Proceedings of Asia-Pacific Microwave Conference 2007

A Novel Radiation Enhanced Split-Ring TriplateAntenna

Anne D. Abeygunasekera, Charles E. FreeAdvanced Technology Institute

University of SurreyGuildford, United Kingdom

Abstract- A novel circular polarized antenna structure capableof high radiation efficiency and suitable for use in multilayermicrowave circuits is presented. The antenna uses a lowpermittivity dielectric layer on top of a high permittivitysubstrate to enhance radiation. The novel split ring designensures compensation for the progressive loss in the travellingwave around the ring. Data are provided for a prototype antennaworking at 20GHz.

I R~~~~~adiating Fatch_t Low Perrmittivity

Sub strta e

A Hiqh PermittivitySubstrate

Figure 1. Configuration of the Layers

Keywords-Microwave, Circular Polarization, Travelling wave,Multilayer

I. INTRODUCTION

The expanding applications for microwave communicationsystems have generated a need for compact, high-performancemicrowave circuits with integrated antennas. In this paper, wepropose a novel antenna structure capable of high radiationefficiency suitable for use in multilayer microwave circuits.The traveling wave feed system offers high quality circularpolarization which is a key requirement for many existing anddeveloping mobile communication systems. The proposeddesign can be fabricated using thick film technology, thussatisfying the demand for high volume microwave circuits atlow cost.

The proposed antenna uses a low permittivity dielectriclayer printed on a high permittivity substrate, as shown in fig.1, to improve the fringing fields at the edges of the radiationpatches. The work is an extension of the concept using stripsof low dielectric constant material underneath the radiationedges of a patch to enhance radiation efficiency [1]. In theproposed design the low dielectric constant material is placedon top of the high permittivity substrate, thus forming amultilayer structure. The antenna comprises three substratelayers with a patch array fed by two half circular microstripchannels, compensating for the progressive loss in signal. Atheoretical investigation based on electromagnetic simulationwas performed to validate the new radiation efficiencyenhanced antenna at 20GHz.

II. ANTENNA THEORY AND STRUCTURE

The structure of the radiation enhanced CP antenna isillustrated in fig. 2. The antenna is composed of 8 planarrectangular radiating patches. Each patch is linearly polarizedand has dimensions Lp and Wp, where Lp is the resonant

Input Port 1 4 Output Port 3r~~~~~~~~Z ii iY

Microstrip Feed Line -=sicrostrip Feed LineBottom Layer Middle Layer

I. ..

... 1IL i g<Distance- PatchI

.... ..centreto Patch

n cent re - ?8Ws2 !L 4

..JLp

Radiating |Patche s - To pN

Layer 'C

Output IPor: 2 ti g Via- Inpujtforthe second

;lel half of the rincg

Figure 2. Configuration of the Antenna

1-4244-0749-4/07/$20.00 w2007 IEEE.

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dimension and Wp is the width of the patch, defining theradiating edges. The patches are excited with a progressivephase lag of 450 because there is an annular spacing of )/8between the patches, thus leading to circular polarization. Thebasic design concept of the antenna was originally establishedby K. Lum and C. Free[2].

The antenna feed is through a circular 50Q microstripchannel, of width Ws, and Ws2, which runs on top of the firstsubstrate and on top of the second substrate respectively. Thefeed is divided into two half rings splitting the power from thefeed to the patches. The power in from the feed on the bottomlayer is fed through to the feed in the middle layer through avia. Each patch was matched to the respective microstrip line.A portion of the signal travelling in the bottom layer feed linewill successively excite each patch. The second half of thepatch array is then successively excited through the middlelayer feed line, therefore compensating for the progressive lossof signal in the feed.

