High Gain Cylindrical DRA Based on Higher Order HEM133 Mode – Kopie

7/23/2019 High Gain Cylindrical DRA Based on Higher Order HEM133 Mode – Kopie

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ENHANCED GAIN CYLINDRICAL DIELECTRIC RESONATOR ANTENNA

BASED ON COMBINATION OF HIGHER ORDER MODES

Michal Mrnka and ZbyněkRaida

Department of Radio Electronics, Brno University of Technology

Abstract:

I INTROD!CTION

Dielectric resonator antennas, frstly proposed by Long et al. [1], have become

very popular radiating elements in microwave and millimeter-wave reuencybands. !o their main advantages belong high radiation e"ciency, compact si#e,

ease o e$citation and relatively large impedance bandwidth when compared to

other resonant antenna elements, to list a ew. %ylindrically shaped resonator,

operating with the low order hybrid electromagnetic mode &'( 11) placed above a

su"ciently large ground plane is probably the most reuently used D*+

confguration [1-]. !his mode generates broadside radiation pattern with linear

polari#ation and gain about di. /everal approaches have been suggested to

increase the gain o the D*+s. +rraying o single element D*+s [0] is probably the

most versatile method in which the gain value can be directly controlled by

number o elements in the array. evertheless, increased si#e, comple$ity andcosts o the resultant antenna are the main disadvantages.

+ltering o a single element D*+ can be used in cases, where medium

gains up to around 12 di are su"cient. 3n general, two tactics to increase the

gain o the single element D*+ e$ist. 4irstly, additional structures are placed in

near vicinity o the resonator operating in the low order mode. !hese can

represent e.g. surace mounted short horns [5], '6 structures [7] or

superstrates [8]. 9articularly, the rectangular hybrid D*+ antenna described in

[8] provided pea: gain o 1. di and gain above 11 di in complete 3/( band

at 52 6&#. ;n the other hand, the common disadvantages o the antennas based

on the frst approach are again higher comple$ity and increased si#e.

/econd strategy is to utili#e higher order radiating modes in single

dielectric resonator. !his approach has been already adopted in both rectangular

and cylindrical D*+. 9etosa and !hira:oune [8] showed the D*+ based on higher

order !')1< and !')1 modes in rectangular resonator can achieve gains o 7.0 di

and12.0 di, respectively. !he structure operating in !')1 mode [8] reuired

ma$imum dimension o the resonator o about 1.1 λ2 when build rom dielectric

material with relative permittivity =r > 12, where λ2 is the ree space wavelength.

6uha et al. [12-10] managed to e$cite higher order &'(10) mode in cylindrical

resonator by introducing an air-flled cavity in the ground plane below the

resonator. !his way pea: gain o about 12 di was achieved but only in relatively

narrow impedance bandwidth.



3n presented letter, we use e$citation o higher order hybrid

electromagnetic &'(1<< mode in single cylindrical dielectric resonator with partial

e$citation o the nearby &'(10< mode in order to accomplish considerable gain

enhancement. !hese modes do not reuire any special eeding schemes nor the

ground plane modifcations and can be e$cited similarly to the well-:nown &'( 11)

mode. !he aperture coupling eed [] is selected and optimi#ed in order to avoide$citation o unwanted !'?!( modes within the resonator. 9arametric study in

%/! (icrowave /tudio [1<] was conducted to see the behavior and to determine

the limits o the proposed D*+. !he antenna was designed to operate over

unlicensed 3/( .7 6&# band @.80 6&# - .78 6&#A, but the concept might be

more suitable at higher reuencies, where the resonatorBs si#e can be less

crucial.

!he letter is organi#ed as ollows. /ection 33 brieCy describes the antenna

concept and design process. !he '-feld distributions within and in close vicinity

o the resonator are discussed. 9arametric study, fnal dimensions and simulation

results are given in /ection 333. (anuacturing o the prototype together with

e$perimental results are summed up in /ection 3. 4inally, the letter is concluded

by short discussion in /ection .

