A Circular Polarization Alterable Square MPA withWide Frequency Tuning

Biswajit DwivedyDepartment of Electronics and Communication Engg.

National Institute of Technology Rourkela, IndiaEmail: biswajit.dwivedy.in@ieee.org

Santanu Kumar BeheraDepartment of Electronics and Communication Engg.

National Institute of Technology Rourkela, IndiaEmail: skbehera@ieee.org

Abstract—A wide-range frequency tunable (2.01 GHz- 2.41GHz) square patch antenna with simultaneous CP alterationability is designed and its performance is analyzed. Frequencytuning is achieved by varying the capacitance of four varactorsconnected between the central portion and four additionalsegments of the main patch. Circular polarization at differentbands are achieved by using a wideband 90 phase shifter anda DPDT switch is used to vary the sense of circular polarizationrotation. The antenna shows very good axial ratio value at allfrequencies with a maximum gain of 2.2 dBc.

Keywords—frequency reconfigurable; circular polarization; var-actor; axial ratio

I. INTRODUCTION

In RF communication major challenges which are becomingworse day by day are spectrum congestion problem arising dueto uninterrupted increase in modern features along with usersand signal quality degradation by multipath propagation basedfading. The earlier problem is handled by frequency tunableantennas using MEMS, p-i-n diodes, RF switches and varac-tors etc [1]. To withstand the later problem, polarization agileantennas are considered, as they can provide better weatherpenetration and enhance information content by their two setsof co-polar and cross-polar data [2]-[3]. So intense importanceis given on development of such antennas which are lesscomplex, compact, efficient and with ability to reconfigureboth frequency and polarization to mitigate above discussedproblems [4]-[5]. In this article, design of a microstrip linefed square patch antenna having both frequency and circularpolarization agile characteristic within 2.01 GHz - 2.41 GHzis discussed and its performance is analyzed.

II. DESIGN AND FUNCTIONAL DESCRIPTION

A. Design Procedure And Frequency Reconfigurability

The design of the proposed antenna starts with a simplequarter wave length transformer matched square patch antennadesigned on 62 mil FR-4 substrate at the centre frequency of2.4 GHz following standard design formulae [6]. The antennaradiates in basic TM10 fundamental linear polarized mode. Toachieve frequency tuning, an annular square shape portion withsome additional lengths on four sides are carefully removed

TABLE I. DIMENSIONAL DETAILS OF THE ANTENNA

Para. Ls Lz Wz Lx Wf g L(λ/4) W(λ/4)

Val.(mm) 60 11.4 1 25 3 1 11.3 0.9

Fig. 1. Layout of the proposed antenna

from the main patch for connection of four varactor diodes.Specific varactor (SMV-2025, Skywork solutions) is selectedfor tuning purpose as it has low series resistance (0.8 Ω) andhigh range of capacitance variation (1.15 pF-8.81 pF). Dueto the simultaneous variation of capacitances of four varactorsoverall electrical length of the antenna is varied causing changein resonant frequency of the antenna. All dimensional detailsof the antenna layout shown in Fig. 1 are presented in Table.I. From Fig. 2, it is revealed that the resonant frequency of theantenna varies between 2.41 GHz to 2.01 GHz for variationof the capacitances from 1.15 pF to 8.81 pF.

B. CP Alteration with Frequency Tuning

To achieve circular polarization the main patch is fedorthogonally with a phase difference of +90 for LHCP or -90 for RHCP respectively. Current distribution of the antennaat 2.16 GHz which is shown in Fig. 3 clearly illustratesRHCP for a phase difference of -90. The circular polarizationperformance of the antenna for different values of capacitances

Fig. 2. S-parameters of the antenna for different values of varactor capacitance

Fig. 3. Current distribution of the antenna at 2.16 GHz showing RHCP

Fig. 4. Axial ratio versus frequency plots for different values of capacitance

are presented in Fig. 4, where it can be clearly visualizedthat the axial ratio values are well below 3 dB at all tunablefrequency bands. To facilitate automated switching of antennaCP states at different operational frequencies, its performanceis checked using a wideband phase shift network consisting ofa Wilkinsons power divider and a Schiffman 90 phase shifter(shown in Fig. 5). S-parameter file of a practical diversityswitch (MASWSS0129) provided by the manufacturer wasalso used to examine the overall performance of the antennawith the feed network using ANSYS Designer-15. The radi-ation performance of the antenna at different frequencies andthe impact of the switching circuit on the antenna are discussedin the next section.

III. RADIATION PERFORMANCE ANALYSIS

The radiation patterns of the antenna at two different fre-quencies (2.41 GHz and 2.04 GHz) corresponding to varac-tor capacitance values 1.15 pF and 6.93 pf evaluated usingANSYS HFSS-15 are given in Fig. 6. From the plots, itis noticed that the difference between the co-pol. and crosspol. components of the antenna is more than 25 dB at boththe frequencies within a very wide beam-width of ±50.Similarly the radiation patterns at same two frequencies usingthe switching network are shown in Fig. 7. In both the cases,it can be seen that the co-pol. is about 20 dB higher thanthe cross pol. component and the realized gain of the antennareduces as the operational frequency lowers due to the decreasein effective aperture of the antenna. It has been verified that theantenna radiation pattern remains unaltered at all frequenciesfor both RHCP and LHCP but only selected cases of RHCPare shown due the limitation of paper length.

IV. CONCLUSION

The speciality of the proposed antenna is its wideband fre-quency tuning range varying from 2.01 GHz-2.41 GHz, though

Fig. 5. Antenna feed network consisting of phase shifter and DPDT swicth

(a) (b)

Fig. 6. Circular polarized radiation pattern of the antenna without using thefeed network at (a) 2.41 GHz (b) 2.04 GHz

(a) (b)

Fig. 7. RHCP radiation pattern of the antenna using the feed network at (a)2.41 GHz (b) 2.04 GHz

the basic square patch shows low bandwidth performance.Along with frequency agility the antenna can be operated inboth RHCP and LHCP mode with a maximum realized gainof 2.2 dBc. Small degradation in gain value is noticed due toreduction of effective aperture of the antenna with decrease inresonant frequency, but the loss may be compensated by usinga wideband LNA.

REFERENCES

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