Recent Advances in Dielectric-Resonator Antenna Technology

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Recent Advances in Dielectric-Resonator AntennaTechnologyA . Petosa, A . Ittipibo n, Y. M. MAntar, D . Roscoe,and M. Cuhaci PCommun ications Researdh Centre3701 Carling Ave. ~PO Box 11490, StationHOttawa, ONCanada K2 H 8S2 ~Tel: (613) 991-9352E-mail: aldo.petosa@crc. oc.caFax: (613) 990-8360 IRoyal Military College0 CanadaKingston ON ~Canada K7K 5LO

Keywords: D ielectric loa jed antennas, dielectric resonatorantennas, antenna arrays ~~

1. Abstract ~This paper features Lome of the recent advance s in dielectric-resonator antenna techn4logy at the Communications Research

Centre. Several novel el+ents are presented that offer significantenhancements to parameders such as imp edance bandwidth, circu-lar-polarization bandwidbh, gain, or coupling to various feedstructure s. Several linear1 and pla nar ar rays are also prese nted, toof dielectric-resonator antenna elements

2. Introduction Iiver the past seven y ars, the Comm unications Research Cen-College (RMC) of Canad , has been pursuing a program to inves-tigate the capabilities of ielectric-resonator antenna (DRA) tech-nology as an alternative tI ore traditional antennas. Much of theinitial work focused on cqaracterizing the basic properties of D RAsfor a variety of simple shhpes and feed configurations, to illustrate0 re (CRC), in clos, collaboration with the Royal Military

men tation, to demonstrate/ the fea sibility of usingDRAs in an arrayenvironment. This paper ummarizes recent work carried out inthis program, focusing oI he novel DRA configurations and thevarious arrays that have bben developed.

3. W hy DRAs? ~asked questions by those first hearingthey? and What do they offer thats

new? This section is intended to answer these questions by illus-trating the salient features of DRAs, and bringing to light some oftheir advantages.A DRA is a resonant antenna, fabricated from low-lossmicrowave dielectric material the resonant frequency of which ispredominantly a function of size, shape, and material permittivity.The basic rectangular DRA-fed by slot coupling, in this case-is

shown in Figure 1. The impeda nce bandwidth is a function of thematerials permittivity and aspect ratio. For this particular DRA, a10 dB return-loss bandwidth of 6% is obtained (Figure 2a). Band-widths of up to 10%can be easily achieved with simp le rectangularDRAs, with relative permittivity values of 10 or less. The rectan-

IMicrostrip Feed Line DRA(& =17.6)Grounded Substrate I

Microstrip Feed LineFigure 1. A basic rectangular DR A fed by a slot-coupledmicrostrip line.

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several feeding mechanisms can be used (probes, slots, microstriplines, dielectric image guides, co-planar lines), making DRAsamenable to integration with various existing technologies;

-1 0ha'S - 1 5CACA

3 . - 2 0U -25

-302-35

9.0 9.5 10.0 10.5 11.0 11.5 12.0Frequency (GHz)

Figure 2a. The return loss of the rectangular DRA.

n92N

0-5-10-1 5-20-25-30-35-40

Figure 2b. The normalized H-plane radiation pattern of therectangular DRA at 10.5 GHz.

gular DRA radiates like a short horizontal magnetic dipole. Thenormalized H-plane pattem, at 10.5 GHz, is shown in Figure 2b.The E-plane pattem is, in theory, uniform, but in practice it isstrongly influenced by the size and shape of the ground plane onwhich the DR A is m ounted. Although the first reported investiga-tion of DRA s dea lt with a linear array [12], most of the initialresearch focused on the c haracterization of the p erformance ofindividual elements of vanous common shapes [13-50]. Thisresearch has dem onstrated that DRAs offer several attractive fea-tures, including:

high radiation efficiency (>95%), due to the absence of con-ductor or surface-w ave losses;various sha pes of resonators can be used (rectangular, cylindrical,hemispherical, etc.), allowing for flexibility in design;

various modes can be excited, producing broadside or conical-shaped radiation patterns for different coverage requirements;a wide range of perm ittivity values can be used (fkom about6 toloo), allowing the designer to have control over size and band-

width (i.e., wide bandw idth is achievable using low permittivity,and com pact size is achievab le with high permittivity);DRAs are not as susceptible to tolerance errors as microstrip

antennas, especially at higher frequencies.These features make DRAs very versatile elements, which can beadapted to num erous applications by appropriate choice of thedesign parameters. Also, as will be shown, many of the techniqu esused for enha ncing microstrip-antenna performance are equallyapplicable to DRAs. A good ove rview of the early work on DRAsis given in [51].

