介质谐振器天线

Compact wideband multi-layer cylindrical dielectric resonator antennas
W.Huang and A.A.Kishk
Abstract:Homogenous dielectric resonator antennas(DRAs)have been studied widely and their
bandwidth have been reached to the possible upper limit.A new non-homogenous DRA,multi-
layer cylindrical DRA(MCDRA),is designed and fabricated to achieve wider bandwidth.The
antennas consist of three different dielectric discs,one on top of the other.Two different excitation
mechanisms are studied here.As much as66%of impedance bandwidth with a broadside radiation
pattern has been demonstrated using a50V coaxial probe placed off the antenna axis.More than
32%of impedance with a broadside radiation pattern has been achieved when the antenna is excited
by an aperture coupled50V microstrip feedline.Mode analysis is carried out to investigate the
natural resonance behaviours of the MCDRA structure.
1Introduction
The dielectric resonator(DR)was used as an energy storage device rather than a radiator in microwave circuits for many years[1].In1983,Long et al.[2]introduced it as an antenna,which is able to offer the advantages of compact size,low Ohmic losses and wider matching bandwidth over the microstrip antenna.The dielectric resonator antenna(DRA)is also simple to fabricate and easy to feed by different coupling mechanisms,such as coaxial probe,microstrip line coupled aperture,slotline,stripline and so on.Moreover,compared with the microstrip antenna,no surface wave losses are suffered because the DRA element is directly placed on the ground plane. However,because of the high dielectric constant and the high Q-factor,it has a limited impedance bandwidth of operation.At the early stage of development,simple shapes of the DRAs,such as a hemispherical DRA[3],a cylindrical DRA[4]and a rectangular DRA[5],were con-sidered.A bandwidth ranging from5to10%was achieved. Later,with improved knowledge of the antenna operation and the numerical tools,enhancements of the bandwidth were achieved using other shapes,such as truncated tetrahe-dron shape[6],split cone shape[7]and half-hemispherical shape DRAs[8].Although the bandwidth of the homo-geneous DRAs was improved to its possible upper limit,a much wider bandwidth was achieved by stacking two differ-ent DRAs[9,10],loading a high permittivity,low-profile dielectric disc on top of a conventional homogeneous DRA in[11]and plugging an inner core into the lower stacked part[12].In addition,in[13],multisegment DRAs are developed to enhance its coupling to a microstrip line by inserting one or more thin segments of different per-mittivity substrates under a DRA of low permittivity. Here,a wideband multi-layer cylindrical DRA (MCDRA)is designed and fabricated by simply placing three different dielectric discs of the same diameter,one on top of the other,as shown in Fig.1.Three dielectric discs are made of standard available dielectric substrate materials in our laboratory:Rogers RT/Duroid6010 (1r¼10.2)with thickness2.5mm,Polyflon POLYGUIDE (1r¼2.32)with thickness 3.35mm and Rogers RT/ Duroid6006(1r¼6.15)with thickness 2.5mm.The shape of the MCDRA can be considered as not physically deformed but electrically deformed because of the different dielectric constant of each disc.Therefore compared with the equivalent homogenous DRA,the MCDRA supports several broadside radiating modes with close resonant fre-quencies,which provide wider bandwidths.Also,the MCDRA resides on a ground plane,which does not support surface waves as multisegment DRAs do,so it will not suffer the surface wave losses.The fabrication is also simple since the thickness of each disc is the same as the materials available in market.
In Section2,MCDRAs with different stack order are per-formed numerically in order tofind the optimal order.A coaxial-probe-fed MCDRA geometry with optimal order is described for both simulation and measurements cases. Also,the measured reflection coefficients and radiation pat-terns are verified with the simulated results.In Section3,an aperture-coupled microstrip-line-fed MCDRA is described and the measured voltage standing wave ratio(VSWR)is verified experimentally.The simulated radiation patterns are also demonstrated.In Section4,mode analyses are dis-cussed to explain the natural resonance behaviour of the MCDRA.In the last section,conclusions are provided.
