Reference 1 - Dr. Tran Thi Huong

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Multi-Band CP Double-Semi-Rings Patch Antenna

Wojciech J Krzysztofik**Wroclaw University of Technology, Institute of Telecommunications,

Teleinformatics Acoustics, Wyspianskiego 27, 50-370 Wroclaw, Poland,[email protected]. pI

Antenna for mobile terminal of satellite navigation GPS and/or communicatione.g. IRIDIUM, GLOBALSTAR systems is proposed. The novel design presentedhere takes two semi-ring patches of different radii, fed by a single microstrip linevia a crossed slots in the ground plane, to produce an antenna with a CP axialratio < 3 dB, and different impedance bandwidth, depending on frequency ofoperation 1.76 5.4 9.2 14.9, 17.3 GHz . Antenna dimensions are determinedapproximately as for annular-ring patch of the TM mode. The antenna can also

be designed as a dual-band antenna with orthogonal linear polarisation.

Introduction

Circular microstrip antennas offer performance similar to that of rectangulargeometries, in some applications such as arrays, however, offer certainadvantages over other configurations. Experimental results have shown thatcircular disc microstrip elements may be easily modified to produce a range ofimpedances, radiation patterns, and frequencies of operation.It is well-known that an annular-ring microstrip antenna has a smaller patch sizeas compared to a circular microstrip antenna for a given frequency. In applicationto arrays, this allows the elements to be more densely situated, thereby reducingthe grating-lobe problem. Secondly, it is possible to combine the annular ring withsecond microstrip element, such as circular disc within its aperture, to form acompact multi-band antenna systems. Thirdly, the separation of the modes can becontrolled by the ratio of outer and inner radii. Finally, it has been found that, byoperating in one of the higher-order broadside modes, i.e. TM 12 , the impedancebandwidth is several times larger than is achievable in other patches ofcomparable dielectric thickness [2]. However, when the annular-ring microstripantenna with a larger inner slot size or higher substrate thickness is excited at itsmode, which respectively has lower resonant frequency and wider impedancebandwidth, the 50 Q input impedance would be difficult to obtain on the patch.Several technologies have been proposed to solve the problems and are

successfully used to produce linear and circular polarization CP [1]-[2], [4],radiations for the ring microstrip antenna using the probe or microstrip-line feed.In most methods a geometrical deformation is used to generate both symmetricand asymmetric modes to cause a CP radiation. These methods are convenient togenerate circular polarization when only one sense of polarization is needed, andcan be implemented by a single feed Fig. 1 However, when a polarizationdiversity is required, one must use two separate feed arrangements. In such case, a

978-1-4244-3647-7/09/ 25.00 2009 IEEE


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a b

Fig. 1 Geometry a and photo o the CP double-semi-rings microstrip antenna.

symmetric patch with two separate feed points and an appropriate phase switchwill be sufficient. Also, to generate circularly polarized radiations with a low axialratio, one needs an antenna with nearly symmetric radiation pattern. The patternsymmetry can be controlled by modifying the ground-plane size and thickness. Toinvestigate the quality o the circularly polarized radiation we present a fewcomputed results for semi-ring patches as in Fig. 1 fed at different locations, witha 90 phase difference.

Design Procedure CP Double Semi Rings ntenna

A circular disc operating in the dominant mode TM is the most prevalentcircular microstrip antenna configuration. The first design step is to select asuitable substrate o appropriate thickness. Bandwidth and radiation efficiencyincrease with substrate thickness, but excess thickness is undesirable theantenna is to have a low profile and be conformal. The three most commonly usedsubstrate materials are duroid r=2.32 , rexolite r=2.6 and alumina r=9.8 .For a known dielectric substrate at a specified operating frequency fr the radius ff slightly larger than the physical a, due to taking into account the fringingfields on the disc edge o the microstrip disc element is [1]:

K 1a ; = = = = = = = =

ef - h HK1 [ I n - 1.7726]

HcrK h

where K = 8.794//,. F and t is in GHz.For a given values o a and b, the inner and outer radius o annular-ring patch, and be are calculated. After solving the characteristic equation, the mn-order moderesonant frequency may be predicted accurately from:

J: = c e{kmn } = mn 2r 2H F:; 2H F:;


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where Xmn=R{kmna}is the real part o kmna , and denotes the root o thecharacteristic equation.For annular-ring microstrip antenna, the radio o outer to inner radius b/a is oftenchosen to be equal 2. In some cases this parameter can be used to control thefrequency separation o the modes excited in the antenna. For convenience some

roots n o equation (2) for different ratios o the ring outer-to-inner radius b/aare presented in table 1.Table 1: Roots o eqn. (2) for different ratios b/a.