4. Advances in DRA technologyThis section features some of the latest developm ents in DRAtechnology achieved at the CRC. The research has been dividedinto two categories: novel DRA elements and array configurations.

4.1 Novel DRA elementsThe research carried out on novel DRAs can be categorizedinto the following groups:

9 wide-band;compact;circular polarized;high gain;active.

This section presents some of the research in each of these fivecategories.

4.1.1 Wide-band DRA sFor many of the existing and emerging commu nication appli-cations, wide-band antenna operation is desirable to acco mmo datethe increasing data rates required for services such as video-conferencing, direct digital broadcast, EHF portable satellite com-munications, local multi-point comm unications, and indoor wire-less. Some of these requireme nts may be m et by existing pnnted-antenna technology, but with the added co st and comp lexity asso-ciated with multi-layer configurations required for achieving broadbandwidths. This section presents some novel DRAs of relativelysimple design, which have de monstrated wide-band perform ance,

and may serve as suitable antenna candidates for these variousapplications.

The notch DRA. Simple rectangular DRAs of low permit-tivity can offer impedance bandwidths of about 10%. For widerbandwidths, a notched rectangularDRA (as shown in Figure 3) hasbeen rep orted (patent pending), offering bandwidths of up to28%36 IEEE Antennas and Propaga tion Magazine, Vol. 40, o. , June 1998


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The multi-segment RA. or integration with printed tech-nology, direct coupling bek en DRA s to microstrip lines is desir-able. In general, to achieve strong coupling, the DRA m ust be fab-ricated from high-permittiJ ity materials. However, to operate over

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h82E -10.s -20Y

a" -15

-25a: -30

Figure 3a. A gotched rectangular DRA.

Slot Aperture/ Microstrio

Centre PortionRemoved (Notch)

Figure 3b . A schematic iagram of the notched rectangularDR A shown in Figure 3a.

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2-15

-20 1 1 12 1 3 14 1 5 1 6requency (GHz)Figure 4a. The return lois of the notched rectangular DRA.

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Figure 4b. The normalized H-pane radiation patterns of th enotched rectangular DR A (Ll = 10 mm and L, = 5 mm).

Figure 5a. A multi-segment DRA.

Dielectric

PermittivitiesMicrostrip Feed Line

Figure 5b. A schematic diagram of the multi-segment DR A inFigure 5a.

a wide bandwidth, the DRA must have a low dielectric constant.To resolve this conflicting requirement, the multi-segmentDRA(MSDRA), shown in Figure 5 , has been reported (patent pending)[55, 61. It consists of a rectangular DRA of relatively low permit-tivity, under which one or more thin segments of higher permittiv-ity are inserted. These inse rts serve to transform the impedance ofthe DRA to that of the microstrip line by concentrating the fieldsundeme ath the DRA , and thus significantly improving the cou-pling performance. In a practical antenna system, the number ofinserts should be minim ized, to reduce the com plexity of the fabri-cation process and ultimately the cost. Research has thus focusedon developing an MSD RA with a single insert.Figure 6depicts thereturn loss of an MSD RA, comp ared to the simple DRA. Couplingis significantly enhanced by the insert, and the MSDRA achievesbandwidths of up to 20%. Th e MSDRA is amenab le to integration

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with printed technology, and is being used as a wide-band arrayelement in a large low-profile array (Section 4.2). A similar con-figuration, using cylindricalDRAs, was reported in [57].

