure Simulator, HFSS). The VSWR and gain againstfrequency are given in Fig. 3, respectively. The results showthat the VSWR of the initial antenna can achieve less than2 in the entire frequency band, in addition, the gain increasesgradually with the increase of frequency, but not smoothlyenough. The gain grows more slowly at 18-30 GHz, whileit grows steeply at 30-38GHz, this is due to the presenceof higher-order modes around 30 GHz. Higher-order modescause the distortion of the radiation pattern and the reductionof peak gain at 26-30 GHz. This is why further modificationon the structure and improvement on the axis radiation patternare needed.

FIGURE 3. The variation of VSWR and gain with respect to frequency ofthe initial antenna.

Fig. 4 shows the normalized E-plane and H-plane radiationpatterns of the antenna at 30 GHz. It can be seen that the mainlobe of E-plane pattern is distorted, resulting in a deterioratedaxis gain. According to the operating principle of the hornantenna, the aperture field determines the pattern of the horn,and the deteriorated axis pattern at a certain frequency point isdue to the disturbance of high-order modes on the phase dis-tribution of the aperture field. The generation of higher-ordermodes is mainly due to the discontinuity of the structure.

FIGURE 4. Normalized pattern of the initial antenna at 30 GHz in theE-and H-planes.

Meanwhile the radiation performance of the initial antennais also examined from the perspective of half powerbeamwidth (HPBW), as shown in Fig. 5. The variation ofthe beamwidth in the entire frequency band is more than30 degrees for both E and H planes, which also indicates thatthe radiation characteristics of the initial antenna still needfurther improvement.

FIGURE 5. Variation of the HPBW of the initial antenna with respect tofrequencies.

From the above analysis, it is obvious that the main prob-lem of the initial DRHA is the deterioration in the axisradiation pattern at some frequency points due to the intro-duction of higher-order modes, which also leads to the unevenvariation of gain and beamwidth against frequency. Moreimportantly, for a standard antenna, the splitting of the mainlobe of the pattern is absolutely unacceptable, since it willcause inaccurate and uncertain magnitude and phase calibra-tion results.

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III. DRHA DESIGN EVOLUTIONIn order to reduce the impact of high-order modes on theDRHA and improve its radiation performance, the initialantenna need to bemodified. Firstly, wedge-shaped structuresare embedded in the coaxial-waveguide transition sectionsto ameliorate the discontinuity of the internal structure, andthen the H-sidewall of the horn is replaced by a metal grid tooptimize the gains at medium and low frequencies.

A. WEDGES EMBEDDED IN THE RIDGE WAVEGUIDECoaxial lines transmitting TEM waves tend to create higher-order modes when exciting ridge waveguides due to struc-tural discontinuity, which affect the impedance matching andradiation characteristics of the antenna. Therefore, adding awedge structure into the feed section can effectively reducethe structural discontinuity. The wedge structure is placedinside the waveguide along the four walls of the ridge waveg-uide, as shown in Fig. 6. The structure was then optimizedinto a modified one where R0 is 19 mm and d is 5 mm whileother parameter values remain unchanged.

FIGURE 6. Detail of the feed section after adding the wedge structure.

The variation of VSWR and gain with respect to frequencyis shown in Fig. 7 and compared with the simulation resultsof the initial antenna. After adding the wedge structure,the VSWR of the antenna basically remains the same, but thegain of the antenna has improved significantly, especially inthe frequency range of about 30 GHz, thereby make the gainin the whole frequency band more balanced.

FIGURE 7. Variation of VSWR and gain of antenna with respect tofrequency after adding wedge structure.

Similarly, the normalized radiation patterns of the antennaat 30 GHz before and after embedding the wedge structure

are compared in Fig. 8. It can be observed that the addition ofthe wedge successfully fixes the deterioration of the E-planeaxis pattern and reduces the back-lobe level to below -20dB.

FIGURE 8. Two-dimensional radiation pattern of the antenna after addingthe wedge structure at 30GHz.

In addition, the wedge has a positive effect on the HPBWof the antenna, especially the H-plane becomes smooth in thewhole frequency band, as shown in Fig. 9. However, it shouldalso be noted that there is still a large gap in the beamwidthbetween the two cutting-planes in the low and intermediatefrequency bands.

