Jerzy Guterman, António A. Moreira, Member, IEEE, and Custódio Peixeiro, Member, IEEE
Abstract—A novel microstrip patch antenna with a Koch pre-fractal edge and a U-shaped slot is proposed for multi-standarduse in GSM1800, UMTS, and HiperLAN2. Making use of an in-verted-F antenna (PIFA) structure an interesting size reduction isachieved. The multi-band behavior has been obtained by broad-ening the lower frequency resonance of the fractal patch to coverGSM1800 and UMTS, and inserting an U-slot dimensioned for theHiperLAN2 band. The effect of small ground plane has been takeninto account. Experimental results have validated the design pro-cedure and confirm the fulfillment of the requirements for multi-standard mobile terminal applications.
Index Terms—Fractal antennas, microstrip antennas, miniatur-ized antennas, multiple-input–multiple-output (MIMO) antennasystems.
I. INTRODUCTION
RECENT publications have shown that the fractal geom-etry concept can be applied to the design of antennas. As
a consequence of the unique properties of fractals new antennaelements with reduced size and multiband behavior can be pur-posed. Further size reduction can be obtained by combining afractal patch design with miniaturization techniques, like the useof shorting pins in a planar inverted-F structure.
In this letter, we introduce a novel compact microstrip fractalpatch. A planar inverted-F antenna (PIFA) configuration withfractal shaped edges along the resonant length is combined witha U-shaped slot. While the fractal PIFA provides a broad lowerband the slot is designed for a complementary higher band.
A. Fractal Antennas
The concept of “fractals” was introduced by Mandelbrot [1]who described objects that were too irregular to fit in traditionalgeometrical settings.
Fractal objects can be generated in an iterative fashion withthe use of collinear transformations, as illustrated in Fig. 1. Forpractical designs the number of iterations is limited, and the re-sult should be referred to as pre-fractals [2]. Due to their method
Manuscript received August 25, 2004; revised October 26, 2004. This workwas supported by the Foundation for the Development of Radiocommunicationand Multimedia Technologies, Poland, and was carried out at the Instituto Su-perior Técnico, Technical University of Lisbon, Portugal, under the frameworkof the IST Project FLOWS (IST-2001-32125), which is partially funded by theEuropean Union.
J. Guterman is with the Institute of Radioelectronics, Warsaw University ofTechnology, Warsaw 00-665, Poland (e-mail: [email protected]).
A. A. Moreira and C. Peixeiro are with the Instituto de Telecomunicações,Instituto Superior Técnico, 1049-001 Lisbon, Portugal (e-mail: [email protected]; [email protected]).
Digital Object Identifier 10.1109/LAWP.2004.840253
Fig. 1. Hilbert curve—fractal line (four initial stages of generation).
of generation, fractals possess unique features such as self-sim-ilarity and space filling properties.
Self-similar objects look “roughly” the same at any scale [3].Thus, in an antenna with fractal shape similar surface currentdistributions are obtained for different frequencies, i.e., multi-band behavior is provided [4].
The space filling property, when applied to an antenna ele-ment, leads to an increase of the electrical length. The moreconvoluted and longer surface currents result in lowering theantenna resonant frequency for a given overall extension of theresonator. Therefore, given a desired resonance frequency, thephysical size of the whole structure can be reduced. The fractalminiaturization technique has already been applied to wires[5], loops, and microstrip patch antennas [6]. Comprehensiveoverview of fractal antenna engineering research has beenpresented in [7].
B. Mobile Terminal Antennas
The recent explosion in the mobile telecommunicationsmarket has forced more strict requirements for mobile terminalantennas. The terminals for 3G mobile communication systemsmust be 2G compatible. The growing importance of wirelesslocal area networks (WLANs) has already demanded specificattention. Therefore, new terminal antennas should support amultisystem operation.
Another important requirement for mobile terminal antennasstems from the terminal size, being the handset the more re-stringing one. Although our design is not restricted to handsetapplications we have considered a handset common size as areference for the overall antenna dimensions.
When using microstrip antennas the patch element sizeshould be reduced to a minimum, while the ground planeshould approximate the handset shape. The design of an an-tenna fulfilling all those requirements calls for novel methods.