Simulated data was obtained for a test antenna made onAlumina ADS-96R with the following parameters: substratethickness h = 0.254 mm, substrate dielectric constant Fr = 9.8.The key antenna dimensions were:Length of the patch Wp = 6.9mmWidth of the patch Lp = 2.9mmWidth of the microstrip line Ws, = 0.248mm Ws2 = 0.578mmDiameter of the circular microstrip line channel = 16.4mmCircuit dimensions = 50.8mm x 50.8mm

As illustrated in fig. 3 a low dielectric constant material wasplaced beneath the patches to improve the efficiency of theradiation. One of the key parameters affecting radiationefficiency is the dielectric constant. The lower the dielectricconstant, the less the concentration of energy in the substrate.Therefore more radiation occurs. The high dielectric constantsubstrate material reduces the radiation. This is because themost of the EM field is then concentrated in the dielectricbetween the conductive patch and the ground plane. The lowdielectric constant material used was KQ150 (Heraeus Inc)with a dielectric constant of 3.9. Dielectric layer thickness wasfabricated to be 50pm. The presence of the low dielectricmaterial increases the intensity of the electric field distributionat the radiating edges of the patch.

The coupling between the two microstrip feed lines wasminimized by a dielectric layer of 100um. The same lowdielectric constant KQ150 (Heraeus Inc) was used to avoidcoupling between the microstrip feed lines whilst contributingto the enhancement of radiation.

RADIATING PATCHES

SUBSTRATE LAYER 3 -KQ150 (50um)

SUBSTRATE LAYER 2 -

KQ150(100um)MICROSTRIP FEED

Ws2

VIA-CONNECTINGWs2TOWsl

MICROSTRIP FEED Wsl

SUBSTRATE LAYER 1 -ALUMINA (254um)

GROUND PLANE

Figure 3. Configuration of the layers

III. RESULTS-ANTENNA ON CERAMIC

The structure was modeled using an electromagneticsimulator Momentum® (AGILENTAdvanced Design System).

0-

freq 26 30GHz20- dB(S(3,l)) -1074

m2-20 -| 1--lfreq=20 30GHz

dB(S(2,j)) 115086230

35 - dB.S( l)-f-r35050GH

-40-19.5 196 197 190 19.9 20O 201 202 202 204 20

freq, GHz

Figure 4. S-Parameters of the Antenna Designm4freq=20 28GHzmag(x)-0 900

0.80-

0o75-

070-

o 65-

0 60-

0.55-

0.50-

040-

0.35-

freq 20 23GHzmag(x) 0 909

M4rrq

mSifreq 20 35r3Hz08.0

195 196 197 198 199 200 201 202 203 204 205

1x-l-Sl J)onbnj(S(1 1))-S(2l)*conj(S(2,l1))-S(3l)*obj(S(3, 1))

Figure 5. Maximum Power Radiation of the Antenna at the Frequency ofOperation

U ~u OI IIIJ,

D: C r-

END uMmE

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The optimum frequency of operation was 20.3GHz. This isslightly different from the designed value of 20GHz because nomatching was included to account for the reactance presentedby the patches to the feed line. As seen from fig. 4, the returnloss of the antenna is -35.58dB at 20.3GHz. In this antennastructure the traveling wave can reflect, radiate or dissipatepower in the substrate. By determining S21 and S31, we canfind the amount of power dissipated in the antenna. If weassume that the amount of signal dissipated in the substrate isnegligible, then S21 and S31 indicate the percentage of powerthat is radiated. The reflection is negligible, as shown from thehigh return loss in fig 4. Therefore most of the signal travelingthrough the feed has been radiated. S21 and S31 were found tobe -15.86dB and -11.07dB, respectively, for the above design.Thus, indicating radiation loss between the input port and therespective output ports.

Furthermore, from fig 5. the radiated power from theantenna design was calculated to be 90% of the input power atthe optimum frequency of operation, 20.3GHz. The bandwidthof the antenna was 120MHz. Therefore the design isconsidered to be useful for practical narrowband applications.

IV. ANTENNA ON POLYMER

This type of antenna structure was also investigated usingpolymer material for all three layers. Simulated data wasobtained for a test antenna with the following parameters:substrate thickness for each layer h = 0.127mm and substratedielectric constant Fr = 2.5The key antenna dimensions were:Length of the patch Wp = 4.6mmWidth of the patch Lp = 4.1mmWidth of the microstrip line Ws, = 0.35mm Ws2 = 0.744mmDiameter of the circular microstrip line channel = 29.6mmCircuit dimensions = 35mm x 50mm

0S4J-

D_J-a_m

_3-

0.J-

mr4

m5 7 freq2DA0GHzfrecq 24G.mag()-0701U m,4

m6freq-21. 4G,Hz

m mag(x-.83.... ... ...... ....