II ANTENNA CONCE"T

!he antenna is composed o a single cylindrical dielectric resonator placed above

the ground plane o circular ootprint. !he resonator is e$cited through

rectangular slot in the ground plane o a microstrip line according 4ig 1. +perture

coupling eeding mechanism was selected in order to minimi#e e$citation o

unwanted lower-order modes in the structure. *elative permittivity o the

resonator in all simulations was 5.1. !his value was simply selected due to

availability o material with given er. /ubstrate +rlon 0 with relative

permittivity <.<7 was used in the eeding structure design.

3nitial dimensions or the resonator @height h and diameter d according to the

4igure <A were ound by magnetic wall method using %/! 'igenmode solver. +ll o

the walls were considered perect magnetic conductors and the resonatorBs

dimensions were obtained so that the resonant reuency o the both target

modes &'(1<< &'(10< lied close to the desired reuency .7 6&#. /ince the

method does not ta:e into account radiation losses o the resonator, considerablylarge inaccuracy is a result. evertheless, the method provided reasonable initial

appro$imation o the dimensions that needed to be tuned and optimi#ed

aterwards. 4ull-wave transient solver in %/! (icrowave /tudio was used or this

purpose.



Fig.1. Geometry of an aperture fed cylindrical DRA

4igure 0 depicts simulated electric-feld distribution o the mode combination

within the resonator. !he '-feld oscillates predominantly in the $-a$is and thus

the radiation is linearly polari#ed in the broadside direction @direction along #-

a$isA. 4igures 0a and 0b show cross sectional views o the '-feld distribution in

two diEerent planes o the resonator according to the 4ig 0c. 9resence o the

desired modes is evident in these cross sections. *elatively high gain radiator

with radiation e"ciency above F2 G can be obtained by careul optimi#ation o

the resonatorBs si#e and the ground plane diameter.

Fig. 2 E-eld distribution in the cross section A a! corresponds to the "E#1$$%in cross section & b! ' mode "E#12$ and the E-eld conguration on the top

(all of the DRA c!

+perture coupling e$citation scheme was selected to e$cite both o the desired

modes in the D*+. !his solution was preerred, since the desired modes are o



higher order and e.g. a probe eed would give rise to e$citation o unwanted

!'?!( modes, deteriorating the near feld distribution. !he aperture was ed by a

2Hmicrostrip line placed on the other side o the ground plane according to the

4ig. <. y placing the aperture symmetrically below the D*+ and tuning its

dimensions it was possible to e$cite both o the desired modes with single eed.

%orrect values o parameters l, w and s had to be oundI initial values werecalculated using general guidelines []J

l=0.4 λ

0

√ εe

,(1)

w=

0.2 l , (2)

s= λg

4,

(3)

where ε

e

=

εr +ε

s

2 Iε

r

andε

s are the relative permittivities o the D*+ and

substrate, respectively and λg is the guided wavelength in the substrate. !he

length in @<A is selected, so that the hal wave resonance o the slot is avoided.

evertheless, in our case it was necessary to wor: with considerably longer slot,

or appropriate mode e$citationI hence the bac: lobe radiation was increased due

to the partial resonance o the slot. &owever, this could be easily solved by using

stripline instead o microstrip line. !he optimum slot dimensions were ound to be

l > 18.8<mm, w ><.Fmm with stub length s > 12.Fmm.



Fig.$ Detail of the antenna geometry

III "ARAMETRIC ST!DY

!he inCuence o two main design parameters o the proposed D*+ on antennaKs

perormance was studied. amely its diameter d and the diameter o the ground

plane a were varied and the resonant reuency, impedance bandwidth and

ma$imum gain were observed. Due to the restrictions o the antennaKsabrication method, its height h could be only an integral multiple o 1.8 mm,

which was the height o the +rlon 522 substrate used or manuacturing @more

details in ollowing sectionsA. !he optimum height h was ound out to be 00.2

mm corresponding to 1 layers o the mentioned substrate. !he desired resonant

reuency was .7 6&# with band o interest covering reuency range .80-

.78 6&# corresponding to relative bandwidth o 0.5 G. !hroughout the

parametric study, the dimensions o the eeding structure @i.e. slot width w, its

length l and stub length lA were :ept constant as well as the height h o the

resonator.

