IET Microwaves, Antennas & Propagation

Research Article

Half-mode dielectric waveguide antenna fedby a micro-strip line with air media for endfireradiation

ISSN 1751-8725Received on 22nd August 2018Revised 4th January 2019Accepted on 29th January 2019E-First on 4th March 2019doi: 10.1049/iet-map.2018.5700www.ietdl.org

Yuefeng Hou1, Yue Li1 , Zhijun Zhang1, Zhenghe Feng1

1Department of Electronic Engineering, Tsinghua University, Beijing 100084, People's Republic of China E-mail: lyee@tsinghua.edu.cn

Abstract: A half-mode dielectric waveguide antenna fed by a micro-strip line with air media is presented. The proposed antennaconsists of two dielectric cuboids and an air-substrate micro-strip line. Due to similar modal field configurations, the dielectriccuboids are excited effectively by the micro-strip line, and they work as two half-mode waveguide antennas working on the TM0mode. With the length of 6.5 wavelength at the centre frequency of 5 GHz, a measured endfire gain of 12.2 dBi is achieved. Themeasured 1 and 3-dB gain bandwidths could reach 17 and 46%, respectively. Good matching is obtained over the entireoperating band from 3.5 to 6.5 GHz. Compared with the antennas mounted on a large conducting plane, the proposed antennahas the advantages of a high endfire gain, a wide endfire gain bandwidth, and a stable endfire radiation pattern.

1 IntroductionDue to the applications on the unmanned aerial vehicle, missile,and reconnaissance vehicle, the endfire antennas mounted on alarge conducting plane have been extensively investigated.Multiple types of antennas have been adopted to realise endfireradiation on a large conducting plane, such as log-periodicantennas [1–3], surface wave antennas [4, 5], horn antennas [6, 7],Yagi antennas [8–15], and leaky-wave antennas [16–19]. However,for the antennas in [1–14], the lengths of radiating apertures are tooshort, which results in a low endfire gain. Moreover, the antennasin [1–5] have the problem of non-uniform energy distribution onthe radiating aperture, because only part of the radiating aperture isused at a single frequency point. The phase constants are notsuitable for the endfire radiation in the antennas [6, 7], which leadsto a sloping-upwards main-beam direction. For the Yagi antennasin [8–15], since the director element couples less energy from thespace when it is far away from the driven element, the increase ofthe director element has a weaker influence on the endfire gain.

For the antennas with a long radiating aperture [16–19], theycould realise a high endfire gain readily. However, the substrateintegrated waveguide leaky-wave antennas with transverse slots in[16, 17] have the problems of the unstable radiation pattern,because the phase constants of the waveguide modes changedrastically with frequency. The high endfire gain and stableradiation pattern could be obtained easily by the leaky-waveantennas which are fed by an air-substrate micro-strip line [18, 19].However, due to the resonant nature of the radiating elements, thegain bandwidths of the antennas in [18, 19] are narrow. Inconclusion, although many researches have made a lot of efforts, itis still a tremendous challenge for the endfire antennas to achieve ahigh endfire gain, a wide endfire gain bandwidth, and a stableradiation pattern, simultaneously.

In this paper, a half-mode dielectric waveguide antenna fed by amicro-strip line with air media is proposed. The half-modedielectric waveguide works on the TM0 mode. With a lowdielectric constant, the half-mode dielectric waveguide has a stableradiation performance. Owing to the similar modal fieldconfigurations, the energy in the micro-strip line with air media isgradually coupled by the half-mode dielectric waveguide, whichimproves the energy distribution on the aperture of the half-modedielectric waveguide. Fed by the air-substrate micro-strip linewhich works on the transverse electromagnetic (TEM) mode, thehalf-mode dielectric waveguide obtains a moderate phase constant.The proposed antenna has the advantages of the high endfire gain,

wide endfire gain bandwidth, and stable endfire radiation pattern. Ithas a prospect to be adopted in the wideband long-distancecommunication applications.