The novel microstrip fractal patch antenna design, describedin this letter, has been developed as part of a feasibility studyfor antenna solutions in the frame of project flexible conver-gence of wireless standards and services, IST-2001-32 125(FLOWS) of the European Union IST program. FLOWS aimsat multiple standard operation, implying the use of terminals
352 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 3, 2004
Fig. 2. Comparison of a rectangular patch and the Koch fractal patch.
with multi-band capability. The selected standards in FLOWSare GSM1800, UMTS, and HiperLAN2.
II. MINIATURIZATION EFFECTS
Miniaturization of microstrip antennas can be achieved dueto the application of convoluted fractal curves along the longi-tudinal electrical edges of the patch [6].
As an example of miniaturization we show in Fig. 2 a com-parison of a rectangular patch with a four-iteration Koch curvepatch designed for the same resonant frequency. The numericalsimulations were performed with Ensemble Ansoft [8]. In thecase shown, the linear dimensions of the fractal patch have beenreduced by 23.8% [9] as compared with a simple rectangularpatch. As expected, a substantial decrease of the (impedance)bandwidth also occurs.
This fractal miniaturization technique can be valuable in thedesign of very small mobile terminal antennas.
Miniaturization effects attained with fractal patch antennascan be combined with other techniques. Aiming at a further re-duction in size, shorting pins can be incorporated as in PIFAstructures roughly halving its size.
III. MULTIBAND FRACTAL PIFA
In this section, we describe a novel antenna which was de-signed for multiband purposes covering GSM1800, UMTS andHiperLAN2 [10]. The higher frequency band was obtained withthe incorporation of a U-shaped slot.
A. Antenna Geometry
The antenna has been implemented in microstrip planar tech-nology. A finite ground plane, of dimensions 100 mm 45 mm,has been chosen to represent a common handset size. The patchelement has been printed on a Duroid 5880 substrate (
, ). To meet the bandwidth requirements a10-mm-thick air gap has been introduced between the substrateand the ground plane. The antenna structure is shown in Fig. 3.
The patch element has been designed based on a simple rect-angular shape where the longitudinal resonant edges have beensubstituted by the fourth iteration of the Koch fractal curve. Toobtain an additional miniaturization effect, the Koch-edge patchhas been implemented in a PIFA configuration [11]. By doingso, the patch element length has been reduced by 62% in com-parison with the simple rectangular patch [9].
The desired upper band (HiperLAN2) antenna behavior hasbeen achieved using a U-shaped slot [12]. The U-shape slothas been cut inside the patch element around the feeding point(Fig. 3).
For the lower frequency bands (GSM1800 and UMTS), thedimensions of the slot are much smaller than the wavelength, so
Fig. 3. Structure of the fractal PIFA with a U-slot (dimensions in millimeters).
Fig. 4. Current distribution on the patch surface (simulation results). a) f =
2:035 GHz. b) f = 5:25 GHz.
the cut does not influence the antenna behavior. In this case, theactive region covers the entire patch shape [Fig. 4(a)]. For theHiperLAN2 band the effectively excited area is limited to theinterior of the U-shaped slot [Fig. 4(b)]. In the upper frequencyband, the antenna works as a simple rectangular patch. It is notnecessary to miniaturize the inner rectangular patch because itfits inside the fractal element. The antenna without U-slot hasbeen analyzed in [13].
The Ensemble Ansoft [8] software tool has been used in thedesign procedure.
B. Antenna Prototype
An antenna prototype has been fabricated with the use of pho-tolithographic printing circuit technology.
The antenna input reflection coefficient (referred to 50 ) hasbeen measured with a network analyzer. The comparison oftheoretical and experimental results is shown in Fig. 5. The an-tenna is matched in the frequency ranges from1.74 to 2.20 GHz and from 5.07 to 5.4 GHz. Therefore, the re-quired GSM1800, UMTS, and HiperLAN2 bands have almostbeen covered. The lower antenna band has suffered a shift of30 MHz, however, the patch can be easily resized to compen-sate for this small shift.
GUTERMAN et al.: MICROSTRIP FRACTAL ANTENNAS FOR MULTISTANDARD TERMINALS 353
Fig. 5. Comparison of computed and experimental S results.
The far-field radiation patterns have been measured in an ane-choic chamber. The E and H planes experimental pattern re-sults, at UMTS and HiperLAN2 central frequencies, are shownin Fig. 6.