Parasitic DRAs. Wide bandw idth can also be achieved withparasitic DRAs, using a similar technique as with microstrippatches. Figure 7 shows a slot-fed DRA with two parasitic ele-ments. The three DRAs are tuned to different frequencies, and thecombined retum-loss performance is shown in Figure 8. The indi-vidual resonators have bandwidths of up to 5.8%, but when com-bined, the three-element antenna exhibits a 17% bandwidth for a10 dB return loss. This co nfiguration remains quite compac t,requiring a single feed with no matching network, and has the

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-30-35-40

5 6 7 8 9 10Frequency (GHz)Figure 6. The return loss of an MSDRA compared to that ofthe simple rectangular DRA.

G r o u n d e dS u b s t r a t e

M ic ro s t r i pFeed Line\

Slot

0

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ha -1 02.i;e0

-15u2

-20

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-304 4.5 5 5 .5 6 6.5 7

Frequency (GHz)Figure 8. The return loss of the slot-fed DRA w ith parasitics.

I E-Field L ines

I \ round P laneFigure 9a. A front view of a short-circuit rectangular DRA.

I

DRAs

Figure 9b. A side view of the short-circuit rectangular DRA inFigure 9a .

Figure 7a. A top view of a slot-fed DRA with tw o parasitic ele-ments. advantage that each DR A can be individually tuned for eitherwide-band or multiple-frequency-b and operation [58, 591.

4.1.2 Compact DRAs

of the slot-fed DRA in Figure 7a./ A nte nna s and Propagation Magazine,Vol. 40, o. 3, June 1998

Since the volume of the DR A increases by a factor of eighteach time the frequency is halved, the use of DRAs at lower fre-quencies becomes qu estionable, due to the increase in their dimen-sions (and thus their weight and cost). The sizeof th e DRAs can besignificantly reduced by fabricating them from materials with very


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high permittivity [IO]. Tde disadvantage of this approach is theaccompanying decrease in bandwidth. An altemative methodinvolves the introduction f a hort circuit, as shown in Figure9.By placing the short circ it at a location of symmetry in the E-an be removed, wh ile still maintain-ation. Alternatively, placing a shortxisting D U ill result in a lowerings approach has been investigated for

and with decreases of as much11. Althoug h, therebandwidth can bea compact antenna

eof the resonant frequeprobe-fed rectangularas 65% of the originalis an accompanying dincreased by using anwith moderate bandwi

4.1.3 DRAs for circular pblarizationrequired in applications such aswhere depolarization due to

I Slot Aperture/

"9'L/Slot Feed b , A '\ GroundedSubstrateI Microstrip Line\

ating circular polarization: A quasi-a slot aperture or a pro be.

50

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Figure l l a . The norm alized radiation pattern of the cross DRAat 11.2 GHz.

4r-

1.5 1

0.510.8 1 1 11.2 11.4 11.6 11.8Frequency (GHz)

Figure ll b . The axial-ratio bandwidth of the crossDRA.propagation effects preclude the use of linearly polarized systems.Unless an tennas are used that are inherently circular polarized (Le.,helices, spirals), there is added comp lexity in the design required toproduce CP radiation. In general, a two-point feed is used, wherethe feed points are spatially 90" apart, and are fed with equal-amplitude signals in phase quadrature. The required power-divid-ing circuit takes up additional real estate, and increases the inser-tion loss (thus decreasing radiation efficiency). This added com-plexity can be avoided by adopting a single-point feed system,which, in the ca se of microstrip-patch antennas, involves designinga patch with a perturbation to excite dual-orthogonal-mode opera-tion. The disadvantage of the single-point-fed microstrip configu-rations is that they usually produce narrow CP pattern bandwidths(1 - 2% for 3 dB axial ratios) [62]. For DRAs, on the other hand,single-point-fed configurations have been designed with up to 7%CP bandwidth [63-661.

Figure 10 shows two configurations: a quasi-square DRA,and a cross-shaped DRA. Both elements generate similar CPIEEE Antennas and Prop,/igation M agazine, Vol. 40, No. , June 1998 39


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Cavity

Slot Feed

Figure 12a. A DRA-fed cavity element.

Cross DR A\

Microstrip LineFigure 12b. A schematic diagram (top and side views) of theDRA -fed cavity element in Figure 12a.

the cross and quasi-square DRAs have been demonstrated to offerwide-band, wide-beam CP performance, which is difficult (if notimpossible) to achieve with a single-point-fed, single-layer micro-strip patch.

