When a leaky rectangular waveguide is used to realize the coverage of radio wave in the small confined spaces, there will be ashadow region, which influences the coverage performance. In this paper, the traditional leaky rectangular waveguide is improvedaccording to the principle of the equivalent circuit, by cutting interdigital slots in the upper wall and adding uniserial metal viasbetween the upper and lower walls of the rectangular waveguide. +us, the right/left-handed transmission line property isintroduced to the periodic leaky-waveguide (LWG), realizing the broadside radiation with relatively high gain (15.7 dBi), goodcross polarization (−50 dB), and narrow half-power beamwidth (10.9°) at 6.97GHz and providing a method for a uniformcoverage of the radio wave in rooms without a shadow region.
1. Introduction
With large numbers of mountain railways and railway tunnelsbuilt in China, the diversification of the information service israpidly developed, which requires the real-time and reliabledata transmission including pictures, sounds, and environ-mental parameters. Leaky rectangular waveguide with itssimple structure, flexible installation, great capacity of trans-mission power, low transmission loss at high frequency, andmany other advantages has been widely researched and appliedin the wireless communication systems [1–10], particularly insome environments where uniform coverage of radio wave isneeded, which can replace the traditional distributed antennas[1–6]. In some specific confined spaces, the leaky rectangularwaveguide shows much more advantages in realizing uniformand stable radio wave coverage [1–3]. Moreover, in the ap-plications of the 5G communication systems, the leaky-wavestructure even shows tremendous advantages in solving theproblems in the aspects of the mutual coupling, isolationdegree, and the antenna bandwidth.
However, when the leaky-wave structure is used in theradio wave coverage in the wireless communication
systems, the inherent forward-radiation property willproduce a shadow region which influences the coverageperformance. In order to realize the broadside radiation andthe continuous beam-steering, people have been graduallyexploring the leaky-wave structures with compositeright/left-handed (CRLH) transmission line characteristicsin the past years, including designing novel structures andintroducing different metamaterial properties, covering themicrowave band and even the terahertz band.
In this paper, a periodic leaky-waveguide (LWG) forbroadside radiation based on the rectangular waveguide isdesigned. +is proposed periodic LWG can realize thebroadside radiation and provide a method to realize theradio wave coverage indoors without a shadow region.
2. Structure and Mechanism
+e classic hollow metal rectangular waveguide with peri-odic transverse slots on the upper wall has the forward-radiation property, whose beam direction θ depends on theratio between propagation constant β and wavenumber inthe free space k0 (sin θ � β/k0), as shown in Figure 1(a). Since
HindawiInternational Journal of Antennas and PropagationVolume 2021, Article ID 8309431, 5 pageshttps://doi.org/10.1155/2021/8309431
this leaky rectangular waveguide is a fast-wave structure andradiates with the fundamental mode, whose propagationconstant is smaller than that in free space, its radiation beamalways points to the first quadrant. +e reason of this in-herent right-handed transmission line property could beexplained from the circuit parameters. As is known, theequivalent circuit of a section of the closed rectangularwaveguide with a length of P (P is smaller than the guidewavelength) consists of a series inductance LR connectedwith a shunt capacitance CR, as shown in Figure 1(b).Generally, the mechanism to realize the right/left-handedtransmission line characteristics is to introduce the seriescapacitance CL and the shunt inductance LL in the rectan-gular waveguide. As a result, the introduced circuit and theinherent circuit can constitute a new circuit structure, whichhas the right/left-handed capacitance and inductance at thesame time. When the circuit parameters meet the balanceconditions, the waveguide can radiate the broadside beam.
In this paper, a novel periodic leaky-wave antenna basedon the rectangular waveguide is designed for broadsideradiation, as shown in Figure 2(a). It is found that, forperiodically slotted rectangular waveguide, the slot itself canbe equivalent to the inductance Ls and the capacitance Cs [6],which are connected in parallel, as shown in Figure 1(c).However, Ls is a series inductance andCs is too small to reachbalance. In order to introduce the left-handed transmissionline property, the interdigital slots are cut on the uppersurface to produce enough series capacitance CL [6], andmetal vias are set between upper and lower walls to produceenough shunt inductance LL [10].
+e transverse interdigital slots and metal vias are pe-riodically set along the waveguide. +e structure and pa-rameters of the unit cell are given in Figure 2(b), which isconstituted by an interdigital slot and a metal via. +iscomposite interdigital slot consists of a group of transverseand longitudinal rectangular slots, which are intensively
arranged to introduce the interdigitated capacitor.+emetalvias are set along one side of the waveguide to produceenough shunt inductance. As a result, the equivalent circuitof the unit cell of the proposed structure can be described asthat in Figure 2(c).
+e dispersion curves of the structure are shown inFigure 3.+e dispersion characteristics of the CRLH unit cellhave been computed by a full-wave numerical method. Acontinuous transition at 6.97GHz is obtained without anygap between the LH and RH portions. +us, the CRLHleaky-wave antenna based on the rectangular waveguide canrealize the broadside radiation.
3. Results
In this paper, a CRLH LWG based on the rectangularwaveguide for broadside radiation is designed based on thestandard WR90 rectangular waveguide; the structural pa-rameters are a� 10.15mm, b� 22.86mm, A� 12.7mm, andB� 25.4mm.+e details of these parameters of the LWG areshown in Figures 2(a) and 2(b), where L is the whole lengthof the waveguide, which is set to 120mm in this work, and Prefers to the period of CRLH unit cell. +e performancecharacteristics of the CRLH LWG were optimized with aseries of CST simulations. For broadside radiation, thestructural parameters for the unit cell are finally obtainedafter several trials, as listed in Table 1.