As expected, due to the small ground plane plate, the an-tenna radiates in both front (angles from to ) andback hemispheres (angles from to and from 90 to180 ). In the lower frequency bands the antenna exhibits low di-rectivity (maximum gain is 3 dBi). In the high-frequency band,the antenna demonstrates moderate small directivity (maximumgain is 6 dBi). Especially in the lower band, but also in the higherband if the two field components are considered, the radiationpattern is almost omnidirectional, which is adequate for mobileterminal applications.
The experimental results have validated the design procedure.Therefore, the proposed fractal PIFA with a U-shaped slot seemsto be an interesting configuration to be used in small mobileterminals.
Fig. 7. Photo of the two-element fractal MIMO antenna prototype.
Furthermore, due to the very small size of the patch element,it turns out that it can be successfully used in compact multi-element antenna arrangements.
IV. TWO-ELEMENT ANTENNA FOR MIMO APPLICATIONS
The use of multiple-input–multiple-output (MIMO) antennatechniques in mobile terminal has become one of the centralpoints of research contributing to the goal of augmenting the ca-pacity of wireless systems. Multiple element antenna systemsare thus an issue of research and these may include diversity,smart antennas, and MIMO or space-division multiplex antennasystems [14]. The performance aspects of MIMO systems havebeen reported [15], showing that a significant throughput in-crease can be obtained even in an antenna configura-tion, when compared to single-input–single-output (SISO) sys-tems. Thus, a two-element antenna for a small handset is worthstudying.
Most requirements for antennas of MIMO handsets are equalto those described in Section I-B. However, MIMO techniquescreate an additional difficulty of housing multiple multibandantennas with low level mutual coupling between elements insmall terminals.
The application of fractal shapes in antenna field, besidesminiaturization, allows reducing mutual coupling in mul-tielement antenna arrangements [6], [7]. In this section, atwo-element multiband antenna arrangement [16] is reported.The structure has been designed to be compatible with a smallhandset size terminal.
A. Antenna Arrangement
A two element arrangement based on the antenna structuredescribed in the previous section has been developed. Thedielectric layers remain unchanged, that is, a 1.57-mm-thickDuroid 5880 substrate is used on top of a 10-mm-thick airgap. A ground plane of dimensions 100 45 mm is again takenas a representative handset size. As shown in Fig. 7, the twoidentical fractal PIFA elements have been disposed to ensurephysical symmetry of the antenna structure.
354 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 3, 2004
Fig. 8. Experimental S-matrix (amplitude) of the two-element antenna.
B. Antenna Prototype
An antenna prototype has been fabricated and tested (photoin Fig. 7).
The measured S-matrix is shown in Fig. 8. As the structureis intentionally symmetrical, the and characteristicsshould be coincident.
The antenna elements are matched inthe frequency ranges from 1.82 to 2.19 GHz and from 4.95to 5.31 GHz. Thus, the UMTS and HiperLAN2 bands havebeen fully covered. Although the GSM1800 band has notbeen entirely covered with this criterion, the structure caneasily be redesigned for all bands of interest. The measuredmutual coupling between elements remains below inGSM1800, in UMTS and in HiperLAN2,which is acceptable for MIMO applications.
The far-field radiation patterns have been measured at severalfrequencies of the bands of interest [16]. The test of each ele-ment has been done with one patch excited and the other loadedwith 50 . The results showed very small changes when com-pared to the single element antenna described in the previoussection.
In spite of the small overall size of the structure, the mu-tual coupling between elements is acceptable for MIMOapplications.
V. CONCLUSION
Microstrip fractal shaped antennas developed to be used insmall mobile terminals with multistandard operation have beenpresented. A novel antenna design, based on a combination of afractal shaped edged patch with a PIFA structure and a U-shapedslot, has been proposed.
The proposed antenna meets the requirements for mobile ter-minals: small overall size (100 45 mm), small size of patch
element (linearly 38% of a simple rectangular patch) and radi-ation pattern without strong directive properties. The structurehas been matched to 50 in the frequencyranges covering GSM1800, UMTS, and HiperLAN2 bands. Anantenna prototype has been fabricated and tested. The experi-mental results validate the antenna design procedure.
An antenna arrangement of two Koch fractal patch PIFA el-ements has been studied. The miniaturized fractal patches havebeen designed to fit into a small handset. In spite of the elements’close distance, the mutual coupling level seems to be acceptablefor MIMO system applications.
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