2Coaxial probe excitation
2.1Antenna geometry and fabrication
The geometry of the probe-excited MCDRA is shown in Fig.1.The antenna with diameter(D1)of14mm resides on afinite square ground plane with side length(D2)of 80mm,which is large enough to assure negligible edge effect on the input impedance.A50V coaxial probe is used to excite the DRA.The probe is located(A)3.7mm off the centre with the length(B)5.845mm and radius 0.3mm.The antenna is simulated using the frequency domain commercial software WIPL-D[14],which is
#The Institution of Engineering and Technology2007
doi:10.1049/iet-map:20070028
Paperfirst received7th February and in revised form24th June2007
The authors are with the Department of Electrical Engineering,University of Mississippi,Oxford,MS,USA38677
E-mail:whuang1@
based on the surface integral equations and method of moments.In order to provide some practical insight into the design of the MCDRA,different stack orders for the dielectric discs with the same dimensions are performed numerically.The calculated bandwidths are shown in Table 1.It was found that the optimal arrangement for the three dielectric discs from top to bottom are:(1r1)10.2,(1r2)2.32,(1r3)6.15,and loss tangent (tan d 1)0.0023,(tan d 2)0.0002,(tan d 3)0.0019.The corresponding thicknesses of the discs are (H 1) 2.5mm,(H 2) 3.35mm and (H 3)2.5mm,respectively.
In the experimental model,as shown in Fig.1b ,a 50-V microstrip feedline is used to connect the probe and submi-niature A (SMA)connector.This requires milling a circular hole on the ground plane side in order to insert the probe through it,as well as connect with the microstrip feedline.The MCDRA is built using the materials as those given pre-viously.The 80mm Â80mm ground plane is built using Rogers RO3010(1r ¼10.2).Fig.2shows photographs of the fabricated probe-excited MCDRA and three dielectric discs individually.2.2
Simulation and measurement results
The simulated VSWRs of the MCDRA are compared with the measured results and simulated equivalent homo-geneous DRA results in Fig.3.Here,the equivalent homo-geneous DRA has the same height and radius as the MCDRA with equivalent permittivity (i.e. 3.2).The matching frequency range of the simulated equivalent homogeneous DRA is from 8.4to 11.5GHz (VSWR ,2),corresponding to a bandwidth of 31.16%.The matching frequency range of the simulated MCDRA is from 6.9to 13.7GHz (VSWR ,2),corresponding to a
Table 1:Calculated bandwidth of different dielectric discs arrangements
1r of top layer 1r of middle layer 1r of bottom layer BW,%
10.2 2.32 6.156610.2 6.15 2.3212.062.3210.2 6.1517.542.32 6.1510.227.46.1510.2 2.3214.056.15
2.32
10.2
13.74Fig.2Photo of the probe-excited MCDRA
a Top view
b Three individual
layers
Fig.1Geometry of the probe-excited MCDRA with finite ground plane
a Simulated
b Fabricated
bandwidth of 66%.The matching frequency range of the measured result is from 6.95to 12.3GHz (VSWR ,2),corresponding to a bandwidth of 60%.Both the simulated and measured MCDRAs have a much wider bandwidth than the homogeneous equivalent DRA.Good agreement is achieved between the simulated and measured MCDRA results.The slightly smaller measured bandwidth compared with the simulated bandwidth may be because of the induc-tance effect of the probe inside of the substrate below the ground plane and the fringing capacitance effect at the end of the microstrip line.To determine the effect of probe feed location (A ),a parametric study of the dis-tance between probe location and the center of MCDRA is shown at Fig.4.It can be found that the location of the probe impacts the antenna’s impedance matching,but has little effect on the resonant frequency.Moreover,the effects of probe length (B )are studied in Fig.5.As expected,longer probe have lower resonant frequencies.In addition,the antenna matching changes,but this can be tuned by adjusting other antenna parameters.