2 Antenna design and analysisFig. 1 shows the geometry of the proposed antenna. The antenna iscomposed of a copper strip, two dielectric cuboids, a ground plane,and two coaxial lines. The two dielectric cuboids have the identicaldimension, they are the radiating structure of the proposed antenna.As plotted in Fig. 1b, the copper strip and the ground plane form amicro-strip line with air media which works on the TEM mode.The micro-strip line is the feed structure of the proposed antenna.The width ‘TW’ of the copper strip is the same as the distance ‘TW’between the dielectric cuboids. The lengths ‘L’ of the micro-stripline and the dielectric cuboids are the same. The micro-strip line isconnected to the coaxial lines from both ends. To mitigate theeffect of the discontinuities between the micro-strip line and thecoaxial lines, tapered structures are added between them. Aspresented in Fig. 1c, by clinging the two dielectric cuboids to thetwo sides of the micro-strip line tightly, the proposed antenna isrealised. The distance ‘GΔ’ between the edge of the dielectriccuboids and the edge of the ground plane is 1λ0. λ0 is thewavelength at the centre frequency of 5 GHz. The detailedoptimised values of each parameter are listed in Table 1.

The two dielectric cuboids of the antenna are placed on theground plane. They form two half-mode dielectric waveguidesworking on the TM0 mode. Since the lower the dielectric constantof the half-mode dielectric waveguide, the more stable theperformance obtained by the half-mode dielectric waveguide overa wide bandwidth [4], the dielectric cuboids should be designed bythe material with a low dielectric constant. To achieve a highradiation efficiency, the dielectric loss introduced by the dielectriccuboid should be low. Based on the analysis mentioned above,Teflon (relative permittivity of 2.1 and loss tangent of 0.002) ischosen as the material of the dielectric cuboid due to the merits ofthe low dielectric constant and low loss. To realise a good radiationcapability, the height of the dielectric cuboid is about λ0/4√εr [4],where εr is the dielectric constant of the dielectric cuboid.

To validate the feasibility of the design strategy, the fieldconfigurations of a micro-strip line with air media working on theTEM mode and a half-mode dielectric waveguide working on TM0mode are presented in Fig. 2 for analysis. As shown in Fig. 2, theelectric vector distributions inside the micro-strip line and the half-

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mode dielectric waveguide on both the yoz-plane and xoy-plane aresimilar to each other. For both of the micro-strip line and the half-mode dielectric waveguide, the maximum magnitude of the electricvector fields along the y-axis exist at the place near the groundplane. It indicates that the energy in a micro-strip line could becoupled by two half-mode dielectric waveguides, if a micro-stripline is placed on the middle of two half-mode dielectricwaveguides. However, as shown in Figs. 2a and c, the energy ofthe micro-strip line mainly concentrates on the place between thecopper strip and the ground plane. To feed the half-mode dielectricwaveguide effectively, the micro-strip line should be long enough.Based on the field configurations analysis above, for the proposedantenna, the micro-strip line and the dielectric cuboids are designedwith the same length ‘L’.

As exhibited in Fig. 3, owing to the similar modal fieldconfigurations between the TM0 mode and TEM mode, the energyin the micro-strip line is gradually coupled by the half-modedielectric waveguide. On the different cross-sections along thepropagation direction (+z-axis), it is found that the energy of theTEM mode in the micro-strip line becomes lower and that of theTM0 mode in the dielectric cuboid becomes higher. Based on theperformance mentioned above, the proposed antenna couldpropagate the energy in both the micro-strip line and the half-modedielectric waveguide, which is different from the traditionaltravelling-wave antennas [20, 21]. For the traditional travelling-wave antennas [20, 21], the radiating structures propagate andradiate the energy along the propagation direction at the same time,due to which the energy distribution on the apertures of thetraditional travelling-wave antennas has the form close to e−jkz,where k = β − jα, β is the phase constant and α is the attenuationconstant. However, because the radiating structure of the proposedantenna could couple with the energy gradually from the micro-strip line, the energy distribution on the aperture is improved,leading to a higher endfire gain.