4.1.4 High-gain DRAsAt higher frequencies, conductor and surface-wave losses

increase significantly for printed technology. In a large planararray, the majority of losses w ill occur in the printed feed-distribu-tion network. These losses could be reduced if fewer elementswere required in the array. This can be achieved for certain fixed-

90- 1 003E

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Figure 13. The normalized radiation pattern of the cross-DRAcavity element at 20GHz.

Coupling Aperture\

i.IMicrostrip Feed Line FRA

Figure 14a. A slot-fed rectangular FRA (top view).Applied Magnetic Bias Fieldsradiation pattems by exciting two spatially orthogonal TE,

modes (which radiate like short horizontal magnetic dipoles) inphase quadrature. Figure 11 shows the radiation pattem and bore-sight axial ratio versus frequency of a cross DRA, designed at Xband. The pattern exhibits a 100" beamwidth over which the axialratio is less than 3 dl3.For wider CP bandwidths, a dual-point-fedring DRA has been designed, with about a 12 % CP bandwidth[67]. Also, a compact 2 x 2 array has been designed with the crossDRAs fed using sequential rotation, to achieve wideband CP per-formance (17% for 3 dB axial ratio) [68]. Cross DRAs have alsobeen designed to operate at frequencies of up to 30 GH z [69].Both40

PermanentMicrostrip Feed Line Magnet

Figure 14b. A slot-fed rectangular FRA (side-view cross sec-tion).IEEE Antennas and Propag ation Magazine, Vol. 40 , No. 3, June 1998


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Yt?r3k Za, 10.5

2 ' 3 1 0s

9.5

95 1 o 1.5pplied Magnetic Bias

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Figure 15. Th e freque nc shift versus applied magnetic bias ofthe FRA.

Microstrip Line I

Irounded Substrate ICoaxial Launcher

DR A

Substrate ( ~ r )Figure 17b . A side-view schem atic of the multi-layer bran ch-l ine l inear arr ay o f MSDRAs in F igure 17a.

Multi-Segment DRAs

Microstrir, Branch-Line FeedFigure 17c. A top-view sc hematic of the multi-layer branch-linel inear arra y o f MSDRAs in F igure 17a.

20 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 '

1 0

0

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-2 0

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t:; I I I 1 1 , I , I , I , I , I , I , I , I I I I I I I I I , 1 , I , i-40-90 -60 -30 0 30 60 9 0

Angle (Degrees)F igure 18 . The rad iat ion pat tern of the b ranch- l ine MSDRAarray .

Figure 16. A l inear D&4 arr ay fed by a microstrip line.

Figure 17a. A multi-I yer bran ch-line linear arr ay ofMSDRAs.F igure 19 . A seven-element array of cross-DRA cavity ele-ments, designed at 30 GHz.

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beam or limited-scan applications by using high-gain elements.ADRA-fed cavity element, shown in Figure 12, has been developedfor high-gain operation at K and Ka bands [69, 701 (patent pend-ing). The antenna consists of a circular cavity machined into ametal block, and is fed by a DRA located at the bottom center ofthe cavity. The DRA is, in turn, fed by slot coup ling from a micro-strip line, located under the cavity. A dielectric cover is placedover the cavity, to provide impedance matching to free space.Inaddition to producing high gain, the cavity was designed for inte-gration with powe r amp lifiers that are attached directly to the metalblock, which serves as an excellent heat sink for the amplifiers[71]. The gain of this element is a function of cavity diameter.Using the cross DRA fo r CP operation, a gain above 13 dB,, hasbeen measured at K band for cavity diameters of two wavelengths.A typical pattern is shown in Figure 13. The c ross-DRA-fed cavityis being used in an element for a reflector feed array [88].