Fig.6provides the simulated and measured co-polarised and cross-polarised radiation patterns of the MCDRA in the E-plane and H-plane at 7,8and 9.5GHz,respectively.The measured co-polarisation E-plane patterns are very similar to the simulated one,but the measured cross-polarisation patterns in the E-plane have higher levels than the simulated
cross-polarisation patterns.This could be related to the con-nector effect and the rigid cables used to mount the antenna on the tower of the anechoic chamber.The backside micro-strip feedline also causes the measured H-plane co-polarisation patterns to have higher back-radiation than the simulated ones,and the measured H-plane cross-polarisation patterns to have some degradation.The com-puted results show good radiation patterns up to 10.5GHz.Some higher-order modes with non-broadside radiation patterns are excited and disturb the radiation pattern axial symmetry when the frequency is higher than 10.5GHz.The calculated peak gain is 5.76dBi at 8.8GHz.3Aperture-coupled excitation 3.1
Antenna geometry and fabrication
In addition to probe excitation,a microstrip-line-coupled aperture can also feed the MCDRA.The geometry of the aperture-coupled MCDRA is shown in Fig.7.The three dielectric discs have the same permittivity and dimensions as in Section 2.The antenna resides on a finite ground plane with dimension 90mm Â120mm.The slot width is (W st )0.6mm,which is much less than l g /20,and its length (L sl )10mm is experimentally adjusted to provide optimum coupling.The appropriate microstrip feedline width (W m )1.72mm was chosen to obtain a 50-V trans-mission line at the design frequency.The open circuit stub is used to improve the impedance matching.The stub length (L st )5.72mm was set to approximately one-quarter of a guide wavelength and is located (L offset )3.45mm off the centre of the slot.The three dielectric discs were glued to each other and to the ground plane over the slot using 3M Spray Mount Artist’s Adhesive.Since the glue is spray-type,its volume is negligible.Also,because the proposed antenna is a wideband antenna,the effect of the glue is not noticeable.The antenna is simulated using the Ansoft high-frequency structure simulator (HFSS)[15],which is based on the finite element method.Fig.8shows the photograph of a fabricated aperture-coupled MCDRA.3.2
Simulation and measurement results
Fig.9shows the simulated and measured VSWR curves.The frequency bandwidth of the simulated
aperture-coupled
Fig.3Comparison of measured and simulated VSWR of probe-excited
MCDRA
Fig.4Magnitude of the reflection coefficients as function of the distance of probe location from the centre of
MCDRA
Fig.5Magnitude of the reflection coefficients as function of the probe length
MCDRA for a VSWR ¼2criteria is from 6.2to 8.5GHz,corresponding to a bandwidth of 31.3%.The bandwidth of the measured result for a VSWR ¼2criterion is from 6.15to 8.5GHz,corresponding to a bandwidth of 32.3%.The calculated and measured values are in good agreement.
It should be noted that there are some fabrication tolerances,such as probe length and probe locations as shown in the parametric study part,affected the impedance matching.Also from the experiment,it has been found that,unlike aperture-coupled microstrip patch antenna,
precise
Fig.6Measured and simulated radiation patterns (10dB /div)of E /H-plane co-polar and cross-polar field of the probe-excited MCDRA
a f ¼7.0GHz
b f ¼7.0GHz
c f ¼8.0GHz
d f ¼8.0GHz
e
f ¼9.5GHz f
f ¼9.5GHz
positioning of the MCDRA antenna over the slot is not required,which means that good coupling can be attained without the requirement for precise aligning the antenna over the slot.