Due to the similar modal field configurations plotted in Fig. 2,when the micro-strip line is loaded by the two dielectric cuboids,the reflection magnitude of the micro-strip line is not affectedseriously. As illustrated in Fig. 4, over the entire operatingbandwidth from 3.5 to 6.5 GHz, the reflection magnitude of themicro-strip line in Fig. 1b and the proposed antenna in Fig. 1c aresimilar to each other, which are both lower than −15 dB. However,since the energy in the micro-strip line is coupled effectively by thehalf-mode dielectric waveguide, the transmission magnitude of theproposed antenna drops dramatically around the centre frequencycompared with the micro-strip line alone.

The normalised leakage constant (α/k0) in the middle of themicro-strip line versus the frequency is exhibited in Fig. 5. Aroundthe centre frequency, due to the better coupling capability betweenthe micro-strip line and the half-wave dielectric waveguides, moreenergy in the micro-strip line is coupled by the half-wave dielectricwaveguides. Therefore, the normalised leakage constant is higheraround the centre frequency. The normalised phase constants(βz/k0) in the middle of the dielectric cuboid and the micro-stripline are also plotted in Fig. 5. The normalised phase constants in

Fig. 1  Schematic drawings of the proposed antenna(a) Two dielectric cuboids(b) Micro-strip line(c) Proposed antenna

Table 1 Optimised dimension of the proposed antenna(Unit: mm)L W H GL GW GH10 10 1 1 39 39

 FL FS FH FD TW TS8 10 1.85 1.75 15.5 3.25

Fig. 2  Schematic electric vector field distributions of the micro-strip lineand the half-mode dielectric waveguide(a) Micro-strip line on the yoz-plane(b) Half-mode dielectric waveguide on the yoz-plane(c) Micro-strip line on the xoy-plane(d) Half-mode dielectric waveguide on the xoy-plane

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the two structures are similar to each other and close to the freespace. Over the entire operating band, the normalised phaseconstant of the antenna changes slightly, which indicates that theantenna could realise stable endfire radiation.

The radiation patterns of the antenna with different sizes of theground plane at the centre frequency are shown in Fig. 6. The GΔ isthe distance between the edge of the dielectric cuboids and the

edge of the ground plane, as presented in Fig. 1c. Due to thediffraction influence by the finite ground plane, the main beamdirection of the antenna is slightly upward [15, 16]. When the GΔchanges from 0.5λ0 to 5λ0, the endfire gain varies from 12.86 to13.11 dBi, and the front-to-back radio varies from 19.67 to 22.89 dB. To mitigate the effect of the finite ground plane, the antennawith an infinite ground plane is simulated. When the antenna isdesigned with an infinite ground plane, the beam points to theendfire direction exactly.

3 Prototype and measurementsA prototype is fabricated and measured to provide a verification ofthe new design method. The geometry of the proposed antenna isillustrated in Fig. 7. The antenna is excited at Port 1. Port 2 isterminated with a matching load. S-parameters were measuredusing an N5071B vector network analyser (300 kHz−9 GHz). Thegains and radiation patterns were measured in a far field anechoicchamber. The simulated and measured S-parameters are shown in

Fig. 3  Simulated electric vector field distributions of the proposed antennaon different cross-sections along the propagation direction (+z-axis) at 5GHz(a) On the cross-section through AA′(b) On the cross-section through BB′(c) On the cross-section through CC′

Fig. 4  Simulated S-parameters of the micro-strip line with or without thedielectric cuboids

Fig. 5  Normalised leakage constant and normalised phase constantsversus frequency

Fig. 6  Simulated radiation patterns at the centre frequency of 5 GHz on E-plane (yoz-plane) with different G▵

Fig. 7  Fabricated prototype of the proposed antenna

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Fig. 8. The simulated and measure S-parameters are lower than−10 dB over the operating band from 3.5 to 6.5 GHz, verifying thatgood matching is achieved by the antenna. The measuredtransmission magnitude is ∼−15 dB at the centre frequency of 5 GHz, which indicates that most of the energy in the micro-strip lineis coupled by the half-mode dielectric waveguides.