!he resonant reuency o the D*+ and its impedance bandwidth were

determined mostly by the dimensions o the resonator whereas its gain was in

addition uite strongly inCuenced by the diameter o the ground plane. 4igure

shows the reuency response o the reCection coe"cient or several d values

@reerence impedance 2 HA with ground plane si#e a>1<0. mm.



Fig. ) *+11* ,s. freuency for se,eral diameters of the resonator

4ollowing 4igure depicts the dependency o reali#ed gain on reuency or

several diameters o the resonator as well as or several ground plane si#es @4ig.

5A. 3t was ound out that the gain was increasing with increasing the ground plane

si#e only to a certain pointI e$tending its si#e beyond this value did not bring anyurther gain improvement. &eight o the resonator h was f$ed to the value 00.2

mm in all o the plots.

Fig. Reali/ed pea0 gain in the broadside direction ,s. freuency for se,eraldiameters of the resonator

Fig. Reali/ed pea0 gain in the broadside direction ,s. freuency (ith groundplane diameter as a parameter



I# "ROTOTY"E AND RES!LTS

!he antenna was built by stac:ing up layers o completely etched +rlon 522

substrate with er > 5.1, loss tangent tand > 2.221 and thic:ness 1.8 mm.

!he layers were held together by double sided duct tape o 5 um thic:ness and

relative permittivity o appro$imately <. !he photograph o the proposedantennaKs fnal prototype is given on 4ig. 8. !he resonant reuency had to be

slightly tuned by altering resonatorKs diameter in order to compensate the eEect

o 1 layers o duct tape in between the dielectric layers. 4inal dimensions o the

resonator were h>00.8 mm corresponding to 1 layers o the +rlon 522

substrate interleaved with 1 layers o the duct tape. Diameter o the resonator

was .1 mm and diameter o the ground plane a>1<0.2 mm.

Fig. . #anufactured circular re3ector (ith a slot and the dielectric resonator

%omparison between simulated and measured magnitude o the reCection

coe"cient is depicted in 4igure 7I reasonable agreement can be observed./imulations were conducted in transient solver o %/! (icrowave /tudio and

measurement was done using a vector networ: analy#er.

Fig. 4 +imulation ,s. e5perimental measurement results for +11

e$t, radiation patterns and gain o the designed antenna were measured

in an anechoic chamber. 4irst, the gainBs reuency response was measured in

the broadside directionI the ma$imum gain 11.F di was obtained at the

reuency .70 6&#. *adiation patterns in two orthogonal principal planes @ xz , yz

in 4ig. 1A were measured at the reuency o ma$imum gain .70 6&# or co- and



cross-polari#ation components @4ig.FAI co-polari#ation component corresponded

to the x a$is and the cross-polari#ation to the y a$is.

!he measured data complied reasonably well with the simulation results.

(oreover, it should be mentioned that the bac: lobe present in the simulations

was not measured accurately. 9robable reason was the nature o the anechoic

chamber in which the antenna was measured, i.e. the line o sight between the

illuminator and the +M! was obstructed by antenna scanner or corresponding

angles.

Fig. 6 +imulated and measured radiation patterns at .42 G"/% E-planecorresponds to xz plane and "-plane to yz plane

Fig. 17 Freuency response of the gain8 simulation ,s. measurement

# CONCL!SIONS

ew concept o dielectric resonator antenna based on combination o higher

order modes in simple cylindrical resonator was presented and e$perimentallyverifed by measurement. Directive radiation pattern with high gain o 11.F di

was measured and impedance bandwidth was su"cient to cover the whole .7

6&# 3/( band. *elatively high side lobe levels in the '-plane @8 d below the gain

in the broadside directionA could be considered the main disadvantage.

!he implementation o the antenna might be more convenient at higher

reuencies, where the dimensions are less crucial. /ince the structure operates

with higher order modes, the electrical si#e is increased when compared to the

operation with low order modes. 4urther miniaturi#ation might be possible using

higher permittivity materials, which was not attempted in this letter.



#I AC$NO%LEDGEMENT

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High Gain Cylindrical DRA Based on Higher Order HEM133 Mode – Kopie

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