FIGURE 9. Comparison of bandwidth of antenna before and after addingwedge structure.

The antenna with wedge structure has achieved the presetgoals in improving the DRHA performances, but there arestill some problems remaining, so it is necessary to makefurther improvements to the structure of the antenna.

B. METAL GRID IN THE H-SIDE WALLThrough the previous analysis, at low and medium frequen-cies, the variation of beamwidth of the E-plane of the antennaradiation pattern is still not smooth enough. The reason isthat as the operating frequency increases, the electromagneticenergy propagated in antenna is more concentrated betweenthe ridges, so the influence of the horn sidewalls on thebeamwidth is also reduced. When the frequency is lower,the sidewalls still have some influence on the antenna perfor-mance. Therefore, the sidewalls in the H-plane are replacedby a metal grid instead of removing them directly. The finalimproved antenna structure is shown in Fig. 10.

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FIGURE 10. The conformation of the proposed final DRHA. (Designparameters: t1 = 6 mm, t2 = 3 mm).

After the EM simulation, the variation of VSWR and gainof the final antenna with respect to frequency is given andcompared with the results of the previous antennas as shownin Fig 11. TheVSWR remains below 2 in thewhole frequencyband and good impedance matching is maintained, whichis assured by carefully optimizing the introduced structuresso that their impact on impedance matching is minimized.Also, the gain variation across the entire band is smoothercompared to the previous antennas. In particular, compared tothe antenna when the wedge structure is added, the gain in thelow frequency is higher, while the gain in the high frequencyis almost the same.

FIGURE 11. Comparison of VSWRs and gains of the three models inrectangular and ridge waveguides.

The normalized radiation patterns of the final antenna atthe E and H planes at ten frequency points throughout thefrequency band are given in Fig. 12. It can be observed thatthere is no axis pattern deterioration at all frequency points,in the meantime, all the back-lobe level is less than -20dB.

Finally, we compared the HPBW of these three designantennas, as shown in Fig. 13. The beamwidth of the finalantenna fluctuates very little throughout the frequency band,and the difference of HPBW between E and H plane patternsis significantly reduced.

IV. FABRICATION AND MEASUREMENTIn order to obtain a prototype with excellent performance,machining and assembly are also crucial to the design.

FIGURE 12. The normalized radiation patterns of the final antenna at tenfrequency points, (a) E-plane, (b) H-plane.

FIGURE 13. HPBW of the three antennas at two cutting planes.

Particularly for the antenna in millimeter wave band,the impact of fabrication error on performances is more sen-sitive. Hence, in this paper, we adopt the block machiningand assembly method, and choose the five-axis machiningtechnology with high accuracy. This can make the proto-type processing and assembly simple and easy to operate.Furthermore, the material of the prototype is brass, and itsassembly is shown in Fig. 14. Where, A1/A2 is a collectionof E-side sidewall, ridge and wedge structure. B1 and B2 areH-side sidewall formed by metal grid, C is a combinationof shorting plate and H-side wedge structure, and D is acoaxial connector for antenna feeding. When the prototypeis assembled, A1 and C, A2 and D are connected with metalscrews respectively, then the two combinations are connected

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and fixed, and finally B1 and B2 are installed. The size of theassembled prototype is about 38 × 30.5 × 40.2mm3.

FIGURE 14. Schematic diagram of DRHA’s block processing and assembly.(a) Exploded 3-D view, (b) Antenna prototype.

The prototype of the antenna was produced accordingto the above machining and assembly procedure and thenmeasured to get the VSWR and gain responses within thefrequency band of interest which are compared with thesimulation results, as shown in Fig. 15. The measured VSWRis obtained using Keysight Technology’s vector network ana-lyzer N5247B PNA-X. The measured results are in goodagreement with the simulated results. Additional measure-ments of the gain and radiation patterns of the prototypeare made in a planar near-field microwave chamber. Regret-fully, since the test and calibration antennas equipped inthe chamber could not completely include the frequenciesfrom 18-54 GHz, it is necessary to replace the measurementantennas to continue the measurements on the subsequentbands after testing the 18-40 GHz band.