Fig.10provides both the simulated and measured co-polarised and cross-polarised radiation patterns of aperture-coupled MCDRA in the E-plane and H-plane at 6,7and 8GHz,respectively.The cross-polarisation is much lower than the probe-fed case because of the suppres-sion of the zero-order mode.The calculated peak gain is 5.5dBi at 8GHz.4
Mode analysis
To investigate the natural resonance behaviours of the MCDRA structure,mode analyses are carried out using Ansoft HFSS eigenmode solver,which consists of three dielectric discs,one on top of the other.The
dimensions
Fig.7Geometry of the aperture-coupled MCDRA with finite ground
plane
Fig.8Photo of the aperture-coupled MCDRA
a Top view
b Bottom
view
Fig.9Comparison of the measured and simulated VSWR of the aperture-coupled MCDRA
and materials are the same as those previous given.In HFSS,the eigenmode solver does not support radiation boundaries or perfect matched layer (PML)boundary because it is difficult to find the resonances for low
Q -factor cases.However,eigensolutions can be obtained when the whole model is enclosed in a closed conducting cavity [15].Since the cavity is about two to three times the size of the DRA resonator [1],the resonant frequencies
of
Fig.10Measured and simulated radiation pattern (10dB /div)of E /H-plane co-polar and cross-polar field of the aperture-coupled MCDRA
a f ¼6GHz
b f ¼6GHz
c f ¼7GHz
d f ¼7GHz
e
f ¼8GHz f
f ¼8GHz
the resonator are close to the isolated case.Therefore the cavity modes are able to distinguish from the DR modes. It should also be noted that eigenmode is the nature mode in the structure and is independent of source.The resonant frequencies of the natural modes and full complex vectorfield can be obtained from the output results of HFSS.Table2illustrates thefirst12 eigensolutions obtained starting at0.1GHz.Thefirst resonance appears around7.14GHz.These modes could be excited by proper sources at the proper locations with different strengths.However,the zeroth-order mode can be coupled by the probe but not by the centred slot. Based on our experiences,more excited modes could increase the antenna matching bandwidth,but the radiation bandwidth could be deteriorated by the presence of the high-order modes.The following mode analyses are pre-sented to explain such performance.Fig.11illustrates the vectorfield distributions of TE-,TM-,HEM-and cavity-type modes existing among thefirst12eigensolutions,Table2:First12eigensolutions obtained starting at 0.1GHz
Eigenmode Frequency,GHz Type
17.14HEM
28.39HEM
38.63TM
48.65TE
59.96cavity
610.15HEM
711.07TM
811.42TE
911.49TM
1012.13cavity
1112.79cavity
1212.91
TE
Fig.11Vectorfield distributions
a,b TE-type at8.65GHz
c,d TM-type at11.07GHz
e,f HEM-type at7.14GHz
g,h Cavity-type at9.96GHz
where Figs.11a,c,e and g are x–y-plane view and Figs.11b,d,f and h are x–z-plane view.From the vector field distributions shown in Fig.11,we can learn which mode could be coupled to probe or slot.For example,the TE-type mode at8.65GHz shown in Figs.11a and b could not coupled to the probe,but to a non-centred narrow slot.The TM-type mode at11.07GHz shown at Figs.11c and d can be coupled to a probe and not to a slot.Based on the vectorfields of all the12modes,we con-clude that more modes can be coupled by coaxial probe than slot,which explains the wider bandwidth due to the probe excitation rather than aperture-coupled excitation. However,only4of the12modes’vectorfields are demon-strated for the sake of brevity.It was also presented to demonstrate that the natural modes existing in the MCDRA structure are more complicated than a simple disc resonator case.For example,for a simple disc resonator case,the ground plane usually suppresses the TE modes. But for the MCDRA case,some TE modes could be sup-ported with the presence of the ground plane,because their electricfield has z-variation which allows the electric field normal to the ground plane also with zero tangential components at the boundary of the ground plane,as shown in Fig.11d.
5Conclusion
A compact wideband MCDRA was designed and fabri-cated.Around60%of bandwidth was achieved using coaxial probe feeding.The simulated and measured results are in good pared with other wide-band DRAs,the MCDRA has much wider bandwidth, surface wave losses above the ground plane and relatively simpler and easier fabrication.Another excitation mechan-ism–aperture-coupled microstrip feedline–was used and more than30%bandwidth was achieved.Simulated and measured results also have good agreement.Mode analysis showed the resonance behaviour of the MCDRA structure and explained that wider bandwidth can be sup-ported by the probe-excited MCDRA than aperture-coupled MCDRA.6Acknowledgment
This work was supported by the National Science Foundation under Grant No.ECS-524293.
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