The simulated and measured endfire gains are presented inFig. 9. The measured endfire gain is better than 10 dBi from 4.1 to6 GHz with a maximum value of 12.2 dBi at the frequency of 5 GHz. The measured 1-dB gain and 3-dB endfire gain bandwidthsof the antenna are ∼17.0 and 46.0%, respectively. The simulatedand measured normalised radiation patterns are plotted in Fig. 10.Three scanned radiation patterns at 4–6 GHz are chosen forpresentation. From Fig. 10, over the entire operating bandwidth, astable endfire radiation pattern is achieved by the antenna. Theantenna also has a high level of front-to-back radio and a low levelof cross-polarisation. The front-to-back radios of the antenna at thethree frequencies are higher than 17 dB. The cross-polarisation atthe three frequencies are lower than 25 dB compared with the co-polarisation. As shown in Fig. 10, the radiation patterns on the E-plane are stable and point to the quasi-endfire direction at the threefrequencies. With the frequency increasing, the half-power beamwidths on the H-plane are gradually narrower [21], as presented inFig. 10. The measured half-power beam widths on the H-plane atthe three frequencies vary from 22° to 16°.

Table 2 shows a comparison between existing designs in theopen literature and the proposed antenna. Compared with theendfire antennas mounted on a large conducting plane whichexplicitly present the endfire gain versus the frequency, theproposed antenna has a wider 1-dB and 3-dB gain bandwidth.Meanwhile, the proposed antenna also has a relatively high endfiregain.

4 ConclusionIn the paper, a half-mode dielectric waveguide antenna fed by amicro-strip line with air media is presented. Fed by the micro-stripline, the proposed antenna has a good matching and a stable endfireradiation pattern. The proposed antenna achieves a high endfiregain and a wide endfire gain bandwidth. Based on the experimental

Fig. 8  Simulated and measured S-parameters of the proposed antenna

Fig. 9  Simulated and measured gains of the proposed antenna

Fig. 10  Simulated and measured normalised radiation patterns at different frequencies on the E-plane and H-plane(a) 4 GHz, E-plane (yoz-plane) and H-plane(xoz-plane)(b) 5 GHz, E-plane (yoz-plane) and H-plane(xoz-plane)(c) 6 GHz, E-plane (yoz-plane) and H-plane(xoz-plane)

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results of the reflection magnitude and the radiation characteristics,the proposed antenna exhibits a good potential to be applied in thewideband long-distance wireless communication systems.

5 AcknowledgmentsThis work was supported in part by the National Natural ScienceFoundation of China under Contract 61525104, and in part byNatural Science Foundation of Beijing Manipulate under Contract4182029.

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[14] Zhang, Z., Cao, X., Gao, J., et al.: ‘Broadband SIW cavity-backed slotantenna for endfire applications’, IEEE Antennas Wireless Propag. Lett.,2018, 17, pp. 1271–1275

[15] Liu, J., Xue, Q.: ‘Microstrip magnetic dipole Yagi array antenna with endfireradiation and vertical polarization’, IEEE Trans. Antennas Propag., 2013, 61,(3), pp. 1140–1147

[16] Liu, J., Jackson, D.R., Long, Y.: ‘Substrate integrated waveguide (SIW)leaky-wave antenna with transverse slots’, IEEE Trans. Antennas Propag.,2012, 60, (1), pp. 20–29

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Table 2 Comparison of performances for various antennas(λ is the wavelength in free space at the frequency when theantennas achieve the maximum endfire gain).Ref. Length, λ Max.

endfiregain, dBi

−1 dB gainbandwidth, %

−3 dB gainbandwidth, %

[2] 2.41 9.7 7.9 15.8[15] 2.97 10.4 3.7 7.6[18] 6.10 11.5 4.0 7.0[19] 5.67 13.3 8.0 17.0ours 6.5 12.2 17.0 46.0

858 IET Microw. Antennas Propag., 2019, Vol. 13 Iss. 6, pp. 854-858© The Institution of Engineering and Technology 2019