4.1.5 Active DRAsSome of the properties ofDRAs can be actively controlled by

using low-loss ferrite materials. When unbiased, these ferrite-reso-nator antennas (FRAs) exhibit similar behavior to DRAs. How-ever, when a dc magnetic bias is applied, the tensor nature of theferrite permeab ility is invoked, and various parameters can be con-trolled electronically. FRAs hav e been designed that exhibit activefrequency tuning a nd polarization agility[72-751.Figure 14 showsa slot-fed rectangular FRA, designed to operate at 10 GHz in itsunbiased state. When a dc magnetic bias is applied, the resonantfrequency of the FRA will shift either up or down, depending onthe direction of the bias field. Measured results are plotted in Fig-ure 15. Frequency shifts of k8% were obtained for this FRA, butmuch wider shifts are possible by u sing ferrite material with highersaturation magnetization. Permanent magne ts were used in the labenvironment to demonstrate this ability, but in practical applica-tions, electromagnets would be more suitable. FRAs have alsobeen d esigned with polarization agility [74].By making use of thetensor nature of the ferrite permeability, the polarization of a cir-cular-disk FRA can be magnetically switched from linear to CP .This may prove useful in applications that would benefit frompolarization diversity.

5. Linear and planar arraysMuch of the work reported on DRAs has focused on the

characterization of single elements. A significant effort has beenundertaken at the CRC to investigate the performance of DRAs inan array environmen t. Numerou s linear arrays have been develop edand, presently, large low -profile two-dimensional arrays are beingdesigned. Some of the research activities are highlighted in thissection

5.1 Linear arraysSeveral linear arrays of DRAs have been investigated,

including probe-fed DRAs with parasitic elements at L band [61],dielectric image-guide-fed DRAs at K band [76], slot-fed arrays atQ band (40 GHz) [77], and several microstrip-line-fed arrays [78-831. Microstrip transmission lines offer a sim ple, practical methodfor feeding linear arrays of DRAs. A typical array is shown in Fig-ure 16. This configuration is a series-fed array, with the DRAsspaced a guided wavelength apart for in-phase excitation. Theposition of the DRAs with respect to the microstrip line is a

parameter that can be varied to adjust the amount of couplingbetween the line and the DRA. Arrays have been successfullydesigned with 20 dB Taylor amplitude distributions, and withbroadband impedance characteristics [79, 821.

There are two disadvantages of the series-fed linear array ofDRAs. The first is the scanning of the main beam with frequency(common to all series-fed arrays), which precludes the use of thisarray in wide-band fixed-beam applications. The second is that, ingeneral, only a small amount of co upling is achievable between themicrostrip line and the DRAs. Thus, to make an efficient array,many DRAs are required to maximize radiated power. To over-come these disadvantages, a multi-layer microstrip-branch-linearray has been developed, as shown in Figure 17 [84]. The arrayconsists of a microstrip branch line, fed in the center by a slot-cou-pled microstrip line, located on a second substrate. MSDRAs areplaced at the ends o f the branches, instead of simple DRAs. This isdone since, as shown previously, they have significantly more cou-pling, and thus higher radiation efficiency can be achieved usingonly a few elements. To avoid beam squint with frequency, themicrostrip branch is center fed. A multi-layer approach wasadopted, to allow for the integration of active devices in a largearray of parallel branches (de scribed in Section 4.2). By using thesecond layer, more area is made available for mounting any activedevices, and good isolation is provided to prevent any spuriousradiation of the devices from interfering with the antenna pattem.The impedance s of the various branches can be design ed to providethe desired amplitude distribution to the elemen ts. The path leng thsof the various branches were chosen to provide equal phase to eachelement at the design frequency. The branch-line array is a com-pact structure, which takes up the same amount of area as an end-fed series array, making it amen able to integration in a larger pla-nar array. Figure 18 shows the measured pattern of a 10 elementMSDRA, designed at C band. The array achieved a peak gain of15.2 dBi, with a 3 dB gain bandwidth of 17%, and boresight cross-polarization levels on the order of 20 dB below the peak co-polari-zation levels.

Top ViewRadiating Boarr\

/BranchedMicrostripFeed

DR A Elements

Ground P

Figure 20. A low-profile active phased array ofMSDRAs.42 IEEE Antennas and Propagation Magazine, Vol. 40, No. , June 1998


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Recent Advances in Dielectric-Resonator Antenna Technology