FIGURE 15. Measured VSWR and gain of the prototype antenna.

The measured 2D radiation patterns of the prototype aregiven in Fig. 16 at ten frequency points in the entire frequency

band. Among them, (a) is the E-plane and (b) is the H-plane.It can be observed that the maximum direction of the far-fieldbeam coincides with the z-axis direction, which means thatthe maximum pointing of the radiation pattern is not offset,and more importantly, there is no distortion or splitting in theaxis patterns.

FIGURE 16. Measured far-field patterns are shown from 18 to 54GHz.The results are given for the (a) E-plane (phi = 0◦) and (b) H-plane(phi = 90◦) cuts.

TABLE 2. Comparison of proposed antenna with relevant antennadesigns.

V. CONCLUSIONIn this paper, a double-ridge horn antenna operating in thefrequency band from 18 to 54GHz is designed, fabricated and

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measured. By redesigning the conventional antenna structure,the radiation characteristics of the antenna are effectivelyimproved, especially the axis radiation pattern splitting prob-lem is completely solved and stable gain is obtained. In addi-tion, the antenna assembly method adopted also provides aneffective and feasible technical solution for these kinds ofsmall volume horn antenna in millimeter band. The advan-tage of this solution is that the prototype can have minimalmechanical errors, easy assembly and feeding, and simpleprocessing. The antenna can be widely used in millimeterwave measurement system either as a standard antenna forcalibration, or a probe antenna for accurate field acquisition.

At the end of this paper, a comparison between the pro-posed double-ridge horn antenna and published designs isshown in Table 2. Several antenna characteristics, includ-ing operation band, gain variation, antenna volumes, patternsplitting are listed. It is clear that the proposed antenna hasadvantages in many aspects.

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[3] C. Bruns, P. Leuchtmann, and R. Vahldieck, ‘‘Full wave analysis andexperimental verification of a broadband ridged horn antenna system withparabolic reflector,’’ in Proc. IEEE Antennas Propag. Soc. Int. Symp. HeldConjunct, Jul. 2001, pp. 230–233.

[4] C. Bruns, P. Leuchtmann, and R. Vahldieck, ‘‘Comprehensive analysisand simulation of a 1-18 GHz broadband parabolic reflector horn antennasystem,’’ IEEE Trans. Antennas Propag., vol. 51, no. 6, pp. 1418–1422,Jun. 2003.

[5] C. Bruns, P. Leuchtmann, and R. Vahldieck, ‘‘Analysis and simulation of a1-18-GHz broadband double-ridged horn antenna,’’ IEEE Trans. Electro-magn. Compat., vol. 45, no. 1, pp. 55–60, Feb. 2003.

[6] M. Abbas-Azimi, F. Arazm, and R. Faraji-Dana, ‘‘Design and optimisationof a high-frequency EMCwideband horn antenna,’’ IETMicrow., AntennasPropag., vol. 1, no. 3, pp. 580–585, Jun. 2007.

[7] B. Jacobs, J. W. Odendaal, and J. Joubert, ‘‘An improved design fora 1–18 GHz double-ridged guide horn antenna,’’ IEEE Trans. AntennasPropag., vol. 60, no. 9, pp. 4110–4118, Sep. 2012.

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[9] A. S. Turk and A. K. Keskin, ‘‘Partially dielectric-loaded ridged hornantenna design for ultrawideband gain and radiation performance enhance-ment,’’ IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 921–924, 2012.

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[16] Electronics Desktop, Version 18.0.0, Ansys, Canonsburg, PA, USA, 2017.[17] C. A. Balanis, Antenna Theory: Analysis and Design. Hoboken, NJ, USA:

Wiley, 2005.

YUQI HE (Student Member, IEEE) received theB.S. degree from Xidian University, Xi’an, China,in 2019, where he is currently pursuing the M.D.degree in electromagnetic wave and microwavetechnology. His research interests includemillime-ter wave array, phased antenna array, and meta-material-based antenna array.