19.SU 19M 19-i; 19-95 2Sal 2D51 2J1E1 21AS 2J2- 20.5 2J1M 2J- 20AU20AS2J. U2JSM 2iJM

Figure 7. Maximum Power Radiation of the Antenna at the Frequency ofOperation

The optimum frequency of operation was 20.44GHz. Thisis slightly different from the designed value of 20GHz becauseno matching was included to account for the reactancepresented by the patches to the feed line. As seen from Fig. 6,the return loss of the antenna is -10.92dB at 20.44GHz. Thereflection is negligible, as shown from the high return loss infig 6. Therefore most of the signal traveling through the feedhas been radiated. S21 and S31 are calculated to be -18.02dBand -13.895dB respectively for the above design. Thus,indicating radiation loss between the input port and therespective output ports.

Furthermore, using the data from fig 7. the radiated powerfrom the antenna design was calculated to be 87% of the inputpower at the optimum frequency of operation, 20.4GHz. Thebandwidth of the antenna is 300MHz.

V. RESULTS -ANTENNA ON POLYMER

The structure was modeled using an electromagneticsimulator Momentum® (AGILENTAdvanced Design System).

d 1 I 0I920ftl I I I I ..i .

15 11 1

<1 l: I

m~~~~~~~~~2

freqi20.44GHzdB(S(31)) 13.895

-. rn3freq 20A44GHzi | i . g ~~~~~dB(6(2,1;))-1&0209

3- X 3. l~~I

(a) Middle Feed Line (b) Bottom Feed Line

i9M AS SS5 199 S19-6 21 MM23 :13.11D 2.1AS 2 25 20 MHX

freq. GHz

Figure 6. S-Parameters of the Antenna Design

Wu.

g ...

OJE -_ .....

M.AD 21145 1MS 21ss 23S

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for the progressive loss of the signal in traveling around thering. This form of antenna structure is particularly suitable forinclusion in highly integrated multilayer transceivers.

REFERENCES

(c) Radiating Patches

Figure 8. Antenna Design

VI. CONCLUSION

The concept of a novel circular polarized patch antenna hasbeen established. In this paper an extension to the basic antennadesign was investigated, and was shown to have beneficialeffects. The benefits resulted from radiation enhancement dueto the low dielectric constant material located between thepatches and the high permittivity supporting substrate.Furthermore, the novel split-ring design ensures compensation

[1] K.M. Lum; T. Tick; C. Free; H. Jantunen, "Design and MeasurementData for a Microwave Dual-CP Antenna Using a New Travelling-WaveFeed Concept", IEEE Trans. On Microwave Theory and Techniques,Vol. 54, no. 6, pp. 2880-2886, June 2006.

[2] K. M. Lum, C. Laohapensaeng, and C. E. Free, "A novel traveling-wavefeed technique for circularly polarized planar antennas," IEEEMicrowave Wireless Components Letters, vol. 15, no. 3, pp. 180-182,March 2005

[3] K. P. Tang, C. E. Free, and G. F. Goldspink, "A novel slotline-fedmicrostrin patch antenna", IEEE Antennas and Propagation Conf:, vol.1, no. 407, pp. 235-238, April 1995.

[4] J. Huang, "A technique for an array to generate circular polarizationwith linearly polarized elements", IEEE Trans. Antennas andPropagation., vol. 34, no. 9, pp. 1113-1124, September 1986.

[5] E. A. Mariani, C. P. Heinzman, J. P. Agrios, and S. B. Cohn, "Slot linecharacteristic", IEEE Trans. Microwave Theory Tech., vol. 17, no 12,pp. 1091-1096, December 1969.

[6] J. B. Knorr, "Slot-line transitions", IEEE Trans. Microwave TheoryTech., vol. 22, no. 5, pp. 548-554, May 1974.

[7] K. M. Lum, and C. E. Free, A Novel Traveling-Wave feed Techniquefor Circularly Polarized Planar Microstrip antennas, IEEE Antennas andPropagation Conf., vol. 2A, pp.250 - 253, July 2005.

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