In this work, a CRLH leaky-wave antenna based on therectangular waveguide with 64 periods is designed for abroadside beam. +e simulated radiation patterns in the H-plane for the LH region, broadside, and RH region areshown in Figures 4(a)−4(c), respectively.
In Figure 4(a), it is found that, by changing the frequencyfrom 6.1 to 6.6GHz, the beam scans from −20° to −5°, withan isotropic gain from 14 dBi to 15.2 dBi and cross polari-zation below −30 dB. Similarly, in Figure 4(b), it is found
y
x
β
k0θ
P
slot
(a)
Unit cell
CR
LR
(b)
Unit cell
CS
LS0.5LR
0.5CR 0.5CR
0.5LR
(c)
Figure 1: Rectangular waveguide: (a) configuration of whole structure with periodic transverse slots; (b) equivalent circuits for a section ofclosed rectangular waveguide; (c) equivalent circuits for one period of leaky rectangular waveguide.
2 International Journal of Antennas and Propagation
B
A
b
y
x
P
a
Interdigital slot
L
(a)
slot_wW_y
Λx
V_y_bV_x
V_y
Lxb W_x
Lyb
x
y
o
(b)
CL
LR
CR LL
(c)
Figure 2: +e proposed periodic leaky rectangular waveguide: (a) perspective view of the whole structure; (b) structure and parameters ofthe unit cell; (c) equivalent circuit of the unit cell.
Freq
uenc
y (G
Hz)
Airlineβ
6.0
6.4
6.8
7.2
7.6
8.0
8.4
50 100 150 200 2500β
Figure 3: Dispersion curves for the proposed CRLH unit cell.
International Journal of Antennas and Propagation 3
that, by changing the frequency from 7.0 to 7.6GHz, thebeam scans from 5° to 18°, with an isotropic gain from15.2 dBi to 14.4 dBi and cross polarization below −35 dB. InFigure 4(c), it shows that when the LWG works at 6.97GHz,the beam points to the broadside and the realized gain canreach to 15.7 dBi with the cross polarization below −50 dBi.
At the same time, the simulated radiation patterns forseven operating frequencies of the LWG show that the half-power beamwidths (HPBWs) of the backward and forwardbeams are wider than this of the broadside beam. For ex-ample, the HPBW is 14° for 6.1GHz and is 10.9° for6.97GHz.
Figure 5 shows the simulated S-parameters as functionsof the frequency. It is seen that the S11 fluctuates around−10 dB in the LH region except for the transition frequency(6.97GHz) and keeps below −10 dB in the RH region. At thesame time, the values of S21 increase in the LH region andbecome almost stable in the RH region. As a result, theradiation efficiency decreases in the LH region and increases
f = 6.1GHzf = 6.4GHzf = 6.6GHz
f = 6.1GHzf = 6.4GHzf = 6.6GHz
Co-polX-pol
–30–25–20–15–10
–505
101520
Abs
olut
e gai
n (d
Bi)
–60 –30 0 30 60 90–90θ (deg)
–65
–60
–55
–50
–45
–40
–35
–30
–25
–20
Abs
olut
e gai
n (d
Bi)
(a)
f = 6.97GHzf = 6.97GHz
Co-polX-pol
–30–25–20–15–10
–505
101520
Abs
olut
e gai
n (d
Bi)
–60 –30 0 30 60 90–90θ (deg)
–60
–40
–20
0
20
40
60
Abs
olut
e gai
n (d
Bi)
(b)
f = 7.0GHzf = 7.2GHzf = 7.6GHz
f = 7.0GHzf = 7.2GHzf = 7.6GHz
Co-polX-pol
–30–25–20–15–10
–505
101520
Abs
olut
e gai
n (d
Bi)
–80
–60
–40
–20
0
20
40
60
80
Abs
olut
e gai
n (d
Bi)
–60 –30 0 30 60 90–90θ (deg)
(c)
Figure 4: Simulated radiation patterns in the H-plane: (a) LH region; (b) broadside; (c) RH region.
Figure 5: Scattering parameters for the proposed CRLH LWG.
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in the RH region. Also, it could be figured out that theradiation efficiency is around 70%within the whole scanningrange.
4. Conclusion
In this paper, a periodic leaky-wave antenna based on therectangular waveguide is proposed to radiate a broadsidebeam for radio wave coverage in the confined space. +einterdigital slots and metal vias are designed to introduce theleft-handed transmission line properties, and the radiatingmechanism is investigated as well. +e LWG can radiate abroadside beam at 6.97GHz, with relatively high gain(15.7 dBi) and narrow half-power beamwidth (10.9°). Per-formance degradation in the scattering parameters is ob-served around the transition frequency. Furtherimprovement may be required to introduce the transitionfeeding structure. Besides, it is not easy to fabricate thecomposite slots accurately on the metal surface. +us, asimilar CRLH LWG based on the dielectric waveguide willbe further studied to replace the metal structure. However,the periodic CRLH LWG based on the rectangular wave-guide in this article shows a low cross polarization (−50 dBi)and relatively high gain (15.7 dBi) compared to those in theprevious work (−10 dBi and 12.5 dBi, respectively) [11],where the LWG is 20mm longer than that in this paper.
Data Availability
+e data used to support the findings of this study are in-cluded within the article.
Conflicts of Interest
+e authors declare that there are no conflicts of interestregarding the publication of this paper.
Acknowledgments
+is work was supported in part by the National NaturalScience Foundation of China (Grant no. 61771038), in partby the National Science Research Project of Department ofEducation in Guizhou Province (Grant nos. KY[2019]125,KY[2019]144, KY[2020]112, and KY[2020]062), in part bythe High-level Talent Starting Foundation of LiupanshuiNormal University (Grant no. LPSSYKYJJ201802), and inpart by the Foundation of LiuPanShui Technology Council(Grant no. 52020-2018-04-04).
References
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