XIAOYUAN ZHAO (Student Member, IEEE)received the B.S. degree from Zhengzhou Insti-tute of Light Industry, Zhengzhou, China, in 2018.He is currently pursuing the M.D. degree in elec-tromagnetic wave and microwave technology withXidian University, Xi’an, China. His researchinterests include millimeter wave antenna and mil-limeter wave measurement technology.

LUYU ZHAO (Senior Member, IEEE) was bornin Xi’an, China, in 1984. He received the B.Eng.degree fromXidian University, Xi’an, in 2007, andthe Ph.D. degree from The Chinese University ofHong Kong, Hong Kong, in 2014.

From 2007 to 2009, he was with the KeyLaboratory of Antennas and Microwave Technol-ogy, Xidian University, as a Research Assistant,where he was involvedwith software and hardwareimplementation of RF identification (RFID) tech-

nologies. From 2014 to 2015, he was a Postdoctoral Fellow at The ChineseUniversity of Hong Kong. From October 2015 to October 2016, he was withWyzdomWireless Company Ltd., where he was a Co-Founder and the CTO.He has been an Associate Professor with the National Key Laboratory ofAntennas andMicrowave Technology, Xidian University, since 2016. He hasalso been with Xi’an Lambda Communication Technology Company Ltd.,since 2019. His current research interests include design and applicationof multiple antenna systems for next generation mobile communicationsystems, innovative passive RF and microwave components and systems,millimeter wave and terahertz antenna array, and meta-material-based orinspired antenna arrays.

Dr. Zhao was a recipient of the Best Student Paper Award of 2013 IEEE14th HK AP/MTT Postgraduate Conference, the Honorable Mention Awardof 2017 Asia-Pacific Conference on Antenna and Propagation, and the BestPaper Award of IEEE ICEICT 2019.

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ZEXING FAN was born in Shaanxi, China,in 1996. He received the B.S. degree in mechanicaldesign and manufacturing, and automation fromShaanxi University of Technology, in 2018. He iscurrently working at Xi’an Lambda Communica-tion Technology Company Ltd., as a StructuralEngineer. His research interest includes millimeterwave measurement. Currently, the research anddevelopment products include millimeter wavemeasurement box LMD-MTS-112 based on single

probe and millimeter wave test system LMD-MTS-211 based on compactrange technology.

JING-KE WANG was born in Henan, China,in 1991. He received the B.E. degree in com-puter science and technology from ZhengzhouUniversity, Henan, in 2014, and the M.S. degreein electronic science and technology from GuilinUniversity of Electronic Technology, in 2019.

He is currently an Antenna Engineer withAntenna Technology Department, Xi’an LambdaCommunication Technology Company Ltd. Hiscurrent research interests include MIMO antennasand microwave components.

LU ZHANG received the Bachelor of Engineer-ing degree from Xi’an University of Posts andTelecommunications, in 2013. He is currentlyworking at Xi’an Lambda Communication Tech-nology Company Ltd., as the Director of test andresearch and development. His research interestis microwave and millimeter wave antenna mea-surement. Currently, the research and developmentproducts include millimeter wave measurementbox LMD-MTS-112 based on single probe and

millimeter wave test system LMD-MTS-211 based on compact rangetechnology.

CHAO NI received the B.Eng. degree in informa-tion engineering from Wuhan University, in 2011,and the M.Eng. degree in communication andinformation systems from the University ofChinese Academy of Sciences, in 2014. He iscurrently pursuing the Ph.D. degree with the Elec-tronic Information School, Wuhan University.

He is currently working with the NationalKey Laboratory on Electromagnetic Compatibil-ity, China Ship Development and Design Center.

His current research interests include circularly polarized antennas, minia-ture antenna design, and electromagnetic compatibility.

WEI-JUN WU was born in Hubei, China, in 1985. He received the B.S.degree in information administration and information system and the Ph.D.degree in electromagnetic field and microwave technology from XidianUniversity, Xi’an, China, in 2007 and 2012, respectively. He currently worksas a Senior Engineer at Science and Technology on Electromagnetic Com-patibility Laboratory, China Ship Development and Design Center, Wuhan,China. His current research interests include filtering antennas, antennaarrays, and EMC.

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