b/a 1.5 1.6 1.7 1.8 1.9 2.0 2.61 2.15 1.83 1.59 1.4 1.25

Authors proposed a technique by which the antenna size could be reduced byhalving or quartering the patch dimensions. Quartering the patch dimensionsreduced the gain significantly, by a margin o 3dB, which is unacceptable formost applications. Conversely, a half patch has similar performance to a full-sizedpatch except for a small reduction to the bandwidth. The novel design [7]presented here takes two half-annular-ring o different radii, fed by a singlemicrostrip line via crossed-slots in the ground-plane, to obtain a CP o theantenna. The cross-slots feed excites each semi-annular-ring equally in amplitudeand phase. However, the two semi-annular-rings resonate at different frequencies.The larger ring resonates at center frequency 1.615 GHz and radiates thehorizontally polarized E-field, and the smaller - is vertically polarized at a muchhigher frequency. In this geometry it couples to the larger annular-ring and bringsabout 90 phase shift at 1.615 GHz, thus producing RHCP.Such solution can be also used to design a dual-band antenna with orthogonallinear-polarization.

Numerical Experimental Results

The numerical analysis is based on the spectral-domain integral equationtechnique with boundary conditions enforced using a Galerkin moment method,MoM, applied in the commercial computer code IE3D and on the finitedifference-time-domain method, FDTD used in FidelityTM code, o ZelandSoftware Ltd .. The implementation o these techniques are described in detail inUser s Manual o Zeland Software Ltd. [11].The IE3D code uses a full wave formulation which enables accurate predictionso the coupling, near-fields, far-fields, radiation patterns, current distributions,impedances etc. Special Green s functions for planar multilayered media are used.The formulation, and the implementation thereof, enables the analysis oarbitrarily oriented metallic surfaces and wires.A surface integral formulation for multiple dielectric/ magnetic volumes can alsobe used to model antennas on a finite substrate. The regions where currents flow Le. metallic surfaces and wires or surfaces o dielectric bodies) are discretizedinto form o wires or a small patches. The integral equation MPFIE areformulated on each wire segment and solved by means o the MoM.

The Fidelity code is based on FDTD technique - a numerical approach that usesdiscrete approximations o Maxwell s time domain equations. The Maxwells


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equations are discretized accordingly to Yee algorithm, in which the wholeobject is divided on number of cubes, called voxels having 3D dimensions 0 1A The resulting algebraic equations can be used to track the time evolution ofthe fields within a given spatial region. The derivation as well as the practicalimplementation of both algorithms are well covered in past literature and as such

will not be covered in this paper.In Fig. 2 the surface current distributions at frequency, in the band of Iridium

satellite system, are presented. To begin with, simulated results for the antennadesign are first presented in Fig. 1 in which typical results of the excited patchsurface currents distributions of TM 11 modes are shown. It should first be notedthat the loading of slots has small effects on the TM 11 mode and can make thedistribution of patch surface current density mode more uniformly distributed inthe center portion of the annular-ring patch. It is also noted that although, in thesimulation results, of the excited patch surface currents are constrained to flowaround the slot.

a

dB

3dB

6 dB

9 dB

12 dB

15dB

18 dB

21 dB

24 dB

27 dB

3 dB

33 dB

36dB 39 dB 42 dB

25

20

155N

10

10 15 20 25 30Y[cm]

b

14-ICNIRP

6 SAR Level: :1< 9 2 [W/kg]

12 = - 3d B 15 18

21 24

27

D

33 36

39

42 45 48

Fig. 2. The surface current a and SAR distribution around the semi-ring patch antenna.

The SAR map, is plotted in Fig. b at P=0.6 [W]; f= 1 621 [GHz], for estimationof the radiation hazard for the human user of hand-held mobile terminal. Fig. 3shows the measured results of the return loss and the impedance for the semi-ringpatch antenna. Results show that good matching could be obtained for manyfrequency-bands in the of range 1.7 to 17 GHz. The antenna power gain at the1 621 GHz, is close to 5 dBi, as is required for Iridium mobile station receiver.

~ ~ ) 1 4 . 9 0 0 0 7 6 5 0 0 0 G H ; I ~ 1M a r k e f _ :

~ ~ J j 1 4 9 7 6 5 G H ; e t MilIker4 _

Fig. 3. Input return loss a , and the impedance plots b o f the semi-ring antenna vs frequency.

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Reference 1 - Dr. Tran Thi Huong